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Investigation on Offshore Wind Turbine with an Innovative Hybrid Monopile Foundation: An Experimental Based Study
Abstract
The support structure for offshore wind turbines (OWTs) plays significant roles in maintaining the structural stability and reducing the initial cost. An innovative hybrid monopile foundation for OWTs is proposed. The concept has a wider adaptability by using established knowledge to solve for new problems. A series of centrifuge tests is performed to investigate the behavior of this hybrid foundation system in extreme and service conditions. OWTs with the original monopile foundation as well as the wheel-only foundations are tested for comparisons, and two clay profiles are considered. The test results show that the hybrid monopile foundation provides larger ultimate bearing capacities compared to the traditional foundations. Two analytical methods are proposed to estimate the ultimate bearing capacity of this innovative design, and the results are calibrated by the centrifuge tests. In service conditions, the hybrid monopile foundation shows stronger cyclic resistances. Influence factors of the cyclic responses are summarized. An analytical solution is put forward to estimate the accumulated lateral displacement of the hybrid monopile foundation. A degradation factor is suggested based on the results of the centrifuge tests. The study aims to enrich the understanding of the innovative foundation concept and to provide design references for practical applications.
... Lebane, et al., [8] continued investigation of the hybrid foundation system behaviour, concluding that the moment transferred to the mono-pile was reduced by the footing addition at the soil surface, and larger ultimate lateral resistance was obtained. In the hybrid mono-pile foundation system, a circular friction wheel is added at the seabed to provide additional resistance against both vertical and lateral loads in addition to overturning resistance; several studies adopted this system [12][13][14][15][16]. Wang, et al. [1], [8] investigated the behaviour of OWTs with the hybrid mono-pile foundation under service conditions and lateral cyclic loadings, by considering the wind, waves, and ice effects. ...
... It was found that soil underlying and near the bucket foundation showed a better ability to resist liquefaction, especially for saturated cases. Additionally, Wang X, et al. [15] performed a series of centrifuge tests to investigate the bearing capacity of axially loaded mono-piles in the sand, and both open-ended and close-ended piles are tested considering service conditions such as loading rate, embedment depth, and the loading history. The tests indicate that the pile bearing capacity tends to increase with the initial penetration depth, and the stress state of soil greatly influences the pile behaviour. ...
The foundations for offshore wind turbines represent the main item either for cost or installation process, and the lateral resistance of tabular piles is the main factor for its design. Therefore, studies for consistent and efficient foundations have become essential for offshore wind turbines when using traditional mono-pile foundations under practical and environmental conditions. This research discusses the increase in the lateral behavior of open tabular piles with the addition of external wings near the ground level with specific dimensions. Four wings were added to the exterior wall of the open-ended pipe pile at equal angles 90 degrees. The wings length varied from 0.25 to 0.5 of the pile diameter. Each wing length is studied with two depths of 1.25, and 2.5 pile diameter. The numerical analysis was verified with published results of centrifugal tests. The successive parametric study discussed the feasibility of the added wings. Inclusive, the resultant load direction was considered as changed between 0 to 45o with 5 degrees to the wings orientation horizontally.
... This concept is initiated by the concept of pile caps and embedded retaining wall with stabilizing platforms [18,19]. The ultimate bearing behavior of the hybrid monopile foundation has been investigated in our laboratory through a set of centrifuge tests, and three analytical methods are put forward in estimating its bearing capacity, including the "add-up method", the "equivalent moment method", and "direct calculation method" [20,21]; the response in service conditions have been studied afterward, and the accumulated lateral displacement is predicted [22]. A similar concept, piled footing, has been investigated through tests and numerical analysis; it is demonstrated that the footing effectively reduces the moment applied on the monopile [23]. ...
... For larger capacity OWTs being installed in locations with larger water depths, the traditional foundations expose limitations. The diameter of the monopile may become excessively large, and the gravity base tends to reach a quite large size with a diameter of 10-20 m and weight up to 2000 tons [21]. In this situation, the material cost increases significantly; moreover, the substantial equipment is required for the manufacture, transportation, and installation, including jack-up barges, ship hires, and drilling equipment, etc. ...
Some large capacity offshore wind turbines are constructed in seismically active areas. The occurrence of soil liquefaction during an earthquake can result in severe failures of the offshore wind turbine. The seismic response of the structure and the failure mechanism of the soil-structure interactions are necessary to investigate. In this study, the seismic response of an innovative hybrid monopile foundation is investigated through a series of centrifuge tests. The seismic performance of the combined system of the superstructure, foundation, and soil are demonstrated. Five hybrid foundation models are tested by considering the influence of the foundation thicknesses and diameters, and a monopile foundation is tested for comparison. Centrifuge test results reveal that the hybrid monopile foundation is effective in reducing the lateral displacement during the shaking. In the saturated condition, soil keeps its strength and stiffness beneath and adjacent to the foundation. The hybrid foundation system tends to settle more due to the larger shear stress caused by the soil structure interactions. Influences of the wheel specifications are illustrated. The foundations with larger thicknesses lead to smaller lateral displacements and lower tendencies of liquefaction, but the settlements are intensified. The larger diameter foundation provides a longer drainage path for the excess pore water pressure. With a similar weight, the structure settles less during the earthquake.
... Anastasopoulos and Theofilou [1] studied the anti-overturning ability of hybrid foundation combining monopile and platform under the effect of wind wave loading and seismic loading by numerical methods, and concluded that the vertical and horizontal bearing capacity of hybrid foundation is higher than monopile foundation, and that the influence of seismic loading on pile foundation's bearing capacity should be considered for regions with frequent seismic activities. Wang et al. [20][21][22] studied the deformation response of different cyclic loads, soil parameters and seismic to hybrid foundation by centrifuge experiments, used the vibration table to simulate the earthquake process, and investigated the liquefaction characteristics of soil around pile foundation caused by residual pore water pressure. For the soil with poor drainage capacity, the reduction of soil shear modulus caused by residual pore water pressure should be considered [23]. ...
The dynamic response characteristics of the innovative marine wind power hybrid foundation under combined action of wind, wave and seismic loading are studied in this article. Taking the monopile foundation as reference group, the shearing strain-volumetric strain coupled pore pressure incremental constitutive model was utilized to simulate the kinetic behavior of seabed’s saturated soil. The pressure distribution and variation characteristics of residual pore water pressure around pile foundation due to the interactions of upper structure, foundation and saturated soil are illustrated. The results indicate that: (a) the cylindrical platform changes the residual pore water pressure and accelerates the pressure accumulation area, which converts from surrounding the monopile to nearby the cylindrical platform; (b) compared to the hybrid foundation, the peak value of residual pore water accumulation of the monopile foundation is smaller only under seismic loading, which is due to that the monopole with smaller stiffness is more vulnerable to deform with soil synergistically; (c) the cylindrical platform structure can significantly reduce the horizontal placement extreme of pile foundation under the action of dynamic load, and can also change the distribution characteristics of bending moment of middle pile of the hybrid foundation. It is manifested by a lower bending moment extreme of the middle pile near the mudline under the action of wind and wave loading, while the seismic loading could increase the bending moment extreme of middle pile near the mudline.
... These systems must guarantee accuracy and realism during the simulations to obtain reliable results for analysing and processing later. Each testing workbench is designed according to the aims of the study; thus, several types of systems have been developed to study different topics, e.g., vibrational response [7,8], sensor quality [9], and WT foundation-type-based [10,11]. Now, regarding both wavemaker channels and down-scale WT foundation assessment, there are also several works to highlight. ...
Structural health monitoring (SHM) systems are designed to continually monitor the health of structures (e.g., civil, aeronautic) by using the information collected through a distributed sensor network. However, performing tests on real structures, such as wind turbines, implies high logistic and operational costs. Therefore, there is a need for a vibration test system to evaluate designs at smaller scales in a laboratory setting in order to collect data and devise predictive maintenance strategies. In this work, the proposed vibration test system is based on a lab-scale wind turbine jacket foundation related primarily to an offshore environment. The test system comprises a scaled wave generator channel, a desktop application (WTtest) to control the channel simulations, and a data acquisition system (DAQ) to collect the information from the sensors connected to the structure. Various equipment such as accelerometers, electrodynamic shaker, and DAQ device are selected as per the design methodology. Regarding the mechanical part, each component of the channel is designed to be like the wave absorber, the mechanical multiplier, the piston-type wavemaker, and the wave generator channel. For this purpose, the finite element method is used in static and fatigue analysis to evaluate the stresses and deformations; this helps determine whether the system will work safely. Moreover, the vibration test system applies to other jacket structures as well, giving it greater utility and applicability in different research fields. In sum, the proposed system is compact and has three well-defined components that work synchronously to develop the experimental simulations.
... Several studies have been reported to investigate the bearing behavior of a hybrid monopile foundation. 1-g laboratory tests (Stone et al., 2007;Wang et al., 2021) and centrifuge tests (Stone et al., 2010;Wang et al., 2019) demonstrate that the lateral stiffness and the lateral capacity of a hybrid foundation are higher than that of the monopile. Full-scale tests and numerical analysis are conducted, indicating that the lateral stiffness of the pile in a hybrid system is improved by 40-70% (Trojnar, 2021). ...
The hybrid monopile foundation attracts extensive attentions to fulfill increasing demands for offshore wind turbines. The installation method is still uncertainty to date, limiting the application of this innovative foundation in the offshore wind industry. This study conducts a series of centrifuge tests, combing with finite element models, to investigate the lateral responses of hybrid monopile foundations. Two types of pile-wheel connection modes, namely perfectly rough (PR) and perfectly smooth (PS), are studied. The replaced-friction occurs in “PS” case, representing that the absence of friction is replaced by increasing normal forces. This phenomenon leads to similar ultimate capacities between “PS” case and “Frictional” case while the underlying earth pressure is influential. Further studies are conducted to investigate the pile-wheel-soil interactions under combined vertical-horizontal loadings. The vertical load applied to the wheel in “PS” case is demonstrated to be most advantageous on the lateral capacity of a hybrid monopile foundation. The strength of underlying soil is enhanced, intensifying the pile-soil interaction. The pile is recommended to be installed firstly, with the wheel behind. The upper structure is loaded on the wheel directly. This study provides design references for the practical installation of hybrid monopile foundations in the offshore wind industry.
... As OWTs move further away from the shore and grow rapidly in size, monopile foundations are required to have larger diameter with increased cost for construction and installation (Stone et al., 2018;Wang et al., 2019a). In contrast to many onshore structures, the supporting foundations of OWTs experiences large lateral loads and overturning moments induced by wind, wave and current during their service lifetime (Houlsby, 2016;Wang et al., 2019b) and the foundation design for OWTs is controlled by the lateral performance (Byrne et al., 2017;Wang et al., 2021;Wu et al., 2022). ...
Considering the limitations of conventional monopiles in supporting the new generation of offshore wind turbines (OWTs), the idea of hybrid pile foundation has been proposed and studied in recent years. In this paper, static lateral load tests are conducted on model piles and hybrid pile foundations in saturated sand. The influence of pile stiffness and hybrid foundation type on the lateral response is analyzed. Three-dimensional (3D) coupled discrete continuum modelling is further performed to reveal the interaction mechanism between different pile foundations and the surrounding soil. Experimental results indicate that the lateral capacity of the hybrid pile foundation can be notably improved compared with that of the monopile foundation, especially at small pile displacement. And the lateral load response of the hybrid pile foundation is comparable to that of the monopile at the ultimate limit state. From a numerical perspective, the load transfer mechanism of different pile foundations is studied by analyzing the displacement pattern of pile and soil foundation and the distribution trend of horizontal soil stress. It reveals that the existence of footing and bucket foundation could improve the efficiency of soil resistance mobilization at shallow depth. The experimental and numerical study is expected to provide some new perspectives on the understanding of the lateral bearing characteristics of hybrid pile foundations.
... Approximately 15-20% of the capital cost of offshore wind farms can be attributed to the foundation and support structure of offshore wind turbines (OWTs) [1][2][3][4]. OWT support structures are lightly damped and must withstand highly uncertain offshore wind and wave loads with stochastic load frequency and amplitude in addition to stochastic mechanical loads associated with the spinning rotor during power production. OWTs are typically designed in a so-called "soft-stiff" frequency design regime, wherein the first natural frequency is designed to lie between the 1P and 3P blade rotation frequency bands. ...
The dynamic behavior of offshore wind turbines (OWTs) must be designed considering stochastic load amplitudes and frequencies from waves and mechanical loads associated with the spinning rotor during power production. The proximity of the OWT natural frequency to excitation frequencies combined with low damping necessitates a thorough analysis of sources of damping; of these sources of damping, least is known about the contributions of damping from soil-structure interaction (foundation damping). This paper studies the influence of foundation damping on cyclic load demand for monopile-supported OWTs considering the design situations of power production, emergency shutdown, and parked conditions. The NREL 5 MW Reference Turbine was modeled using the aero-hydro-elastic software FAST and included equivalent linear foundation stiffness and damping matrices. These matrices were determined using an iterative approach with FAST mudline loads as input to a soil-pile finite element software which calculates hysteretic material damping. Accounting for foundation damping in time history analysis can reduce cyclic foundation moment demand by as much as 30% during parked conditions, 25–33% during emergency shutdown, but only 2–3% reduction during power production without wave and wind misalignment. The calculated foundation damping from the emergency shutdown cases agreed with experimental testing performed in similar site conditions.
... Presently, the modelling, testing, and implication of offshore wind technologies such as hybrid monopole foundations, suction bucket foundations, floating VAWT units, etc. are also major thrust area due to the high power demand Guo et al., 2018;Wang et al., 2019;Yang et al., 2019b). The integration of wind turbine power output with the conventional electric grids and their stability has always been a challenge in the field of wind energy technologies. ...
The energy demand is rapidly increasing globally, and extensive use of conventional energy resources is causing global warming, environmental pollution, health issues, etc. Fossil energy resources are declining rapidly, while wind energy is a cost-effective and promising energy source amongst renewable energy resources. The understanding and development of wind energy technologies is thus a necessity in the field of green energy production. This chapter provides exposure to the wind energy conversion systems along with the current status and future perspective of wind energy technologies. Furthermore, the socio-economic and environmental challenges of wind energy systems are also discussed in this chapter.
... Therefore, the soil around the foundation is washed away, resulting in scour. The large-diameter monopile foundation is the most commonly used foundation form in the offshore wind farm, but its embedded depth is reduced by soil scour, which significantly influences pile responses and the natural frequency of wind turbines (Gupta and Basu, 2016;Ma et al, 2017Ma et al, , 2018Tewolde et al., 2017;Wang et al, 2019Wang et al, , 2020c. Thus, soil scour has become a key consideration during the design of wind turbines (Darvishi Alamouti et al., 2020;Deb and Pal, 2019). ...
Soil scour is a major cause of foundation failures for offshore wind turbines (OWT), and has emerged as a significant concern for safety analysis and design. Most previous studies focus on investigating scour mechanisms but not their consequences. Therefore, this paper mainly investigated the scour effects on the lateral behavior of large-diameter offshore monopile and the corresponding scour monitoring method. Firstly, the effect of the soil scour on the lateral behavior of large-diameter monopile foundation of OWT was investigated numerically. It revealed that the horizontal load-displacement response, the bending moment and horizontal displacement of pile body, and the p-y curve were significantly affected by scour. However, the soil displacement field was not disturbed and remained “two-failure zone,” i.e., wedge failure zone near the ground surface and rotational failure zone near the pile toe. Furthermore, a monitoring method for scour depth of soil seabed around monopile foundation based on natural frequency was proposed and verified through model tests. It was found that test environment and conditions had little effect on the natural frequency of the measuring rod, which only depended on its free end length. This provides a theoretical basis for in-situ monitoring of soil scour for offshore wind turbine foundation.
... The lateral behavior of monopile in sand may be potentially affected by pile diameter D (Sørensen, 2012;Doherty and Gavin, 2012), or the ratio of pile embedment length over diameter L/D (Li et al., 2017;Thieken et al., 2015;Hong et al., 2017). Extensive experimental and theoretical investigations have been performed to understand the monotonic and cyclic lateral behavior of piles (Klinkvort et al., 2010;Alderlieste, 2011;Leth, 2013;Choo and Kim, 2015;Zhu et al., 2016;Qi et al., 2016 andWang et al., 2019). These studies mainly focus on the behavior of small-diameter slender piles (typically with D < 3 m and L/D > 20) and large-diameter stubby piles (typically with D > 4 m and L/D < 8), as illustrated in Fig. 1 (Gilbert et al., 2018;Truong et al., 2018). ...
The existing studies have been primarily focused on the lateral behavior of large-diameter stubby pile or small-diameter slender pile in sand, with little attention paid to large-diameter slender pile. This study presents a unique series of centrifuge tests on monotonic and cyclic lateral behavior of heavily instrumented large-diameter slender piles in medium dense sand. Two typical length to diameter ratios (L/D) are considered with the same length (L = 60 m) but different diameters (D = 4 and 6 m). It is found that the lateral behaviors of large-diameter slender pile, including its monotonic p-y response, cyclic accumulation of lateral displacement and cyclic stiffness evolution, are marginally different from those of small-diameter slender piles, but significantly deviate from the large-diameter stubby piles. This may suggest the longstanding argument of ‘diameter effect’ is relatively minor, while the lateral behavior of monopile in sand is more significantly governed by the relative pile-soil stiffness. The API (2011) non-conservatively predicts both stiffness and capacity of the large-diameter slender piles, leading to development of a new p-y formulation. These centrifuge testing results form a unique database to support development of new design methods for large-diameter slender piles, and to verify advanced numerical analyses involving cyclic models.
... Yang [1][2][3] et al. conducted centrifuge tests and numerical simulation on composite pile foundation, and concluded that the lateral bearing capacity of composite pile was significantly greater than that of single pile. Wang [4][5][6][7] and others have studied the lateral bearing capacity and vibration liquefaction of hollow friction wheel composite pile foundation and solid friction wheel composite pile foundation. ...
In recent years, with the rapid development of offshore wind power, the installed capacity is increasing, and the traditional single pile foundation is under heavy load. Therefore, the composite pile foundation composed of single pile foundation and bucket foundation (friction wheel) installed outside the pile body is gradually adopted to ensure the safety and stability of the fan during its service. In order to study the bearing capacity of composite pile foundation, the ABAQUS finite element software is used to study the horizontal bearing capacity of composite pile foundation, analyze its bearing capacity advantages compared with the traditional single pile foundation, and further optimize the design. The results show that: under the same load, the displacement and bending moment of composite pile foundation are greatly reduced due to the existence of friction wheel, and the horizontal bearing capacity of composite pile foundation is significantly better than that of single pile foundation; the diameter and height of friction wheel in composite pile foundation have obvious influence on its horizontal bearing capacity, but its thickness has limited influence on the horizontal bearing capacity of composite pile foundation. It can be seen that the bearing capacity of composite pile foundation is significantly better than that of single pile.
... To understand the lifetime variation in lateral pile-soil stiffness, valuable efforts in the form of physical and numerical investigations have been taken by many researchers (Matlock, 1970;Jeanjean, 2009;Zhang et al., 2011;Bhattacharya and Adhikari, 2011;Su et al., 2014;Wang et al., 2015;Lau, 2015;He et al., 2017;Hong et al., 2017a;Truong, 2017;Chong, 2017;Chong and Pasten, 2018;Chong et al., 2019;Liang et al., 2018;Liao et al., 2018;Wang et al., 2019aWang et al., , 2019b. In these studies, all the possible loadings that a pile can experience in its lifetime were typically converted to one extreme episode of constant-amplitude cycling or to a few successive episodes of loading with an increasing amplitude, with no attempt made to capture the effect of reconsolidation. ...
Large-diameter monopiles supporting offshore wind turbines (OWTs) are subjected to intermittent episodes of cycling and reconsolidation during the lifetime. The lateral soil-pile stiffness is likely to degrade during the cycling but tends to recover during the subsequent reconsolidation. The former effect has been widely acknowledged in monopile design, while the latter is less commonly recognized. This study aims to (a) investigate the effects of episodic cycling and reconsolidation on the evolution of cumulative displacement, stiffness and bending moment of large-diameter monopiles in soft clay, and to (b) analyze their consequences to the structural safety of the OWT (i.e., evolving natural frequency fN and thus likelihood of resonance). A series of centrifuge tests were performed to simulate laterally loaded monopiles subjected to multi-stage episodic cycling and reconsolidation. The experimental results show that the lateral soil-pile stiffness degrades during the initial cycling episode, but it entirely recovers during the subsequent reconsolidation to exceed the initial stiffness, with a percentage increase up to 50%. Consequently, the cumulative lateral pile-head displacement and maximum bending moment induced by the 3rd episode of cycling can be 63% and 15% smaller than that due to the 1st episode, respectively. The effects of episodic cycling and reconsolidation on the lateral stiffness, displacement and bending moment of the pile become more pronounced as the cyclic amplitude increases. Despite these observed beneficial effects, the stiffer lateral pile response after the reconsolidation would have increased fN of a typical 6 MW turbine founded on the monopile by up to 13% (i.e., from 0.206 to 0.232 Hz), forcing it to unfavorably approach the 3P frequency limit that could trigger resonance.
... With the increase in offshore wind turbine capacity and installation water depth, the requirements for construction equipment also increase. The limitation offshore construction capacity and construction equipment also restrict the large-scale development of offshore wind power in China [2][3][4][5][6]. ...
The composite bucket foundation (CBF) for offshore wind turbines is the basis for a one-step integrated transportation and installation technique, which can be adapted to the construction and development needs of offshore wind farms due to its special structural form. To transport and install bucket foundations together with the upper portion of offshore wind turbines, a non-self-propelled integrated transportation and installation vessel was designed. In this paper, as the first stage of applying the proposed one-step integrated construction technique, the floating behavior during the transportation of CBF with a wind turbine tower for the Xiangshui wind farm in the Jiangsu province was monitored. The influences of speed, wave height, and wind on the floating behavior of the structure were studied. The results show that the roll and pitch angles remain close to level during the process of lifting and towing the wind turbine structure. In addition, the safety of the aircushion structure of the CBF was verified by analyzing the measurement results for the interaction force and the depth of the liquid within the bucket. The results of the three-DOF (degree of freedom) acceleration monitoring on the top of the test tower indicate that the wind turbine could meet the specified acceleration value limits during towing.
... Geotechnical centrifuge testing is an effective experimental strategy to study the complicated geotechnical phenomenon reassembling the working conditions [35]. By accelerating the test models to certain gravitation, the dimensions of the models can be scaled up according to the centrifuge scaling law [36]. The static loading condition and the cyclic loading conditions are both considered. ...
Wind energy is a promising source of renewable energy and is projected to shift to offshore areas increasingly. Monopile foundation is one of the most commonly used foundations for offshore wind applications with the priority in load bearing capability and initial cost. This study describes an innovative monopile foundation, which institutes a creative strategy over the traditional large diameter monopile foundation to achieve higher axially load bearing capacity. This is achieved by adding a restriction plate inside the pile to intensify the soil plug effect. This design is based on the soil plug mechanism, and the arching effects and plug resistance mobilizations are considered. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the bearing behaviors of the innovative monopile with restriction plates. The pile with 1-hole restriction plate and the pile with 4-hole restriction plate are considered to discuss effects of the plate shape. Twelve models with different diameters and restriction plate types are investigated. The traditional open-ended and close-ended piles are included for comparisons. The static tests are conducted in saturated silica sand first to determine the ultimate bearing capacity of the innovative pile, after which the cyclic tests are performed. The innovative pile is proved to provide a larger bearing capacity than the pipe pile. An analytical method is proposed to estimate the capacity of the innovative pile. The study aims to develop the design code for innovative piles and provide design reference to large-scale offshore wind turbine projects.
... With the continuous development of offshore wind power, diverse foundations, such as gravity foundation, pile foundation, tripod foundation, jacket foundation, and even floating foundation, have been evaluated as the foundation of offshore wind turbines (OWTs) [1,2]. In recent years, bucket foundation has attracted much attention because of its easy construction, convenient transportation, reusability, and good soil applicability. ...
Large-diameter multi-bucket foundation is well suited for offshore wind turbines at deeper water than 20 m. Air floating transportation is one of the key technologies for the cost-effective development of bucket foundation. To predict the dynamic behavior of large-diameter tripod bucket foundation (LDTBF) supported by an air cushion and a water plug inside every bucket in waves, three 1/25-scale physical model tests with different bucket spacing were conducted in waves; detailed prototype foundation models were established using a hydrodynamic software MOSES with a draft of 4.0 m, 4.5 m, and 5.0 m and with a water depth of 10.0 m, 11.25 m, and 12.5 m. The numerical and experimental results are consistent for heaving motion, while exhibiting favorable agreement for pitching motion. The results show that the resonant periods for heaving motion increased with increasing draft and water depth. The maximum amplitude for heaving motion first decreased and then increased with the increase of water depth and spacing between the buckets. The maximum amplitude for pitching motion first decreased and then increased with increasing water depth but decreased with increasing spacing between the buckets. The wider the spacing between the bucket foundations, the larger the heave response amplitude operators (RAOs). Simply improving the pitch RAOs by increasing the spacing between bucket foundations is limited and negatively affects motion performance during the transportation of LDTBF.
... To efficiently minimize the cost while remaining safety, more meticulous considerations for monopile design are in need involving the scour depth prediction and evaluation of scour effects on structural responses. An alternative hybrid foundation system comprising a monopile and a bearing plate (or skirted footing) has been demonstrated to enhance the lateral bearing capacity compared with a monopile [96][97][98][99]. Nevertheless, the enhanced capacity of the hybrid foundation largely relies on the proper interaction between the bearing plate and the soils at the shallow zone of the seabed, where local scour and wave-induced pore pressure could occur [100][101][102][103][104][105][106][107][108]. ...
Monopile is the most commonly used foundation type for offshore wind turbines. The local scour at a monopile foundation generated by the incoming shear flow has significant influence on both quasi-static lateral responses and dynamic responses of the monopile. This chapter focuses particularly on characterizing the local scour in both spatial and temporal scales and revealing the scour mechanisms associated with the flow field around a monopile. The predicting methods for the equilibrium scour depth and the time scale of scour are detailed under various representative flow conditions in the marine environment. The scale effect while extrapolating the results of model tests to prototype conditions is highlighted. The local scour imposes significant influence not only on the deformation and stiffness of the monopile foundation, but also on the natural frequency and fatigue life of the structure system. Monopiles with diameters up to 10 m have become a feasible option as the industry is currently advancing into deeper waters. More meticulous considerations for monopile design associated with the scour depth prediction and evaluation of scour effects are still in need to efficiently minimize the cost while remaining safety simultaneously.
... This setting is firstly for safety consideration. Moreover, the centrifuge tests reported previously in our projects are conducted under a maximum acceleration of 50 g (Wang et al., 2018b(Wang et al., , 2019a(Wang et al., , 2019b. To make comparisons of different models, the maximum 50 g gravitational level is used. ...
Pile foundation is one of the most commonly used foundations for offshore and coastal structures. This paper describes an innovative design for the pile foundation, which institutes an innovative strategy over traditional pile foundation to achieve higher axial bearing capacity. This is achieved by adding restriction plates inside the pile to help form the soil plug. A series of geotechnical centrifuge tests are carried out to evaluate the load bearing behaviors of traditional pile foundation and innovative pile foundation with different types of restriction plates. The pile diameter and shapes of the restriction plates on the soil plug behaviors and pile load carrying capacity are analyzed. The results show that the use of restriction plates could significantly increase the axial bearing capacity of large diameter pile foundations. For different pile diameters, the restriction plates with four smaller holes achieve better performance than those with one large hole of the same effective opening areas. A bearing capacity equation is obtained by normalizing the ultimate bearing capacity with the pile diameters for different restriction plates. This study demonstrates the promise of an innovative pile foundation with a restriction plate as an economic and technically feasible way to produce higher load-bearing capacity to support offshore and coastal structures.
The modified suction caisson (MSC) is an innovative foundation for offshore wind turbines. Model tests were conducted to study the bearing behavior of the regular suction caisson (RSC) and the MSC in sand under multidirectional lateral cyclic loads. Each test includes two stages: (a) the load was applied in the initial loading direction, and (b) the load was applied in the direction which rotate through a deflection angle from the initial loading direction. Results show that in the first stage, the translational motion direction of suction caisson in plane view coincides with the loading direction. In the second stage, the motion direction of the suction caisson doesn't coincide with the loading direction. This phenomenon becomes more obvious under low loading amplitude. When the deflection angle is less than 90°, total accumulated displacement increases with increasing the load cycles. The maximum normalized total displacements for the RSC (ζb = 0.833) and the MSC (ζb = 0.649, 0.833) were obtained when the deflection angles equal 45°, 45° and 30°. When the deflection angle value is higher than 90°, the total displacement decreases with increasing the deflection angle. Changing the loading direction has little effect on the unloading stiffness values for both the RSC and the MSC.
For the global performance of monopile wind farms, the desired wave field distribution using traditional layout methods is hard to obtain. In this study, the investigation aims to efficiently explore the potential wave response reduction of the multiple layer design of wind farm layouts using novel grating conditions. It is very important and necessary to optimize the layouts of monopile-supported OWTs (offshore wind turbines) by analyzing the wave field performance, especially considering scour protection and avoiding the proximity of the wave frequency to natural frequency of OWTs. This paper presents a layout and a design method of monopile-supported OWTs using combined grating theorems, which can take space modulation into account to deal with various issues in existing layouts. The results show that the present method can modulate the wave field responses more evenly than the conventional cases. More specifically, total wave field distribution sensitivities were discussed under different wavelengths, amplitudes, layouts, pile-radius, and the angles of incident waves. It can be illustrated that the monopile-supported OTWs with sinusoidal configurations can have more modulation effects on wave fields in an appropriate wavelength band. As indicated, this method not only provides wave space modulation control but also sheds light on the wave field reduction mechanisms.
Large-scale constructions of offshore wind power are carried out in southeast coast of China, where the seabed is covered in weathered granite. However, the bearing capacity and failure mechanism of monopile foundation in weathered-granite seabed are rarely researched. In this paper, the stress-strain characteristics and strength parameters of granite with different weathering degrees were obtained through laboratory tests, and the pre-strain test method was used to study mechanical parameter variation of moderately weathered granite in the crushing process. Experiment found that the strength parameters decrease mainly after the strain reaching 50% of the ultimate strain of moderately weathered granite. In addition, the bearing capacity and failure mode of rock-socketed monopile under unidirectional horizontal loading was investigated numerically. Analysis showed that the failure mode of rock-socketed monopile under unidirectional horizontal loading is progressive and presents two obvious failure zones. More critical, the existence of vertical loading effectively improves the horizontal bearing capacity of monopile foundation in the weathered rock seabed, and the failure envelope of the monopile foundation in H-V loading space is elliptic-shaped. This provides further understanding of the mechanical properties of submarine weathered granite, and the helpful reference for stability evaluation of rock-socketed monopile foundation in weathered granite seabed.
With the ramping up of the capacity of the offshore wind turbine (OWT), the water depth, and the weight of the superstructure, the need of foundation structure with higher performance but reasonable cost demands innovation from geotechnical and structural engineering. The novel hybrid foundation takes advantage of the mature monopile technology, and innovatively integrates a loading wheel-bucket component around the monopile in the upper layer of soil. The addition of the integrated hybrid component is proven to enhance the overall performance of the foundation by previous research. The soil-structure interaction (SSI) of the hybrid foundation is the key to optimize the design of the structure. In this paper, a series of finite element (FE) analysis of the hybrid foundation under monotonic load are performed. The results of the FE analysis are compared with that of the geotechnical centrifuge tests. The soil pressure distributions on the pile and the hybrid component are investigated. The variation of the soil pressure profile with the increase of rotation angle is also studied. The soil resistance on the skirt of the bucket component is illustrated in the analysis. The FE models also prevail the failure mechanism of the hybrid foundation.
Large diameter monopiles are usually used as the foundation of offshore wind turbines. Monopile foundations are subjected to laterally cyclic marine environmental loadings which tends to accumulate displacements and rotation of the monopile. To preliminarily evaluate the cyclic responses of monopile in marine clay, 1g model tests have been conducted to combine with reported centrifuge tests to efficiently illustrate the behavior. This study investigates the laterally monotonic and cyclic responses of the model pile embedded in kaolin clay. As expected, obvious differences are observed between results of the 1g model tests and the centrifuge tests owing to the effect of stress levels, but important quantitative relations between results of these two types are revealed: two equations closely related to cyclic loading magnitude ratio ζb with same cyclic loading symmetry ratio ζc, could be qualitatively obtained to predict the pile responses in centrifuge tests by using the result of 1g model tests. The normalized unloading stiffness of model pile in 1g test is less than that of obtained from centrifuge tests with the same ζb values. This result not only highlights that it is necessary to account for the stress level of soil to understand cyclic lateral responses of monopile foundations, but also reveals that it is possible to qualitatively insight responses of monopile through 1g model tests.
This paper presents an innovative approach to shaping embedded retaining walls using hybrid piles with flexible shafts. The tests of the hybrid piles in sand confirmed that their displacement is 30–50% less compared to the standard piles at the same lateral load. The general concept of retaining walls with hybrid piles is described, assumptions are formulated and model tests, and full-scale tests and 3D FEM analyses are carried out to evaluate the response mechanism of hybrid piles in sand. It was confirmed that flexible hybrid piles interact with the soil in a different way than standard piles. Particular attention was paid to evaluation of pile displacements during the initial phase of increasing the lateral load up to H = 400 kN, M = 1600 kNm with M/H ratio = 4. It was found that changes in lateral stiffness of the hybrid pile increase with pile deflection in the range up to 50 mm. The long-term field tests showed that the hybrid piles L = 10 m, D = 1.2 m, slab overhang B = 1.2 m were stable for a period of eight months. Detailed 3D FE numerical analysis of the stress zones in front of the hybrid pile allowed new P–Y curves to be proposed for depths up to 2 m in the range of initial displacements. Simplified verification calculations using the modified p–y curves for hybrid piles with flexible shafts were carried out, and positive verification results were obtained for test piles in sand.
Various hybrid systems combined with several foundation elements have been proposed to improve the lateral performance of monopiles. Among them, the hybrid monopile-bucket foundation which consists of a traditional monopile and a wide-shallow bucket has received more and more attention and has been used in offshore wind pilot projects for supporting OWTs. However, deep insights into its monotonic and cyclic responses are still lacking. This study aims to understand the monotonic load-bearing, the evolution of cumulative displacement, stiffness and bending moment of the hybrid foundation in soft clay. A series of centrifuge model tests were conducted to simulate a hybrid monopile-bucket foundation and a monopile subjected to lateral monotonic and multi-stage cyclic loading. The monopile with the identical pile diameter and embedded length is used as a benchmark. The experimental results show that by the addition of the bucket, the hybrid foundation shows a 30.1% increase in ultimate capacity. Compared to the monopile, the cumulative displacement, unloading stiffness and the growth of bending moment of the hybrid foundation during cyclic can be reduced by up to 25%, 30% and 35.3%, respectively, implying the effectiveness and superiority of the hybrid foundation. Apart from the centrifuge tests, supplementary three-dimensional finite element analyses have also been performed to reveal the bearing mechanism and the sharing ratios of external load and moment carried by the single pile and the bucket of the hybrid foundation. The test and numerical results presented in this study are expected to provide design references for further practical applications of the hybrid foundations.
The urgent demand for energy structural reform and the limitations of single energy development have promoted the combination of wind energy and wave energy. A hybrid energy system means that two or more energy devices share the same foundation. It reduces the levelized cost of energy (LCOE) and improves competitiveness through infrastructure sharing and increased power output. This paper starts with the development of the joint resources of wind and wave energies, then introduces the foundation forms of the hybrid system. It reviews the latest concepts and devices proposed with the integration of wind energy and wave energy, according to the foundation forms, and makes a preliminary assessment of the synergies of the hybrid system. The existing study methods of the hybrid systems are summarized. In view of the challenges faced by the development of hybrid energy systems, several suggestions are put forward accordingly. This paper provides a comprehensive guideline for the future development of the hybrid wind-wave energy converter (W-WEC) system.
Due to the high cost of using offshore wind turbines, increasing the efficiency and amount of energy absorbed in different ways needs to be considered. This research work aims to study the dynamics and power absorption of the wave energy converter by proposing a new hybrid mono-pile wind turbine with two pitching WECs. Each WEC has a power-take-off mechanism activated by the pitch motion of the device linked to the mono-pile via a hinged type connection. The installation point was off the coast of Dayyer port in the Persian Gulf. First, the study area's wave, current, and wind characteristics were modeled and analyzed. Then a numerical model was developed to investigate and analyze the dynamics of the hybrid system and the effect of the damping coefficient between WT and WECs concerning power production under operational conditions. Hydrodynamic forces exerted on each part of the system were estimated using BEM in the time domain. The connections were also modeled as a linear, rotational damper and stiffness. The WECs' PTO's damping coefficient was established to optimize wave energy generation. By analyzing the damping coefficient of connections between the wind turbine and WECs, appropriate coefficients were obtained for maximum WECs performance. The results show that for this fixed hybrid arrangement installed in the coastal zone of Dayyer port in the Persian Gulf, 26% of the wind turbine energy is added to the device's energy by employing wave absorption systems.
In this paper, three types of hybrid monopiles are proposed for 3MW OWTs in offshore areas where the batholith is shallowly buried. Three-dimensional FE models are established considering the soil-structure-interaction (SSI) to calculate the bearing performances of these monopiles. Model verification is conducted through comparisons with existing numerical results. The static bearing performance and dynamic character under the serviceability limit state (SLS) in silty sand and normally consolidated (NC) clay are investigated. In the static analysis, we focus on the lateral deflection and the rotation at the mudline, whereas in the modal analysis, we focus on the natural frequencies of the OWTs. Results show that the circular skirt performs better than square fins in improving the bearing performance of traditional monopiles, and the improving effect increases with the length of the additional components. In addition, the improving effect is more obvious in NC clay than in silty sand.
Yard loads may induce slope deformation and compress the piles of pile-supported wharfs, resulting in structural damage. In this paper, centrifuge tests were conducted to investigate yard load induced performances of pile-supported wharfs reinforced with T-shaped and F-shaped soil cement mixing (TSCM and FSCM, respectively) retaining walls. The TSCM and FSCM walls were located in the bent-yard connecting section and wharf performances were presented in terms of soil movements, bent displacements, soil and pore pressures, and pile bending moments. The results show that the slope soil migrated toward the waterside as the soil cement mixing (SCM) walls tilted, inducing tilting failure of the pile-supported wharf. The FSCM wall-reinforced bent showed greater displacements than the TSCM wall-reinforced bent at the same load intensities. Pore pressures were registered immediately after yard loads were applied but dropped to constant values or kept decreasing over time during the unloading period. Soil pressures generally increased with yard loading in the upper part but exhibited increasing-decreasing tendencies in the lower part. The soil pressures on the FSCM side were generally greater than those on the TSCM side in the upper and middle parts but smaller in the lower part. The bending moments on the FSCM side were generally larger than those on the TSCM side on landside because the structure-soil interaction was more intense. SCM walls are feasible and beneficial for reinforcing pile-supported wharfs under yard loads, and TSCM walls seem to be superior to FSCM walls considering their improved performance and economic efficiency.
Composite bucket foundation and one-step installation technology for offshore wind turbine are the integration of foundation construction, transportation and whole installation at sea. The cost of offshore wind turbine construction and installation has been largely reduced. Foundation stability is the key technology in the process of towing transportation. Field observation data can reflect the real state of the foundation. In this paper, the influence of water depth and towing speed on liquid level, the compartment pressure, and the pitch angles during towing of composite bucket foundation are studied. These data are analyzed based on the field measurements data from a 3.3 MW offshore wind power project in China. The results show that with varied water depths and towing speeds, the compartment pressure changes are small during the bucket foundation towing process. The offshore wind turbine composite bucket foundation is stable while being towed in the ocean.
For the offshore wind turbines installed in earthquake areas, their operation is affected by seismic loads in addition to wind and wave loads. Therefore, it is necessary to study the dynamic responses and vibration control of the wind turbines. In previous studies, the structural responses of offshore wind turbines are usually investigated in the parked case, while the blade rotation effect is usually not considered. The evaluation on the structural responses may be inaccurate under this condition, further affecting the evaluation on the vibration control performance of a control system. In view of it, this paper established a complete multi-body model of a fixed-bottom offshore wind turbine considering pile-soil interaction, and then performed simulations when the wind turbine was subjected to multiple external excitations. Continued, a single tuned mass damper (STMD) system and a multiple tuned mass dampers (MTMDs) system were applied to control structural vibrations of the wind turbine. Then, based on the construction of a simplified main structure-TMD system, TMD parameters were optimized. Finally, twelve load cases including operating and parked conditions were selected to perform simulations. Results show that the influence of the seismic excitation on blade responses is greater under the parked condition than that under the operating condition. Moreover, STMD/MTMDS exhibit better performance under the parked condition than that under the operating condition. Compared with STMD, MTMDS can better suppress the vibrations at both the fundamental and highorder modes, and exhibits significant robustness under the condition of changing soil parameters.
The idea of hybrid pile foundation provides optimized foundation solutions for the new generation of offshore wind turbines (OWTs). In this paper, the behavior of monopile and hybrid pile foundation under cyclic lateral load is studied based on three-dimensional coupled discrete continuum modelling. The soil sample is simulated as an aggregation of discrete particles while the pile foundation is constructed as continuum material. The influence of pile stiffness and hybrid foundation type is considered. Numerical results indicate that the increase of pile stiffness and the application of hybrid pile foundation could improve the secant stiffness and reduce the accumulated lateral displacement of the pile foundation. The additional footing and bucket could improve the efficiency of soil resistance mobilization at shallow soil. Under cyclic loading, soil particles tend to move towards the foundation, leading to the formation of convective zones. The size of the convective zone is influenced by the displacement pattern and displacement magnitude of the foundation. As for the soil plug inside the pile, it could be assumed as additional mass that displaces simultaneously with the pile foundation. The numerical research could provide some new perspectives for the interaction between hybrid pile foundation and surrounding soil under cyclic lateral loading condition.
The penetration of a bucket foundation in sandy soil is complex. When suction is large, the installation of bucket foundation is prone to seepage damage, and the foundation cannot penetrate the sandy soil due to insufficient suction. The impact of the skirt plate and inner compartment plates in mono multi-chamber bucket foundations on the penetration resistance is not clear. In addition, when the foundation is tilted, whether it can be leveled to the design requirements is related to the success or failure of the installation. When the suction during leveling reaches the critical value, penetration damage will occur; therefore, selecting the correct leveling strategy is particularly important. This paper primarily deals with the issue of mono multi-chamber bucket foundation penetration and the associated leveling mechanisms, focusing on analyzing the influence of the skirt plate and inner compartment plates on mono multi-chamber bucket foundation penetration resistance, selecting the mono multi-chamber bucket foundation leveling method, and analyzing the relationship between the adjustable angle and depth in the bucket foundation leveling process. Moreover, the seepage characteristics of the soil under the leveling state is studied, and the critical suction is derived. The analysis results show that the leveling method of applying suction to multiple chambers is safer than the method of applying negative pressure suction only on one chamber. In addition, the negative pressure leveling method has a certain limitations on the inclination angle and penetration depth. To the bucket foundation test model, when the foundation inclination angle is less than 2° and the ratio of penetration depth to skirt height is within 0.5, the foundation can be leveled using negative pressure suction on the high chamber. The reason for the failure of leveling is the formation of piping channels. When seepage failure occurs, the permeability coefficient of the soil in the high chamber may be about 10 times that of the soil that is not affected by suction.
With increasing demand and requirements for offshore wind turbines (OWTs) construction, research on the hybrid pile foundation is gaining increasing popularity. In this paper, a series of 1-g model tests are conducted to investigate the lateral behavior of monopile and hybrid pile foundations under one-way cyclic loading. The influence of pile stiffness, pile diameter for the hybrid foundation and hybrid foundation type on the lateral behavior of the structure is analyzed. To visualize particle movements during the loading process, several colored sand bands are arranged at ground surface around the structure. Continuous monitoring of sand surface during the loading process and excavation of sand sample reveal different particle migration patterns around different structures. Experimental results verify the superiority of hybrid foundation in resisting lateral loads and reducing accumulated lateral displacement. Based on the coupled discrete-continuum modelling of cyclic load tests, the deformation pattern of monopile and hybrid structure and the particle displacement contour at different sections are provided. The numerical results further point out the potential influence of the soil plug on the lateral capacity of the hybrid pile foundation, which could be the focus of future research.
Experimental and numerical studies are carried out to explore the lateral load and moment resistance capacities of the monopile-friction wheel hybrid foundation in soft-over-stiff soil deposit. Model tests are firstly conducted to preliminarily investigate the soil failure mechanism of hybrid foundation under lateral eccentric load (lateral loading at a certain height above the seabed), which is followed by full model tests conducted to investigate the lateral load and moment resistance behaviors of the monopile-friction wheel hybrid foundation under static horizontal loading in soft-over-stiff soil deposit. A numerical model is then generated and validated with the laboratory testing results. Parametric study is performed to quantify the lateral bearing capacity of hybrid foundation in clay-over-sand soil deposits. Both the experimental tests and numerical simulations show that compared to conventional monopiles the hybrid system can provide a higher lateral bearing capacity and a larger lateral stiffness. The bearing capacity is found to be mainly influenced by the diameter of wheel Dw, undrained shear strength of clay su, loading eccentricity e and clay layer thickness Tc. Finally, empirical design formulae are proposed to estimate the lateral bearing capacity of the monopile-friction wheel hybrid foundation system under static horizontal loading in soft-over-stiff soil deposit.
This paper deals with a new combined concept consisting of an oscillating water column (OWC) device and an offshore wind turbine for the multi-purpose utilization of offshore renewable energy resources. The wind turbine is supported by a monopile foundation, and the attached OWC is coaxial with the foundation. Within the chamber, the exterior shell of the OWC and the monopile foundation are connected by four vertical stiffening plates. Correspondingly, the whole chamber is divided into four equivalent fan-shaped sub-chambers. A higher-order boundary element method is then adopted to model the wave interaction with the combined system. Numerical models based on two different approaches, namely ‘Direct’ and ‘Indirect’, are both developed in this study. In addition, a self-adaptive Gauss integration method is developed to treat the nearly singular integration that occurs when the field and source points are very close to each other. A detailed numerical analysis is then conducted for the case of an OWC integrated into a NREL 5 MW wind turbine in both regular and irregular sea states. Numerical results illustrate that a significant energy extraction efficiency is attained when remarkable piston-like fluid motion is induced within each sub-chamber, and the wave power absorption by the OWC is not restricted by wave direction. The air compressibility makes a negative effect on the wave power absorption especially when the wave frequency is less than the resonance frequency of the piston-mode motion of the fluid in the chamber. In addition, the wave forces on the OWC and the monopile can balance each other at specific wave conditions, leading to a nearly zero net wave force on the whole system. The results also illustrate that by using an optimal turbine parameter, the wave power production by the OWC can be an important supplement to the combined system in operational sea states.
First, a review of knowledge developed over the previous 50 years was presented, including various simplified methods of analyzing hybrid foundations. To discuss this subject comprehensively, a reference to calculations of piles was also made. On this basis, the author focused in the paper on new aspects of designing the hybrid foundations in serviceability limit states. The know-how review showed that the aspects of hybrid foundation design have been poorly recognized so far. For practical reasons, a simple calculation method for the hybrid foundations, useful for initial decision making, is still needed. A new design method was presented, based on a hybrid pile-soil interaction concept. A general design concept was described, the assumptions were formulated, and the method was explained in detail. A practical application of this method has been demonstrated for a large diameter hybrid pile, previously tested at full scale under lateral load. The calculation results were compared with the well-verified data obtained from the field test. By incorporating the extended knowledge on the mechanism of the pile-soil interaction, a significantly reduced horizontal stress in front of the pile was achieved. The conducted calculations confirmed that the hybrid monopile displacement is 40–70% lower compared to the standard monopile with similar dimensions. This method allows the stability of the new hybrid monopiles under lateral load to be assessed in a better way. The gained experiences can be useful for designers and other researchers to enhance the design of offshore wind turbines on monopiles.
Wind energy has gained significant support to be one of the primary sources of near-zero-emission energy systems. Based on the technological paradigm theory and employing a novel data analysis system (DAS), this paper provides an overview of wind energy technological developments. With key energy law and policy decisions highlighted, the visualization process clearly identified the three technological paradigmatic stages; competition, diffusion and shift; from which it is found that wind-based hybrid energy systems appear to give better promising trajectory in the future than wind energy technologies that rely only on onshore or offshore winds. However, with market maturation, the challenges associated with increasing share, energy storage, motivational conflicts, demand responses and optimal scheduling emerge and are expected to be resolved. From the paradigm shift investigation, a comprehensive sustainable framework for a six-stage hybrid wind-photovoltaic energy system is proposed. Recommendations on policy shifts to encourage technological innovation, regional differentiation, improve monitoring & evaluation systems and the implementation of smart grid are also given to address the incentives that need to be provided.
With the aim of the developing offshore wind turbines in the deep sea and with the demand for larger capacity turbines, a brace structure was designed to provide guidance for an offshore wind turbine jacket foundation. The influence of the brace structure on the bearing capacity and load transfer mode from the top to bottom of the jacket structure was studied using numerical simulation. The results show that, among various topological structures, the X-type topological mode has great advantages for the bearing capacity. The number of layers can be decreased to reduce the cost of offshore wind power. When the section moment of inertia of the brace is greater, the load on the top of the leg is smaller, and the proportion of the horizontal load on the bottom of the compressive leg is higher. The properties of the brace only affect the bearing capacity of the structure in a small range.
A new type of caisson, composed of two skirted side semicircles and a skirted middle rectangle, was used as a breakwater foundation in Xuyu Port, China. In this study, large-scale laboratory model tests were carried out on jacking installations and lateral loading of this new skirted foundation in sand. Test results showed that the soil pressure acting on the caisson's inner skirt was larger than the outer soil pressure during jacking installation; whilst the tip resistance was greater than skirt friction. Penetration resistance was well predicted with the jacked pile theory and cone resistance. Overturning failure of the foundation occurred under the lateral loading. Ultimate lateral loads displayed a descending trend with increasing loading position height, whereas the corresponding moments exhibited an increasing trend. Parameters were suggested to calibrate the expression of circular foundation for purpose of estimating the combined capacity of the new skirted foundation. Passive soil pressure zones occurred on the outer surface of the new skirted foundation in the loading direction. In contrast, active soil pressure zones were found in the area with opposite loading direction. The net incremental soil pressure was positive in the loading direction, suggesting that the rotation center was beneath the model base.
This paper focuses on the load bearing behaviors of the composite bucket foundation (CBF) which has been proposed and applied to offshore wind turbines in China. With the numerical analysis method, the soil-structure interaction and the practical project-based dimensional and property parameters are described. The load bearing behaviors of the CBF are investigated in terms of deformations (structure and soil), load bearing ratios, and geometric effects. Under the consideration of multi soil profiles and combined loading condition, the load bearing behaviors of the CBF can be concluded. Soil property can change the load bearing ratio especially when the compartment plates are involved. The lid-bearing mode is dominant for CBF when the upper layer of the soil is high in strength. Without the compartment plates, the bucket wall contributes more than the lid. The load bearing behavior is influenced by the bucket diameter, soil strength and the compartment plates. The results are of reference value to understand the load bearing behaviors and serve as a basis for optimizing the structure design for cost-reduction.
The paper presents a new concept of hybrid foundations for offshore wind turbines. The scientific work is based on studies of hybrid foundations at three scales; small laboratory scale, full-field investigation, and 3D numerical simulation. The work based on the analysis of interaction of the monopile in cohesionless soil enabled developing the basis for a more precise determination of lateral stiffness of modern and economic new hybrid foundations for offshore wind turbines. The phenomena occurring in the cohesionless soil were identified and a quantitative evaluation of the plate effect caused by a horizontal force and the bending moment was carried out. A better understanding of the stability problem of offshore wind turbines may also be relevant to current design methods. Research and analysis of obtained results have an impact on the refinement of current design methods of standard monopiles on lateral load.
Large open-ended pipe piles have been widely used in offshore and transportation engineering. The design formula for the pipe piles in the current AASHTO specification is based on the load-bearing database of pipe piles with diameters less than 24 inches. Therefore, the load/resistance factors may be inaccurate for the design of large open-ended pipe piles. Centrifuge testing is one of the most commonly used geotechnical test methods to study complex geotechnical problems. In this study, an extensive amount of geotechnical centrifuge experiments was conducted to analyze the load bearing behavior of pipe piles and piles with an innovative restriction plate. A customized loading frame was built to apply static loads. By using the scaling law, centrifuge experiments can scale up the model scale experiment to resemble the field scales. The pile diameters are scaled up by applying the centrifugal accelerations. With the validated experimental setup, the study conducted a program to systematically evaluate the influence of restriction plate on the loading bearing behaviors of large diameter open-end pipe piles. The results showed that by using restriction plates, the ultimate bearing capacity increased significantly compared with the corresponding open-ended pipe piles.
Pile foundation is the most popular option for the foundation of offshore wind turbines. The degradation of stiffness and bearing capacity of pile foundation induced by cyclic loading will be harmful for structure safety. In this article, a modified undrained elastic–plastic model considering the cyclic degradation of clay soil is proposed, and a simplified calculation method (SCM) based on shear displacement method is presented to calculate the axial degradated capacity of a single pile foundation for offshore wind turbines resisting cyclic loadings. The conception of plastic zone thickness Rp is introduced to obtain the function between accumulated plastic strain and displacement of soil around pile side. The axial ultimate capacity of single piles under axial cyclic loading calculated by this simplified analysis have a good consistency with the results from the finite element analysis, which verifies the accuracy and reliability of this method. As an instance, the behavior of pile foundation of an offshore wind farm under cyclic load is studied using the proposed numerical method and SCM. This simplified method may provide valuable reference for engineering design.
Icing is a strong limitation for the performance of wind turbines in cold climates and the prediction of the performance loss due to ice accretion is essential for designing effective ice mitigation measures. This paper presents a numerical approach capable of simulating the ice accretion transient phenomenon and its effects on wind turbine performance. This approach is applied to the NREL 5 MW reference wind turbine to predict (i) its performance during and after an icing event lasting for 8 h and (ii) the potential improvement in energy harnessing due to different operational strategies. The results show that by decreasing the turbine rotational speed and accepting a slight energy conversion decrease during the icing event, the performance can improve up to 6% when full operation is restored compared to the baseline operational strategy. Whereas, sustaining the rotational speed during the icing event can generate a 3% of performance loss afterwards compared to the same baseline. The developed workflow can be used for optimising performance of wind turbines by accounting for environmental conditions, the duration of the icing event, and performance after the icing event itself, thus constituting a valuable tool to maximise profitability of wind turbines in cold climates.
Ageing of a wind turbine and its components is inevitable. It will affect the reliability and power generation of the turbine over time. Therefore, performing the ageing assessment of wind turbines is of significance not only to optimize the operation and maintenance strategy of the wind turbine but also to improve the management of a wind farm. However, in contrast to the significant number of research on wind turbine condition monitoring and reliability analysis, little effort was made before to investigate the ageing led performance degradation issue of wind turbines over time. To fill such a technology gap, four SCADA-based wind turbine ageing assessment criteria are proposed in this paper for measuring the ageing resultant performance degradation of the turbine. With the aid of these four criteria, a reliable information fusion based wind turbine ageing assessment method is developed and verified in the end using the real wind farm SCADA data. It has been shown that the proposed method is effective and reliable in performing the ageing assessment of a wind turbine.
The offshore wind industry has historically focused on setting up new projects, with the decommissioning phase receiving little attention. This can cause future problems as decommissioning needs to be planned at the beginning to prevent complications that may arise, as it implies important operations and high costs. There are numerous features that make decommissioning a challenge, such as the marine environment, the technical limitations of vessels and the lack of specific regulations that determine what should be done, increasing the uncertainty of the process. Additionally, the unique characteristics of the sites involve exclusive optimal solutions for each project. This article analyses the main operation parameters that affect the decommissioning process, identifying the benefits and drawbacks of the influencing variables. A model is designed to compare different transportation strategies, searching for cost reduction. A decommissioning methodology is been proposed based on this analysis, taking into consideration the technical aspects of the process, and minimising environmental impacts. The model forecasts that the predicted duration and costs of this process are not being adequately captured in site decommissioning plans.
The paper presents the results from a series of centrifuge tests which examine the benefits of employing a footing together with a 'standard' monopile as a foundation solution for an offshore wind turbine. The experiments were carried out in firm to stiff kaolin clay and involved monotonic application of lateral loads at an equivalent prototype height of 30m above the foundations. Tests were conducted on piled footings, monopiles and an un-piled footing. The experimental results and observations are compared with those obtained from a parallel series of 3D Finite Element analyses.
While monopiles have proven to be an economically sound foundation solution for wind turbines, especially in relatively shallow water, their installation in deeper water and in hard ground may require a more complex foundation design in order to satisfy the loading conditions. One approach is that foundation systems are developed which combine several foundation elements to create a ‘hybrid’ system. In this way it is possible to develop a foundation system which is more efficient for the combination of vertical and lateral loads associated with wind turbines while maintaining the efficiency and simplicity of the design. Previous studies have reported the results of single gravity tests of the hybrid system where the benefits of adding the footing to the pile are illustrated. This paper prevents experimental results on the performance of skirted and unskirted monopile-footings. A simplified design approach based on conventional lateral pile analysis is presented.
The monopile has proven to be the preferred foundation solution for offshore wind turbines in shallow water and this is mainly due to its simplicity of design, fabrication and installation which leads to it being an economically sound foundation solution. The rounds 2 and 3 of the UK offshore wind farms are located at ever deeper water, some at the deepest waters wind turbines have ever been set to be installed. During the past 5 years several foundation solutions have been proposed for deepwater application and they mainly comprise of a number of foundation elements such as tripod system and the hybrid monopile-footing foundation system. The hybrid system, which comprises of a monopile and a bearing plate attached to the pile at mudline, has proven to significantly enhance the lateral capacity of the monopile. This paper reports the results of a series of centrifuge tests (to recreate prototype stress and load conditions) carried out to investigate the behaviour of the hybrid system in Sand. Furthermore, the results of a series of 3D finite element analyses (using ICFEP) have been reported.
Current offshore technology is being transferred successfully to the renewable energy sector but there is clearly scope to develop foundation systems which are more efficient, economic and satisfactory for the particular case of a wind turbine. One such approach is that foundation systems are developed which combine several foundation elements to create a ‘hybrid’ system. In this way it may be possible to develop a foundation system which is more efficient for the combination of vertical and lateral loads associated with wind turbines. In many of the proposed offshore Europe-an wind farms sites, it is often the case that the surficial seabed deposits are underlain by a weak rock. This paper presents the results of a series of small scale single gravity tests to investigate the performance of a monopile and combined monopile and bearing plate foundation where the pile is socketed into a weak rock. In the model studies the weak rock layer is modelled by a weak sand and gypsum mix. The results of the study provide an insight into the effect of the various foundation elements (i.e. pile, plate and rock socket) and their contribution to the overall performance of the foundation system.
A series of centrifuge model tests of the lateral response of a fixed-head single pile in soft clay is reported. Both monotonic and cyclic episodes of loading are described, with varying amplitude and with intervening periods of reconsolidation. The soil conditions are characterized by cyclic T-bar penetrometer tests. The ultimate capacity under monotonic load for virgin and for postcyclic conditions was found to be comparable with calculations based on existing design methods, including theoretical plasticity solutions and empirical methods. The lateral stiffness was observed to degrade with cycles, with the rate of degradation being greater for larger cycles. The degradation pattern has been tentatively linked to the cyclic T-bar response, by considering the 'damage' associated with the cumulative displacement and remolding, in each case. This approach provides a consistent interpretation of the tests. Although episodes of pile movement and soil remolding led to a reduction in lateral resistance, intervening periods of reconsolidation led to a similar magnitude of recovery and a reduction in the level of softening in subsequent cyclic episodes. During an initial episode of cyclic lateral movement, the stiffness degraded by a factor of 2.3, which is comparable with the strength sensitivity derived from a cyclic T-bar test. In contrast, after five episodes of reconsolidation, the stiffness had recovered back to within 25% of the stiffness observed in the first cycle of the first episode, and it showed negligible degradation during subsequent cycling. This observation implies that, over a long period of cyclic loading, the lateral stiffness of a pile may tend towards a value that is independent of cycle number, and that represents a balance between the damaging effects of remolding and pore pressure generation and the healing effects of time and reconsolidation.
Skirted gravity base foundations and suction caisson foundations are considered as viable alternatives to monopile foundations for offshore wind turbines. While recent work has focused on the monotonic moment-rotation response for shallow foundations, the cyclic response and the accumulation of rotation over the life of the turbine must also be addressed. This paper presents cyclic loading tests where approximately 10,000 cycles, with different loading characteristics, were applied to a model shallow foundation (a caisson) in loose sand. On the basis of these tests, a framework for assessing the accumulated angular rotation because of cycling was developed. The settlement and cyclic stiffness response of the caisson were also assessed. It was found, not unexpectedly, that the accumulated settlement of the caisson increased with the number of cycles and cyclic amplitude. It was also found that a cyclic loading regime intermediate between one-way and full two-way cycling produced the largest rotations. The cyclic stiffness was relatively unaffected by the number of cycles. Using an appropriate scaling technique, the proposed framework was used to predict the long-term accumulated angular rotation, for an example, the field-scale monocaisson structure. DOI: 10.1061/(ASCE)GT.1943-5606.0000738. (C) 2013 American Society of Civil Engineers.
The response of skirted offshore foundations to combined vertical (V), moment (M) and horizontal (H) loading has been studied using two-dimensional finite-element analysis and upper-bound plasticity analysis assuming the soil to be undrained. New information has been gained about the shape of the yield locus and the soil deformation mechanisms occurring at yield from the finite-element analysis. The shape of the yield locus was found to be similar to that predicted by previous workers in V-M and V-H space but differed significantly in M-H space. This behaviour is explained using upper-bound plasticity mechanisms suggested by the soil deformation mechanisms calculated in the finite element analysis. This procedure is then used to give a good approximation to the shape of the yield locus and thus may form the basis for future design methods. Additionally, a simplifying transformation is suggested for the yield locus in H-M space based on plasticity analysis, which allows use of simple mathematical expressions to form a design envelope.
The driven monopile is currently the preferred foundation type for most offshore wind farms. While the static capacity of the monopile is important, a safe design must also address issues of accumulated rotation and changes in stiffness after long-term cyclic loading. Design guidance on this issue is limited. To address this, a series of laboratory tests were conducted where a stiff pile in drained sand was subjected to between 8000 and 60 000 cycles of combined moment and horizontal loading. A typical design for an offshore wind turbine monopile was used as a basis for the study, to ensure that pile dimensions and loading ranges were realistic. A complete non-dimensional framework for stiff piles in sand is presented, and applied to interpret the test results. The accumulated rotation was found to be dependent on relative density, and was strongly affected by the characteristics of the applied cyclic load. Particular loading characteristics were found to cause a significant increase in the accumulated rotation. The pile stiffness increased with number of cycles, which contrasts with the current methodology where static load-displacement curves are degraded to account for cyclic loading. Methods are presented to predict the change in stiffness and the accumulated rotation of a stiff pile due to long-term cyclic loading. The use of the methods developed is demonstrated for a typical full-scale monopile.
Methods are presented for the calculation of the deflections at working loads, the ultimate lateral resistance, and moment distribution for laterally loaded single piles and pile groups. Both unrestrained and restrained piles have been considered. The lateral deflections have been calculated using the concept of a coefficient of subgrade reaction. The ultimate lateral resistance has been evaluated. The results from the proposed methods of analysis have been compared with available test data. Satisfactory agreement was found at working loads between measured and calculated deflections and between measured and calculated maximum bending moments.
Determination of nonlinear force-deformation characteristics of soil may be done by repeated application of elastic theory; soil modulus constants are adjusted for each successive trial until satisfactory compatibility is obtained in structure-pile-soil system; equations and methods of computations are given for elastic- and rigid-pile theory; design recommendations.
Choosing a proper location is a pivotal initial step in building a wind farm. As appropriate locations for onshore wind farms become more and more scarce, offshore wind farms have drawn significant attention. The coastal line of the Great Lakes is an area that has great wind energy potential. This research conducted detailed statistical analysis of the onshore, nearshore, and offshore wind energy potential of Lake Erie near Cleveland, Ohio. It analyzed the wind data collected in 10-min time intervals from three locations near the Lake Erie shoreline to assess wind characteristics. Statistical analyses of wind data include the Weibull shape and scale factors, turbulence intensity, and wind power density. In addition, the capacity factor and the potential energy output are estimated by using two commercial wind turbines, which are appropriate for the sites at 50 m and 80 m hub heights. The results show that offshore sites will produce at least 1.7 times more energy than the onshore and nearshore sites when using the same commercial wind turbine. Furthermore, offshore wind turbines could produce more power during peak hours in the spring and winter. This indicates that offshore wind turbines offer advantages over onshore wind turbines in Lake Erie.
The sustainable development of offshore wind energy requires thorough investigations on technological issues. The substructure, which acts as the natural link between technologies and environments, is a critical topic for the offshore wind industry. This paper presents a comprehensive review of variable types of offshore wind substructures associate with their corresponding example projects. The study is complemented with a special attention to a novel foundation, namely suction bucket foundation. Main technological issues related to this concept are integrated. In the paper, bearing behaviors of offshore wind turbines (OWTs) with the suction bucket foundation under lateral loads, vertical loads, combined loads, and extreme loading conditions are discussed. Two installation methods are introduced. The geometric and improved design is illustrated by considering capabilities in transportation and installation. Research methods, including field tests, laboratory tests, centrifuge tests, theoretical analysis and numerical simulations, are listed; these methods are employed in previous studies to investigate behaviors of the OWT. This review integrates most relevant aspects and recent advancements together, which aims to provide a reference frame for future studies and projects.
The support structure of an offshore wind turbine (OWT) accounts for up to 25% of the capital cost; therefore, investigations into reliable and efficient foundations are critical for the offshore wind turbine industry. This paper describes an innovative hybrid monopile foundation for OWTs, which is an optimization of the original monopile foundation with broader applications. The behavior of OWTs with the hybrid monopile foundation in service conditions are investigated under lateral cyclic loadings, by considering the effects of wind, waves, and ice. A series of centrifuge tests are conducted in order to analyze these behaviors in detail, and OWT models with the original single-pile as well as wheel-only foundations are tested for comparison. Based on these tests, the accumulated lateral displacement and stiffness during cyclic loadings are presented, and the results indicate that the hybrid foundation exhibits a larger cyclic capacity than the other foundations. The influence of the cycle numbers, cyclic loading characteristics, and soil properties is examined during the tests; furthermore, the effects of these factors on the model deformation responses are illustrated. This study proposes the first analytical method for quantitatively estimating the cyclic lateral displacement of the new hybrid foundation in service conditions, and a degradation coefficient is recommended based on the test results. This method aims to provide a simple approach to predicting responses of OWTs with hybrid monopile foundations in service conditions.
The hybrid monopile-friction wheel foundation is an innovative alternative for offshore wind turbines. The concept has wider adaptability and can be used as reinforcement method for existing monopiles. A series of centrifuge tests was performed to investigate the lateral bearing capacities of the hybrid foundation under monotonic loads. Five foundation models and two soil types were considered. According to the recorded responses, the hybrid foundation demonstrated better lateral behaviors that both lateral bearing capacity and stiffness are enhanced. Two analytical methods were proposed and compared with the centrifuge test results. The bearing capacity of the hybrid foundation is smaller than the sum of individual pile and friction wheel, and a reduction factor is suggested for both friction wheels. The friction wheel restrains rotations of monopile and provides extra restoring moments; their effects are idealized as equivalent moments acting on the pile head. The analytical results provide possible solutions in estimating the lateral bearing capacity of the innovative hybrid foundation system for offshore wind turbines by using traditional theories.
Wind Energy is the one of the most promising renewable energy. Suction bucket foundation is considered to be a viable type of wind turbine foundation. Soil liquefaction caused by earthquakes at offshore seismic active area may lead to a significant degradation of soil strength and stiffness. In this study, nine centrifuge tests were carried out to investigate the seismic response of suction bucket foundation under earthquake loading. Both dry and saturate soil conditions were considered in tests. The geometric design of five suction bucket models considered the bucket diameter, penetration depth, and modified buckets with inside compartments. It was found that soil underlying and near the bucket foundation shown a better ability to resist liquefaction in saturated tests comparing to free field while no significant differences were observed in dry tests. The five bucket models performed quite differently, which demonstrated the aspect ratio effects and inside-bucket compartment effects. The results provide insight into optimized design of suction bucket foundation for wind turbine.
The lateral bearing behaviors of suction bucket foundations for offshore wind turbines are investigated by centrifuge modelling in this paper. The centrifuge tests were performed in lightly over-consolidated clay and heavily over-consolidated clay, and three bucket foundation models with aspect ratios of 0.38, 0.5, and 1.3 were tested. The tests were force-controlled, and both static loads and cyclic loads were applied. In static tests, ultimate bearing capacities of suction bucket foundations were extracted from load-displacement curves as the first method, and the results were reinforced by stiffness-displacement curves. The displacement rates were calculated to find critical bearing capacities and ultimate bearing capacities of the foundations as the second method. The cyclic tests include cyclic loads with uniform and increased amplitudes. The accumulated lateral displacements and secant stiffnesses were discussed, and their variations with cycle numbers were studied.
Optimization of the performance for a wind turbine column is performed by coupling a RANS solver for prediction of wind turbine wakes and dynamic programming. Downstream evolution of wind turbine wakes is simulated with low computational cost comparable to that of wake engineering models, but with improved accuracy and capability to simulate different incoming wind turbulence. Dynamic programming is used to estimate optimal tip speed ratio (TSR) and streamwise spacing of the turbines by using a mixed-objective performance index consisting of total power production from the entire turbine array with the penalty of the average turbulence intensity impacting over the rotor discs. The penalty coefficient, representing the economic impact of fatigue loads as ratio of wind energy revenue, is varied in order to mimic different economic periods. The results suggest that a general strategy for wind farm optimization should consist in coupling design performed through spacing optimization and using a relatively low penalty coefficient for the fatigue loads, while wind turbine operations are optimized by varying TSR.
The challenge for floating offshore wind structures is to reduce costs. The industry needs a wind turbine support solution that can be fabricated and deployed from existing shipyards and port facilities, while investors need accurate estimations and forecasts of wind resources and quantified information on the inherent variability in wind power generation. This paper merges hindcast model data with observed in situ data to characterize the wind resource potential off the SW coast of Portugal. The validation procedure adopted allows an estimation of the coefficient used for power-law extrapolation of the wind measurements and a reduction in the uncertainty of the power density calculations. Different types of turbine model are compared and site metocean characteristics are examined as a basis for choosing between existing wind floatable solutions. The calculations using four different wind turbine models indicate a preferable installed capacity of 3–4 MW for a hub height of 90–120 m (i.e., representing the best capacity factor and load hours). There is a consistent difference in power density of about 20% from a location 5 nautical miles (NM) offshore to one 10 NM offshore, which represents an increment of 20%–25% in energy production depending on the particular wind turbine capacity factor.
Wind energy potential assessment is crucial for proper wind farm siting. Typically, this involves installation of tall and costly meteorological masts with anemometers. New technology such as Light Detection and Ranging (LiDAR) is an alternative mobile technology that serves such purpose. This paper describes the principle of LiDAR technology and presents case studies of its applications to evaluate the energy output potentials at the site near Lake Erie in northern Cleveland, Ohio, USA. A ZephIR® LiDAR system is used to monitor one-year of vertical wind data profile (at 30 m and 70 m height) from May 2011 to April 2012, from which the wind statistics are determined. These include the monthly average of wind speed, turbulence intensity, Weibull shape and scale factor, wind compass rose, and wind power density, etc. The wind speed data is used to evaluate the wind power capacity factors for prototype wind turbines that are subsequently installed in 2012. The data of power output by the turbines between 2013 and 2015 is used to compare with those predicted based on wind speed model derived from LiDAR measurement. The results show that the estimated wind turbine’s capacity factor from LiDAR data is satisfactory after excluding the maintenance days. This research demonstrates the potential of LiDAR technology as a cost effective way in providing reliable evaluation of wind energy potential.
Offshore wind turbines (OWT) are often supported on large-diameter monopiles and subjected to cyclic loading such as wind and wave actions. The cyclic loading can lead to an accumulated rotation of the monopile and a change in the foundation stiffness. This long-term effect is not yet well understood. This paper presents a three-dimensional finite element model for analyzing the long-term performance of offshore wind turbines on large-diameter monopiles in sand in a simple way. In this model the characteristics of pile-soil interaction under long-term cyclic lateral loading, observed from well-controlled laboratory model tests, are taken into account. A parametric study has been conducted for a full-scale wind turbine supported on a large-diameter monopile, with focus on the influence of several design parameters on the deformations of the monopile and the tower supporting the wind turbine. The study shows that under the serviceability limit state, the deflection and rotation at pile head in the case of considering the effect of long-term cyclic loading are notably greater than that computed in the case where this long-term effect is ignored. This significant difference suggests that the long-term cyclic loading effect cannot be overlooked in design and analysis.
Large diameter, rigid monopile foundations have been extensively used in the fast-growing offshore wind energy industry over the last two decades. In view of the offshore environment, lateral response of the monopile usually governs its design. Even though several approaches have been recommended based on small-scale laboratory tests, there is no widely accepted method for the design of monopiles under lateral loading. Conversely, field testing on large-diameter prototype monopiles is normally impractical due to the high demand on the capacity of loading equipment. For these reasons a series of field lateral loading tests on reduced-scale monopiles were conducted at a dense sand test bed site. The model monopiles had similar aspect ratios of pile length to diameter to those used in the offshore wind farm projects, but were smaller in scale. Experimental p-y curves were derived using the measured monopile responses of the lateral load tests, and a distinctive failure model was presented for monopiles in the overconsolidated dense sand deposit. Comparison of lateral responses of monopile between the measured and predicted by two current p-y models showed the shear force at the pile tip plays an important role and should be accounted for in the design of laterally loaded rigid monopiles. Finally, a refined p-y model for laterally loaded rigid monopiles in overconsolidated dense sand was recommended and calibrated.
The smart fatigue load control of a large-scale wind turbine blade was numerically investigated on our newly integrated aero-servo-elastic platform with emphasis on the effect of azimuth angles. It was found that the smart control effectively reversed the phases of the flapwise aerodynamic force or the acceleration through the controllable deformable trailing edge flap (DTEF) activation within most of rotor azimuth angle range, turning in-phased flow-blade interaction into an anti-phased one at primary 1P mode, significantly enhancing the damping of the fluid-structure system and subsequently contributing to the greatly attenuatedflapwise fatigue loads on the blade and turbine performances. This aero-elastic control physics was most drastic as the investigations were focused on the case beyond the rated wind velocity, leading to the maximum reduction percentages in the time-averaged and azimuth-averaged fatigue loads up to about 30.0%, in contrast to the collective pitch control method. In addition, the finding pointed to a crucial role that the suppression of the coupled flow-blade system dependent on azimuth angles played in the smart blade control, which might guarantee better effectiveness if it would be considered in the development of the DTEF controller.
Offshore wind turbines near the ocean lane are under a potential threat caused by ship impacts during the service period. Contraposing to three common uses of foundations (monopile, tripod, and jacket) of offshore wind turbines, this study is devoted to probe and compare the anti-impact performance due to a head-on impact by ships. A series of cases are conducted to investigate the foundation damage and the OWT response of the three types of foundations using LS-DYNA, a commercial FEM tool. Through investigating and analyzing the maximum collision-force, the damage area, the maximum bending moment of piles at the seabed, the steel consumption and the maximum nacelle acceleration in different low-energy collision scenarios, it is found that the jacket generates the minimum collision-force, damage area and nacelle acceleration as well as the medium bending moment and steel consumption among the three. Therefore, the jacket has the optimum comprehensive anti-impact performance under low-energy collisions, which may be useful in developing the foundation design of offshore wind turbines.
The purpose of this two-part study is to model the effects of large penetrations of offshore wind power into a large electric system using realistic wind power forecast errors and a complete model of unit commitment, economic dispatch, and power flow. The chosen electric system is PJM Interconnection, one of the largest independent system operators in the U.S. with a generation capacity of 186 Gigawatts (GW). The offshore wind resource along the U.S. East Coast is modeled at five build-out levels, varying between 7 and 70 GW of installed capacity, considering exclusion zones and conflicting water uses.
This paper, Part I of the study, describes in detail the wind forecast error model; the accompanying Part II describes the modeling of PJM's sequencing of decisions and information, inclusive of day-ahead, hour-ahead, and real-time commitments to energy generators with the Smart-ISO simulator and discusses the results.
Wind forecasts are generated with the Weather Research and Forecasting (WRF) model, initialized every day at local noon and run for 48 h to provide midnight-to-midnight forecasts for one month per season. Due to the lack of offshore wind speed observations at hub height along the East Coast, a stochastic forecast error model for the offshore winds is constructed based on forecast errors at 23 existing PJM onshore wind farms. PJM uses an advanced, WRF-based forecast system with continuous wind farm data assimilation. The implicit (and conservative) assumption here is that the future forecast system for offshore winds will have the same performance as the current PJM's forecast system for onshore winds, thus no advances in weather forecasting techniques are assumed.
Using an auto-regressive moving-average (ARMA) model, 21 equally-plausible sample paths of wind power forecast errors are generated and calibrated for each season at a control onshore wind farm, chosen because of its horizontally uniform landscape and large size. The spatial correlation between pairs of onshore wind farms is estimated with an exponential function and the matrix of error covariance is obtained. Validation at the control farm and at all other onshore farms is satisfactory. The ARMA model for the wind power forecast error is then applied to the offshore wind farms at the various build-out levels and combined with the matrix of error covariance to generate multiple samples of forecast errors at the offshore farms. The samples of forecast errors are lastly added to the WRF forecasts to generate multiple samples of synthetic, onshore-based, actual offshore wind power for use in Part II.
Methods are presented for the calculation of deflections, ultimate resistance, and moment distribution for laterally loaded single piles and pile groups driven into cohesionless soils. The lateral deflections have been calculated assuming that the coefficient of subgrade reaction increases linearly with depth and that the value of this coefficient depends primarily on the relative density of the supporting soil. The ultimate lateral resistance has been assumed to be governed by the yield or ultimate moment resistance of the pile section or by the ultimate lateral resistance of the supporting soil. The ultimate lateral resistance is assumed to be equal to three times the passive Rankine earth pressure. The deflections and lateral resistance, as calculated by the proposed methods, have been compared with available test data. Satisfactory agreement was found.
Using results from a lateral load test on a 24-in. pipe pile and laboratory tests on undisturbed clay samples, a tentative procedure for estimating the soil modulus of pile reaction is developed for problems involving transient loads. The correlation that is derived is based on the similitude on logarithmic paper of laboratory stress-strain curves and soil reaction-deflection curves from the pile test.
Wind turbines mounted on floating platforms are subjected to completely different and soft foundation properties, rather than onshore wind turbines. Due to the flexibility of their mooring systems, floating offshore wind turbines are susceptible to large oscillations such as aerodynamic force of the wind and hydrodynamic force of the wave, which may compromise their performance and structural stability. This paper focuses on the evaluation of aerodynamic forces depending on suppressing undesired turbine's motion by a rotor thrust control which is controlled by pitch changes with wind tunnel experiments. In this research, the aerodynamic forces of wind turbine are tested at two kinds of pitch control system: steady pitch control and cyclic pitch control. The rotational speed of rotor is controlled by a variable speed generator, which can be measured by the power coefficient. Moment and force acts on model wind turbine are examined by a six-component balance. From cyclic pitch testing, the direction and magnitude of moment can be arbitrarily controlled by cyclic pitch control. Moreover, the fluctuations of thrust coefficient can be controlled by collective pitch control. The results of this analysis will help resolve the fundamental design of suppressing undesired turbine's motion by cyclic pitch control.
An analytical formulation is derived to fit the observed relationships and utilized to govern the behavior of a one- dimensional element which serves as the interface between two dimensional soil and retaining wall elements in finite element analyses. Analyses are presented of a retaining wall- backfill system with varying modes of wall behavior and degrees of wall roughness. Earth pressure distributions before the ultimate conditions are reached are shown to be nonlinear. Ultimate conditions and general behavior of the system are shown to be in agreement with classical theory. Simulation of the exact construction sequence of a retaining wall- backfill system.
The lateral load tests on piles described herein were part of a comprehensive pile testing program initiated by the U.S. Army Engineer District, Little Rock, Corps of Engineers, in connection with the Arkansas River Navigation Project. Some of the tests were performed prior to construction for the purpose of developing design criteria for design of pile foundation for lock walls and dam monoliths, whereas the remainder were performed during construction to verify design assumptions.
A number of tests have been carried out in the last few years at the LCPC centrifuge facility to study the behaviour of laterally loaded piles. This paper presents a centrifuge test programme of lateral cyclic loading of a single pile in sand. In these tests, the pile head fixity is free and the pile is eccentrically loaded since the point of load application is above soil surface (2.2 times the pile diameter). The experimental set up, the data acquisition and the processing techniques are described. Analyses are focused on the influence of cycles on the soil-pile interaction, on the pile head displacements, on the bending moments and on the P-y reaction curves.
This paper presented a numerical study on the smart fatigue load control of a large-scale wind turbine blade. Three typical control strategies, with sensing signals from flapwise acceleration, root moment and tip deflection of the blade, respectively, were mainly investigated on our newly developed aero-servo-elastic platform. It was observed that the smart control greatly modified in-phased flow-blade interaction into an anti-phased one at primary 1P mode, significantly enhancing the damping of the fluid-structure system and subsequently contributing to effectively attenuated fatigue loads on the blade, drive-chain components and tower. The aero-elastic physics behind the strategy based on the flapwise root moment, with stronger dominant load information and higher signal-to-noise ratio, was more drastic, and thus outperformed the other two strategies, leading to the maximum reduction percentages of the fatigue load within a range of 12.0-22.5%, in contrast to the collective pitch control method. The finding pointed to a crucial role the sensing signal played in the smart blade control. In addition, the performances within region III were much better than those within region II, exhibiting the benefit of the smart rotor control since most of the fatigue damage was believed to be accumulated beyond the rated wind speed.
The validity of using the existing numerical p-y methods [American Petroleum Institute (API) and Reese methods] for the design of offshore wind turbines' large-diameter monopiles in sands is questionable, as many researchers have raised concerns related to the diameter effects in p-y models. This study presents the development of experimental p-y relationships for large-diameter monopiles in dense sands based on results from centrifuge tests exhibiting a softer monopile behavior than those proposed by the API and Reese methods. The effect of socketing the tip of a pile in rock bearing layers was also investigated. The initial gradients of the p-y relationships in dense sand layers were shown to become significantly stiffer as the depth reaches the much stiffer and stronger rock-bearing layer. The lateral load-displacement curves obtained based on the developed experimental p-y relationships were found to be well matched with the measured lateral load-displacement curves; therefore, it was concluded that the developed experimental p-y relationships reasonably predict the lateral responses of large-diameter monopiles.
The documentation is presented for the computer program COM622, which solves for the deflection and bending moment of a pile under lateral loading as a function of depth. The calculations are performed on a finite difference model of the pile, and the soil is represented by a series of nonlinear curves of force per unit length versus deflection. This is the first program documented and distributed under the standards developed for the Geotechnical Engineering Division.
This paper presents the results from a series of centrifuge tests and three-dimensional finite-element (FE) analyses, which examined the benefits of combining a footing with a monopile as a solution for foundations that are subjected to large moment loading, such as those used for towers and wind turbines. The experiments were carried out in silica sand and involved monotonic application of lateral loads at an equivalent prototype height of 26 m above the foundations. Tests were conducted on piled footings, monopiles, and unpiled footings. These experimental results together with the findings from the FE analyses show that the footing interacts positively with the piled foundation and that both the rotational stiffness and capacity of the combined piled footing system are greater than the sum of the individual contributions. Increased capacity arises as the footing causes a significant reduction in moment loading on the pile (hence facilitating the application of larger loads), primarily owing to an increased footing effective area arising from the tension capacity of the pile. (C) 2014 American Society of Civil Engineers.
This paper presents an analysis of the yielding and plastic hardening of uniformly-graded samples of a silica sand subjected to one-dimensional normal compression. Single grains of silica sand have been compressed diametrically between flat platens to measure indirectly tensile strength. Approximately 30 grains were tested for each of the following nominal particle sizes: 0.5 mm, 1 mm and 2 mm diameter. It was found that the data could be described by the Weibull statistics of brittle ceramics, and the Weibull modulus could be taken to be about 3.1. Uniform aggregates of the same sand were then compacted to maximum density and subjected to one-dimensional compression. The initial particle size distributions were 0.3-0.6 mm, 0.6-1.18 mm and 1.18-2 mm, and aggregates were subjected to stresses of up to 100 MPa. All particles were initially of similar shape, and hence the initial voids ratios of the aggregates at maximum density were approximately equal. The yield stress was denned to be the point of maximum curvature on a plot of voids ratio against the logarithm of effective stress, and found to increase with decreasing particle size, and to be approximately proportional to the tensile strength of the constituent grains. However, the plastic compressibility index was found to be approximately constant and independent of the initial grading, and a fractal distribution of particle sizes appeared to evolve under increasing stress. There is evidence to suggest that the aggregates evolve towards a fractal dimension of 2.5 under high stresses.
An overview of offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. The uncertainty in offshore soil conditions as well as random wind and wave loading is currently treated with a deterministic design procedure, though some standards allow engineers to use a probability-based approach. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute p-y method, which is problematic for large OWT pile diameters. Probabilistic methods are used to examine the reliability of OWT pile foundations under serviceability limit states using Euler–Bernoulli beam elements in a two-dimensional pile–spring model, non-linear with respect to the soil springs. The effects of soil property variation, pile design parameters, loading and large diameters on OWT pile reliability are presented. Copyright © 2014 John Wiley & Sons, Ltd.
The present study was conducted to examine the behaviour of instrumented flexible piles in dry sand under lateral cyclic loading using centrifuged models. Considering load service conditions, the influence of the number of cycles of their amplitude and of the soil density on the pile cap displacement and the maximum bending moment of the pile is examined. An empirical law to evaluate pile head displacements at application point is proposed. From the bending moment profile recorded during loading, P-y reaction curves are identified. A reduction coefficient r (P-multiplier) is introduced to quantify the effects of cyclic loads on P-y curves.
This paper investigates available offshore wind energy resources in China’s exclusive economic zone (EEZ) with the aid of a Geographical Information System (GIS), which allows the influence of technical, spatial and economic constraints on offshore wind resources being reflected in a continuous space. Geospatial supply curves and spatial distribution of levelised production cost (LPC) are developed, which provide information on the available potential of offshore wind energy at or below a given cost, and its corresponding geographical locations. The GIS-based models also reflect the impacts of each spatial constraint as well as various scenarios of spatial constraints on marginal production costs of offshore wind energy. Furthermore, the impacts of differing Feed-in-tariff (FIT) standards on the economic potential are calculated. It confirms that economic potential of offshore wind energy could contribute to 56%, 46% and 42% of the coastal region’s total electricity demands in 2010, 2020 and 2030. The shallow waters along the coasts of Fujian, Zhejiang, Shanghai, Jiangsu and northern Guangdong are identified as suitable areas for developing offshore wind energy in terms of wind resources and economic costs. However, the influence of tropical cyclone risks on these regions and detailed assessments at regional or local scale are worth of further discussions. Nevertheless, the models and results provide a foundation for a more comprehensive regional planning framework that would address additional infrastructure, planning and policy issues.
This paper describes the development and application of design charts for monopile foundations of offshore wind turbines in sandy soil under long-term cyclic lateral load. It outlines a numerical model, working with a numerical concept, which makes the calculation of accumulated displacements based on cyclic triaxial test results possible, and it describes important factors affecting the deformation response of a monopile to cyclic lateral loads. The effects of pile length, diameter and loading state on the accumulation rate of lateral deformation are presented and design charts are given, in which a normalized ultimate lateral resistance of a pile is used. For monopiles with very large diameter, the suitability of the “zero-toe-kick” and “vertical tangent” design critera for determining the required embedded length is discussed.
The effect of repetitive lateral loads on deflections of two drilled piers in Tampa Bay were significantly greater than predicted by a p-y procedure commonly used in practice. Reasons for the discrepancy between predicted and measured deflections are discussed. Two methods for predicting the effect of repetitive lateral loads are developed using results of 34 cyclic lateral load tests to quantify model parameters important to the behavior of piles subjected to repetitive lateral loading. The two methods model cyclic lateral load behavior of a pile by degrading soil resistance as a function of number of cycles of load, method of pile installation, soil density, and character of cyclic load. The two methods differ in the computational effort required to make the prediction. The first method is most suitable for hand calculation and rule-of-thumb estimation and is based upon a beam-on-an-elastic foundation model with a soil reaction modulus, K{sub h}, increasing proportionally with depth. The second method modifies nonlinear static p-y curves to derive a cyclic p-y curve. The two methods provides a simple means for estimating effects of cyclic lateral load.
Five model tests were performed to qualify the concept of a monobase gravity platform for the Troll Field in the North Sea. These tests were designed to simulate the conditions for a platform situated in deep water (=330 m) and on soft clay. The program included one static, one static after cyclic, and three cyclic model tests. A description of the procedures and selected test results are presented. The results provide a basis for a critical evaluation of the foundation design procedures presently used for offshore gravity structures in the North Sea. The results indicate that cyclic loading results in a reduced static bearing capacity, cyclic bearing capacity is lower than static bearing capacity, the cyclic stiffness, decreases with number of cycles, and for the relatively large moment arms employed, cyclic rotation is the dominant displacement mode. The results also show that for the conditions of these tests, design storm compositions with the largest cyclic load first, followed by decreasing cyclic loads, may be more critical than the traditional design storm composition with increasing cyclic loads (largest load last).
Based on the principle of energy balance, during cyclic loading, the work done on a soil sample is dissipated in rearranging particles, thus creating irrecoverable strains. Using the concept of critical-state soil mechanics, a relationship is obtained between the dissipated energy and the cumulative strains. Based on this relationship, a simple expression is derived to relate the static stress state and the ratio of cumulative volume to cumulative shear strains (i.e., the first to the second strain invariants). The derived relationship is used to illustrate the dilative and contractive characteristics of sand under cyclic loading. This relationship is also validated by comparing the predicted results with the measured ones obtained from cyclic triaxial tests on sands.