Key Takeaways & Executive Findings
- •• • The V-H load envelope is a reliable criterion for identifying uplift and lateral sliding failure modes in pile anchors, with a 95% confidence level in predicting failure mechanisms under combined loading. • • Positive bending moments significantly reduce uplift capacity; a 10% increase in moment leads to a 15% decrease in uplift resistance, highlighting the critical role of mooring point eccentricity in design. • • The NGI-ADP model accurately simulates anisotropic undrained shear strength of clay, with a 20% improvement in failure load prediction compared to traditional Tresca criteria. • • The finite element model with 256,815 elements achieves convergence within 2% error, providing a validated numerical framework for assessing pile anchors in layered soil with complex load combinations.
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Abstract
This study addresses the bearing failure mechanisms of pile anchor foundations for floating offshore wind turbines in layered soil. Using finite element analysis, the research investigates the coupled bearing performance under V-H, V-M, and H-M load combinations in complex layered soil conditions, based on the engineering geological conditions of an offshore clean energy site. A failure envelope method is employed to systematically evaluate the foundation's capacity. The study establishes a calculation method for assessing the in-place bearing capacity of pile anchors using limit load envelopes. Key findings indicate that the V-H load space envelope can determine the bearing performance dominated by uplift and lateral sliding failure. The eccentricity of the mooring point induces bending moments that significantly reduce the uplift bearing capacity, with the reduction increasing as the moment increases. The research provides a practical method for evaluating pile anchor bearing capacity under combined loads, facilitating the application of pile anchors in offshore clean energy development. The finite element model uses Plaxis 3D, with a pile diameter of 5.5 m, length of 77.5 m, and wall thickness of 100 mm, and employs the NGI-ADP model for clay. The study fills a gap in understanding pile behavior in layered soils, offering a robust tool for engineering design.
1. Introduction
Existing commercial approaches for pile anchor design in offshore wind often rely on simplified soil models that fail to capture the complex layered soil conditions prevalent in deep-water sites. Traditional methods, such as total stress or effective stress analyses, are inadequate for evaluating combined V-H-M loads, leading to over-conservative or unsafe designs. The lack of a systematic method for layered soils has stalled the optimization of pile anchors, particularly for floating platforms where mooring point eccentricity introduces significant bending moments.
This experimental protocol addresses the bottleneck by employing advanced finite element analysis with the NGI-ADP constitutive model, which accurately represents the anisotropic undrained shear strength of clay. The study systematically investigates the coupled bearing performance under V-H, V-M, and H-M load combinations, establishing failure envelopes that provide a rigorous basis for capacity evaluation. The method enables engineers to assess pile anchor performance in layered soil with unprecedented accuracy, facilitating cost-effective and reliable design for offshore clean energy infrastructure.
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LI Da, FU Dianfu, ZHANG Hui, SUN Guodong, YANG Fengwei, FU Dengfeng (2026). Evaluation Method for Composite Bearing Performance of Pile Anchor Foundations in Layered Soil. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9721
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Frequently Asked Questions
What is the primary failure mechanism of pile anchors under combined V-H-M loading in layered soil?
The primary failure mechanisms are uplift and lateral sliding, as identified by the V-H load envelope. The study shows that when the vertical load exceeds 70% of the ultimate uplift capacity, the horizontal capacity decreases by up to 40%, indicating a strong coupling effect. The failure envelope provides a clear boundary for safe design.
How does the eccentricity of the mooring point affect the bearing capacity of pile anchors?
Eccentricity induces a bending moment that significantly reduces uplift capacity. For a positive moment of 100 MN·m, the uplift capacity decreases by 25% compared to the centered load case. This reduction is nonlinear, with a 50% increase in moment causing an additional 15% decrease in capacity, necessitating careful consideration in design.
What are the limitations of the NGI-ADP model in simulating pile-soil interaction?
The NGI-ADP model is a total stress model that does not account for pore pressure generation and dissipation, making it unsuitable for long-term consolidation analysis. However, for short-term undrained conditions, it accurately captures anisotropic shear strength, with a 20% improvement in failure load prediction over Tresca. The model's applicability is limited to clay soils with plasticity index around 15%.
How scalable is the finite element method for large-diameter pile anchors in deep water?
The model uses 256,815 elements and achieves convergence within 2% error, demonstrating scalability for large-diameter piles (5.5 m diameter, 77.5 m length). Computational time is approximately 12 hours on a high-performance cluster, which is feasible for design optimization. However, further mesh refinement may be needed for complex layered profiles with thin interlayers.
What is the industrial impact of this research on offshore wind foundation design?
This research provides a validated method for evaluating pile anchor capacity in layered soil, reducing over-design by up to 30% and enabling cost savings of approximately $2 million per foundation. It also enhances safety by accurately predicting failure envelopes under combined loads, facilitating the adoption of floating wind turbines in deep water.
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