Seismic activity often triggers liquefaction in sandy soils, which coupled with initial vertical tensile loads, poses a significant threat to the stability of suction bucket foundations for floating wind turbines. However, there remains a notable dearth of studies on the dynamic response of these foundations under combined seismic and vertical tensile loads. Therefore, this study developed a numerical method for analyzing the dynamic response of suction bucket foundations in sandy soils under such combined loading conditions. Through numerical simulations across various scenarios, this research investigates the influence of key factors such as seismic intensity, spectral characteristics, as well as the magnitude and direction of tensile loads on the seismic response of suction buckets. The results revealed that the strong earthquake may cause the suction bucket foundation of floating wind turbines to fail due to excessive vertical upward displacement. This can be attributed to that the accumulation of excess pore water pressure reduces the normal effective stress on the outer wall of bucket, and consequently decreases the frictional resistance of bucket-soil interface. Additionally, the above factors significantly influence both the vertical displacement of the suction bucket and the development of pore pressure in the surrounding soil. The findings can provide valuable insights for the seismic safety assessment of suction bucket foundations used in tension-leg floating wind turbines.

Prolonged lateral cyclic loading leads to soil stiffness degradation around offshore wind turbine (OWT) foundations, which reduces the system's natural frequency and increases the accumulation of foundation rotation angle. Proper evaluation of natural frequency and rotation angle is crucial for the design of OWT foundations. This study develops a two-stages numerical approach to calculate the fundamental frequency of OWT systems considering the foundation stiffness degradation by integrating a stiffness degradation model of soft clays with a simplified three-spring model. Subsequently, it investigates the evolution of accumulated rotation and natural frequency for three foundation types-monopile, monopod bucket, and hybrid monopile-bucket-throughout their service life. It is observed that the hybrid foundation shows the smallest rotation in the first cycle, attributable to its relatively high initial stiffness compared to the other two foundations with the same steel consumption. However, it also exhibits the highest rate of cumulative rotation growth. At the same load level, the monopod bucket foundation exhibits the largest cumulative rotation angle due to its lower bearing capacity, and the degradation of natural frequency is most pronounced for monopod bucket OWT. For all three foundation configurations, increasing either the pile diameter or the bucket diameter is the most effective approach to reduce the cumulative rotation angle and improve natural frequency degradation, while maintaining the same steel consumption. These findings should be considered in the design of OWT foundations.

This study investigated the seismic performance and assessed the seismic fragility of an existing pentapod suction-bucket-supported offshore wind turbine, focusing on the amplification of earthquake ground motions. A simplified suction bucket-soil interaction model with nonlinear spring elements was employed within a finite element framework, linking the suction bucket and soil to hypothetical points on the OWT structures at the mudline. Unlike conventional approaches using bedrock earthquake records, this study utilized free-field surface motions as input, derived from bedrock ground motions through one-dimensional wave theory propagation to estimate soil-layer-induced amplification effects. The validity of the simplified model was confirmed, enabling effective assessment of seismic vulnerability through fragility curves. These curves revealed that the amplification effect increases the vulnerability of the OWT system, raising the probability of exceeding damage limit states such as horizontal displacement of the tower top, tower stress, and horizontal displacement at the mudline during small to moderate earthquakes, while decreasing this likelihood during strong earthquakes. Comparisons between the Full Model and the simplified Spring Model reveal that the simplified model reduces computational time by approximately 75%, with similar seismic response accuracy, making it a valuable tool for rapid seismic assessments. This research contributes to enhancing seismic design practices for suction-bucket-supported offshore wind turbines by employing a minimalist finite element model approach.

The suction bucket foundation equipped for offshore wind turbines was a promising solution for sandy seabed locations. However, its typically short embedment depth presented additional challenges when installed in seismic zones. These challenges pertained not only to structural response but also to the seismic motion itself, which was strongly influenced by soil characteristics. This study examined the uncertainty of equivalent shear-wave velocities to explore the variability in input seismic motion characteristics and investigated their impact on the structural response in terms of tower-top displacement, mudline displacement, and acceleration amplification factor at the hub height of 3 MW and 5.5 MW suction bucket-supported offshore wind turbines (OWTs). Additionally, the influence of equivalent shear-wave velocities on the exceedance probabilities of various damage states, using fragility curves for tower-top and mudline displacement, was analyzed. The results indicated that equivalent shear velocities of soil significantly impacted the seismic performance of suction bucket-supported offshore wind turbines. These effects were closely related to the intensity of the seismic motion, highlighting the importance of carefully considering the correlation between site-specific shear velocities and earthquake intensities.

Suction bucket jackets have been used as foundations for offshore wind turbines in intermediate water depths where layered soil stratigraphies are often encountered. Although suction installation in layered soils has been studied, experimental data on the in-service response is scarce. During installation in stratigraphies containing a low permeability layer underlain by a high permeability layer, suction is transferred to the underlying layer when the pressure at the lid invert is sufficient to uplift the low permeability plug. This suction-transfer mechanism also affects the in-service response, albeit the load-sharing mechanism is not well understood. This paper presents data from centrifuge tests of suction buckets subjected to constant amplitude and varying amplitude cyclic vertical loading in two stratigraphies-a sand with an overlying clay layer and in a sand with a sandwiched clay layer. These experiments show that tensile stresses exceeding the vented tensile resistance can be withstood without significant uplift of the bucket in both stratigraphies, even under a zero mean stress. Plug uplift was shown to have an important effect on the amount of stress transferred to the skirts, with the load-sharing mechanism depending on the stratigraphy. Additionally, the load-sharing mechanism and the bucket in-service resistance was shown to depend on the effectiveness of the clay in sealing the soil plug within the bucket, with a more effective seal resulting in higher tensile resistance and therefore better performance. A limiting loading condition was not identified in the sand with a sandwiched clay layer, with the data indicating that the suction pressure to cause plug uplift during cyclic loading may be much higher than during suction installation.

This study assesses the performance of a memory surface constitutive model (SANISAND-MS) in capturing vertical cyclic loading on a suction bucket foundation in sand. The model has been calibrated against drained cyclic triaxial responses and validated against corresponding centrifuge experiments on suction buckets. The model was found to satisfactorily capture the effects of increasing accumulated strain with increasing mean stress level and reducing density. The performance of the model was further investigated through a parametric study on suction buckets at different mean stress levels, densities and loading sequences. The insights gained from investigating the strain and stress responses, along with the movement of the memory surface, reveal that the model can satisfactorily capture the strain accumulation and ratcheting effects under different load histories.

Traditional suction bucket foundations incur high maintenance costs and are susceptible to corrosion, resulting in a diminished bearing capacity over prolonged service. The suction bucket foundation, constructed with a glass fibre -reinforced polymer (GFRP), introduces a novel approach to iteratively optimise conventional steel bucket foundations. In this study, three-dimensional finite element models of the GFRP bucket -soil interaction were established using the VUMAT subroutine, which incorporates the stress -strain damage relationship of GFRP materials. The mechanical response during installation was analyzed for different fibre -laying angles( A ) and wall thicknesses( t ) of the GFRP bucket, and the results were compared with those of a steel bucket. The results indicated increased circumferential stress, radial deformation, and out -of -roundness of the GFRP bucket as the fibre laying angle increased. Deformation and stress of the bucket skirt remained low at A of 0 - 45 degrees . When A >= 60 degrees , the matrix ' s damage area significantly increases, with the minimum damage occurring at 45 degrees . For A <= 30 degrees , it approaches the maximum radial deformation of an equivalent -sized steel suction bucket. As the wall thickness increased, the circumferential stress, radial deformation, and out -of -roundness of the GFRP bucket skirt gradually decreased. When the GFRP bucket t was four times that of the steel bucket, its radial deformation was approximately equal.

Suction-induced seepage flow can significantly reduce the soil resistance during the installation of suction buckets, thereby ensuring their intended penetration depths and the designed in-service capacities. However, the lack of analytical models describing seepage flow behavior in the literature poses a significant challenge, primarily due to the complexity of seepage boundary conditions and the anisotropy and spatial variation of soil permeability. This paper addresses this gap by presenting a novel analytical solution for analyzing suctioninduced seepage flow around buckets, with a particular focus on the general multilayered soil profile featuring anisotropic permeability. The method of separation of variables is used initially to derive general solutions, and the final solutions are subsequently obtained by combining continuous conditions with the orthogonality of Bessel functions. The accuracy of the solutions is confirmed through comparisons with the results obtained from finite element analysis. Furthermore, this study discusses the seepage flow behavior in several typical scenarios, including permeability anisotropy, increased permeability within the bucket, and the presence of an overlying low-permeability layer. The analytical solutions presented in this paper provide a rapid and accurate method for the analysis of suction-induced seepage flow during suction-assisted installations across a wide range of complex soil permeability conditions.

The tripod foundation (TF) is a prevalent foundation configuration in contemporary engineering practices. In comparison to a single pile, TF comprised interconnected individual piles, resulting in enhanced bearing capacity and stability. A physical model test was conducted within a sandy soil foundation, systematically varying the length-to-diameter ratio of the TF. The investigation aimed to comprehend the impact of altering the height of the central bucket on the historical horizontal bearing capacity of the foundation in saturated sand. Additionally, the study scrutinized the historical consequences of soil pressure and pore water pressure surrounding the bucket throughout the loading process. The historical findings revealed a significant enhancement in the horizontal bearing capacity of the TF under undrained conditions. When subjected to a historical horizontal loading angle of 0 degrees for a single pile, the multi-bucket foundation exhibited superior historical bearing capacity compared to a single-pile foundation experiencing a historical loading angle of 180 degrees under pulling conditions. With each historical increment in bucket height from 150 mm to 350 mm in 100 mm intervals, the historical horizontal bearing capacity of the TF exhibited an approximately 75% increase relative to the 150 mm bucket height, indicating a proportional relationship. Importantly, the historical internal pore water pressure within the bucket foundation remained unaffected by drainage conditions during loading. Conversely, undrained conditions led to a historical elevation in pore water pressure at the lower side of the pressure bucket. Consequently, in practical engineering applications, the optimization of the historical bearing efficacy of the TF necessitated the historical closure of the valve atop the foundation to sustain internal negative pressure within the bucket. This historical measure served to augment the historical horizontal bearing capacity. Simultaneously, historical external loads, such as wind, waves, and currents, were directed towards any individual bucket within the TF for optimal historical performance.

As an alternative to monopiles, suction bucket jacket foundations are gaining increasing popularity in China for supporting offshore wind turbines. One of the major design challenges and governing factors for foundation sizing is the long-term tilt caused by the differential settlement between the buckets due to a combination of prevailing wind directions and soft seabed conditions. The assessment of the long-term consolidation settlement is complicated and subjected to uncertainties, such as the load sharing between the skirt and the bucket lid, and load re-distribution with time as the soil response transits from short-term undrained behaviour to long-term drained behaviour. This paper presents an attempt to understand the short-term and long-term load sharing mechanisms for the suction bucket foundation by means of finite element analysis. Without modelling the large deformation installation process explicitly, the initial (undrained) load sharing mechanism and induced additional stress (or excess pore water pressure) in the soil body is first examined. The re-distribution of the load between the skirts and the lid as the excess pore pressures dissipate is subsequently investigated. The study examines a range of soil conditions and foundation aspect ratios. It is found that the undrained skirt wall friction capacity relative to the load level has an important impact on the initial load sharing mechanism. As consolidation takes place, significant load redistribution occurs, with the loads carried initially by the lid partially or fully transferred to the internal and external skirt wall frictions. The load sharing at completion of consolidation heavily depends on the drained skirt friction capacity relative to the load level. Guided by the numerical findings, a tentative analytical model for practical design purpose is proposed.