Wind Turbine Foundations Research Articles

A conceptual basis for surveying fouling communities at exposed and protected sites at sea: Feasible designs with exchangeable test bodies for in-situ biofouling collection

The enhanced inertia load caused by biofouling on device components, such as the foundations of wind turbines or other structures at sea, modifies the hydrodynamic properties, and increases the stress to structures, predominantly in upper water layers with high impact from wave dynamics. This compromises the stability, functioning, operation as well as the durability of these devices especially in exposed environments. A main challenge is the quantification of the impact of hydrodynamic forces on irregular bodies being overgrown by soft- and hard-bodied biofouling organisms. Therefore, test bodies from the upper 1–5 m water depth and thus exposed to the strongest wave actions close to the surface shall be overgrown by biofouling and used in measurement trials in a wave and current flume. These measurements shall shed light on the varying roughness and its influence on the load bearing capacity of foundation piles. Consequently, the main aims of the present work were the development of two independent test stations as holding devices for artificial test bodies for the collection of biofouling organisms during field studies: a carrying unit floating at the surface in an exposed area (System A) and a sampling device with access from a land-based facility (System B). Both systems are relatively easy to access, exhibit straightforward handling, and are reasonable cost-effective. A Test Body Support Unit (TBSU, System A) was designed and mounted on a spare buoy to carry the test bodies (cylinders), which serve as substrate for the fouling. The system was sufficiently robust to withstand several periods of rough sea conditions over the first two years. This system can only be accessed by vessels. System B (MareLift) provided the robustness and functionality needed for areas exhibiting harsh conditions but can be operated from land. The here used test bodies (steel panels) exhibited a sound basis for the monitoring of succession processes in the biofouling development. System B offered the possibility to analyse two habitats (intertidal and subtidal) and revealed clear differences in the composition and development of their fouling communities.Overall, both systems provide advantages in obtaining standardized biofouling samples compared to previous approaches. Such test stations play an important role in the risk management of marine sectors as they could help characterising biofouling communities over different geographical areas. System A and B provide a sound basis for biofouling research but potentially also for other potential research approaches in exposed areas as they provide space for future developments.

Numerical Analysis of Local Scour of the Offshore Wind Turbines in Taiwan

Rapid expansions of the offshore wind industry have stimulated a renewed interest in the behavior of offshore wind turbines. Monopile, tripod, and jack-up wind turbines support most offshore wind turbines. These foundations are sensitive to scour, reducing their ultimate capacity and altering their dynamic response. However, the existing approaches ignore the seabed’s rheological properties in the scour process. This study focuses on the scour development around the wind turbine foundation in the Changhua wind farm in Taiwan. The simulation results explain the influence of different hydrodynamic mechanisms on the local scours in a cohesive fluid, such as regular waves, random waves, and constant currents. A newly non-Newtonian fluid model, the Discontinuous Bi-viscous Model (DBM), reproduces closet mud material nature without many empirical coefficients and an empirical formula. This new rheology model is integrated and coupled into the Splash3D model, which resolves the Navier–Stokes equations with a PLIC-VOF surface-tracking algorithm. The deformation of the scour hole, the backfilling, and the maximum scour depth are exhibited around the wind turbines. Waves, including regular and irregular waves, do not increase the scour depth compared with currents only. In the case of random wave–current coupling, the results present a signal of scour evolution. However, the scour depth is shallow at 0.033≤S/D≤0.046.

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.

Numerical investigation of concrete-filled double skin steel tubular (CFDST) structure subjected to underwater explosion loading

With the increasing applications in the offshore industry such as oil and gas jackets, submarine pipelines and wind turbine foundations, concrete-filled double skin steel tubular (CFDST) structures are encountering the ever-increasing risk of threats to underwater explosions (UNDEX). This study presents a systematical investigation on the structural behaviors and design recommendations of the CFDST structures subjected to UNDEX loadings through finite element analysis (FEA) approaches. Finite element models have been developed, where the non-linear material properties of the constitutive steel and concrete parts and the composite actions in-between have been considered. The FEA models are verified against the experimentally determined shock wave pressure history, the deformation shapes, and the residual strength. The full-range analyses were firstly carried out on the structural responses of CFDST structures, including the typical damage patterns and residual strength of the specimen after UNDEX. Then, the parametric studies show that the cross-section hollow ratio, charge weight, and explosion distance play great roles in determining the residual strengths. Thereafter, damage indexes considering the parameter of the hollow ratio and the scaled explosion distance has been formulated, and design recommendations have been suggested accordingly.

Offshore Wind Energy and Marine Biodiversity in the North Sea: Life Cycle Impact Assessment for Benthic Communities.

Large-scale offshore wind energy developments represent a major player in the energy transition but are likely to have (negative or positive) impacts on marine biodiversity. Wind turbine foundations and sour protection often replace soft sediment with hard substrates, creating artificial reefs for sessile dwellers. Offshore wind farm (OWF) furthermore leads to a decrease in (and even a cessation of) bottom trawling, as this activity is prohibited in many OWFs. The long-term cumulative impacts of these changes on marine biodiversity remain largely unknown. This study integrates such impacts into characterization factors for life cycle assessment based on the North Sea and illustrates its application. Our results suggest that there are no net adverse impacts during OWF operation on benthic communities inhabiting the original sand bottom within OWFs. Artificial reefs could lead to a doubling of species richness and a two-order-of-magnitude increase of species abundance. Seabed occupation will also incur in minor biodiversity losses in the soft sediment. Our results were not conclusive concerning the trawling avoidance benefits. The developed characterization factors quantifying biodiversity-related impacts from OWF operation provide a stepping stone toward a better representation of biodiversity in life cycle assessment.

Re-purposing of shallow wind turbine foundations for power capacity increase

Re-purposing the foundations of older wind turbines to install new and larger wind turbines can reduce the investment costs and carbon emissions of wind energy development projects. In this paper, detailed numerical modeling is used to assess the behavior of a shallow foundation designed for a 5 MW wind turbine under the aerodynamic loads and seismic motions of larger replacement wind turbines. The finite element modeling scheme used for the soil under rocking foundations is validated based on experimental data. Results of this study indicate that maximum gapping and maximum tilting have similar trends of increase with increasing turbine power. Relaxing the no-uplift rule for normal operation conditions allows for installing an 8 MW wind turbine on the foundation designed for 5 MW, without compromising the foundation safety or serviceability requirements. However, the foundation does not perform satisfactorily with the 10 MW wind turbine. The no-uplift rule of design regulations for operational conditions is overly conservative. The limits of capacity increase for individual projects can be determined using the procedure presented in this paper.

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.

Ecological effects of offshore wind farms on Atlantic cod (Gadus morhua) in the southern North Sea

Evaluating the ecological effects of the rapid expansion of offshore renewables at local, regional and ecosystem-wide scales is essential to understand the overall socio-ecological trade-offs also for other sectors such as fisheries. Hence, little is known about the ecological impact on demersal fish. To shed light on this topic, we studied the effects of an offshore wind farm in the southern North Sea on different life stages of Atlantic cod (Gadus morhua) using a combination of sampling methods at varying spatial and temporal scales. Our investigations of diet composition and trophic niches indicate that cod utilizes wind turbine piles with scour protection as feeding grounds. Furthermore, collected information on cod adults and early life stages during winter spawning season suggest that spawning activity occurred in winter across the wider wind farm area. We conclude that wind turbine foundations with a scour protection can function as artificial reefs that have local positive effects on the resilience of local cod populations. With our study we contribute to urgently needed observational evidence regarding the ecological impact of offshore wind farm installations to inform area-based management and future monitoring activities.

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.

Study of the Interaction between Regular Waves and a Wind-Fish Combined Structure

The combination of a wind turbine and fish cage (wind-fish) is able to solve the complex issue of power supply for deep-sea aquaculture. Nevertheless, the hydrodynamic coupling between the fish cage and the wind turbine foundation is increasingly complex, and there is little research on the related hydrodynamics. In this paper, the hydrodynamic research on the wind turbine foundation and fish cage structure is carried out using numerical simulation, in which the porous medium model is adopted to simulate the effects of the fish cage on the flow field. By analyzing the hydrodynamic characteristics of the combined structure of an offshore wind turbine and aquaculture cage under the action of regular waves, the load on both the cage in the combined structure and the wind turbine foundation, and the overall load on the combined structure of offshore wind turbine and cage are obtained. The effects of wave parameters, structural dimensions, cage submergence depth, and biological attachment on cage load, wind turbine foundation load, and overall load of the combined structure are analyzed. The results demonstrate that the wind turbine foundation is persistently the central part of the combined structure. The wave height, water depth, and degree of attachment of marine organisms all have different effects on the amplitude of the structural load. In contrast, the wave period, cage geometry, and cage submergence depth have no significant effects on the amplitude of the structural load. In addition to the water depth, other factors will affect the proportion of the load on the wind turbine foundation and the total load as well.

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.

Ensemble of feature extraction methods to improve the structural damage classification in a wind turbine foundation

Ensemble of feature extraction methods to improve the structural damage classification in a wind turbine foundation

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.

Model Tests on Cyclic Responses of a Laterally Loaded Pile Considering Sand Anisotropy and Scouring

Monopile is a common foundation for offshore wind turbines. Long-term cyclic loading from winds and currents may affect the normal operation of offshore wind turbines. For offshore wind farms, natural soils are usually inherently anisotropic, and scouring often occurs as a removal of soil around a monopile. Their influence on pile behavior is seldom studied. To investigate the effects of anisotropy of sand and scouring on a cyclic laterally loaded pile, a series of 1 g model tests are conducted, in which three deposition angles of sand, two scour depths, and two cyclic amplitudes are considered. It was found that for a monopile in sandy soil, the accumulated displacements at the pile head increased with the increased deposition angles of sand and scour depth. The effects of sand anisotropy are more profound for piles with a smaller cyclic amplitude. However, the results for the pile with and without scouring did not give a consistent tendency as the factors of anisotropy of sand. Based on test results, an explicit expression is advised to assess the long-term accumulated residual displacement at the pile head, in which factors of deposition angle of sand, scour depth, and cyclic amplitudes are incorporated.

A numerical methodology to predict the lateral load response of monopiles installed in SAND considering soil stiffness degradation

Most of the routine work concerning the design of laterally loaded monopile foundations is conducted using simplified one-dimensional (1D) models based on p-y curves. However, many of these curves have been derived from slender piles typically used in the oil and gas industry, which behave differently than rigid monopiles used as offshore wind turbine foundations. Moreover, these curves usually represent the soil response without considering its degradation, which may significantly modify the monopile lateral response. Hence, this work proposes a numerical methodology based on the finite element method to derive suitable curves for 1D models of monopiles installed in sand. First, a three-dimensional (3D) finite element model (FEM) of the monopile and the surrounding soil was constructed. In this model, the monopile had an elastic and linear material behavior. The soil response was addressed using the Mohr-Coulomb criterion combined with three different soil stiffness degradation models. For extracting the p-y curves from the 3D FEM, a 10th-order polynomial was used to fit the bending moment profiles; consequently, an 8th-order polynomial described the soil reaction profiles. Results obtained using the proposed 3D FEM and a 1D model based on the derived p-y curves were then verified against experimental results obtained through centrifuge modeling. Furthermore, the proposed 1D model adequately matched the experimental results, especially for small displacement conditions.

An innovative method of assessing yield strength of floater hull for semi-submersible floating wind turbine in whole life period

The concepts of external and internal corrosion damage of materials have been proposed, and the improved corrosion damage models considering marine corrosion zone have been established. Then, a new method of assessing yield strength of floater hull for semi-submersible floating wind turbine considering the effect of corrosion damage in whole life period is proposed based on the equivalent design wave and structural quasi-static analysis method. Taking the OC4 floating wind turbine foundation for example, this new assessment method has been conducted. The results show that the stresses and displacements of structures will be increased under the influence of corrosion damage in whole life period and the risk of structural failure is also increased due to the corrosion damage. The structures which locate in the high stress level region will up to the yield failure critical point (yield strength margin is close to zero) by the end of service period if just considering the external corrosion damage, and these structures may fail due to yield after 18–22 years of service if considering the external and internal corrosion damage simultaneously. Therefore, the corrosion damages (especially the internal corrosion damage) have produced an important effect on the structural yield strength. In addition, the value of material factor γm = 1.1 (given by DNVGL-ST-0119) is small, and the suggested value is 1.25–1.3 for the structures in high stress level region.