Offshore Wind Turbine Foundations Research Articles

Coupled stability of offshore wind monopile foundations and submarine slopes under periodic loads

Long-term cyclic loading on offshore wind monopiles will lead to a decrease in the mechanical properties of the seabed near the monopile's foundation, therefore, causing instability of the submarine slope and the monopile foundation. To capture the accumulation strain caused by high cycle numbers and plastic deformation of seabed sands, the high cycle accumulation (HCA) model is complied with Mohr-Coulomb constitutive models. The shear strength reduction (SSR) method is used to obtain the safety factor of submarine slopes. The coupled numerical model simulates a 6 MW offshore monopile foundation with a single periodic lateral load applied on the monopile at the sea level, which represents concentrated wind and wave loads in storm conditions. The results show that the plastic zones begin to form at the monopile foundation, then extends towards the slope toe and slope top, and eventually across the entire slope, leading to slope instability. In this process, the safety factor of the slope is significantly reduced with each stage of the process. It was found that both the sharp changes of load amplitude and large average load also decreases the safety factors. Moreover, at the beginning of the monopile installation, the safety factor will increase by nearly 10%, but it will drop by nearly 30% due to long-term periodic lateral loads. Therefore, these have important implications for the installation of monopile foundations for offshore wind turbines (OWTs) since the slopes we consider are within the range of slopes at planned sites for OWTs. An increase in monopile diameter and embedded depth can enhance the overall slope stability. However, an increase in embedded depth could potentially instigate more extensive seabed landslides despite enhancing overall stability.

Hydrodynamic Analysis of a Multi-Pile-Supported Offshore Wind Turbine Integrated with an Aquaculture Cage

The integrated development of offshore wind power and marine aquaculture is becoming increasingly important. However, the impact mechanism of integrating a net cage on the dynamic characteristics of offshore wind turbines remains unclear. This paper presents a design scheme for a multi-pile-supported offshore wind turbine integrated with an aquaculture net cage and conducts a preliminary theoretical analysis of the influence of an additional net cage on the wind turbine. The analysis reveals that the primary effect is an increase in hydrodynamic loads on the wind turbine foundation, while the structural frequency of the wind turbine remains largely unaffected. Furthermore, computational fluid dynamics (CFD) numerical models, whose accuracy is verified by physical experiments, are utilized to compare the hydrodynamic characteristics of the offshore wind turbine foundation with and without the net cage, considering different net solidities. The simulations identify significant changes in the flow field surrounding the foundation due to the presence of the net cage, resulting in a considerable increase in the overall hydrodynamic load on the foundation. Moreover, the mutual interference between the netting and the foundation amplifies their respective hydrodynamic loads and concentrates these loads at the upstream section of the structure. The maximum increase in hydrodynamic load for a single pile reaches 6.32 times its original value, posing significant risks to the structure. Finally, a preliminary feasibility analysis of the scheme was conducted. The results presented in this article can serve as a theoretical basis for the design of such innovative structures.

Study on bearing performance and reliability of modified monopiles for OWTs using deterministic and probabilistic analysis methods

Existing studies mainly focus on the bearing capacity of monopiles, with less emphasis on reliability analysis, which is an important aspect of the performance of offshore wind turbine (OWT) foundations. This paper investigates the bearing capacity and reliability of traditional and hybrid monopiles using deterministic and probabilistic analyses. The three-dimensional finite element model (FEM) considering the structure-soil interaction is established and its accuracy is verified through comparisons with existing research findings. The responses of monopiles under wind and wave loads are compared in detail, including the rotation angle at the mudline, structural stress, and soil displacement. The environmental design parameters are obtained based on the 30-year annual extreme data, and the correlation between these environmental parameters is considered to obtain a large amount of random environmental parameter data. Then, the back propagation neural network (BPNN) and the response surface method (RSM) are combined to construct the failure response surface. Research shows that the reliability index of the plated monopile does not improve significantly, while the skirted-monopiles perform much better than the traditional monopile. The proposed FEM-BPNN-RSM coupled methodology provides a more comprehensive understanding of the monopile bearing capacity, and can be used for the future monopile structure design of OWTs.

Hydrodynamic analysis of the combined structure of offshore monopile wind turbine foundation and aquaculture cage

Reliable prediction in hydrodynamics is crucial for the design of marine structures, especially for columnar structures such as offshore wind turbine foundations. A combined structure of fixed monopile wind turbine foundation and bottom-seated circular aquaculture cage is proposed for efficiently utilizing the offshore space and reducing the wave forces on the monopile wind turbine foundation. A further developed volume-averaged numerical model is adopted to simulate the pressure drop due to clean and fouled cages by introducing the porous media theory, in which the coefficients of source terms are determined by the least square method based on hydrodynamic force on cage from existing experimental data. The Stokes fifth-order wave theory is considered reliable in simulating the nonlinear waves on slopes and corresponding inline wave force by comparing with multiple existing experimental data. Then the effects of incident wave steepness, seabed bottom slopes and cage diameters on simulation results are analyzed. Results reveal the fouled cage can effectively reduce inline wave forces relative to the clean cage of the combined structure, accompanying a significant resonance effect of wave motion inside the cage.

Proposal for the suitable choice of material to avoid brittle fracture on Jacket foundations

AbstractIn the past years offshore wind turbines have been established as a source of renewable energies. Especially installations at greater distances from the shore and thus at increasing sea depths offer great potential. The foundations of these structures are exposed to complex load configurations while simultaneously exhibiting difficult environmental conditions. Although, there are currently no harmonized regulations with regard to the suitable choice of material to avoid brittle fracture for the hollow section joint structure of a Jacket foundation. In accordance with EN 1993‐1‐10 appropriate rules are given in terms of onshore structural engineering. Although, due to the differing conditions, these rules can not be applied to offshore foundations without further modifications. To achieve a suitable application, investigations regarding the material toughness requirements for Jacket foundations of offshore wind turbines were carried out. A parametric study regarding stress concentration was conducted to identify a reference geometry. Afterwards a fracture mechanics approach was applied to define the material toughness requirements. The obtained results served as a basis upon which a proposal to avoid brittle fracture for Jacket foundations was drafted.

SCALE EFFECT IN LOCAL SCOUR AROUND AN OFFSHORE PILE

Accurately predicting scour is important for its prevention around the foundations of offshore wind turbines. Many studies on local scour around piles have been conducted. Chen et al. (2022) conducted scour analysis using largescale experiments and examined the maximum scour depth. However, the relationship between scour depth and time has not yet been investigated. Furthermore, only a few studies have comprehensively examined the scale effects and similarity laws. In this study, experiments for analyzing local scour around a pile were conducted at different scales, 1/13, 1/65, and 1/130, to verify the scale effects and corresponding similarity laws regarding scouring time.

Evaluation of Seismic Performance of Offshore Wind Turbine Foundations with Slip Joints by Large Shaking Table Tests

AbstractIn Europe, a monopile is the most commonly used foundation type for offshore wind turbines. Existing connections between the monopile (MP) and transition piece (TP) are generally bolts or grout, but another new method is the slip joint. In this method, the joints between the MP and TP are conically shaped and are simply joined together, which is expected to improve workability and reduce maintenance and management costs. In order to apply this structure in Japan, it is necessary to verify its seismic resistance. This paper reports the results of shaking table tests using a scaled specimen to verify the structural safety of the structure against earthquakes.

Research on the Effect of Burial Depth on The Bearing Characteristics of Three Helical Piles Jacket Foundation for Offshore Wind Turbines

With global offshore wind power gradually moving to deep and distant waters, there is a clear trend towards larger-scale units, posing significant challenges for future offshore wind power foundations. In this paper, a helical pile jacket foundation type is proposed by combining the advantages of the current jacket foundation, which is suitable for deep and distant seas and offers high stiffness, with the excellent bearing performance of helical pile foundations. The influence of buried depth on the bearing characteristics of this foundation type is discussed through a physical model test. The results obtained from the study reveal the distinct bearing characteristics and damage modes exhibited by the foundation under deep and shallow burial conditions. These findings clearly indicate that the overall bearing characteristics and damage modes are superior in deep burial conditions compared to shallow burial conditions. Furthermore, it is observed that the damage and displacement of the foundation are more concentrated in localized areas when subjected to shallow burial.

Experimental research on the lowering process of the three and four-bucket jacket foundation of offshore wind turbine

Whether or not the top of the multi-bucket jacket foundation is open during the lowering process affects the air-water replacement process in the bucket. The bucket's layout impacts the skirt effect, the shielding effect under the wave load, and the force during the lowering process. For four types of multi-bucket jacket foundations, the method of the experiment is adopted to study the influence of the foundation structure type, the lowering velocity, the perforation ratio of the top, and the initial air storage in the bucket on the foundation dynamic characteristics and response mechanism during the lowering process under the two construction methods of ventilation and active air storage. During the lowering operation of the top ventilation, the air pressure response trend of the three-bucket and four-bucket jacket foundation with the same size are consistent. The effect of the lowering velocity on the air cushion height in the bucket is greater than that of the perforation ratio. During the lowering process of active air storage, before the top cover is completely submerged, the air cushion height in the bucket basically increases linearly, and is stable when it continues to be lowered, and increases slightly at the landing on the seabed stage. The dynamic amplification factor (DAF) value and failure factor of the sling during the lowering process of the four kinds of multi-bucket jacket foundations all meet the design requirement. The initial air storage in the bucket can be between 5% and 15%.

Reliability analysis for monopile foundation of offshore wind turbine considering correlated wind and waves and spatially varying soils

The monopile-supported offshore wind turbines (OWTs) are increasingly installed in more challenging seabed environments, resulting that the reliability of the monopile foundation attracts more attention. The reliability of the monopile foundation is largely uncertain due to uncertainties in correlated wind and waves and spatially varying soils. To address this issue, a reliability analysis framework is suggested to assess the annual probability of failure of the monopile foundation, that considers the correlated wind and waves, and spatial variability of soil properties. In this framework, a two-stage fragility surface of the monopile foundation is constructed. A joint probability distribution is calibrated using copulas to describe the correlation between wind and wave conditions. Spatially varying soils are modeled by the random field method. An example is provided to illustrate the application of the framework. The results suggest that the reliability of the monopile foundation may be overestimated if the spatial variability of soil properties is ignored. Additionally, the assumption that wind and waves are fully correlated may lead to an over-conservative reliability analysis.

Seismic response of monopile foundation of offshore wind turbines under near-field and far-field ground motions

The tendency to use renewable energy sources has been increased due to the problems associated with utilization of the fossil fuels such as their instability in the future and environmental issues. Offshore wind turbines (OWTs) have become a major renewable energy source due to their merits. However, many of the offshore wind farms planned recently are located in the seismic prone areas, which introduces some challenges regarding their foundation design while, the most common OWTs’ foundation by far, is monopile. OWTs are tall infrastructures with a longer natural period than the predominant period of most earthquakes. While far-field earthquakes have a large predominant period which might amplify the structure response and cause significant damage to OWTs, near-field earthquakes with large pulse period might lead to large residual displacements, especially in OWTs that wind could exert an eccentric load. In this study, seismic behavior of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine on monopile foundation embedded in liquefiable soil is investigated under near-field and far-field earthquakes using finite element computer program OpenSees. The seismic response of the structure was also evaluated under bidirectional excitation corresponding to the Kobe earthquake, Japan. Considering the lateral displacement of the superstructure, rotation, settlement, and bending moment of the monopile and shear stress/strain, and excess pore water pressure in the ground, a comparison between the seismic behavior of the monopile foundation of the offshore wind turbine under near-field and far-field earthquakes was performed. The results of this study showed that the seismic response of the monopile-superstructure system under near-field earthquakes was more critical despite the shorter duration and the smaller arias intensity of those earthquakes. It highlighted the remarkable effect of the other parameters such as near-field pulse which contained a lot of energy. Furthermore, the seismic response of the monopile was enhanced under the bidirectional earthquake loading.

Modeling Lifetime Performance of Monopile Foundations for Offshore Wind Applications

This paper explores the application of a numerical method for modeling pseudorandom cyclic loading, at very large cycle numbers, to the design of offshore wind turbine foundations. The work expands the development of a novel constitutive modeling framework, the hyper-plastic accelerated ratcheting model (HARM), for which the key constitutive equations and the calibration method are presented. HARM captures both the nonlinear hysteretic behavior during cycling and the accumulation of permanent deformation (ratcheting) with large cycle numbers in a rigorous, yet computationally efficient manner, enabling the computation of foundation response over a lifetime of loading. This paper demonstrates how the approach can be applied to the cyclic pile field testing from the pile soil analysis (PISA) project. Following calibration, the model is used to assess pile response to three load signals representative of operational and extreme loads throughout the lifetime of a full-scale wind turbine foundation: (1) a short storm, (2) a 35-h storm, and (3) lifetime loading. The paper discusses how computational efficiency can be achieved while maintaining a high level of calculation accuracy.

Centrifuge testing of improved monopile foundation for offshore wind turbines

Large fixed vertical offshore wind turbine (OWT) tower structures are typically used to transfer complex loads to the foundations from a combination of wind, waves, and self-weight. These loads must be accommodated within a very small rotation envelope and natural frequency bands to allow the turbines to operate effectively. These challenging loading conditions and strict operational requirements can lead to extremely costly foundation designs. Several foundation options are available to support these turbines, with monopiles currently accounting for 80% of the installed capacity with routinely used diameters of 5–8 m and depths of penetration of 30–80 m. To limit monopile diameters and penetration depths, an improved monopile design: the ‘hybrid foundation’ comprising a plate and centrally located pile, is proposed as an alternative to monopiles. A series of scaled physical model centrifuge tests were conducted to investigate the benefits of the hybrid foundation system and compare its behavior with the typically used monopiles. This type of foundation system can enhance the performance of an OWT since the turbines are subjected to high lateral loads and overturning moments. Centrifuge modeling has been used to investigate the lateral capacity and stiffness of the foundations under lateral monotonic and cyclic loading conditions. Two models were tested: a standard monopile (MP) and a hybrid foundation (HF). Lateral loads were applied with eccentricity for both models to replicate prototype (field) conditions. Models were tested at 50g in over-consolidated clay beds. These soil samples were prepared using inflight consolidation and subjected to a sand surcharge, to increase the shear strength in the zone of influence of the model foundations. Monotonic lateral loading results indicated that the addition of a plate improves the relative lateral ultimate capacity, whilst enabling a reduction of monopile penetration depth and diameter for similar capacities. Specifically, similar capacity/stiffness was realized for the HF system compared to the MP. One-way cyclic lateral loading indicated both the HF and the MP had a shakedown response under fatigue loading of up to 10000 cycles, which indicates the potential use of this novel system for future developments.

Engineering critical assessment (ECA) for monopile foundation of an offshore wind turbine subjected to pitting

An engineering critical assessment (ECA) framework for the monopile foundation of offshore wind turbines subjected to pitting is proposed in this paper. In the framework, a finite element (FE) analysis is conducted based on a 3D model of pitting, and the extended finite element method (XFEM) is utilized to predict the propagation behavior of pitting cracks. The critical size of pitting cracks is then assessed by a failure assessment diagram (FAD) in terms of the BS7910. Compared with the traditional ECA, the proposed ECA framework is more effective in assessing the effects of pits on crack propagation and thus the excessive conservatism of fatigue failure assessment for pitting structures is avoided. An engineering application example is conducted to demonstrate the capability of the proposed ECA framework. The influence of the pit depth and the initial location of the pitting crack on the foundation's fatigue life are further investigated. The results show that the fatigue life of a monopile foundation can be significantly decreased by pitting, and fatigue life can be decreased by almost 30% even a pit with a depth of 2.3 mm.

Preliminary Test for 3D Surface Strain Measurement in the Tower and Foundation of Offshore Wind Turbines Using DOFS.

Subjected to the relentless impacts of typhoons and rough seas, offshore wind turbines' structures, particularly the tower, foundation, and blade, are at constant risk of damage. Full-field strain monitoring helps to discover potential structural defects, thereby reducing disasters caused by overall structural failure. This study introduces a novel method for assessing strain and temperature fields on these kinds of 3D surfaces of cylindrical structures. The method harnesses the capabilities of a high spatial resolution (0.65 mm) Optical Frequency Domain Reflectometer (OFDR)-based Distributed Optical Fiber Sensor (DOFS) in conjunction with a unique helical wiring layout. The core process begins with mapping the fiber optic path onto a plane corresponding to the unfolded cylinder. Fiber optic signals are then differentiated on this plane, deriving a two-dimensional strain distribution. The plane strain field is subsequently projected onto the 3D side of the cylinder. An experiment was carried out in which a 3.5 m long optical fiber was helically wound with a 10 mm pitch on the surface of a cantilever beam of a cylinder shell with a diameter of 36 mm and a length of 300 mm. The experiment collected about 5400 measurement points on the cylindrical surface of 340 cm2, approximately 15.9 measurement points per square centimeter. The reconstructed results successfully reveal the strain field of the pipe cantilever beam under bending and torsional loads, as well as the palm-shaped temperature field. This experimental validation of the method's efficacy lays the theoretical groundwork for its application to real wind turbines.

Effects of strain rate and strain softening on the installation of helical pile in soft clay

Helical piles are alternative foundation for offshore wind turbine as a green foundation. When a helical pile is screwed into soft clay, the soils exhibit strain-rate dependency as well as strain softening effect when sheared and remolded. In this study, large deformation finite-element (LDFE) model is established to investigate the penetration behavior of helical pile, where the soil flow mechanism and penetration resistance are studied considering strain rate effect and strain softening effect in soil. The numerical model is validated by comparing with available centrifuge results. Parametric studies are then conducted covering a wide range of strain rate and strain softening parameters including the strain-rate parameter, the sensitivity and ductility of the soil, and the original shear strength. It is found that the penetration behavior of helical pile is significantly influenced by the strain rate and strain softening of soil. The difference of the torque profiles between the ideal Tresca soil and the rate-dependent, strain-softening soil is 0–30%. Based on the numerical modelling results, the theoretical model to predict torque requirements for the installation of helical piles is extended by accounting for the strain rate effect and strain softening effect.

The offshore prefabrication and semi-wet towing of a bucket foundation for offshore wind turbines

In recent years, the bucket foundation as a new form of supporting structure of offshore wind turbines (OWTs) has attracted increasing attention with the rapid development of offshore wind power. A synthetical prefabrication and transportation (P&T) technique is proposed in this paper to enable a broader application of bucket foundations efficiently and economically in the offshore wind market. The novel P&T technology consists of two key auxiliary pieces of equipment, i.e., an offshore prefabrication platform (OPP) and a U–K barge. The process of the offshore prefabrication is elaborated, and the requirement that need to be met for the monopod-bucket foundation with subdivisions (MBFS) to achieve offshore prefabrication utilizing the OPP is analyzed. Then, the concept of the semi-wet towing with a U–K barge is proposed, which can provide sufficient buoyancy and stability to ensure the safety of the MBFS during the towing operation. To investigate the seakeeping of the combined semi-wet system in moored and towing scenarios, a series of experiments were carried out to prove the reliability of the U–K barge design. Sensitivity analysis regarding towing speed, wave height, and wave period for the disintegration risk is performed, and three methods are proposed to avoid the disintegration of the U–K barge and MBFS at extreme sea cases. The results show that the proposed offshore prefabrication is feasible, and it is suggested that as the MBFS floats up, the water depth of the construction site should not be less than 6 m. In addition, it is proved that the combined semi-wet towing system composed of the U–K barge and the MBFS has an exceptional seakeeping capability with regard to the pitching angles, accelerations, and towing resistances, and that the wave height and the towing speed have greater effects on the disintegration risk than the wave period.

Characterization of sand motions around vibrating vertical flat plates under saturated conditions

Monopile foundations of offshore wind turbines undergo continuous vibrations due to the cyclic lateral loads induced by waves and winds. To focus on the dynamics of sand behavior around vibrating foundations, researchers often conduct two-dimensional experiments using flat plates as representations of monopile foundations. This approach enables convenient observation of sand motions and bed deformation around the structure, facilitating a better understanding of how the sand responds to the cyclic vibrations. Previous studies have investigated sand motions around vibrating flat plates under dry conditions and found sand convective and subsidence motions around them. However, it remains unclear whether the same convective and subsidence motions occur under saturated conditions and if there are any differences in sand motions between dry and saturated conditions. Thus, this study with a simplified two-dimensional experiment is presented to further investigate the characteristics of sand motions under dry and saturated conditions. The experimental results indicate that the compacted sand surrounding the structure reduces the vibration amplitude of the monopile. But the vibration amplitudes are more significantly attenuated under saturated conditions compared to dry conditions. Moreover, the convective motions of sand under the saturated condition are weaker than those under the dry condition, which suggests that the viscous dissipation effect may weaken the convective motions around the vibrating monopile foundations.

The Study of the Failure Envelopes of Hollow Shaft Single-Plate Helical Piles in Clay

Helical piles are utilized worldwide as foundations for onshore infrastructure, providing fast installation and high capacity. Their unique advantages have led to their potential as an alternative to monopile foundations for offshore wind turbines. One challenge in their service life is dealing with combined loading caused by wind, waves, current, and the weight of the structure itself. While research has focused on helical piles’ capacity for uniaxial tensile loads, there is limited knowledge regarding their performance under combined loads. This study used FEM analysis to investigate the impact of aspects such as helix-to-shaft diameter ratios and helix position on the capacity and failure mechanisms of hollow shaft single-plate helical piles in clay. With 561 analysis cases under both uniaxial and combined loading, failure envelopes were evaluated for various helix-to-shaft diameter ratios and positions. The study revealed a linear positive correlation between helix-to-shaft diameter aspect ratios and load-bearing ability, while the effect of helix positioning on failure envelopes was more complex and nonlinear. The outcomes from this comprehensive analysis enabled the development of a formula to predict the bearing capacity of helical anchors under combined loading.

Numerical modeling of the effect of loading rate on the tensile load capacity of offshore caisson foundations

Suction caissons are skirted foundations frequently used in offshore environments supporting various structures. Among those lie the offshore wind turbines (OWT), which are currently one of the most common renewable energy systems. In this paper we study the tensile capacity of caisson foundations of OWT in dense sands, where the generation of excess pore pressure is analyzed. The tensile capacity of suction buckets under various loading rates is investigated using a series of quasi-static axisymmetric finite element analyses with coupled flow-deformation formulation. DeltaSand model is utilized for the soil, which is an elasto-plastic state-dependent model capturing the response of sands at varying relative densities. Numerical results are validated with available test data of scaled physical model experiments. Particular emphasis is placed on the interaction between suction-bucket and the soil. Parameters governing the soil-caisson interface playing a key role in the uplift response are identified. Lastly, a case study that can be observed in offshore foundations is simulated. Results show that the developed numerical model is applicable to capturing the uplift response of bucket in an ocean environment. The outcome of this study will contribute to closing the gap in the design of suction caissons of OWT in existing standards.