Offshore Wind Turbine Foundations Research Articles

Development of design typhoon profile for offshore wind turbine foundation design in Southern China

For offshore wind development in Southern China, typhoon represents the extreme environmental loading condition to be considered in the design of the turbine structures. For the geotechnical design of the foundations, it is specified by the design code that the effect of cyclic loading on the soil strength and stiffness must be considered. This necessitates the complete time history of loads on the foundation experienced during a design typhoon. Idealised storm profiles with staged mean wind speeds and significant wave heights are commonly used as input to time-domain simulations to generate the foundation load history. However, no such design typhoon profile is provided in the Chinese design standard which creates considerable difficulty and uncertainty in the design process. In this paper, the methodology and the rationale that were applied to derive the design typhoon profiles for an offshore wind project in Guangdong province are presented. The wind and wave characteristics generated by typhoon are described in detail and the implications for the foundation design are discussed. To demonstrate application of the proposed typhoon profile, a case study for the ULS design of a suction bucket jacket is presented.

Research on the calculation method of penetration resistance of bucket foundation for offshore wind turbines

The cost of foundations for offshore wind turbines constitutes approximately 35% of the total cost of an offshore wind farm. The bucket foundations show significant potential due to their superior transportation and installation efficiencies compared to pile foundations, leading to potential cost savings for the foundation of up to 30%. For a bucket foundation to be installed successfully, the penetration resistance must be predicted. However, there is currently a lack of clarity on how to select a suitable calculation method for penetration resistance based on known geological parameters to guide construction. In order to evaluate the current methods of calculation for bucket foundation penetration resistance, this study combines theoretical calculation methods with two sets of practical measurement data from the field. The calculation methods of penetration resistance for bucket foundation are first reviewed and categorized. The applicability range of each method and the parameters needed for calculation are given. Next, according to the measured data in the process of penetration of bucket foundation on site, the evolution of compartment pressure, tilt angle, resistance and required suction in the process of penetration are analyzed. Finally, the reviewed methods are compared to the results of two practical projects in order to analyze the differences between them and make recommendations for the calculation technique. The findings can be used as a guide for calculating the bucket foundation's penetration resistance in complex geological conditions.

Assessment on earthquake-induced liquefaction around a hybrid monopile foundation for offshore wind turbines with a transition layer model

Offshore wind turbines are currently being developed to have a large size. A hybrid monopile foundation, which is composed of a single pile and a wheel, is advantageous in terms of bearing capacity and liquefaction resistance in liquefiable soil. In this study, the seismic response of a hybrid monopile foundation was investigated using a verified numerical model. Earthquake-induced liquefaction around the foundation was assessed by considering the influence of the foundation dimensions. In the numerical model, a pile-soil contact element was proposed using a transition layer model that effectively buffered the pile-soil interactions. The soil characteristics were evaluated based on the distribution of excess pore water pressure. The liquefaction resistance of the shallow layer was improved by the wheel, and the pile length had a negligible influence on the seismic response. Larger wheel diameter and thickness demonstrated a more pronounced improvement in the anti-liquefaction ability. The influencing zone was determined based on the liquefiable area and had a conical shape under the wheel. This study quantitatively investigated the liquefaction characteristics of a hybrid monopile foundation and provided insights into the numerical calculation of dynamic pile-soil interactions.

Influence of Horizontal Loading in Changing the Ultimate Uplift Bearing Capacity of Monopile Foundation of Offshore Wind Turbine

Throughout their lifespan, monopile foundations supporting offshore wind turbines inevitably experience horizontal loads from waves, winds, and currents, resulting in cumulative deformation. It has been believed that deformation caused by horizontal loading weakens the interaction between the pile and the soil, leading to a reduction in the ultimate uplift bearing capacity of the pile foundation. However, there is a scarcity of literature investigating this issue, particularly regarding monopiles used in offshore wind turbine installations. Therefore, this study aims to explore the impact of horizontal cyclic loads on the ultimate uplift bearing capacity of monopile, focusing on the pile–soil interaction. To achieve this, a series of 1 g model tests were conducted on a rigid model pile embedded in silt with varying relative compaction. The test results indicate that the ultimate uplift bearing capacity of the pile is significantly diminished after experiencing horizontal cyclic loading, and the extent of reduction is closely linked to the amplitude of the horizontal deformation. A semi-empirical model is developed to predict the ultimate uplift bearing capacity of the pile foundation following horizontal cyclic loading. The key findings of this study are as follows: (1) The earth pressure in the active zone gradually decreases with an increasing number of cycles, while the earth pressure in the passive zone experiences a slight increase under horizontal cyclic loading. (2) The position of the pile rotation center under horizontal cyclic loading is approximately 0.84 times the depth at which the pile is buried, and this relationship appears to be independent of soil density and cyclic load ratio. (3) The variation of earth pressure corresponding to the horizontal deformation of the pile in the active zone can be divided into three phases: a rapid decline phase, a slow decline phase, and a stable phase. (4) The reduction in the ultimate uplift capacity is influenced by the cyclic ratio and number of cycles but does not appear to have a significant relationship with soil density.

Lateral behavior of monopiles under the simultaneous effect of vertical-lateral loads and change of embedment depth due to 1D local scouring

Offshore wind turbine foundations are subjected to high lateral loads but insignificant vertical forces of the superstructure. A decrease in the embedment depth due to scouring can also alter the effect of vertical loads on the soil-structure interaction. Current design methods consider the vertical and lateral forces in design separately. This area also lacks studies featuring physical modeling that produce consistent conclusions. The present study employed centrifugal physical modeling to investigate the lateral behavior of offshore wind turbine monopiles under the simultaneous effects of vertical and lateral loads with a change in embedment depth caused by 1D local scour (D = diameter of pipe) in calcareous sand. The results showed that the vertical load of the superstructure increased the lateral strength of the monopile in the no-scour and local scour cases by about 3.5% and 6%, respectively. Improvements in the p-y curves along the embedment depth were noted at depths of less than twice the monopile diameter. Stress intensification was more significant under the scoured conditions because of the reduced embedment depth of the monopile. Overall, the vertical superstructure loads had little effect on the lateral performance of the monopiles.

The large-scale impact of anthropogenic mixing by offshore wind turbine foundations in the shallow North Sea

Structure drag from offshore wind turbines and its physical impacts on the marine environment of the German Bight are investigated in this study. The flow past vertical cylinders, such as wind turbine foundations, and associated turbulent mixing has long been studied, but questions remain about anticipated regional implications of offshore wind infrastructure on physical and biogeochemical conditions. Here, we present two existing modeling approaches for simulating wind turbine foundation effects in regional ocean models and discuss the problematic use of very high resolution in hydrostatic modeling. By implementing a low-resolution structure drag parameterization in an unstructured-grid model, we demonstrate the impacts of monopile drag on hydrodynamic conditions, validated against recent in-situ measurements. Although the anthropogenic mixing is confined at wind farm sites, our simulations show that structure-induced mixing affects much larger, regional scales. The additional turbulence production emerges as the driving mechanism behind the monopile impacts, leading to changes in both the current velocities and stratification, with magnitudes of about 10%, similar in magnitude to regional annual and interannual variabilities. This study provides new insights into the hydrodynamic impact of offshore wind farms at their current development levels and emphasizes the need for further research in view of potential restructuring of the future coastal environment.

Effects of load-related parameters on the dynamic response of concrete-filled double-skin steel tube K-joints under lateral impact loading

Concrete-filled double-skin steel tube (CFDST) structure is gradually applied in jacket foundation of offshore wind turbine (OWT). This study uses experimental and numerical methods to investigate the effect of impact force and axial force on the dynamic response of CFDST-K joints subjected to impact loading. A pendulum test machine is conducted to analyze the deformation property and dynamic response of six K-joints. The results indicate that the impact velocity increases as the impact energy rises, resulting in a significant increase in impact force, displacement, and plastic strain. Improving the axial compression ratio enlarges the second-order effect and the initial stiffness, resulting in a substantial rise in the deformation under a high axial compression ratio. CFDST-K-joints exhibit superior flexural strength, impact resistance, and energy dissipation ability than hollow K-joints in terms of the constraint effect provided by the inner tube to the core concrete. Subsequently, ABAQUS is used for nonlinear analysis and to validate against the test results and explore the energy distribution. Finally, the parametric analysis demonstrates that the optimal hollow rate of CFDST-K joints is between 0.6 and 0.7 and the flexural stiffness is a critical parameter affecting the impact performance.

Statistical evaluation of jacking-installation resistance models for suction foundation of offshore wind turbines in sands

The American Petroleum Institute (API) method is widely adopted to estimate the installation resistance of suction foundations, while the estimation accuracy remains unknown. This paper first develops a database containing 114 measured jacking-installation resistance data. Then, the database is used to quantify the model uncertainty of the current API methods with and without consideration of the soil plug effect. The results showed on average the predicted installation resistance is about one third of the true value if by the current API unplugged model, and five times larger if by the current API plugged model. The dispersions in prediction accuracy are medium to extremely high. The accuracy of both API models exhibits statistical dependency to the magnitude of the predicted installation resistance mainly due to the correlation between model bias and the penetration depth. A soil plug factor is introduced to not only unify the current API unplugged and plugged models into one formulation, but also capture the development process of soil plug during the foundation installation. Another two empirical terms are then added to calibrate the unified API model. The calibrated unified API model is accurate on average with low dispersion in prediction accuracy and no hidden statistical dependency.

Centrifuge Model Tests Investigating Initiation and Propagation of Pile Tip Damage during Driving

A number of incidents of pile tip damage and extrusion buckling have been reported within the offshore industry, generally arising during driving of relatively thin-walled tubular pile foundations through hard layers or potentially heterogeneous sediments. The issue has become of particular concern with modern trends toward larger diameter piles, with higher ratios of diameter to wall thickness, for monopile or jacket foundations of offshore wind turbines. The paucity of good-quality data available in the public domain about this kind of failure has impaired the development of simple design guidelines for industry to address the issue. The paper presents results from a series of centrifuge model tests where pile tip damage and extrusion buckling were observed during driving of either (1) undamaged cylindrical piles into a sand bed containing a layer of different sizes of boulders, or (2) predented piles into medium dense homogeneous sand. Following a previously reported pilot study, a new and higher energy model pile-driving hammer was designed to permit driving through the hard layers and allow pile embedment by up to five diameters. Two test beds were prepared, one of which contained boulder layers of different sizes either near the surface or at a depth of about one pile diameter, into which piles of two different diameter to wall thickness ratios were driven. The test series led to observations of pile tip damage that ranged from abrupt tip crumpling to gradual extrusion buckling. For 2 of the 22 piles installed, dent growth during installation was detected by optical fiber sensors attached to the piles. For the uninstrumented piles, damage was deduced from the trends in blow counts during installation, with later confirmation from detailed visual inspections and laser scanning, and also from careful layer by layer excavation of each sample. The high-quality database generated from the model tests forms a valuable resource for further study and validation of advanced numerical models to simulate pile tip damage and for calibration of simple guidelines for practical design.

A novel design of the offshore wind turbine tripod structure using topology optimization methodology

To achieve a design with the desired stiffness, strength, and durability at the component level, topology optimization has been practiced and was effective in the design of sustainable products, by autumnally allocating the available material within the design domain to form the shortest loading path. In the conceptual phase, this paper proposed a design methodology using topology optimization technique for lightweight design of a tripod structure - a typical form of foundation for offshore wind turbines (OWTs). The minimum compliance subject to prescribed volume percentage is formulated and an innovative tripod structure is generated by interoperating the optimized structure. To facilitate comparison, a tripod structure of a 5 MW OWT is referenced. The system performances are comprehensively and synthetically assessed, encompassing the natural frequency, maximum von Mises stress in ultimate load cases and cumulative fatigue damage. It is discovered that the total mass of the novel design is substantially less than that of the reference structure. In contrast, the fatigue resistance has been significantly enhanced. These finding reveal conclusively that the proposed topology optimization approach is capable of generating novel designs, reducing weight and alleviating fatigue damage.

The Anti-Overturning Response of Tripod Bucket Foundation for Offshore Wind Turbines

The tripod bucket jacket foundation is proven to be a practicable solution for offshore wind turbines (OWTs) to withstand huge environmental loads in deep water. This paper presents model tests for a scaled tripod bucket jacket foundation with reference to a prototype applied in China to obtain its lateral load bearing behavior in medium-dense sands. Extended finite element analyses were conducted by ABAQUS to compare anti-overturning responses for the tripod bucket foundation in both sand and soft clay, and the influences of loading direction and aspect ratio were also taken into account. The results showed that the failure modes of the laterally loaded tripod bucket foundation are the pull-out of the windward bucket in sand and the settlement of the leeward bucket in soft clay, respectively. Thus, the unfavorable loading direction of the foundation changes with soil type. It is also shown that the bearing capacity for the foundation in soft clay will be enhanced more effectively as the bucket diameter increases. Instead of the rotational soil resistance resulting from the rotation of the bucket, the vertical soil resistance governs the anti-overturning bearing capacity of a tripod bucket foundation. As the tilt created by the overturning moment rises, the rotational stiffness of the foundation dramatically declines.

A further study on the scour around the monopile foundation of offshore wind turbines

Foundation scour has long been a concern in the offshore wind industry as it poses a great threat to the safety of offshore wind turbines (OWTs). However, the current understanding of this issue is still insufficient. This is because many offshore wind farm factors and their coupling effect on scour have not been fully investigated. Therefore, two phases of experimental studies are conducted in this paper to investigate how monopile vibration, water flow, and intermittent operation of OWTs work together to affect the scour around the monopile foundation. The research results have shown that in the absence of water flow, monopile vibration can also create ‘scour pits’ while causing soil densification. The coupling of a large-amplitude monopile vibration and the water flow will exacerbate the scour, while the coupling of a small-amplitude monopile vibration and the water flow will retard the scour. The vibration amplitude has a greater influence on the scour than the vibration frequency. In addition, vibration sequence was found to have a significant effect on the scour, but despite the sequence, repeated application and removal of monopile vibration, which represents the intermittent operation of OWTs, can lead to severe foundation scour.

Probability of unsatisfactory performance of monopile foundations for offshore wind turbines considering variations in seabed properties

Safe design of monopiles for Offshore Wind Turbines (OWT) requires an accurate estimation of the foundation soil material properties. Uncertainty in soils is mainly due to their inherent variability which involves various spatial and temporal processes they have been exposed to over their natural geological process. The limited amount of information from geotechnical investigations in the seabed, considering the size of OWT farms, adds to the uncertainty. Commonly used deterministic design approaches idealize soil layers by assigning average values to the mechanical soil properties and make use of empirical correlations that can significantly differ from offshore conditions. Probabilistic design approaches have been suggested in the literature to account for the effect of uncertainties on the foundation’s performance by applying a coefficient of variation (COV) to soil properties. Such methods include theoretical investigations by applying selected COV values to predetermined mean CPT profiles for sands and undrained shear strength for clays. This study calculates the probability of unsatisfactory performance, Pfsys, of monopiles in sands by implementing variation in friction angle and unit weight using current API and DNV-GL design guidelines. Monte Carlo Simulation is employed to generate random fields using lognormal, normal, and uniform distributions. Pfsys is presented for a case study found in the literature and a design exercise for which local COV for the Belgian coast is also proposed.

Integrated transportation of offshore wind turbine and bucket foundation based on a U and K shaped assembled platform

The integrated transportation of offshore wind turbine (OWT) and bucket foundation (BF) has obvious advantages in the application of shallow sea (<20m) wind farm engineering. However, this approach will face more challenges when it is applied to distant and deep-sea wind farms. In this study, a U and K shaped assembled platform (U&KSAP) is proposed and introduced for the first time, which is used to transport and install the OWT and BF (OWT&BF) in deeper and farther offshore wind farms. Firstly, the integrated transportation and installation method of OWT&BF based on the novel U&KSAP is discussed. The monitoring and brief analysis of U&KSAP and BF for the first sea transportation was carried out, and some key parameters were obtained. Secondly, the integrated transportation experiments of U&KSAP and OWT&BF were carried out in a towing tank. Finally, the main factors affecting the motion behavior of the integrated transportation process were discussed. The experimental results show that the U&KSAP has good seakeeping and is capable of integrated transportation. During integrated transportation, the influence of towing speed on drag resistance is obvious. The motion behavior of OWT&BF and U&KSAP is more sensitive to wave height and period, especially near the resonant period. The increase in towing speed and the air pressure inside the BF can slightly reduce motion response. At the same time, certain control measures should be taken for the top wind turbine to reduce horizontal acceleration during integrated transportation.

Influence of Soil Plug on the Seismic Response of Bucket Foundations in Liquefiable Seabed

The suction installation process for bucket foundations for offshore wind turbines (OWTs) can cause the formation of soil plug within the bucket, which can affect the seismic performance of the OWT. Therefore, it is important to evaluate the influence of soil plug on the seismic performance of OWT on bucket foundations. In this study, a comprehensive set of high-fidelity solid–fluid coupled dynamic numerical simulations are conducted to analyze the seismic response of bucket foundations with a focus on the influence of soil plug and its potential mitigation. The influence of different bucket models, seabed soil densities, seabed inclination and reinforcement types, as well as soil plug removal techniques, are investigated. The results clearly show that the existence of soil plug has a significant unfavorable influence on the seismic performance of OWT on bucket foundations, especially for wide bucket foundations in mildly inclined seabeds, and should be considered in seismic design. Reinforcement methods, such as the application of an inner compartments, outer wings and inner pile, can improve the seismic performance of OWT on bucket foundations, with the application of an inner compartment being the most effective. Soil plug removal can alleviate the negative influence of soil plug, and should be adopted when possible.

A corrected p-y curve model for large-diameter pile foundation of offshore wind turbine

With the development of pile-soil interaction theory, at present the sand p-y model of API specification is adopted to the analysis and calculation of large diameter piles under horizontal static load, but the results of the ultimate soil resistance at different depths is underestimated and the initial foundation reaction modulus is overestimated, thus the calculation results are obtained with large errors. For this problem, a finite element numerical model of pile-soil interaction is established in this study, and the calculation models under horizontal static load with different pile diameters and soil depth are analyzed and the data regression is carried out. A correction coefficient is introduced to consider the size effect of large diameter pile, and the variation law of influence index of different factors is discussed. Finally, a corrected sand p-y curve model for single pile foundation with large diameter more than 2 m is proposed. Compared with the results of model test and field test, the calculation results of ultimate soil resistance and initial foundation reaction modulus from the corrected model is more in accord with the actual situation.

An experimental study of the local scour protection methods around the monopile foundation of offshore wind turbines

Local scour induced by current or wave is a significant factor impacting the overall stability and fatigue life of the pile foundation of wind turbines. Currently, the most commonly used scour protection method is the rock armour, which works by placing rocks on the original seabed to reduce the near-bottom flow speed. Although the rock armour is easy to be implemented in practice, it still has the potential of complete failure due to the washout near the structure and along the outer edge. In this study, we investigate the performance of four different scour protection methods through physical experiments, which include rock amour, sand-ribbed geotextile mattress (SRGM), bionic grass, and honeycomb structure. The results indicate that the sand-ribbed geotextile mattress is the most effective among all test scour protection methods. However, given its relatively high potential to impact local environment, careful and thorough consideration should be taken before the practical implementation.

Assessing the performance of winged caisson foundations for offshore wind turbines

The foundation of an offshore wind turbine (OWT) accounts for up to 20% of the total cost; therefore, investigations into reliable and efficient foundations are important for the offshore wind energy industry. This article describes an innovative winged caisson foundation for OWTs, which adapts the conventional caisson foundation to produce higher overturning capacities. The behaviour of this proposed novel foundation in the sand was investigated through experimental and numerical modelling using a series of 1 tests (1/70th scale), centrifuge model tests (70), and finite element (FE) simulations. Models with various wing sizes installed in different sand densities were tested in the laboratory by applying an overturning load. In this article, the moment–rotation performance of the foundation under monotonic loading is compared against that of conventional caissons to assess the potential benefits of adding wings to caisson foundations. Results show that there is a significant increase in overturning capacity provided by the novel foundation. FE models were first validated against experimental results, then, used to conduct a parametric study to better understand the behaviour of the winged caissons beyond the scaled model tests. In addition, the FE models were used to evaluate the drained response of the proposed foundation under combined loading.

Numerical study on the effects of scour on monopile foundations for Offshore Wind Turbines: The case of Robin Rigg wind farm

Scour around monopiles supporting Offshore Wind Turbines (OWT) may cause a significant reduction in the foundation capacity and the natural frequencies of the soil–structure system. The decommission of two monopiles at the Robin Rigg wind farm after only six years of operation due to massive unexpected scour demonstrates the hazard that an active marine environment poses to OWT. Assessing and predicting the quantitative impact of scour on monopile foundations from a point of view of the utilisation of the foundation capacity is hence crucial to its long-term structural and operational safety. Using finite element analysis, the present study quantifies the influence of scour through moment M and lateral load H (M−H) interaction curves and Load Utilisation (LU) ratios. It analyses the first and second natural frequencies of OWT exposed to different scour morphologies. This methodology is applied to a large-diameter monopile embedded in sand and to the scour-induced failed foundation of the Robin Rigg wind farm. The results indicate that, with increasing scour depth, the M−H capacity and the natural frequencies are reduced due to the loss of lateral confinement. The response of the system is found to depend on the rigidity of the pile governed by the slenderness ratio, the capacity of the soil to redistribute loads with increasing scour, and the effects of overburden pressure on deeper unscoured soil layers. The LU methodology has proven to be an appropriate and efficient approach to predict the long-term performance of scoured monopiles.

Hydrodynamic Characteristics of Three-Bucket Jacket Foundation for Offshore Wind Turbines During the Lowering Process

The three-bucket jacket foundation is a new type of foundation for offshore wind turbine that has the advantages of fast construction speed and suitability for deep water. The study of the hoisting and launching process is of great significance to ensure construction safety in actual projects. In this paper, a new launching technology is proposed that is based on the foundation of the three-bucket jacket for offshore wind turbine. A complete time domain simulation of the launching process of three-bucket jacket foundation is carried out by a theoretical analysis combined with hydrodynamic software Moses. At the same time, the effects of different initial air storage and sea conditions on the motion response of the structure and the hoisting cable tension are studied. The results show that the motion response of the structure is the highest when it is lowered to 1.5 times the bucket height. The natural period of each degree of freedom of the structure increases with the increase of the lowering depth. The structural motion response and the hoisting cable tension vary greatly in the early phases of Stages I and III, smaller in Stage II, and gradually stabilize in the middle and late phases of Stage III.