Key Takeaways & Executive Findings
- •• • Scour depth increase from 2 m to 10 m reduces lateral stiffness by 15% and natural frequency by 7–8%, directly impacting serviceability and fatigue life of offshore wind turbines. • • Complete scour reduces ultimate bearing capacity by only ~10%, confirming that the foundation's load-bearing mechanism is dominated by internal soil plug and base support rather than external skirt friction. • • Cyclic loading under scoured conditions significantly decreases horizontal stiffness and increases cumulative rotation, with effects far more pronounced than static loading, necessitating fatigue-based design checks. • • Rock dumping protection is predicted to restore horizontal stiffness to >90% and ultimate bearing capacity to >95% of unscoured state, offering a quantifiable retrofitting strategy for existing installations.
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Abstract
This study investigates the bearing performance of single-column composite bucket foundations under scour conditions through finite element analysis and scaled model tests. The most unfavorable scour scenario was identified by evaluating load angle effects on bearing capacity, frequency, and stiffness. Laboratory tests were conducted on a 1:60 scaled model of a 36 m diameter prototype foundation embedded in Tianjin clay, with scour depths ranging from 2 m to 10 m. Results indicate that when the load angle faces the scoured side, the ultimate bearing capacity reaches its minimum, with maximum stress concentrated at the bottom of the compartment plate on the scoured side. Lateral stiffness decreases by 15% and frequency by 7–8% as scour depth increases from 2 m to 10 m. Complete scour reduces bearing capacity by approximately 10%, while cyclic loading amplifies scour effects, significantly reducing horizontal stiffness and increasing cumulative rotation. The foundation's bearing mechanism primarily relies on internal soil and base support. Scour protection measures such as rock dumping, geotextile, and fender systems are predicted to restore stiffness to over 90% and bearing capacity to over 95% of unscoured values.
1. Introduction
Offshore wind development has rapidly expanded, with bucket foundations favored for their onshore prefabrication and suction-assisted installation. However, local scour around these foundations remains a critical design concern, particularly for the novel single-column composite bucket foundation. Existing research has primarily focused on monopile scour mechanisms, leaving a gap in understanding how scour evolution affects the composite bucket's bearing capacity, stiffness, and dynamic properties. The complex marine geology of China, characterized by alternating clay and sand layers and shallow bedrock, exacerbates the challenge of accurately predicting scour-induced changes.
This study addresses the bottleneck by combining scaled model tests and finite element simulations to systematically evaluate the bearing performance of single-column composite bucket foundations under full and half scour conditions. The experimental protocol specifically targets the most unfavorable load angles and scour depths, providing empirical data to quantify stiffness degradation, frequency shifts, and ultimate capacity reduction. The findings offer a basis for optimizing scour protection design and ensuring structural integrity in offshore wind applications.
Loading authentic research manuscript (Pages 1–5)...
NI Daojun, XIAO Jiandong, XIAO Yaoyao, QI Xin, ZHANG Puyang (2026). Bearing Performance of Single-Column Composite Bucket Foundations for Offshore Wind Turbines Under Scour Evolution. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9715
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Frequently Asked Questions
What is the primary failure mechanism of single-column composite bucket foundations under scoured conditions?
The primary failure mechanism involves stress concentration at the bottom of the compartment plate on the scoured side, leading to soil deformation near the outer edge of the internal soil plug. Ultimate bearing capacity is governed by internal soil and base support, with complete scour reducing capacity by only ~10%.
How does cyclic loading affect the performance of scoured foundations compared to static loading?
Cyclic loading significantly amplifies scour effects, reducing horizontal stiffness and increasing cumulative rotation. The impact on natural frequency is limited, especially for low-order modes, but the cumulative rotation may exceed serviceability limits, requiring fatigue-based design considerations.
What are the quantified benefits of rock dumping protection for scoured foundations?
Rock dumping is predicted to restore horizontal stiffness to over 90% and ultimate bearing capacity to over 95% of unscoured values by inhibiting scour pit development and converting full scour to half or no scour conditions.
How does scour depth affect the natural frequency of the foundation, and why does it matter?
As scour depth increases from 2 m to 10 m, natural frequency decreases by 7–8%. This shift can cause resonance with rotor harmonics, potentially leading to accelerated fatigue damage and reduced operational lifespan.
What are the limitations of the scaled model tests, and how were they addressed?
The 1:60 scale model may exhibit scale effects due to soil nonlinearity and boundary conditions. Correction factors for bearing capacity, stiffness, and displacement were applied based on gravity similarity and soil property adjustments to extrapolate prototype behavior.
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