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Assessment of offshore wind turbine with an innovative monopile foundation under lateral loading
Abstract
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 soil properties are identical to those of the centrifuge test. The dilation angle, representing the ratio of plastic volume change against the plastic shear strain, is defined as 5 • (Lancelot et al., 2006;Li et al., 2021). A uniform Poisson's ratio of 0.3 is used in terms of several drained compression tests (Mina et al., 2016;Tang and Graham, 2000). ...
... where p N (x) refers to the vertical earth pressure, which is presented in Fig. 10, x 1 and x 2 are two edges of the wheel, and D w is the external wheel diameter. It is assumed that the earth pressure under the wheel along the circumferential direction is uniformly distributed, and a coefficient α c is assumed to be 0.8 to represent the shape effect (Li et al., 2021;Prasad and Chari, 1999). α D refers to the reduction coefficient, representing the circular ring shape of the wheel, which is 0.95 by assessing the area reduction. ...
The hybrid monopile foundation for offshore wind turbines demonstrates an enhancement of bearing capacities. This innovative foundation is composed of an embedded pile and a gravity footing (namely wheel). In this study, centrifuge tests are conducted to investigate the lateral capacity of hybrid monopile foundation in cohesionless soil. Pile-wheel-soil interactions are studied through validated finite element models. Load transfer mechanisms between the wheel and the pile and failure modes of each component in a hybrid monopile foundation are demonstrated. Functions of wheel are categorized into a friction resistance and additional rotational constraints at the pile head, which are quantitively assessed in terms of the vertical earth pressure generated beneath the wheel. A reduction factor is suggested to determine the effective contact area. The addition of the wheel effectively reduces bending moments at the embedded pile, and the maximum earth pressure of the equivalent monopile is similar to an original monopile. A simplified calculation method is proposed to estimate the lateral capacity of hybrid monopile foundation in ultimate condition. Previously reported tests were used to calibrate the analytical method, demonstrating good consistency. This method is applicable for performing an initial design for hybrid monopile foundation and enhancing calculation accuracy.
... The behavior of monopile foundations has been extensively studied under lateral loading. Due to structural movement, particle migration would occur in the soil around the pile [9][10][11]. Offshore monopiles are subject to the influence of water for long periods of time, which is different from traditional pile foundations [12][13][14]. ...
The analysis of the behavior of soil and foundations when the piles in offshore areas are subjected to long-term lateral loading (wind) is one of the major problems associated with the smooth operation of superstructure. The strength of the pile-soil system is influenced by variations in the water content of the soil. At present, there are no studies carried out analyzing the mechanical and deformational behavior of both the material of the laterally loaded piles and soil with groundwater level as a variable. In this paper, a series of 1-g model tests were conducted to explore the lateral behavior of both soil and monopile under unidirectional cyclic loading, based on the foundation of an offshore wind turbine near the island. The influence of underground water level and cyclic load magnitude on the performance of the pile–soil system was analyzed. To visualize the movements of soil particles during the experimental process, particle image velocimetry (PIV) was used to record the soil displacement field under various cyclic loading conditions. The relationship curves between pile top displacement and cyclic steps, as well as the relationship curves between cyclic stiffness and cyclic steps, were displayed. Combined with fractal theory, the fractal dimension of each curve was calculated to evaluate the sensitivity of the pile–soil interaction system. The results showed that cyclic loading conditions and groundwater depth are the main factors affecting the pile–soil interaction. The cyclic stiffness of the soil increased in all test groups as loading progressed; however, an increase in the cyclic load magnitude decreased the initial and cyclic stiffness. The initial and cyclic stiffness of dry soil was higher than that of saturated soil, but less than that of unsaturated soil. The ability of the unsaturated soil to limit the lateral displacement of the pile decreased as the depth of the groundwater level dropped. The greater the fluctuation of the pile top displacement, the larger the fractal dimension of each relationship curve, with a variation interval of roughly 1.24–1.38. The average increment of the cumulative pile top displacement between each cycle step following the cyclic loading was positively correlated with fractal dimension. Based on the PIV results, the changes in the pile–soil system were predominantly focused in the early stages of the experiment, and the short-term effects of lateral cyclic loading are greater than the long-term effects. In addition, this research was limited to a single soil layer. The pile–soil interaction under layered soil is investigated, and the results will be used in more complex ground conditions in the future.
Monopile-supported offshore wind turbines (OWTs) inevitably undergo the coupled effects of long-term cyclic loads and scour, resulting in cumulative deformation and the migration of the natural frequency of the foundation-superstructure system and the simultaneous development of scour holes around monopiles, which brings notable challenges to the design of OWTs. Hence, a series of model tests on monopile-supported OWTs in sands were conducted in a flume equipped with a custom long-term cyclic loading device. The mechanical responses of the OWT system and the scour process under separate and coupled effects of long-term cyclic loading and scour were studied, with special attention given to the cumulative rotation, natural frequency, ultimate bearing capacity of the OWT system, and scour hole development. A preliminary investigation of the armoring effect of the overlying coarse-grained soil under combined action was also conducted. The experimental results revealed that the coupled effect of long-term cyclic loading and scour not only has a great impact on the mechanical response of the foundation-superstructure system, but it also resulted in an acceleration of the initial scour rate and reduced the equilibrium scour depth. Moreover, the coupled effect reduced the scour resistance of coarse-grained soils under the layered soil conditions.
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.
This paper describes a one-dimensional (1D) computational model for the analysis and design of laterally loaded monopile foundations for offshore wind turbine applications. The model represents the monopile as an embedded beam and specially formulated functions, referred to as soil reaction curves, are employed to represent the various components of soil reaction that are assumed to act on the pile. This design model was an outcome of a recently completed joint industry research project – known as PISA – on the development of new procedures for the design of monopile foundations for offshore wind applications. The overall framework of the model, and an application to a stiff glacial clay till soil, is described in a companion paper by Byrne and co-workers; the current paper describes an alternative formulation that has been developed for soil reaction curves that are applicable to monopiles installed at offshore homogeneous sand sites, for drained loading. The 1D model is calibrated using data from a set of three-dimensional finite-element analyses, conducted over a calibration space comprising pile geometries, loading configurations and soil relative densities that span typical design values. The performance of the model is demonstrated by the analysis of example design cases. The current form of the model is applicable to homogeneous soil and monotonic loading, although extensions to soil layering and cyclic loading are possible.
Improved design of laterally loaded monopiles is central to the development of current and future generation offshore wind farms. Previously established design methods have demonstrable shortcomings requiring new ideas and approaches to be developed, specific for the offshore wind turbine sector. The Pile Soil Analysis (PISA) Project, established in 2013, addresses this problem through a range of theoretical studies, numerical analysis and medium scale field testing. The project completed in 2016; this paper summarises the principal findings, illustrated through examples incorporating the Cowden stiff clay profile, which represents one of the two soil profiles targeted in the study. The implications for design are discussed.
As an introduction to more sophisticated model pile tests where radiographic techniques will be used to study the deformation pattern around a pile driven into a clay bed, simple preliminary studies have been conducted on a 'quarter-space' of clay. A specimen of clay is prepared in a D-shaped sample tube. The flat (vertical) surface is exposed and marked with a uniform grid of lines before covering the face with a perspex plate. A semi-circular pile may then be driven into the clay flush with the perspex plate, enabling the soil displacements to be monitored by successive photographs of the grid of lines. The difference in the displacement fields set up by driving open and closed-ended tubular piles, demonstrates that the type of pile must be a major consideration when choosing design parameters for piled foundations. Refs.
Offshore wind turbines are typically founded on single large diameter piles, termed monopiles. Pile diameters of between 5m and 6m are routinely used, with diameters of up to 10m, or more, being considered for future designs. There are concerns that current design approaches, such as the p-y method, which were developed for piles with a relatively large length to diameter ratio, may not be appropriate for large diameter monopiles. A joint industry project, PISA (PIle Soil Analysis), has been established to develop new design methods for large diameter monopiles under lateral loading. The project involves three strands of work; (i) numerical modelling; (ii) development of a new design method; (iii) field testing. This paper describes the framework on which the new design method is based. Analyses conducted using the new design method are compared with methods used in current practice.
Experimental and finite element analyses were conducted on the bearing capacities of modified suction caissons (MSCs) in saturated marine fine sand under static horizontal loading. Results show that the MSC can provide higher horizontal and moment bearing capacity and limit lateral deflection in comparison to a regular suction caisson. The bearing capacity of the MSC increases with increasing external skirt dimensions and the internal compartment aspect ratio, however, decreases with increasing the loading eccentricity. Several equations are proposed to calculate the maximum moment bearing capacities, which take the external skirt width, the internal compartment aspect ratio and the loading eccentricity into account. The deflection and the net earth pressure distribution methods are used to determine the rotation center position and these two methods are proved to be satisfactory. It is found that the rotation center moves downwards and forwards with increasing horizontal loading, then tends to be a stable position when the maximum load is approaching. The distance between the rotation center and the caisson lid at failure decreases with increasing external skirt dimensions, the internal compartment aspect ratio and the loading eccentricity. Based on the simulation results, an expression to estimate the combined capacity of the MSC was proposed.
Designing foundations for Offshore Wind Turbines (OWTs) are challenging as these are dynamically sensitive structures in the sense that natural frequencies of these structures are very close to the forcing frequencies of the wind, wave and 1P (rotor frequency) and 2P/3P (blade shadowing frequency) loading. Typically for the widely used soft-stiff design (target frequency of the overall wind turbine is between 1P and 2P/3P), the ratio of forcing frequency to natural frequency is very close to 1 and as a result is prone to dynamic amplification of responses such as deflection/rotation which may enhance the fatigue damage, thereby reducing the intended design life. Therefore, a designer apart from accurately predicting the natural frequency of the structure, must also ensure that the overall natural frequency due to dynamic-soil-structure-interaction does not shift towards the forcing frequencies making the value of ݂ /݂ even closer to 1. Therefore foundations are one of critical components of Offshore Wind Turbines not only because of the overall stability of the structure but also due to financial viability of the project. The article highlights technical challenges associated with foundation design for offshore Wind farm.
SYNOPSIS Offshore wind farms will contribute significantly to the renewable generation of electricity for the UK. The economic development of windfarms depends, however, on development of efficient solutions to a number of technical issues, one of these being the foundations for the offshore turbines. We review here the results of a recent research programme directed towards the design of caisson foundations as an option for wind turbine foundations. The possibilities of using caissons either in the form of monopod foundations or in the form of a tripod/tetrapod arrangement are discussed.
The paper describes a theoretical study of the stress and strength changes in clay due to the installation of driven tubular piles with open and closed ends. The clay has been idealized as a work hardening elastoplastic material. Pile installation has been modeled as the expansion of a long cylindrical cavity. Numerical results are presented showing how the stress and strength changes in the soil are affected by the consolidation history of the soil prior to pile driving, and by the amount of soil displaced by the pile. It is shown that the final shaft capacity of a closed-ended pile driven into a particular clay deposit may be of the order of 25% greater than that for a similar but open-ended pile.
This paper details a programme of centrifuge tests (100g) on the monotonic and cyclic lateral behaviour of large-diameter semi-rigid monopiles in kaolin clay. Piles of different diameters were used, with a maximum prototype value of 5·9 m. The pile–soil interaction was investigated in terms of the overall load–displacement behaviour, distribution of bending moment and pile deflection, p–y curves and excess pore pressure response. For monotonic loading, procedures were developed to fit the monotonic p–y curves obtained from different pile and soil properties using a hyperbolic tangent model. In addition, an explicit model was derived to assess the development of excess pore pressure within the surficial soil, which was found to approach the vertical effective soil stress at large displacement. For cyclic loading, the accumulation of lateral pile displacement could be accompanied by either increase or decrease in soil resistance, which strongly depends on the loading rate. Strong correlation between the development of peak soil resistance and a dimensionless velocity group was obtained, and was supported by the measured excess pore pressure. Under undrained conditions, the peak soil resistance decreases almost linearly with the accumulation of excess pore pressure.
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.
Renewable energy sources (RES) can play a significant role in economic growth.
This article examines the development of the wind energy market in the EU. The applicable regulations, directives, and legal acts are reviewed, and the State's role in the development of the renewable energy market is discussed. The analyses were carried out based on the available literature and Eurostat data. The authors of the paper used basic statistics to summarize the changes in wind energy and make a prognosis. The data analysis uses the Autoregressive Integrated Moving Average Method (ARIMA). The survey found that the highest average wind cumulative installed capacities were in Germany (31026 MW), Spain (18602 MW), United Kingdom (7894 MW) and France (6108 MW). Our analysis indicates that the biggest average annual installed wind capacity was in Germany (2952.6 MW), Spain (1224.5 MW) and the United Kingdom (1363.8 MW). These same countries achieved the highest average wind gross electricity production in the years 2004–17: Germany (50.9 TWh), Spain (39.3 TWh) and United Kingdom (18.9 TWh). The study finds that wind cumulative installed capacity will increase in most countries of the EU.
Pile foundations in offshore engineering are usually subjected to vertical-horizontal load combinations. Previous researches have discovered that the lateral behaviour of soil-pile system could be influenced by axial load due to P-Δ effect and the change of soil stiffness, but the mechanism and influencing factors are still unclear. The current p-y curve method for laterally loaded piles is independent on vertical loads, which is not often applicable for offshore piles. In this study, numerical analyses with the Hypoplastic constitutive model were employed to simulate the vertical-horizontal combined loading pile in sand. The results of soil-pile system under different vertical-horizontal load combinations were compared to confirm and further investigate the change of soil stiffness due to vertical loads. The mechanism and the influencing factor of the change of soil stiffness were clarified by analysing the change of soil stiffness under pure vertical loading conditions. Based on the above studies, a new p-y curve model for vertical-horizontal combined loading piles in sand was established. The parameter determination method was proposed. The effectiveness of the new p-y model and the parameters were verified by comparing the back-calculated results with those from numerical simulations.
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.
Following careful consideration of long‐term economic crises and ecological impact of fossil resources, green and sustainable energy resources have gained preference. Wind energy has been endorsed as an emission‐free, green, and sustainable, thus supported by state appropriations. However, many avian fatalities at utility-scale wind energy amenities, particularly along forested and habitations globally. These mortalities raise serious concerns about the increasing pace of projected wind energy development on ecological beings. This mini overview discusses current developments of wind energy developments, its increasing trend, and the adverse ecological impacts i.e., noise, visual, deforestation, and land erosion. Moreover, possible solutions to those mentioned above adverse ecological impacts on wind power production facilities are also given. In this context, considering the research needs, decision-makers, developers, and other stakeholders should work closely to standardize the policies to minimize the negative environmental impact of this natural source of energy.
Offshore wind turbines in shallow coastal waters are typically supported on monopile foundations. Although three-dimensional (3D) finite-element methods are available for the design of monopiles in this context, much of the routine design work is currently conducted using simplified one-dimensional (1D) models based on the p–y method. The p–y method was originally developed for the relatively large embedded length-to-diameter ratio (L/D) piles that are typically employed in offshore oil and gas structures. Concerns exist, however, that this analysis approach may not be appropriate for monopiles with the relatively low values of L/D that are typically adopted for offshore wind turbine structures. This paper describes a new 1D design model for monopile foundations; the model is specifically formulated for offshore wind turbine applications, although the general approach could be adopted for other applications. The model draws on the conventional p–y approach, but extends it to include additional components of soil reaction that act on the pile. The 1D model is calibrated using a set of bespoke 3D finite-element analyses of monopile performance, for pile characteristics and loading conditions that span a predefined design space. The calibrated 1D model provides results that match those obtained from the 3D finite-element calibration analysis, but at a fraction of the computational cost. Moreover, within the calibration space, the 1D model is capable of delivering high-fidelity computations of monopile performance that can be used directly for design purposes. This 1D modelling approach is demonstrated for monopiles installed in a stiff, overconsolidated glacial clay till with a typical North Sea strength and stiffness profile. Although the current form of the model has been developed for homogeneous soil and monotonic loading, it forms a basis from which extensions for soil layering and cyclic loading can be developed. The general approach can be applied to other foundation and soil–structure interaction problems, in which bespoke calibration of a simplified model can lead to more efficient design.
Monopiles used as foundations for offshore wind turbines can be installed using different methods including jacking, vibratory driving and impact driving. Significant research efforts have been dedicated to the characterisation of monopile−soil interaction under lateral loading, mainly using p–y curves. There has also been extensive research in quantifying the effect of different installation methods on the axial response using numerical modelling and physical modelling techniques. Little attention has been paid to the effect of the installation method on the subsequent lateral response of a monopile under the in-service condition. In this paper, a purpose-designed apparatus is described that allows in-flight installation using different installation methods followed directly by lateral loading without stopping the centrifuge and thus retaining the installation-induced stress state. Test results from three lateral loading tests are discussed, with the piles either jacked at 1g and Ng or impact driven at Ng into a dry medium dense sand, allowing the effect of the installation method on the initial stiffness and ultimate capacity to be examined. The successfully conducted tests illustrate the capabilities of the new apparatus for centrifuge testing of laterally loaded driven piles.
This paper provides a summary of the PIle Soil Analysis (PISA) project, completed in the UK during the period 2013 to 2018. The research led to the development of a new, computationally efficient, one dimensional design model for laterally loaded monopile foundations, particularly for offshore wind turbine support structures. The current form of the design model is applicable to monotonic loading only, but it could form a basis for extensions to cyclic loading. This short paper describes the background to the project, outlining the key research elements completed, as well as the main impacts that have been achieved. A number of publications describing the research in further detail are highlighted.
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.
A shallow suction bucket is a new foundation type for offshore wind turbines. Due to its large size and bulky shape, the bucket and the soil within the bucket do not necessarily deform as a whole. Moreover, limited research has been conducted on the hydrodynamic wave influence on the shallow bucket bearing response. These factors pose great challenges to the shallow bucket foundation design. This paper presents a set of centrifuge tests of a shallow bucket model subjected to monotonic and dynamic lateral loads to study the lateral bearing response of shallow bucket foundations in the field under combined loads induced by wind, waves, etc. In addition to the routine measurements (e.g., load-displacement), the soil pressures on the bucket and the distribution and evolution of the excess pore pressures in the surrounding soils are also obtained. The deformation pattern of the bucket (e.g., rotation center) is revealed through displacement measurements. Finally, the proposed easy-to-use analytical equations using the limit equilibrium to assess the bearing capacity of bucket foundations, taking into account the influence of the soil strength degradation caused by hydrodynamic wave loadings, are found to yield good results upon comparison with the centrifuge data, providing useful guidelines for the design of shallow bucket foundations.
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 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.
Suction bucket foundations are promising foundation options for offshore wind turbines, which are usually subjected to a relative high lateral-moment loading. To investigate the lateral-moment loading capacity of bucket foundations, a group of three-dimensional finiteelement simulations with different bucket dimensions in both loose and dense saturated sand was carried out based on centrifuge tests. The numerical methods were validated by comparisons with the results of centrifuge tests and assessed by sensitivity analyses regarding the influences of soil properties and soil-foundation interface parameters. The interaction between the bucket and surrounding soil was illustrated in order to investigate the bearing behavior and failure mechanism of the bucket foundation. It was shown that in the ultimate state, the maximum passive earth pressure acting on the external skirt in the loading direction is approximately four times larger than that on the internal skirt. Furthermore, parametric studies on the L:D ratios (L is the skirt length and D is the bucket diameter) and loading eccentricity were conducted and discussed. Consequently, a modified calculation method was proposed to predict the ultimate lateral-moment loading capacity of bucket foundations and the method was validated by field and laboratory data.
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 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.
This paper presents a new approach for the optimization of neighbouring offshore wind farms. Offshore wind energy is one of the most promising and developed low-carbon generation technologies. However, the high capital costs, which are strongly dependent on seabed depth, currently limit the geographical expansion of this technology to areas with relatively shallow waters and appropriate wind resource. This, along with the advantages of sharing a submarine transmission system among several projects, leads to a high concentration of offshore wind farms in certain zones, as happens, for example, in the North Sea. The presence of other neighbouring offshore wind farms has to be taken into account when a developer plans a new project, since the wake effect of wind turbines belonging to other neighbouring wind farms will affect the annual energy production and, consequently, the profitability of the project under study. However, not only already operating or installed neighbouring projects have to be borne in mind, but also the possible design of future neighbouring wind farms yet to be developed. In order to tackle this issue, an innovative co-evolutionary algorithm is proposed in this paper with the objective of determining a Nash equilibrium solution that would provide the best possible configuration of the wind farm under study by taking into account and limiting the disturbance introduced by other neighbouring projects. The performance of the proposed methodology has been successfully tested through the analysis of a realistic case and compared with other collaborative approaches and the classic single-project optimization methods already existing in the literature.
This is the first joint reference paper for the Ocean Energy Systems (OES) Task 10 Wave Energy Converter modeling verification and validation group. The group is established under the OES Energy Technology Network program under the International Energy Agency. OES was founded in 2001 and Task 10 was proposed by Bob Thresher (National Renewable Energy Laboratory) in 2015 and approved by the OES Executive Committee EXCO in 2016. The kickoff workshop took place in September 2016, wherein the initial baseline task was defined. Experience from similar offshore wind validation/verification projects (OC3-OC5 conducted within the International Energy Agency Wind Task 30) [1], [2] showed that a simple test case would help the initial cooperation to present results in a comparable way. A heaving sphere was chosen as the first test case. The team of project participants simulated different numerical experiments, such as heave decay tests and regular and irregular wave cases. The simulation results are presented and discussed in this paper.
Wind Energy is the one of the most promising renewable energy. Suction bucket foundation is considered to be a viable type of wind turbine foundation. Soil liquefaction caused by earthquakes at offshore seismic active area may lead to a significant degradation of soil strength and stiffness. In this study, nine centrifuge tests were carried out to investigate the seismic response of suction bucket foundation under earthquake loading. Both dry and saturate soil conditions were considered in tests. The geometric design of five suction bucket models considered the bucket diameter, penetration depth, and modified buckets with inside compartments. It was found that soil underlying and near the bucket foundation shown a better ability to resist liquefaction in saturated tests comparing to free field while no significant differences were observed in dry tests. The five bucket models performed quite differently, which demonstrated the aspect ratio effects and inside-bucket compartment effects. The results provide insight into optimized design of suction bucket foundation for wind turbine.
The 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.
Results of finite-element analyses calculating the undrained capacities of skirted circular foundations under uniaxial vertical, horizontal, and moment loading are presented. Parallel finite-element analyses using both the Tresca failure criterion and the modified cam clay (MCC) model are reported. The variations in capacity are presented as functions of embedment and soil-strength heterogeneity as a series of graphs. Closed-form formulas of uniaxial capacities are also provided. To account for the complexity in the results, a novel method of separating the contributions of soil above and beneath the skirt tip and for the constant and linearly varying components of the undrained shear-strength profile is outlined. This method can be used to estimate the undrained ultimate uniaxial capacity factors of skirted circular foundations in clay for any given embedment ratio and strength heterogeneity. Its versatility is also shown through validation against previously published embedded strip footing data. The results provide both equations for engineers to use in design and a database that can be used in future research to calculate and express the consolidated undrained capacity under combined loading.
Offshore wind industry is having a great development. It requires progress in many aspects to achieve the sustainable progress of this technology. One of those aspects is the design of foundation, sub-structures and support structures. The most used at present, with more than 80%, is the monopile. Typical piles used in quays in maritime engineering have a maximum diameter about 2 or 3 m. In offshore wind, the diameter can be more than double. There is a risk associated with the difference in scale. Some formulas used for the design of typical piles with diameter less than 2 m can be unsuitable for larger diameter piles. This paper is focused on giving a first estimate of length and weight of piles for knowing its diameter. There are formulas for that for piles with diameters up to 2 m, but there are doubts about whether they can be used for piles with larger diameters. To achieve it, a database gathering offshore wind farms in operation with monopiles is prepared in order to obtain simple formulas relating those parameters. Furthermore, the results of that formula are compared with traditional formula used in maritime engineering for piles with diameters less than 2 m.
This chapter first provides a general overview of different offshore wind power plant (OWPP) designs considering the use of both high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) transmission links to deliver the generated power. It then focuses on the conventional AC wind power plants, by introducing some possible wind power plant topologies and briefly describing the required technologies that encompasses the collection grid. The chapter also deals with the description of the electrical design methodology performed in the conventional offshore wind power plants. The methodology focuses on explaining in detail the cable selection process and how the technical assessment of an OWPP is performed, considering only the collection grid area. Finally, the chapter presents a short presentation of future OWPPs based on DC technologies, as well as other proposals for AC wind power plants connected to the onshore grid through an HVDC transmission links. © 2016 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved..
The purpose of this two-part study is to model the effects of large penetrations of offshore wind power into a large electric system using realistic wind power forecast errors and a complete model of unit commitment, economic dispatch, and power flow. The chosen electric system is PJM Interconnection, one of the largest independent system operators in the U.S. with a generation capacity of 186 Gigawatts (GW). The offshore wind resource along the U.S. East Coast is modeled at five build-out levels, varying between 7 and 70 GW of installed capacity, considering exclusion zones and conflicting water uses.
This paper, Part I of the study, describes in detail the wind forecast error model; the accompanying Part II describes the modeling of PJM's sequencing of decisions and information, inclusive of day-ahead, hour-ahead, and real-time commitments to energy generators with the Smart-ISO simulator and discusses the results.
Wind forecasts are generated with the Weather Research and Forecasting (WRF) model, initialized every day at local noon and run for 48 h to provide midnight-to-midnight forecasts for one month per season. Due to the lack of offshore wind speed observations at hub height along the East Coast, a stochastic forecast error model for the offshore winds is constructed based on forecast errors at 23 existing PJM onshore wind farms. PJM uses an advanced, WRF-based forecast system with continuous wind farm data assimilation. The implicit (and conservative) assumption here is that the future forecast system for offshore winds will have the same performance as the current PJM's forecast system for onshore winds, thus no advances in weather forecasting techniques are assumed.
Using an auto-regressive moving-average (ARMA) model, 21 equally-plausible sample paths of wind power forecast errors are generated and calibrated for each season at a control onshore wind farm, chosen because of its horizontally uniform landscape and large size. The spatial correlation between pairs of onshore wind farms is estimated with an exponential function and the matrix of error covariance is obtained. Validation at the control farm and at all other onshore farms is satisfactory. The ARMA model for the wind power forecast error is then applied to the offshore wind farms at the various build-out levels and combined with the matrix of error covariance to generate multiple samples of forecast errors at the offshore farms. The samples of forecast errors are lastly added to the WRF forecasts to generate multiple samples of synthetic, onshore-based, actual offshore wind power for use in Part II.
Offshore Wind Farms: Technologies, Design and Operation provides the latest information on offshore wind energy, one of Europe's most promising and quickly maturing industries, and a potentially huge untapped renewable energy source which could contribute significantly towards EU 20-20-20 renewable energy generation targets. It has been estimated that by 2030 Europe could have 150GW of offshore wind energy capacity, meeting 14% of our power demand. Offshore Wind Farms: Technologies, Design and Operation provides a comprehensive overview of the emerging technologies, design, and operation of offshore wind farms. Part One introduces offshore wind energy as well as offshore wind turbine siting with expert analysis of economics, wind resources, and remote sensing technologies. The second section provides an overview of offshore wind turbine materials and design, while part three outlines the integration of wind farms into power grids with insights to cabling and energy storage. The final section of the book details the installation and operation of offshore wind farms with chapters on condition monitoring and health and safety, amongst others. Provides an in-depth, multi-contributor, comprehensive overview of offshore technologies, including design, monitoring, and operation Edited by respected and leading experts in the field, with experience in both academia and industry Covers a highly relevant and important topic given the great potential of offshore wind power in contributing significantly to EU 20-20-20 renewable energy targets.
Foundations for offshore wind farms involve significant technical challenges, including design requirements to withstand the harsh marine environment, prolonged impact under large wave loading and wind turbulence. These foundations are subjected to a combination of axial loads, low-amplitude cyclic lateral loads, bending and torsional moments generated by the offshore wind turbine (OWT) structure and various environmental factors. Monopiles are the most widely adopted foundation choice for OWT structures worldwide, in terms of ease of installation, economy and logistics. For monopiles, the applied loads and moments must be resisted by earth pressures mobilized in the surrounding soil, with an adequate factor of safety provided. Compared with the axial loading case, the cyclic lateral loads are considered governing for serviceability requirements. This chapter presents details on the environmental loading, geometric dimensions and geotechnical design considerations for OWT foundations, with a focus on monopile fondations. Existing methods for modelling soil under cyclic loading are reviewed, with emphasis on strain accumulation models that take into consideration the monopile–soil interaction. Inherent limitations and shortcomings of these models for the analysis/design of OWT monopile foundations are discussed, along with recommendations for future research needs.
When it comes to the general design of laterally loaded piles in offshore environments, bedding resistance is usually modelled by the p-y method recommended in the offshore guidelines (OGL). Several investigations presented in the literature indicate that the head displacements of large-diameter monopiles are underestimated for extreme loads but overestimated for small operational loads. An extensive evaluation of the OGL method is presented here using three-dimensional numerical simulations. The evaluation has shown that the OGL method is not applicable for the design of large-diameter piles. Moreover, modified p-y formulations presented in the literature accounting for the effect of the pile diameter are also not generally suitable for piles with arbitrary dimensions and load levels. Therefore, the derivation of a new p-y approach is presented in detail. The new approach consists of "basic p-y curves" that are valid for a pile of infinite length exhibiting a constant horizontal deflection. In an iterative scheme, these basic curves are adapted depending on the pile deflection line and the pile length to account for a more realistic bedding resistance along the pile shaft. A comprehensive parametric study with 250 pile-soil systems reveals that the new p-y approach is able to predict the horizontal loadbearing behaviour as well as the local pile-soil interaction quite realistically. © 2015 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
Under static loading in compression, open-ended piles may fail in a plugged mode, with the soil plug moving with the pile, or in an unplugged mode, with shear failure occurring between the soil plug and the pile shaft. It may be shown that, under drained loading conditions, the former mode of failure will generally occur, because arching action within the pipe pile leads to high frictional capacity of the plug. However, under faster rates of loading relevant to the offshore environment, the increase in effective stresses within the soil plug is limited and the plug capacity is significantly lower. The Paper presents a simple one-dimensional analysis of the soil plug under partially drained conditions. The analysis has been implemented numerically, and the resulting program used to derive design charts which give the plug capacity as a function of the soil plug parameters and the rate of loading. These design charts are presented in appropriate non-dimensional form, with example calculations included for typical offshore piles in calcareous soil.
Bucket foundation has been used for offshore wind turbine in Qidong city, China. In order to transport and install bucket foundations together with the upper offshore wind turbines, a non-self-propelled integrated transportation and installation vessel is designed. Wet tow of the dedicated vessel with two bucket foundations and the upper offshore wind turbines, one of the key issues of the integrated transportation and installation techniques, has been studied through a series of field experiments with different air pressures inside the bucket foundations and different ballasts in the vessel. The motion behaviors of the dedicated vessel are investigated. The results show that the dedicated vessel is quite stable during wet tows. In addition, with the air pressures inside the bucket foundations increase, the vessel becomes more stable. However, larger draft makes the vessel less stable and the ballast in the vessel may have little effect on the stability.
The analysis and computation of the ultimate lateral capacity of rigid piles is usually based on a simplified soil pressure distribution along the pile length. Actual soil pressure distributions were measured in a rigid model pile along its length and across the diameter and it was found that the simplified theoretical assumptions of pressure distribution needs a modification. A method is suggested in this paper to predict soil pressure distribution and ultimate lateral capacity for rigid piles in cohesionless soils. Field and laboratory data from published literature are used to validate the proposed method.
Piles are required to withstand large lateral loads compared with the imposed vertical loads in certain applications in the offshore environment, such as for foundations for offshore wind turbines or as anchors for floating facilities. Although typically the soil strength increases with depth, close to the sea bed, the lateral capacity is often low. The requirement to limit pile head deflections necessitates the design of relatively long piles. Increasing the effective pile cross-section through "wings" close to the pile head is shown here with centrifuge model tests to reduce pile head deflections by approximately 50% compared with regular monopiles without "wings" for the same load level. The stiffer initial response of the winged pile also leads to smaller pile head deflections under cyclic loading, although the relative rate of accumulation is similar to that of a monopile. Simple methods for extrapolating from the monotonic pile head deflection to the deflection after thousands of cycles are compared with the results, and are shown to work equally well for piles with and without "wings". DOI: 10.1061/(ASCE)GT.1943-5606.0000592. (C) 2012 American Society of Civil Engineers.
This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging. DOI: 10.1061/(ASCE)GT.1943-5606.0000528. (C) 2011 American Society of Civil Engineers.
This paper presents an analysis of the yielding and plastic hardening of uniformly-graded samples of a silica sand subjected to one-dimensional normal compression. Single grains of silica sand have been compressed diametrically between flat platens to measure indirectly tensile strength. Approximately 30 grains were tested for each of the following nominal particle sizes: 0.5 mm, 1 mm and 2 mm diameter. It was found that the data could be described by the Weibull statistics of brittle ceramics, and the Weibull modulus could be taken to be about 3.1. Uniform aggregates of the same sand were then compacted to maximum density and subjected to one-dimensional compression. The initial particle size distributions were 0.3-0.6 mm, 0.6-1.18 mm and 1.18-2 mm, and aggregates were subjected to stresses of up to 100 MPa. All particles were initially of similar shape, and hence the initial voids ratios of the aggregates at maximum density were approximately equal. The yield stress was denned to be the point of maximum curvature on a plot of voids ratio against the logarithm of effective stress, and found to increase with decreasing particle size, and to be approximately proportional to the tensile strength of the constituent grains. However, the plastic compressibility index was found to be approximately constant and independent of the initial grading, and a fractal distribution of particle sizes appeared to evolve under increasing stress. There is evidence to suggest that the aggregates evolve towards a fractal dimension of 2.5 under high stresses.
The fatigue life of offshore wind turbines strongly depends on the dynamic behaviour of the structures including the underlying soil. To diminish dynamic amplification and avoid resonance, the eigenfrequency related to the lowest eigenmode of the wind turbine should not coalesce with excitation frequencies related to strong wind, wave and ice loading. Typically, lateral response of monopile foundations is analysed using a beam on a nonlinear Winkler foundation model with soil–pile interaction recommended by the design regulations. However, as it will be shown in this paper, the guideline approaches consequently underestimate the eigenfrequency compared to full-scale measurements. This discrepancy leads the authors to investigate the influence of pore water pressure by utilising a numerical approach and consider the soil medium as a two-phase system consisting of a solid skeleton and a single pore fluid. In the paper, free vibration tests are analysed to evaluate the eigenfrequencies of offshore monopile wind turbine foundations. Since the stiffness of foundation and subsoil strongly affects the modal parameters, the stiffness of saturated soil due to pore water flow generated by cyclic motion of monopiles is investigated using the concept of a Kelvin model. It is found that the permeability of the subsoil has strong influence on the stiffness of the wind turbine that may to some extent explain deviations between experimental and computational eigenfrequencies.
Of all the renewable energy sources (RESs)―except direct solar heat and light―wind energy is believed to have the least adverse environmental impacts. It is also one of the RES which has become economically affordable much before several other RESs have. As a result, next to biomass (and excluding large hydro), wind energy is the RES being most extensively tapped by the world at present. Despite carrying the drawback of intermittency, wind energy has found favor due to its perceived twin virtues of relatively lesser production cost and environment-friendliness.
But with increasing use of turbines for harnessing wind energy, the adverse environmental impacts of this RES are increasingly coming to light. The present paper summarizes the current understanding of these impacts and assesses the challenges they are posing. One among the major hurdles has been the NYMBI (not in my backyard) syndrome due to which there is increasing emphasis on installing windfarms several kilometers offshore. But such moves have serious implications for marine life which is already under great stress due to impacts of overfishing, marine pollution, global warming, ozone hole and ocean acidification. Evidence is also emerging that the adverse impacts of wind power plants on wildlife, especially birds and bats, are likely to be much greater than is reflected in the hitherto reported figures of individuals killed per turbine. Likewise recent findings on the impact of noise and flicker generated by the wind turbines indicate that these can have traumatic impacts on individuals who have certain predispositions. But the greatest of emerging concerns is the likely impact of large wind farms on the weather, and possibly the climate. The prospects of wind energy are discussed in the backdrop of these and other rising environmental concerns.
This document provides a summary of a 236-page NREL report that provides a broad understanding of today's offshore wind industry, the offshore wind resource, and the associated technology challenges, economics, permitting procedures, and potential risks and benefits.
The response of the soil plug within driven, open-ended pipe piles is very different under the dynamic conditions of installation and the static conditions of loading during service. This paper addresses the latter aspect and describes a combined experimental and numerical study of the response of soil plugs in open-ended pipe piles. The work focuses on the partially drained (static) loading relevant to offshore applications, and the experimental work is conducted using calcareous sand from Bass Strait, Australia. Model tests are conducted in pipe piles of 25-mm and 100-mm internal diameters, with loading rates as great as 6 MPa/s, using a downward hydraulic gradient to achieve appropriate effective stress profiles in the soil plug. The experimental results are assessed within the framework of analytical solutions of the drained and undrained response of the soil plug and numerical studies of the partially drained problem that allow the results to be extrapolated to prototype conditions. Example applications are given for pile geometries and loading rates typical for Bass Strait.
A series of model pile tests have been performed in the geotechnical centrifuge at the University of Western Australia to study the plugging behaviour of piles in sand. Open and sleeveended piles have been driven and jacked by a miniature pile driving actuator into silica flour of varying densities. The progression of the soil column has been measured during installation and static loading. It was found that the plug length increased with increasing relative density during driving and decreased with increasing relative density during jacking. During installation, the jacked piles exhibited a greateer tendency to plug than the driven piles. Piles fitted with an internal driving shoe provided significant stress relief within the soil plug durling jacking, leading to longer plug lengths compared with internally flush piles. All static load tests at high embedments behaved in a completely plugged manner. An annlysis of the plug capacity is conducted and a model for the variacapacity is conducted and a model for the variation of the earth pressure coefficient inside the pile is proposed. Differential base pressure on the pile annulus and the internal soil plug is postulated and validated by means of the experimental data.