Combined bearing capacity of bucket foundations in soft-over-stiff clay

Elastic stiffness coefficients of a skirted spudcan foundation in clay

As a form of clean and low-carbon green energy, offshore wind power occupies an important position in the global energy structure. With the rapid development of the wind power industry, wind power projects gradually develop from offshore to far-offshore sea areas. The multi-bucket jacket foundation is a considerable foundation type for far off-shore projects, but high installation costs involving ship transportation with parted components and field installation has significantly hindered its wide application. In this study, based on a 6.7-MW triple-bucket jacket foundation (TBJF) project of a deep-sea wind farm in China, a new integrated wet-towing method of “jacket + triple-bucket foundation” composite structure was proposed, which is suitable for far long-distance transportation of far-offshore applications. The static-model test of both self-stability and wet-towing stability was conducted. Based on the test and the numerical results, the natural period of the foundation for different draft depths in hydrostatic water and the stability for different eccentric loads were first evaluated. Then, the effects of different wet-towing modes and sea conditions on the stability of the TBJF were investigated. Finally, the optimal wet-towing mode and applicable sea conditions for the TBJF structure were proposed.

Laboratory Study of Integrated Wet-Towing of a Triple-Bucket Jacket Foundation for Far-Offshore Applications

Spudcans are commonly installed to support jack-up vessels in construction process of offshore wind turbines (OWTs), which may induce significant soil deformation that may impact existing foundations of established OWTs. Large-deformation finite-element (LDFE) analyses were performed to investigate movements of an adjacent skirted foundation induced by spudcan penetration. A LDFE model was generated and compared with a centrifuge test and good agreement was achieved. A detailed parametric study was then undertaken to examine the influence of spudcan penetration depths, spudcan–skirt clearance, soil shear strength, and foundation skirt length ratios. The outcome of this study leads to a conclusion that the behavior of the impacted skirt is closely related to soil flow mechanisms around the skirted foundation induced by spudcan penetration. The clearance distance between the spudcan and the skirt is the key factor. Significant skirt rotation occurs when the spudcan is less than one diameter distance from the skirt, especially for a short skirt installed in stiff clay. The soil flow mechanisms around the spudcan and skirted foundation are presented and approximating expressions for estimating the maximum rotation of skirt during spudcan penetration are proposed based on intensive numerical modeling results. 

Influence of Spudcan Penetration on Adjacent Skirted Foundation in Uniform Clay

A composite bucket foundation (CBF) is a new foundation type of offshore wind turbine. The investigation of the bearing characteristic of CBF is a crucial part of the structural design and safety assessment of offshore wind turbines. This study investigates the bearing characteristic of CBF in sand and clay foundations using centrifuge model test and numerical simulation. Analysis revealed that as load increases, the rotation center of the CBF moves toward the central line and shallow area. The buried depth of the CBF rotation center in a clay foundation is shallower than in a sand foundation when the CBF is under ultimate loading. Due to the rotation and translation of CBF, the soil pressure–depth curves on the skirt wall in different directions will resemble parabolic, line, and triangle shapes, respectively. Under ultimate loading, the relationship between the depth and horizontal coordinate of the rotation center and embedment ratio is linear. The CBF's bearing capacity trend presents quadratic function characteristics as the embedment ratio increases. However, the bearing capacity ratio of the CBF in the clay foundation is faster than that in the sand foundation. The findings provide guidelines for offshore wind turbines. 

Investigation of composite bucket foundation bearing characteristics in various soil properties

Evaluation of capacities of bucket foundations in soft-stiff-soft clays under combined loading

This study presents a range of numerical modeling to evaluate the pullout capability of suction anchors subjected to inclined mooring loading in a clay-over-silty-sand deposit. Comprehensive numerical models were generated, which were validated against existing centrifuge testing and numerical modeling results. A parametric study was then carried out to assess the effects of the anchor length, padeye location, mooring angle, and layered deposits on the pullout capacity of suction anchors. According to the numerical simulation results, the following key design factors were obtained: (1) the optimal padeye position, (2) the ultimate horizontal and vertical pullout abilities, and (3) the horizontal load distributions at different padeyes. A simple framework of calculating the combined VH surfaces of the anchors was developed for preliminary prediction of the pullout capacity of suction anchors in a clay-over-silty-sand deposit that provides the optimized location of padeyes and more accurate prediction for the holding capacity of anchor. The design method proposed by this study can contribute to optimize and improve the current design guideline for more accurately predicting the bearing capacity of anchors in clay-over-silty-sand soil deposits. 

Ningxin Yang

Papers

Evaluation of capacities of bucket foundations in soft-stiff-soft clays under combined loading

Inclined Pullout Capacity of Suction Anchors in Clay over Silty Sand

Bearing Capacities of Buried Bucket Foundations in Marine Tidal Flat Subjected to Combined Loading