Article

Lateral bearing capacity of hybrid monopile-friction wheel foundation for offshore wind turbines by centrifuge modelling

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Abstract

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.

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... Thus, a new type of single pile-friction wheel composite foundation has therefore been proposed, which combines the monopile and circular footing foundations. Compared to the traditional monopile foundation, this composite foundation provides higher bearing capacity and stiffness with less construction time and expense [10][11][12], as is shown in Figure 1. This composite foundation is initiated from the concept of pile caps and embedded retaining walls with stabilizing platforms, in which the addition of the circular wheel increases the shear stress and restoring moment against the lateral deflection [13][14][15]. ...
... A number of studies have been carried out on the bearing capacity of the composite foundation through centrifuge model tests, detailed three-dimensional finite element analyses and theoretical derivations. Most of these studies are only limited to the behaviors of these foundations in pure sand or clay [10,11,[24][25][26][27][28][29][30][31][32][33][34][35][36]. ...
... For the composite system, the interaction effect between the monopile and the wheel has been mostly conducted in two cases: (i) the composite foundation with a wheel diameter (D w ) exceeds the pile embedment depth (L) (D w /L > 1); and (ii) with a ratio of D w /L = 0.5~1. When the wheel diameter is larger than the pile embedment depth as in case (i), Wang et al. (2018) [10] and [11] conducted centrifuge tests, in which it was found that the lateral bearing capacity of the composite system was actually smaller than the summation of the individual capacities of monopile and wheel. To quantify the reduction, Wang et al. (2018) [10] and Li et al. (2020) [35] proposed an add-up method with a reduction factor to estimate the lateral bearing capacity of the composite foundation. ...
Article
Full-text available
This paper presents numerical modelling to investigate the bearing capacities and failure mechanisms of single pile-friction wheel composite foundation in sand-overlying-clay soil conditions under combined V-H-M (vertical-horizontal-moment) loadings. A series of detailed numerical models, with validations of centrifuge testing results, are generated to explore the potential factors influencing the bearing capacity of this composite system. Intensive parametric study is then performed to quantify the influences of the foundation geometry, soil properties, sand layer thickness, pre-vertical loading and lateral loading height on the failure envelopes in the V-H-M domain. Last but not least, an empirical design procedure is proposed based on a parametric study to predict the bearing capacity of this composite foundation under various loading conditions, which can provide guidance for its design and application.
... As a result, two common options are proposed to enhance the bearing capacity of the monopile: increasing the pile diameter and increasing the pile embedment depth to the bedrock. However, these solutions significantly increase the material costs and installation difficulty (Byrne et al., 2015;Doherty and Gavin, 2011;Pérez-Collazo et al., 2015;Wang et al., 2018b). In Europe, the Pile Soil Analysis (PISA) project was launched to study the behavior of large-diameter monopiles for offshore wind turbines, aiming to improve the design of monopile foundations for offshore wind turbines in a variety of soil conditions. ...
... Several studies have presented methods to improve the original monopile. Wang et al. (Wang and Li, 2020;Wang et al., 2018b) proposed a hybrid monopile foundation with an external plate added at the mud line. The lateral capacity of the foundation significantly improved, with the lateral ultimate load (679 kN) of the solid wheel foundation being 4.4 times larger than that of the single pile foundation (153 kN), and the lateral ultimate load (453 kN) of the gravel wheel foundation is enhanced by 3.0 times. ...
... The addition of the restriction plates is effective in enhancing the vertical bearing capacity, and the installation is easier than that of the close-ended pile. Common methods used to enhance the pile capacity are increasing the pile diameter or enlarging the embedment depth, resulting in more complicated installation procedures and initial costs (Byrne et al., 2015;Doherty and Gavin, 2011;Pérez-Collazo et al., 2015;Wang et al., 2018b). An innovative pile with a one-hole restriction plate was successfully installed in Ohio, USA. ...
Article
The increasing demand for offshore wind turbines with larger capacity brings challenges concerning their original foundation design. This study introduces a new type of monopile foundation with internal restriction plates that aim to enhance the bearing capacity of the entire structure. Two types of improved monopiles are proposed through the addition of one-hole and four-hole restriction plates. A series of centrifuge tests are performed to study the response of the improved piles under lateral loading conditions in saturated sand. The pile-soil interactions are characterized using finite element analysis. A sensitivity study and a parametric study are performed using numerical tests. In offshore applications, the improved pile can provide larger lateral resistances than the open-ended pile foundation. This improvement is proportional to the pile diameter. The four-hole restriction plate is more effective than the one-hole plate for larger diameter piles. The rotational axis is located at 80 % of the embedment depth, and the distribution of earth pressure is determined. A theoretical method is proposed to calculate the ultimate lateral capacity of the novel monopile containing restriction plates.
... The coupled and uncoupled hybrid systems demonstrated different failure mechanisms during external loads. The feasibility study of the hybrid monopile foundation was conducted in our laboratory, and the behavior of the hybrid monopile foundation for offshore wind turbines was checked both under static and cyclic loading conditions (Wang et al., 2018a;Yang et al., 2018). The initial design chart had been summarized. ...
... The lateral load increased from 0 to 1000 kN linearly in 200s monotonically, as demonstrated in Fig. 4. The wind turbine models were loaded to failure, referring to the extreme condition. The lateral displacements were recorded by an LVDT that was fixed at the same level as the actuator, as depicted in Fig. 5. Details of the loading system and the sensor system are described previously (Wang et al., 2018a). ...
... Afterward, the sharing factor of the pile part increases with the continuous development of the lateral deflection, exceeding the wheel part quickly. The platform represents the transformation from wheel bearing to pile bearing during the external load (Wang et al., 2018a). On the contrary, the curve of the monopile foundation is much smoother. ...
Article
The hybrid monopile foundation is an alternative for offshore wind turbines. The parametric study has been performed through a series of centrifuge tests for the optimal design of the hybrid foundation. The wheel diameter, wheel thickness, and pile length are considered in the analysis. The lateral capacity of the hybrid monopile foundation increases with the wheel diameter and tends to accelerate; it increases linearly with the wheel thickness and pile length. The influence of the wheel diameter is more pronounced compared to the other parameters. The hybrid monopile demonstrates an enhanced performance compared to the monopile and the single-wheel. The improvement is more significant at small pile lengths. The hybrid monopile shows its advantages in reducing the pile length. It has great potential in reducing capital costs. An analytical method is proposed by scaling the individual capacity of the pile and the wheel. A design chart for the scale factor is suggested. The calculation is applicable for determining the initial dimension of the hybrid monopile foundation, and the ultimate lateral capacity is assessed.
... Currently, the new generation of offshore structures has higher technical requirements due to their larger capacities or the more harsh working conditions, which will certainly result in more challenges for the design of the foundations. Increasing the pile diameter has been found to be an economic and efficient solution for difficult geotechnical and hydrological conditions in transportation projects (Byrne et al., 2015;Doherty and Gavin, 2011;Wang et al., 2018b). Besides using as a single foundation, the pipe pile is used as the anchors for several foundations, such as the group-pile foundation and the jacket foundation, which are likely to resist very large axial loadings. ...
... 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. ...
... The control function for the load was preprogrammed and input into the control terminal. The signal went through a servo amplifier to mobilize the load actuator, and the load cell reading was used to control the amplitude of the electric current to achieve the target load by the actuator (Wang et al., 2018b). ...
Article
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 hybrid monopile foundation has shown a larger ultimate bearing capacity compared to the monopile foundation from previous researches, which can provide twice of the lateral capacity with a similar vertical load [22,43]. The lateral resistance is the governing factor in designing the foundation supporting earth retaining structures. ...
... It is suggested that the hybrid foundation system is to be more effective if slight sliding is allowed between the pile and the wheel [24]. It has been proved by a series of numerical simulation that the loading capacity of the hybrid monopile foundation is not a simple algebraic summation of the capacities of a single pile and a single wheel [43]. During the lateral force, the wheel resists most of the lateral load while the pile does not play a major role in the initial stage; in the subsequent ultimate stage, the lateral resistance of the pile increases significantly and exceeds the portion of the wheel [57]. ...
... In contrast, displacements of the hybrid monopile foundations accumulate in much milder rates than the single pile, and the total deformations are smaller. The friction wheel introduces additional restoring moment to resist the static and dynamic shear stress induced by the superstructure, and the passive earth pressure acting on the embedded part of the wheel contributes more resistances to the lateral failure [43]. The hybrid monopile foundations demonstrate better lateral stabilities during the earthquake event. ...
Article
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.
... The tensile shaft friction is smaller compared to the compression test. Li et al. [16] [17], investigated the bearing behaviour of an innovative pipe pile with the addition of a restriction plate inside the pile, compared to the traditional system under both static and dynamic loads through a series of geotechnical tests. The proposed plates either having one hole or four holes with different pile diameters have been investigated, Figure 3. ...
... The improvement depends on the pile lateral resistance using external wings to increase the effective passive stresses resist the pile lateral deformation. Figure 3 The different-shape restriction plates: (a) Schematic models, (b) Test Models [17] ...
Article
Full-text available
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.
... Monopile-friction wheel hybrid foundations have attracted much attention in recent years (EI-Marassi 2011; Arshi and Stone 2012;Anastasopoulos and Theofilou 2016;Stone, Arshi, and Zdravkovic 2018;Doherty 2010, Lehane et al. 2014;Pedram 2018;Wang et al. 2018Wang et al. , 2020Yang et al. 2018Yang et al. , 2019Trojnar 2021). However, the previous studies were mainly focused on the bearing capacity and stability of the hybrid foundations in sand or clay soil profiles, with no attention paid on the behavior of foundations in layered soil deposits. ...
... The interaction between components of hybrid foundations subjected to lateral load is studied, undertaken through centrifuge tests, reported by Lehane et al. (2014) and Yang et al. (2018), and they found that the friction wheel interacts positively with the bearing capacity of monopile in sand, and the capacity of the hybrid foundation is greater than the sum of the individual capacities in isolation. However, the findings of Wang et al. (2018) shows that the lateral bearing capacity of hybrid foundation is smaller than the sum of individual monopile and friction wheel. For the effect of wheel on the contribution for the increasing of the bearing capacity of monopile, Yang et al. (2019) also found that, for hybrid foundation, a positive effect for the bearing capacity of monopile is observed, which is due to the additional moment imparted from the friction wheel and the increase of pile lateral earth pressure generated from the bearing pressure underneath the wheel. ...
Article
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.
... The growing demand for wind power, which is acknowledged as a promising energy resource, has promoted the development of offshore wind turbines (OWTs). Most of the existing OWTs are supported by monopile foundations due to their low construction cost and high bearing capacity (Zhang et al., 2021a,b;Wang et al., 2018). 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). ...
Article
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.
... The most common offshore wind turbine substructures are monopile [7][8][9], gravity based foundation [10,11], tripod foundation [12][13][14], jacket foundation [15][16][17], suction bucket [18][19][20], floating foundation [21][22][23] and hybrid structures [24][25][26][27]. A full review of offshore wind turbine substructures and some design recommendations can be found, e.g., in [7,28,29]. ...
Article
Full-text available
With the accelerating progression of global climate change, switching to renewable energy sources is inevitable. Wind energy is a fast-growing branch of this industry, and according to the 2021 Global Wind Report, this trend must continue in order to limit the increase in global average temperature. While onshore wind turbines still dominate and account for most recent growth, offshore wind turbines are becoming a promising alternative for geographical, power density-related or even aesthetic reasons. Offshore wind turbines are subjected to more complex loading conditions and proper foundation design is very challenging, however, this is crucial for ensuring and maintaining the structure’s reliability. Soil dynamic tests are one of the bases for wind turbine foundation design. Technical regulations in many countries require such tests to be carried out in a Resonant Column (RC). In this study, a modification of the RC sensors and data acquisition system was introduced in order to conduct in-depth analysis of vibrating soil specimens. The new set of sensors contained five additional accelerometers (Analog Devices ADXL345) attached to the surface of a soil specimen that was subjected to dynamic loading. These accelerometers sent the data to a new data acquisition system, an ARM microcontroller with software developed by authors. The software was able to process test results synchronously with the original software of the RC device. Additionally, the load control system was supplemented with a current pulse generator, which makes it possible to observe the propagation of high-frequency mechanical waves in the tested materials. The modified dynamic testing equipment allowed for the measuring of accelerations and displacements at specific selected points located along the height of the sample, with sampling frequency more than three times higher than that offered by the sensors originally built into the RC device. As a result, some additional dynamic phenomena (i.e., disturbances in the uniformity of vibrations of non-cohesive materials, specimen–device contact imperfections) were observed in the tested soil specimens which remained undetected in standard RC test.
... One key factor that would limit the economic development of offshore wind turbines lies in the foundation, which takes up to 30-40% of the total project cost (Byrne and Houlsby, 2003;Sun et al., 2012). Of all different foundation types, the monopile originating from the laterally loaded single pile in oil and gas industry is the most widely adopted for its relatively low fabrication and installation cost (Doherty and Gavin, 2012;Wang et al., 2018). By 2018, more than 81% of 5258 installed offshore wind turbines in European countries are supported by the monopile foundations (The EWEA, 2020). ...
Article
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.
... However, very few studies have been conducted on tripod foundations, which have different mechanisms compared to conventional foundations (i.e., monopile and monopods). In addition, design guidelines for tripod foundations have not been established [6][7][8][9][10]. ...
Article
Full-text available
In this study, the cyclic responses of an offshore wind turbine with a tripod foundation installed on an actual site were evaluated in a centrifuge. To understand the behavior of the turbine at the site, the site soil conditions, environmental loads, and real offshore wind turbine structure installed at the actual site were modeled by considering the centrifuge scaling law. From a series of cyclic loading tests, the cyclic responses of the tripod foundation were evaluated in terms of temporary/permanent displacements and cyclic stiffness. Moreover, the long-term behavior of the tripod foundation was predicted from the experimental results. The test results showed that the initial stiffness of the soil–foundation system decreased as the loading amplitude increased and that the stiffness increased with the number of cycles due to soil densification. The findings revealed that the cyclic behaviors of the tripod were more affected by the load amplitude than the number of cycles. In addition, the permanent rotation increased logarithmically with the number of cycles. A simple method to predict the displacement and change in the foundation stiffness of the actual wind turbine is proposed based on the results of the model tests. The results of this study also provide key insights into the long-term cyclic behavior of tripod foundations for offshore wind turbines.
... The new concept of the hybrid monopile for OWT is shown in Fig. 2. The hybrid foundations with a horizontal round bearing plate are intensively researched and analyzed in various scientific centers (Arshi, 2016;Arshi and Stone, 2011, 2012a,b, 2015aArshi et al., 2013;Buslov and Bakulina, 2012;Haiderali and Madabhushi, 2015;Ma and Yang, 2020;Mathus, 2013;Pedram, 2015b;Powrie and Daly, 2007;Stone et al., 2018;Trojnar, 1995, 1997, Trojnar, 2009b, Trojnar, 2013a, 2013bYang et al., 2018Yang et al., , 2019Wang et al., 2018aWang et al., ,b, 2019. It has been pre-confirmed that they allow for better stability, greater safety and cost-effectiveness for OWTs (Abdelkader, 2015;Abdelkader and El Naggar, 2018;Arshi, 2016;El-Marassi et al., 2008;El-Marassi, 2011, Mahiyar andPatel, 2000;Maharaj, 2004;Lehane et al., 2010;Lehane et al., 2014;Stone et al., 2010a,b;Pedram, 2018;Yang et al., 2018Yang et al., , 2019. ...
Article
Full-text available
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.
... [2][3][4] A number of studies have been conducted on the behaviors of monopile or hybrid monopile and addressed in the many technical papers. [5][6][7][8][9] Small-scaled laboratory model tests and field trials have been conducted for extensive studies on suction bucket foundations for wind turbines installed in sand. [10][11][12] In addition, Zhu et al. 4 recently conducted a large-scale model test of a monopod bucket foundation. ...
Article
Full-text available
To be competitive in offshore wind energy production, safe and economical foundation design is essential. In recent years, tripod suction bucket foundations have been considered as an alternative to conventional foundations owing to their unique features suitable for offshore construction environments, economic installation, and high overturning resistance. However, it is difficult to accurately predict the behavior of tripod foundation because the load acting on the tripod is complex in HVM (i.e., horizontal, vertical, and moment loads) and the response varies depending on the size and direction of the load. Moreover, it is harder to analyze because the effects of cyclic loads must be considered in an offshore environment. This study, therefore, has investigated the behavior of the tripod suction bucket foundation under cyclic loadings. To analyze the complex responses of the tripod foundation in detail, the overall behavior of the tripod foundation system was observed based on the compression–pullout behavior of a single bucket. Moment–rotation responses, the cyclic stiffness, and permanent displacements of the tripod foundation are evaluated by analyzing the vertical behavior of the single‐bucket foundations as well as the rotational behavior of the tripod foundation. A number of centrifuge model tests were carried out with different loading conditions (i.e., loading amplitudes and directions). It was confirmed that the cyclic behavior of the tripod bucket foundation is significantly affected by loading amplitudes and directions. Furthermore, this study emphasized the importance of considering load characteristics when designing the tripod foundation.
... Many of the challenges that plague onshore wind are considerably eased in offshore farms and emerging as an alternative or supplement to the onshore wind energy sector in the last ten years. 38 Wind speeds are consistent and higher, and turbines are optimized to operate at very high efficiencies compared to onshore wind turbines. Offshore wind speed is higher in the afternoon and will contribute more energy to the grid and meet peak load. ...
Article
Full-text available
Offshore wind is at its infancy in terms of technology and capacities in India. The Ministry of New and Renewable Energy (MNRE) announced medium and long term offshore targets of 5 GW and 30 GW by 2022 and 2030, respectively. The location of the first offshore wind park has recently been identified, and the Solar Energy Corporation of India (SECI) has signed a contract with the Government of Gujarat to establish the 1000 MW of offshore wind energy capacity by 2019. To achieve the ambitious targets, India will require demand and resources planning, and policy support at an unprecedented scale. The MNRE notified the country’s offshore wind energy policy in 2015, and draft offshore wind energy lease rules in 2019. In this paper, several offshore wind energy challenges have been identified, and a clear policy road map and effective support schemes required to trigger offshore wind development activity for medium to long term are suggested. The environmental consequences of European offshore wind farms are assessed to optimize future monitoring of offshore wind programmes in India. Furthermore, the occupational health and safety management requirements are highlighted to ensure that the accidents, vulnerabilities, and hazards are avoided. The research and development (R&D) considerations are provided to assist policymakers, potential investors, stakeholders, designers and manufacturers, contractors, professional advisers, and wind farm developers in their decisions and planning.
... 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. ...
Article
Full-text available
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.
... Some concepts of hybrid foundation combining monopile with other integrated components have been proposed. An additional friction wheel on a monopile foundation can enhance the bearing capacity of the traditional monopile foundation, and the increment could be up to 2 times (Wang et al., 2018a(Wang et al., , 2018b. A 1-g model test shows the lateral bearing capacity and initial stiffness are enhanced by adding a footing or skirt to the monopile, and the improvement depends on the size of the footing and the soil condition (Stone et al., 2007). ...
Article
In this study, a hybrid monopile-friction wheel-bucket (MFB) foundation for offshore wind turbines is proposed. A bucket and a friction wheel are integrated with a monopile. The friction wheel is filled with scattered material to provide distributed surcharge loads to the subsoil. A series of geotechnical centrifuge tests was performed under monotonic load and cyclic load to investigate the bearing capacity of the MFB foundation. Five hybrid foundations with variable dimensions were tested in four types of sandy soil conditions. The centrifuge test results show that the ultimate bearing capacity of the hybrid MFB foundation could be 4 times of the monopile foundation. Under cyclic load, the final displacement of MFB foundation is significantly smaller than that of the monopile foundation. The MFB foundation is stiffer in the reloading process but shows slightly more plastic characteristic in the unloading process. The size of the add-on friction wheel-bucket structure is positively related to the performance of the MFB foundation. The bucket height tends to be a more effective factor. It is illustrated that the MFB foundation tends to demonstrates more improvements in the saturated loose sand. A simplified method is proposed to predict the bearing capacity of the hybrid MFB foundation.
... The vertical displacement is measured by the linear variable displacement transducer (LVDT). The load control function was programmed and input into the control terminal [46]. ...
Article
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.
... The offshore monopiles are typically hollow steel pipe piles with larger diameters (Negro et al., 2017). In China, more than 5000 pipe piles are used in the construction of Hangzhou Bay Bridge (Yu and Yang, 2011), and the monopile-supported intertidal wind turbine contributes for 56.69% of the installed wind capacity; in Europe, the monopile foundation is the most widely used offshore wind foundation with a market share of 87% by the end of 2017 (Wang et al., 2018a). The monopiles are driven into the ground and provide considerable bearing capacities in service conditions. ...
Article
The monopile has been widely used to support offshore and coastal structures. A series of centrifuge tests has been performed to investigate the bearing capacity of large diameter monopiles in sandy soil. Both static tests and cyclic tests have been conducted for open-ended and close-ended model piles, and the effects of influence factors, such as loading rates, embedment depths, and loading histories are considered. The piles are then loaded by a sequence of compressive-tensile loadings to estimate the tension capacity, from which the shaft friction is derived. The cyclic load tests are performed with five varying load intensities, and the accumulated settlement is assessed. The centrifuge 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 behavior. The tensile shaft friction is smaller compared to the compression test. The capacities of the piles reduce significantly under the axial cyclic load, and the maximum cyclic load intensity should be limited to 75% of the ultimate bearing capacity. The API design method is used to calibrate with the centrifuge test results. The method overestimates the bearing capacity at larger depths, and a conservative reduction factor is required.
... They found that both the ultimate soil resistance and the initial modulus of subgrade reaction were below the conventional p-y curves. Different types of offshore foundations for wind energy were suggested, and the behavior of such foundations have been investigated by many researchers [34][35][36][37] In spite of recent efforts to understand the lateral behavior of offshore monopiles, a lot of research topics are still remaining. The conclusions from the various literatures may be consistent or mutually contradictory. ...
Article
In this study, the cyclic lateral behaviors of an offshore monopile in saturated dense sand under cyclic loading was investigated using centrifuge model tests. The soil used for testing was Jumunjin sand, which was deposited with a relative density of 80 %. A static loading test was carried out to obtain the static lateral capacity of the monopile, from which the magnitudes of cyclic load were determined at 30 %, 50 %, 80 %, and 120 % of the lateral capacity. A hundred cycles were applied to the pile head with the frequency of 0.125 Hz. Experimental cyclic p-y curves were obtained at 2, 5, and 7-m depths, from which equations for cyclic p-y curves for saturated dense sand were proposed. The proposed p-y curve was compared with the conventional p-y curves; it was found that the proposed equations overestimate the ultimate soil resistance compared with the conventional ones, whereas the initial modulus of subgrade reaction was only 35 % the conventional ones.
... The State of Ohio, USA is among the pioneers in promoting renewable wind energy at both onshore and offshore sites [54][55][56][57]. Particularly, Ohio possesses significant potential wind energy resources near the Lake Erie area. ...
Article
Wind shear models are commonly used to predict the wind speed at wind turbine hub heights from the wind data collected at the elevation of the monitoring station. In many cases, the model parameters are based on empirical values recommended by design specifications that reflect the site conditions. This paper evaluates the benefits of incorporating site-specific wind data to calibrate the wind shear model parameters. Wind speed data collected by a ZephIR® Light Detection and Ranging (LiDAR) system over a 2-year (Oct. 2010–Sept. 2012) period are used for the analyses. The atmospheric stability is found to have appreciable effects on the wind shear parameters, i.e. wind shear coefficients (WSC) for the power law model and roughness lengths for the logarithmic law wind shear models. The calibrated wind shear model parameters by the monitored wind data during the first year are presented in the format of a contour map to demonstrate the spatio-temporal variations, which shows daily and seasonal variations. The calibrated wind shear models are then validated by the wind data collected during the second year, which demonstrates decent performance. The accuracy and performance of incorporating site-specific wind shear model calibration to predict the wind energy resource is evaluated, where six different methods are compared. The results show that the consideration of spatio-temporal variations of wind shear model parameters achieved improve performance over the application of the empirical or yearly-averaged wind shear model parameters in extrapolating the wind speed. It is also found that the performance of considering spatio-temporal wind shear parameters are even better at higher elevations. Furthermore, the analyses find that the use of empirical wind shear model parameters underestimates the wind energy output at the studied sites. Site-specific calibration of the wind shear models could further improve the accuracy of wind energy assessment by considering the site condition and the variability in the atmospheric stability.
... However, offshore wind energy development in the U.S. is in its early stages, with several projects currently in the planning phase, including the Cape Wind project (Massachusetts), the Blue-water Wind project (Delaware), the LIPA offshore wind park (New York), and the Galveston offshore wind project (Texas) [3,10,11]. Offshore wind energy has several advantages over onshore wind energy [12,13]. First, offshore wind allows for development in populous regions where there is little land available, but the local energy demand is high. ...
Article
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 monopile foundation is constructed onshore and transported to designated location; it is installed by the pile driving or drilling, and seabed preparations are not necessary [37]. Recently, some improved concepts of the monopile foundation has been proposed and tested [63,71]. The monopile foundation has been applied in many major offshore wind projects, such as Horns Rev 1-3 in Denmark [72][73][74][75][76], London Array in the United Kingdom [77,78], Dantysk in Germany [79,80] and Anholt in Denmark [81,82]. ...
Article
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.
Article
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.
Article
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.
Article
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.
Article
In this study, a series of centrifuge tests are performed to investigate the ultimate lateral capacity of a pile group for offshore wind turbines. The validated Finite element models (FEMs) are established. The rotation center and the maximum bending moment occur at 9 D and 3 D of the pile length. A sensitivity study is performed by considering the soil parameters through numerical tests. The internal friction angle is the most influential factor by introducing 32% change in lateral capacity. The key factors of pile number, pile diameter, loading height, and pile spacing are investigated in the following parametric study. Increasing the pile number and the pile diameter is effective for enhancing the lateral capacity. In contrast, a larger eccentricity has a negative effect on the foundation stability. The failure mode of the pile group foundation tends to change from strain softening to strain hardening when the loading height reaches a threshold magnitude. The p-multiplier is calculated to demonstrate the shadowing effect in a pile group foundation. An optimal design method is proposed by integrating the pile number and the pile spacing factors. This study provides some instructions for the application of pile group foundations in the offshore wind industry.
Article
This paper compares the lateral performance of the two types of monopile foundations, including steel wheel foundation and concrete-filled double skin steel tubular (CFDST) for that portion of the monopile. To achieve this goal, a conventional monopile foundation is chosen and modeled through the three-dimensional (3D) finite element (FE) method. In this paper, to validation of numerical models, the centrifuge test results are compared with the FE model. The findings of FE analyses show that when the monopile was equipped with a steel wheel and an ultra-high performance concrete (UHPC), the lateral load-bearing capacity of the monopile was improved. From the FE analyses, the installation of a steel disk to a short stiff monopile increases the lateral load-bearing capacity of the monopile and decreases the internal body forces in the monopile. If the wheel diameter increase, the lateral load-bearing capacity enhances considerably. By increasing the length of monopile, the lateral stiffness of the system decreased because of the effect of the slenderness of monopiles and unsupported length.
Article
In recent, suction anchor foundations have been encouraged as an alternative for supporting offshore wind turbines in the deep sea. The foundation for offshore wind turbine foundation should be designed considering the cyclic forces, which cause an accumulated displacement of the structure and degradation of the stiffness of the ground-foundation system. However, previous researches for suction anchor behaviors have mostly focused on static loads, although the forces applying on the offshore structure are cyclic in the sea. Moreover, since the suction anchors are always subjected to sustained pullout loads combined with cyclic loading due to the buoyancy forces of the structure, evaluating the in-service performance of suction anchor foundations considering cyclic loadings with sustained pullout loads is also required. In this study, the responses of a suction anchor foundation under cyclic loading along with sustained pull-out loads were investigated via centrifuge model tests. Consequently, it was observed that the behavior of the suction anchor are dependent on the sustained pullout loads, as well as the cyclic loading. This study highlights that the ratio of sustained pull-out loads as well as the cyclic load effect must be considered when analyzing design loads to accurately evaluate the bearing resistance of the suction anchor foundation.
Article
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.
Article
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Here, the results of a three-dimensional finite element study of the complex interaction of horizontal and moment loads (HM) on offshore monopiles as failure envelope, are reported. A new design criterion is described which is based on critical length, ultimate limit states, load characteristics and Eigen-frequency to ensure stable behavior of laterally loaded monopiles. Numerical analyses were performed to examine nonlinear interaction of a soil-pile system for 10,000 load cycles. The resulting framework can predict angular rotation due to cyclic loading. According to the loading level and duration of a load, elastic strains accumulate in the vicinity of a pile. Fairly intermediate two-way cyclic loading induced the largest rotations irrespective of the analysis performed (i.e., drained versus partially drained). Based on the regression coefficients of the non-dimensional frameworks used, accumulating rocking deformations of a pile at seabed level appear to be dependent on cyclic load ratio, drainage condition, and duration of loading. For safe design, sensitivity of the natural frequency of offshore wind turbine (OWT) at a monopile critical length as well as shorter lengths were also examined. The analytical model proposed here for determining the natural frequency of an OWT considers that soil-structure interaction (SSI) can be represented by monopile head springs characterized by lateral stiffness, KL, rotational stiffness, KR, cross-coupling stiffness, KLR, and parabolic soil stiffness variation with depth.
Article
A wide-shallow bucket is connected to a monopile for offshore wind turbines to withstand the severe loading conditions in marine environments and this innovative pile-bucket foundation has not been investigated comprehensively. In this paper, a monopile, wide shallow mono-bucket and pile-bucket were firstly tested under lateral loading via geotechnical centrifuge to examine the performance of the hybrid foundation. The results of the centrifuge tests and extensive finite element analyses with dimensions of an actual wind turbine foundation show that the addition of the bucket significantly enhances the lateral bearing capacity and stiffness of the monopile in both sand and soft clay. The load transfer mechanism, failure mode and bearing behavior are illustrated to study how the bucket and pile component contribute to the performance of the foundation system and the interactions of the pile-soil-bucket. Finally, parametric studies about the loading eccentricity and geometry of the bucket are carried out to provide references for the engineering practice.
Article
This paper presents a physical model study of the feasibility and performance of a monopiled gravity base structure (MGBS) for offshore foundation applications. The research study builds on the current interest in hybrid foundation systems such as monopiled footings. The proposed MGBS is essentially a conventional gravity base structure (GBS) with a projecting monopile or caisson. The system relies on the self-weight of the GBS to drive the projecting monopile into the seabed. Once installed, additional ballast can be added to the GBS, and if necessary installation of the projecting monopile can be enhanced through the use of suction. The test programme has concentrated on the response of the system once installed and has investigated a range of geometries for circular monopiles. The results clearly demonstrate a significant enhancement to lateral resistance offered by the addition of a monopile to a GBS.
Article
Considering the deficiencies of the traditional monopile foundation for offshore wind turbines (OWTs) in severe marine environments, an innovative hybrid foundation is developed in the present study. The hybrid foundation consists of a traditional monopile and a wide–shallow bucket. A series of numerical analyses are conducted to investigate its behavior under the static and dynamic loading, considering various loading eccentricities. A traditional monopile with the same steel volume is used as a benchmark. Although the monopile outperforms the hybrid foundation in terms of the ultimate moment capacity under each loading eccentricity, the latter can achieve superior or the same performance with nearly half of the pile length in the design loading range. Moreover, the horizontal load and moment are mainly resisted by the bucket and the single pile in the hybrid foundation respectively. The failure mechanism of both the hybrid foundation and the monopile is excessive rotation. In the rotation angle of 0.05 rad, the rotation center is located at the depth of approximately 0.6–0.75 times and 0.65–0.75 times the pile length for the hybrid foundation and the monopile respectively. The increasing loading eccentricities can lead to increasing moment bearing capacity, increasing initial stiffness and upward movement of the rotation center of the two foundations, while decreasing load sharing ratio of the single pile in the hybrid foundation. Three scenarios are considered in investigating the dynamic loading behavior of the hybrid foundation. Dynamic response results reveal that addition of the bucket to the foundation can restrain the rotation and lateral displacement effectively. The superiority of the hybrid foundation is more obvious under the combined wave and current loading.
Article
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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.
Article
Existing tripod suction bucket foundations, utilised for offshore wind turbines, are required to resist significant lateral loads and overturning moments generated by wind and currents. This paper presents an innovative type of tripod bucket foundation, ‘hybrid tripod bucket foundation’, for foundations of offshore wind turbines, which has the ability to provide a larger overturning capacity compared with conventional tripod buckets. The proposed foundation consists of a conventional tripod bucket combined with three large circular mats attached to each bucket. A series of experiments were conducted on small-scale models of the proposed foundation subjected to overturning moment under 1g conditions in loose sand. Different circular mat diameter sizes with various bucket spacings were considered and the results were compared with conventional tripod bucket foundation. Finite element models of the proposed foundation were developed and validated using experimental results and were used to conduct a parametric study to understand the behaviour of the hybrid tripod bucket foundation. The results showed that there is a significant increase in overturning capacity provided by the novel foundation. The results of this work can significantly improve lowering the costs associated with installation of foundations to support offshore wind turbines.
Article
The hybrid foundation of combining a friction wheel with a monopile is an innovative solution for offshore structures subjected to large lateral-moment loading. In this paper, the lateral-moment response of the monopile-friction wheel foundation in saturated sand are investigated via centrifuge tests and three-dimensional finite element method (FEM). A series of tests on the monopile, hybrid foundations with wheels of different diameters and thicknesses, and single wheel foundation were conducted. The results show that the lateral bearing capacity and stiffness improve significantly by adding a wheel to monopile, and the improvement follows the diameter or thickness of the wheel. An extensive experimental research regarding to the influential factors such as the embedment of the wheel and the vertical load is also presented. By means of FEM, the load transfer mechanism, interaction between the foundation and soil, and the bending moment in the pile are illustrated to study how the wheel contributes to the performance of the foundation system. Moreover, the effects of load eccentricity and vertical load are investigated by FE analyses.
Conference Paper
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.
Article
This paper evaluates the lateral performance of a monopile reinforced by a gravel wheel for offshore structures via centrifuge tests and three-dimensional finite-element (FE) modelings. The gravel wheel comprises a ring frame placed on the pile head and filled with large particles to potentially utilize gravel or crushed stone in offshore areas. The results of centrifuge tests and FE analyses demonstrate that the lateral loading capacity of the monopile increases when combined with a gravel wheel, and the improvement depends on the diameter and thickness of the wheel. By means of FE methods, the interaction between the pile and surrounding soils and gravel fill are illustrated to explain how the gravel wheel contributes to the lateral resistance of the hybrid system. Furthermore, an equivalent layer method adopting the conventional p-y curves is suggested to predict the lateral response of the hybrid foundation. This method is validated by comparisons with the centrifuge tests results. Finally, a case study of the monopile-gravel wheel foundation indicates that the gravel wheel is less efficient in configurations where the ultimate capacity of the hybrid system is dictated by the bending capacity of structures rather than the strengths of soils.
Article
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.
Conference Paper
Offshore wind farms that are constructed in seismic active areas have high requirements for their foundation designs. The earthquake induced soil liquefaction may cause severe damage to the whole structure and consequently influence the operation of wind turbines. Monopile foundation has been widely used currently, and based on this type of foundation, an innovative foundation is proposed. The concept of hybrid monopile-friction wheel foundation is a combination of a monopile and a gravity base, which is regarded as an improved and reinforced design of the original monopile. A series of centrifuge tests was performed to investigate the seismic response of the hybrid foundation system in sandy soil. Two types of hybrid monopile-friction wheel foundations, which contain solid wheel or gravel wheel, were tested, and the results were compared with the original monopile foundation. Accelerations, pore water pressure ratios, and settlements were recorded to illustrate the results. Each tested model exhibited distinct behaviors during the earthquake. It was found that the friction wheel further reinforced the foundation soil, and the liquefaction tendency is lower. The results validate this new concept of offshore wind hybrid foundation systems.
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This paper describes the feasibility analysis of an innovative, extensible blade technology. The blade aims to significantly improve the energy production of a wind turbine, particularly at locations with unfavorable wind conditions. The innovative ‘smart’ blade will be extended at low wind speed to harvest more wind energy; on the other hand, it will be retracted to its original shape when the wind speed is above the rated wind speed to protect the blade from damages by high wind loads. An established aerodynamic model is implemented in this paper to evaluate and compare the power output of extensible blades versus a baseline conventional blade. The model was first validated with a monitored power production curve based on the wind energy production data of a conventional turbine blade, which is subsequently used to estimate the power production curve of extended blades. The load-on-blade structures are incorporated as the mechanical criteria to design the extension strategies. Wind speed monitoring data at three different onshore and offshore sites around Lake Erie are used to predict the annual wind energy output with different blades. The effects of extension on the dynamic characteristics of blade are analyzed. The results show that the extensive blade significantly increases the annual wind energy production (up to 20% to 30%) with different blade extension strategies. It, therefore, has the potential to significantly boost wind energy production for utility-scale wind turbines located at sites with low-class wind resource.
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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.
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Accurate and reliable prediction of wind energy production is important for the operational management of wind farm as well as ensuring the stability of electrical grid integrated with renewable wind energy. This paper describes a new method that aims to reliably predict wind energy production based on weather forecast data. With this new method, an aerodynamic model is firstly used to predict the distribution of wind speed with elevation by use of 24-hour ahead forecasted wind speed data at three-hour intervals (which is available with common weather forecast provider). The aerodynamic model considers the influence of factors such as ground topology, types of land cover, etc. on the wind speed distribution. Based on this model, wind speed at 10-minutes time interval is simulated, which is subsequently used together with the turbine production curve to predict energy production within the next 24 hours. The model and procedures for forecasting wind energy production are validated on a 100kW utility scale wind turbine. Comparison with alternative wind energy forecast procedures show that this new model-based forecast method provide more accurate prediction of wind energy production for different seasons. Major advantages of this model include that it is based on data commonly available from weather forecast providers and is applicable to different terrain conditions.
Conference Paper
Full-text available
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.
Conference Paper
Full-text available
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.
Conference Paper
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The vast and advantages of offshore wind resources continuously motivate the development of offshore floating wind turbines. This paper describes a model and proceeds preliminary design of a 60 meter deep offshore Tension Leg Platform (TLP) floating foundation for the NREL 5 MW prototype wind turbine and analysis the reliability of the wind turbine under different working conditions. The TLP floating foundation is one of the most optimal floating foundations for offshore wind turbines. A finite element model is used to calculate the initial stability and buoyancy of the designed wind turbine floating foundation. The model established in Moses software considered wind, wave and current load at the same time. Three different working conditions are tested by defining different wind, wave and current magnitude. It was found that compared with alternative floating foundation, the TLP floating foundation is more stable and keep operating at certain working condition. This research can give a guide and reference for further offshore wind turbine development. © 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Article
The results of full-scale lateral load tests on three groups of six piles were published earlier in January, 1976 as Proc Paper 11849. Additional full-scale lateral load tests were performed on the same pile groups and piles after removing 4 in. of soil from under the pile caps to study the effect of the soil directly under the pile caps on the lateral capacity of pile groups. In the range of 16.66 kips/pile to 33.33 kips/pile lateral loads, the absence of the soil under the pile caps increased significantly both the lateral deflections of the pile groups and bending moments in piles.
Article
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.
Article
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.
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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.
Article
This paper describes the beginning and evolution of the now worldwide growing offshore wind energy industry. In particular, the current renewable energy policy in the UK is described. The characteristics of the environmental loads offshore, type of seabed soils and foundations commonly adopted are explained. The type of structure and loading regime establish new conditions from a Civil Engineering point of view. Suction caissons are introduced as an alternative foundation for offshore wind turbines. Suction caissons are currently an accepted alternative to pile foundations in applications for the oil and gas industry. However, this is not yet the case in applications for offshore wind turbines.
Article
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.
Article
The purpose of this paper is to evaluate the improved suction bucket foundation (ISBF) for the offshore wind turbine. The ISBF included the design of two types of internal compartments inside buckets with different aspect ratios (AP). A series of centrifuge tests were performed to investigate the lateral bearing behavior of ISBF in four types of sandy soils. Both static and cyclic lateral loads were applied by an electrical actuator in a load-controlled system. The first method interpolated the lateral bearing capacity from the load-displacement curve and reinforced the results with the stiffness-displacement relationship. Moreover, displacement rate was calculated and plotted to obtain the lateral critical and ultimate bearing capacity as the second method. During cyclic tests, the first several cycles changed the lateral displacement and stiffness significantly, whereas the last several cycles did not obviously affect them. It was demonstrated that the geometric designs of ISBF had limited influence on its lateral bearing behavior, but the soil conditions of the foundation did bring about an obvious difference. The lateral capacity of ISBF was further compared to original suction bucket foundation (OSBF), thereby illustrating the advantages of the ISBF in offshore wind turbine constructions.
Article
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.
Article
Suction bucket foundation is a promising alternative for offshore wind turbine foundations. The lateral bearing behavior and failure mechanism are significant topics for optimizing its design criteria. In this study, a group of centrifuge tests were performed to investigate the lateral bearing capacity of suction bucket foundation with three aspect ratios. Force-controlled lateral static and cyclic tests were performed at a centrifuge acceleration of 50 g. Four soil conditions were considered in centrifuge tests with a combination of loose/dense and dry/saturated sand. The load-displacement and the stiffness-displacement relationships are presented in the paper, and it is concluded that the lateral bearing capacity can be reached when the normalized lateral displacement (displacement divided by the bucket diameter) reached 3% as the first method. Displacement rates are plotted against the lateral load to give the second method for defining the ultimate lateral capacity. In cyclic tests, the lateral displacement and stiffness are correlated to the cycle numbers in the first 5 cycles, but the change is less apparent in the rest. The results demonstrated the behavior of bucket foundation in service conditions. Finally, a simplified calculation method is used as the third method and checked with tests results.
Article
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.
Conference Paper
Bucket foundation has been used extensively in offshore facilities to resist combined lateral and moment loading. The lateral loading capacity and interaction between the soil and foundation is of great interest to geotechnical engineers. In this paper, finite element models of a bucket foundation with two aspect ratios (L/D, where L is the skirt length and D is the foundation diameter) of 0.5, 1.33, were created using the ABAQUS program. Centrifuge model tests were also conducted on the two models. The load-displacement curves of the numerical analysis and test results had good agreement, which verified the reliability of the finite element analysis method for this type of problem. In addition, several 3D numerical models of aspect ratio varied from 0.2 to 2.0 were built to evaluate the effect of the aspect ratio on the lateral loading capacity. The development and distribution of stress and plastic strain were used to study the failure mechanism of bucket foundation for different L/D ratios. The influence of the position of the lateral load applied was analyzed. Sensitivity analysis for the parameters of material properties such as the internal friction angle, the Young’s Modulus, and the friction coefficient between the bucket and soil were performed using non-linear analysis with the Mohr-Coulomb soil model.
Article
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.
Conference Paper
Objective/Scope Offshore wind turbines (OWTs) are getting taller and heavier, and medium water depths are utilized in recent and future offshore wind farms. The increasing turbine size brings with it the increasing size of monopile foundations according to current design guidelines. The increasing diameter of monopiles increases the costs of the foundation, that is material, manufacturing, transportation and installation costs. In addition, it poses engineering challenges, as well as environmental problems due to the high levels of noise emitted by pile driving. The strictest requirement on the monopile diameter is imposed by Serviceability Limit State requirements, which include allowable maximum deflection, allowable maximum initial- and accumulated rotation at the pile head (mudline). These seem to derive from manufacturer requirements for the safe operation of wind turbines and from visual concerns, and may originate from onshore wind technology. However, no obvious reasons were found by the authors for the strict requirements in offshore wind technology considering the additional costs they add. Therefore, the objective of this paper is to critically review the SLS requirements of OWT monopile foundations and investigate the possible reasons for the strict requirements. In addition, in light of the much higher allowable tilt prescribed for floating OWTs, alternative scenarios are investigated with adjusted SLS requirements on pile head deflection and rotation. Methods, Procedures, Process In order to confirm that the SLS requirements typically dominate design with the current guidelines, a simple framework assuming a thin walled pile is given to compare the monopile dimensions required for the Ultimate, Fatigue and Serviceability Limit States. Possible reasons for the strict requirements are investigated, and using the presented simple framework, monopile dimension requirements are recalculated for adjusted less strict SLS requirements. Results, Observations, and Conclusions The SLS requirements are confirmed to give the highest dimension requirements for monopiles. Slightly higher allowable tilt and deflections already show reduced dimension requirements for monopiles. The investigation into reasons for strict requirements suggest that a critical review of these requirements for design codes is necessary and would be beneficial in terms of costs and applicability of monopile foundations for large turbines and medium waters. Novel/Additive Information The investigation reveals possible opportunities to review design guidelines for offshore wind turbine foundations. Adjusting the design requirements in offshore environments with a cost-benefit approach in mind could save substantial costs in material, manufacturing, transportation and installation of monopiles as well as extend the applicability of this type of foundations.
Article
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.
Article
A simplified design procedure for foundations of offshore wind turbines is often useful as it can provide the types and sizes of foundation required to carry out financial viability analysis of a project and can also be used for tender design. This paper presents a simplified way of carrying out the design of monopiles based on necessary data (i.e. the least amount of data), namely site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil stiffness at one diameter depth). Other data that may be required for final detailed design are also discussed. A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Where possible, validation of the proposed method is carried out based on field data and references/guidance are also drawn from codes of practice and certification bodies. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of foundations for London Array wind farm is taken to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one actually used in the project.
Article
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.
Article
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.
Article
The results of full-scale lateral load tests on three groups of six piles were published earlier in January , 1976 as Proc Paper 11849. Additional full-scale lateral load tests were performed on the same pile groups and piles after removing 4 in. of soil from under the pile caps to study the effect of the soil directly under the pile caps on the lateral capacity of pile groups. In the range of 16. 66 kips/pile to 33. 33 kips/pile lateral loads, the absence of the soil under the pile caps increased significantly both the lateral deflections of the pile groups and bending moments in piles.
Article
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.
Article
To reach rational solutions for problems of laterally loaded piles, the non-linear force-deformation characteristics of the soil must be considered. This may be done by repeated application of elastic theory. Soil modulus constants are adjusted for each successive trial until satisfactory compatibility is obtained in the structure-pile-soil system. The computations are facilitated by non-dimensional solutions. Basic equations and methods of computation are given for both elastic-pile theory and rigid-pile theory. Several forms of soil modulus variation with depth are considered. Typical solutions are presented and recommendations given for their use in design problems.
Article
With the rapid development of offshore wind energy, the seismic stability of foundation structures is a concern, especially if the soil is sandy soil that is susceptible to liquefaction. One of the commonly used foundation structures for offshore wind is the bucket foundation. A group of centrifuge tests were performed to study the seismic behavior of the foundation. Acceleration, pore water pressure, settlement, and lateral displacement were recorded to analyze the seismic response of both the upper structure and foundation. The influences of design parameters such as size of the foundation, depth of penetration in the ground, compartments within the foundation were evaluated. The experimental data show that the bucket foundation is capable of resisting failure under seismic event.
Article
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.
Article
The future of offshore wind energy will be in deep waters. In this context, the main objective of the present paper is to develop a sensitivity analysis of a floating offshore wind farm. It will show how much the output variables can vary when the input variables are changing. For this purpose two different scenarios will be taken into account: the life-cycle costs involved in a floating offshore wind farm (cost of conception and definition, cost of design and development, cost of manufacturing, cost of installation, cost of exploitation and cost of dismantling) and the most important economic indexes in terms of economic feasibility of a floating offshore wind farm (internal rate of return, net present value, discounted pay-back period, levelized cost of energy and cost of power). Results indicate that the most important variables in economic terms are the number of wind turbines and the distance from farm to shore in the costs’ scenario, and the wind scale parameter and the electric tariff for the economic indexes. This study will help investors to take into account these variables in the development of floating offshore wind farms in the future.
Article
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.
Article
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.
Article
Series of laboratory tests are crried out on the friction between steel and air-dried sands with a simple shear apparatus. The significance of factors on the frictional coefficient are examined with the use of the experimental design method by orthogonal array table. The influence of sand type (Fujigawa Sand, Fukushima Sand, Toyoura Sand and Glass Beads) and the surface roughness of steel (Rmax=1.5∼3 μm and 10∼15 μm) are found to be significant, while the effects of normal stress (98 kPa and 980 kPa) and mean grain size (0.55∼0.62 mm and 0.15∼0.19 mm) are of poor significance. The sliding displacement at the contact surface and the displacement due to shear deformation of sand mass were obtained distinctively. The shear deformation of sand mass was found to be unaffected by the surface roughness of the steel.
Article
Offshore wind turbines are subjected to multiple dynamic loads arising from the wind, waves, rotational frequency (1P) and the blade passing frequency (3P) loads. In the literature, these loads are often represented using a frequency plot where the Power Spectral Densities (PSDs) of wave height and wind turbulence are plotted against the corresponding frequency range. The PSD magnitudes are usually normalised to unity, probably because they have different units and thus the magnitudes are not directly comparable. In this paper a generalised attempt has been made to evaluate the relative magnitudes of these four loadings by transforming them to bending moment spectra using site and turbine specific data. A formulation is proposed to construct bending moment spectra at the mudline, i.e. at the location where the highest fatigue damage is expected. Equally, this formulation can also be tailored to find the bending moment at any other critical cross section, e.g. the Transition Piece (TP) level. Finally, an example case study is considered to demonstrate the application of the proposed methodology. The constructed spectra serve as a basis for frequency based fatigue estimation methods available in the literature.
Article
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.
Article
Wind energy, commonly recognized to be a clean and environmentally friendly renewable energy resource that can reduce our dependency on fossil fuels, has developed rapidly in recent years. Its mature technology and comparatively low cost make it promising as an important primary energy source in the future. However, there are potential environmental impacts due to the installation and operation of the wind turbines that cannot be ignored. This paper aims to provide an overview of world wind energy scenarios, the current status of wind turbine development, development trends of offshore wind farms, and the environmental and climatic impact of wind farms. The wake effect of wind turbines and modeling studies regarding this effect are also reviewed.
Article
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).
Article
The results of a parametric study, conducted using the finite element method and treating the soil as an elastic continuum with a linearly varying soil modulus, are presented as simple algebraic expressions. These expressions are similar in form to those which arise from a Winkler idealization of the soil, enabling immediate estimates to be made of the active length of the pile; the ground level deformations; and the maximum bending moment down the pile. Application of the solutions is demonstrated by the analysis of lateral loading tests on single piles and on groups of piles.
Article
Sand dilates with shearing at a rate that increases with increasing relative density (D R) and decreases with increasing effective confining stress (σ c′). The peak friction angle of a sand depends on its critical-state friction angle and on dilatancy. In this paper, we develop a simple correlation between peak friction angle, critical-state friction angle, and dilatancy based on triaxial compression and plane-strain compression test data for sand for a range of confining pressures from very low levels to approximately 196 kPa.
Article
A pure fixed-head (zero-rotation) condition at the top of a group of laterally loaded piles is seldom achievable in the field, even when piles are installed in a group that is "rigidly" constrained by a stiff concrete pile cap. Assuming complete fixity during design (zero rotation at the pile head) can result in underestimated values of pile-head deflection, and incorrect estimates of the magnitude and the location of maximum bending moments. A simple and practical approach is presented for estimating the moment restraint that is provided by the pile cap at the top of a pile group. The moment restraint, represented by the rotational restraint coefficient (K-Mtheta), serves as a boundary condition for analyzing groups of laterally loaded piles. Full-scale field tests performed on two pile groups with concrete pile caps show that the proposed method for estimating rotational restraint provides results that are in good agreement with measured field performance.
Article
Although pile caps have considerable ability to resist lateral loads, this resistance is often neglected in design. Published cases involving a variety of pile and cap sizes, soil conditions, and loading conditions indicate that the lateral-load resistance of pile caps can be significant, but it is difficult to generalize on the basis of these results because of the variations in conditions involved in the tests. To develop a more systematic basis: for evaluating cap resistance, a field test facility was constructed to perform full-scale lateral-load tests on single piles and pile groups, with the pile caps embedded in the stiff natural soil at the site and with the pile cops backfilled with granular soil. Thirty-one tests were conducted to evaluate the lateral-lend resistance of pile caps by comparing the response of pile groups with caps fully embedded and with soil removed from around the caps. The results of the tests show that pile caps provide significant resistance to lateral load. This resistance depends primarily on the stiffness and strength of the soil in front of the cap and the depth of cap embedment.