Wind Turbine Foundations Research Articles

On seismic protection of wind turbine foundations founded on liquefiable soils

Wind turbine structures can be susceptible to damage from earthquake loading due to large overturning moments at the foundation level. As the wind energy market expands to seismically active parts of the world, it becomes important to evaluate the seismic behaviour of these structures. Especially, earthquake-induced liquefaction can be important when wind farms are located on sites with marginal soil conditions such as loose, saturated sands or non-plastic silts. Reduction of stiffness during liquefaction can cause excessive foundation settlement and/or rotation. Thus, suitable mitigation methods should be investigated. This paper discusses the seismic behaviour of the wind turbine on liquefiable soil and the effectiveness of the stone column-based liquefaction mitigation method. Two geotechnical centrifuge tests were conducted with and without stone columns for a model wind turbine on a circular raft foundation. The results show that stone columns can significantly reduce both the settlement and rotation of the raft.

Model Test Study of Offshore Wind Turbine Foundation under the Combined Action of Wind Wave and Current

The wind turbine system in the offshore area will be subjected to the combined action of complex marine environment dynamic loads. At present, there is a certain advantage to using traditional mechanical devices to simulate marine environmental loads. In this study, the self-developed complex dynamic load test loading system is adopted to carry out a series of model tests for pile foundations. The theoretical calculation results of wind, wave and current load are applied to the wind turbine system, and the responses of the pile and soil are analyzed according to the actual model test. The main conclusions are as follows: the development of pile displacement is basically linear with the logarithm of cycle times, and pile–soil interaction is directly affected by the form of load, the results of dynamic response obtained in this test are closer to that under the actual marine environmental load.

The use of backfilling to remediate sand motion issues around vibrating monopile foundations

Backfilling, as a common remedial countermeasure, is widely used in ocean engineering to address the scour or subsidence problem around offshore-structure foundations. However, the performance of the backfilling in solving subsidence problems around vibrating monopiles has not been studied, and the mixing mechanisms of the backfilling sand with the bed sand around the vibrating monopile remains unknown. To investigate these issues, an experimental study on characteristics of sand motions around vibrating monopile foundations after backfilling process is conducted. The experiments were designed in two-dimensional conditions and different particle sizes of backfilling sand were used. The time-dependent characteristic geometrical dimensions of the sand bed deformation around the vibrating monopile were measured, and the mean convective velocity around the vibrating monopile was measured by Particle Imaging Velocimetry (PIV) technology. The experimental results indicate that backfilling sand with a larger particle size can effectively reduce the depth of the subsidence hole and mitigate the sand convective velocity around the vibrating monopile, compared with backfilling material of smaller or the same size as the original sand bed. The findings of this study could help to develop effective remedial countermeasures to deal with the soil subsidence problems around the monopile foundations of offshore wind turbines.

Effect of pile arrangement on lateral response of group-pile foundation for offshore wind turbines in sand

This study presents a series of centrifuge tests to investigate the lateral response of group-pile foundation for offshore wind turbines in ultimate conditions. The pile arrangement is specifically assessed together with the influence of pile number and soil property. The numerical analysis is conducted in ABAQUS and validated. The soil-pile interactions are demonstrated, and a parametric study is performed. The analysis shows that the axial load effect and the shadow effect simultaneously influence the behavior of the pile group. The tensile failure of the back-row pile dominates the total failure. The influence zone of the group-pile falls into 10D (D refer to pile diameter) in front of the front-row pile horizontally and 8D vertically. The front-row pile provides a majority of the lateral resistance while the contribution decreases sequentially to the very back-row. Adding rows to the pile group is more effective than adding columns to enhance the overall capacity, and the middle pile is less effective than the corner pile. A fitted formula is proposed to estimate the Pm by integrating the pile arrangement, pile number, and pile spacing. The reliability is checked against the field and centrifuge tests.

A Numerical Analysis on Lateral Resistance of Pile–Bucket Foundation for Offshore Wind Turbines

The large-diameter single pile has been widely used in marine wind turbine foundations. In order to improve its lateral bearing capacity, the suction bucket foundation (BF) can be used as a reinforcement method, which surrounds the periphery of the monopile foundation (PF), forming a new wind turbine foundation, i.e., pile–bucket foundation (P–BF). By using the general finite element software ABAQUS, in this paper, several numerical models are established to investigate the influence of pile diameter and bucket diameter on the lateral bearing capacity of large-diameter monopiles. The numerical results show that the internal force distribution along the pile shaft for the case of P–BF is similar to the case of PF. Compared to bucket height, bucket diameter has more of an effect on lateral capacity and the p-y curve in the P–BF. In the combined P–BF, the buckets can provide lateral bearing capacity, and the piles can provide anti-overturning moments, resulting in higher lateral bearing capacity. In this paper, the p-y curve in the API specification is modified based on the results from the finite element simulation. The modified p-y curve fits well with the results of the finite element calculation and can be used as a reference for the design of the P–BF in actual engineering.

Calculation Method of Foundation-Subgrade Dynamic Interaction for Large Wind Turbine

Taking a 3MW large wind turbine foundation as the research object, the dynamic interaction model of wind turbine foundation and subgrade is established. By intercepting the subgrade in a limited range and setting spring-damper elements at the boundary of the subgrade, the semi-infinite space of the subgrade is simulated. In order to facilitate the use of structural designers, the calculation methods of the stiffness coefficient and damping coefficient of the spring-damper element are simplified. The simplified calculation formulas for the stiffness coefficient and damping coefficient of the spring-damper unit are obtained when the intercepted subgrade range is 5 times and 10 times the wind turbine foundation.

Modeling Tests on Lateral and Overturning Resistance according to Pile Arrangement for Wind Turbine Foundation

In this study, the lateral resistance and bending moment of a wind turbine foundation reinforced with piles on the bedrock were evaluated through modeling tests considering the cross-sectional size and presence of piles. The lateral resistance of a small foundation (16 m) with piles was found to be 1.84 times greater than that of the existing wind turbine foundation (20 m). After installing eight piles in the wind power turbine foundation, the outer and inner circles were compared, and the lateral resistance at the outer circle with piles was found to be the largest. Moreover, the lateral resistance force was similar for the series, parallel, and diagonal arrangements of six piles. The bending moment increased based on the order of the front, right, and back rows. The maximum bending moment of the pile for the front row was observed at the interface between the wind turbine foundation and rubble bed, while that for the right and back rows was observed at the interface between the rubble layer and bedrock.

A simple rotational spring model for laterally loaded rigid piles in sand

Monopiles are the most popular foundation for offshore wind turbines. These foundations typically have a low length to diameter ratio and undergo a rigid body rotation when subjected to lateral load. This paper presents results from an extensive numerical investigation involving 3D finite element analyses to demonstrate that the lateral moment-rotation response of a monopile in sand can be represented using a single non-linear rotational spring located at a depth of about 0.75 times the pile embedment. Expressions for the elastic rotational stiffness of a monopile under very low rotations are developed and these combined with observations from measured non-linear variations of rotational stiffness, that are supported by the numerical analyses, are used to develop a simple approximate expression that can be used to determine the response of a monopile to a monotonic lateral load in sand.

Metamodel-based generative design of wind turbine foundations

Wind turbines play an integral role in energy transition agendas. The optimized design of wind turbine foundations is a complex and intricate task that requires iterative running of computationally-intensive and time-consuming finite element models. However, given the popularity of these structures over the past two decades, there is a wealth of data from the designs of the past projects that can be used for the data-driven modeling of these structures. Given the demonstrated accuracy and success of metamodels as an alternative approach for other computationally-intensive simulation-based problems, this study aims to develop a generative-design framework for the optimization of wind turbine foundations using a metamodel, as a complementary step to more accurate finite element modeling, to reduce the overall design time without compromising the accuracy. To this end, first, the random forest method is used to develop a multi-output metamodel for the wind turbine foundations based on a set of historical data. Then, a metaheuristic method, i.e., NSGA II, is adopted to optimize the design process based on the developed metamodel. In a case study, a wind turbine foundation was designed using the proposed framework and the accuracy of the output was assessed in terms of the ultimate bending moment. The results of the case study indicate that the proposed method provides a significant time gain (i.e., 99.93%) without compromising the accuracy (i.e., 1.75% for the percent error). Besides, the conducted study also offers designers a better understanding of the importance of each design variable and how certain design variables influence the moment-rotation behavior of the wind turbine foundation

Influence of Groundwater Depth on Pile–Soil Mechanical Properties and Fractal Characteristics under Cyclic Loading

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.

Mechanical Characteristics of Prestressed Concrete Curved Transition Section of Composite Bucket Foundations for Offshore Wind Turbines

The composite bucket foundation (CBF) consists of a concrete curved transition section, a concrete beam-slab system, and a suction caisson and is increasingly used as the foundation for offshore wind turbines. The curved transition section transmits the upper load from the tower to the foundation, and its force and transmission characteristics are related to the safety of the entire wind turbine structure. The arced transition section has the characteristics of complex geometry, load conditions, and large curvature. It is difficult to determine its bearing characteristics and force transmission system. In this paper, the boundary conditions and loading device of the CBF model test are designed, and three 1:20 arced transition section model specimens are made. The mechanical characteristic experiments of CBF are used to analyze the failure process, failure characteristics, and seismic performance of the structure. Results show that the cracking effect of the arced transition section after prestressing is obviously better than that of a reinforced concrete arced transition section structure. The arced transition section specimens equipped with prestressed tendons can increase the structural cracking load ratio by about 35% for reinforced concrete members. The energy dissipation capacity of the specimens has been significantly improved, and the material properties can be fully utilized. The failure mode of the arced transition section structure under horizontal reciprocating load shows the characteristics of bending and shear failure.

Influence of geological uncertainty on the design of OWTF monopiles

This paper presents a methodology to assess the influence of geological uncertainty (stratigraphic uncertainty) on designing an offshore wind turbine foundation (OWTF). To consider the stratigraphic uncertainty, we create a synthetic stratigraphic model (SSM) and then sample multiple sets of boreholes to develop a series of stratigraphic models, each with a different degree of uncertainty (entropy). Using Markov random field (MRF) theory, these stratigraphic models were simulated, and their entropies were evaluated. The results indicate the entropies of the stratigraphic models decrease as the number of boreholes increases. The OWTF monopile is then designed based on these stratigraphic models with different entropy levels. Next, Monte Carlo simulation (MCS) is used to evaluate the failure probability of the design, considering uncertainties of the stratigraphic model and soil properties. Then, two monopile designs simulating piles located at different stratigraphic configurations are analyzed to assess the effects of stratigraphic uncertainty and strength parameter variability on their failure probabilities. Finally, insights gained regarding the effect of stratigraphic uncertainty on the OWTF monopile design are discussed.

Undrained capacity of circular shallow foundations under combined VHMT loading

Wind turbine (WT) foundations are generally subjected to large combined vertical, horizontal, moment, and torsional (VHMT) loads. The available investigations of the ultimate limit states of WT foundations focus predominately on unlimited-tension soil-foundation interfaces that are more appropriate for offshore settings. However, the interfaces for onshore WT foundations are generally unable to resist tensile loads. To address this omission, a zero-tension interface is used to investigate the VHMT failure envelope of circular shallow foundations under undrained soil conditions using finite element analysis. The effects of soil strength heterogeneity and foundation embedment were investigated. The results show that torsional loads reduce the VHM bearing capacity of circular foundations. The foundation embedment is also found to significantly increase the foundation bearing capacity. A full 4-D analytical expression for the VHMT failure envelope has been proposed based on the calculated failure envelopes.

Numerical research on the interaction of multi-directional random waves with an offshore wind turbine foundation

Real sea waves are multi-directional, but most studies on wave–structure interaction have focused on unidirectional waves. In this study, a coupled model between a potential flow solver and OpenFOAM wave basin is adopted to investigate multi-directional wave loads on an offshore wind turbine foundation. The coupled model is extended to multi-directional wave states based on the single summation method combined with the JONSWAP spectra. The numerical results are validated against the experimentally obtained values. Multi-directional waves are short-crested, the elevations are random in the wave field, and the wave pressures induced on the two sides of the cylinder are not symmetric about the principal wave direction, leading to a non-zero transverse force. In addition, the effects of wave directionality and relative ratio of foundation diameter and wavelength on wave loads are analysed. For multi-directional waves, the effects of wave directionality on transverse wave force are more sensitive than those on normal force. Wave run-up on the up-wave side increases as the directional spreading parameter decreases. In addition, wave directionality plays a complex role in the wave run-up load induced on the back side, which is found to be controlled by the relative ratio of the cylinder diameter and wavelength.

A semi-analytical one-dimensional model for offshore pile foundations considering effects of pile diameter and aspect ratio

A semi-analytical 1D model with the eight types of soil reactions is derived from the fishbone frame model for pile foundations of wind turbines, in which the coupling between axial and lateral resistances that matters for large pile diameters and small pile aspect ratios is modeled as the rotational soil reactions. The only correction factor for the calculation of the ultimate bearing capacities of the lateral and rotational soil reactions on the pile shaft is then identified by a curve-fitting approach and validated using a series of experiments and 3D FE analyses for both sand and clay. The proposed model shows favorable agreements with the experiments and 3D FE analyses in terms of the load-displacement curves at the ground level and the rotation and moment responses for the pile foundations with various pile diameters and aspect ratios. Finally, the proposed model is further verified by monopiles in layered soils and applied to investigate the effect of the coupling between axial and lateral resistances on the modal damping of monopile supported wind turbines. Without considering the coupling between axial and lateral resistances, the modal damping ratios for the monopile supported wind turbine are underestimated significantly.

Research on Underwater Topography Monitoring of Offshore Wind Turbine Foundation Based on Multi-Beam Data

Research on Underwater Topography Monitoring of Offshore Wind Turbine Foundation Based on Multi-Beam Data

Resistance to skirt-tip with external bevels of suction caissons penetrating clay

Suction caisson is an alternative to the foundation for offshore wind turbines due to easy installation and retrieval for reuse. It is partially installed by self-weight followed by insertion to the final depth by applying suction. During suction penetration, the soil accommodated by skirt wall will move into the caisson inside, leading to soil plug heave that prevents the caisson from penetrating to the desired depth. To mitigate the effect of the soil plug heave, suction caissons are fabricated with external bevels, so as to force the more expelled soil to the outside of the caisson. This paper presents failure mechanisms for calculating the penetration resistance to skirt-tip with various external bevels under two limit conditions. For the smooth base, the failure mechanism consists of two active shear zones below the smooth base and two radial and passive shear zones that are identical to those of a deep foundation. For the rough base, two active shear zones formed below the smooth base are replaced by the radial shear zones. According to the slip-line theory, the penetration resistance of skirt-tip with various external bevels is deduced in terms of su and weighted average of σi and σo (where su is undrained shear strength, σi and σo are equivalent overburdens inside and outside the caisson at the skirt-tip level). Theoretical result shows that the resistance factor Nc of skirt-tip increases with increasing αi and αo and decreasing δ (where αi and αo are adhesion factors, and δ is the angle of external bevel from the horizontal). The difference of resistance factor Nc between rough and smooth bases identically equals 0.57. Finally, numerical simulations borrowed from the literature are used to testify the rationality of the analytical solution presented in this study.

Preliminary Design and Dynamic Response of Multi-Purpose Floating Offshore Wind Turbine Platform: Part 1

Floating offshore wind turbine foundations are based on platforms operated by the oil and gas industry. However, they are designed and optimized to meet the wind turbines’ operating criteria. Although Malaysia is considered a low-wind-speed country, there are some locations facing the South China Sea that are found to be feasible for wind energy harnessing. The average daily wind speed may reach up to 15 m/s. Therefore, designing a cost-effective platform that can operate in Malaysian waters which has less severe environmental conditions compared to the North Sea would be a prudent undertaking. In this study, a new design of a multi-purpose floating offshore wind turbine platform (Mocha-TLP) is presented. In addition, the dynamic response of the platform to wave loads was investigated using the Navier–Stokes code STAR CCM+ developed by CD-adapco. Moreover, free-oscillation tests were performed to determine the natural periods of the platform. Three approaching wave cases and two wave conditions (WC) were considered. The results show that the natural periods of the platforms were within the recommended range for pitch, roll, yaw, heave, sway and surge motions. The platform was stable in rotational motion within the three cases. However, it experienced a noticeable surge motion which was more critical with wave condition one (WC1) since the wavelength equalled the length of the structure. The dynamic response of the platform to wave loads wase minimal and within the operational requirements for wind turbines.

Three-dimensional scour hole model and scour effects on the ultimate capacity of lateral loaded rigid piles

The rigid pile with a large diameter and a small slenderness ratio is the most widely used foundation for offshore wind turbines. Scour around piles seriously threatens the safe operation of offshore wind turbines, so its effects on rigid piles need to be studied urgently. In this paper, scour tests under steady current and static lateral load tests after scour are conducted. According to the results of scour tests, a three-dimensional scour hole model suitable for engineering reality and its analytical expression are proposed. Then, the results of load tests indicate that scour reduces the ultimate lateral capacity of foundations, and the percentage of reduction is related to the current direction, whereas does not change with load eccentricity. Finally, a preliminary formula for evaluating the ultimate lateral capacity of rigid piles after scour is presented, which can easily and quickly determine the upper and lower limits of scour effects.

The remedial performance of suction caisson foundations for offshore wind turbines under seismically induced liquefaction in the seabed: Shake table testing

The assessment of liquefaction hazards to offshore wind turbines (OWTs) requires an improved understanding of the interplay among the foundation geometry, ground shaking, and liquefaction-induced deformations. In this note, shaking table model tests were used to evaluate the behavior of suction caissons due to seabed liquefaction. Spectral accelerations and settlement of OWT foundations of varying skirt lengths on liquefiable sand were analyzed and compared.Insights into the performance of caisson skirts as a liquefaction countermeasure relative to the benchmark models are provided. The experimental results identified the two controversial mechanisms controlling shear-induced caisson settlement; the liquefaction-induced bearing capacity failure as the skirt length increases, whereas the larger net excess pore-water pressures within skirts contribute more significantly to reducing seismic demand.Overall, the skirted foundation with the longest skirt length experienced the least settlement and acceleration, which may be ascribed to the interplay of various mechanisms, i.e., the confining effect of the skirt and a slower pore-water pressure dissipation that limits the possibility of drainage.