Numerical Simulation of Vertical Bearing Capacity of Large-Diameter Steel Pipe Pile Foundations for Offshore Wind Power

Literature Overview

This paper by Luo Guangjie, Zhou Maoqiang, Zhang Qiang, Shen Xiaolei, Zhan Yide, and Shen Jiayi, published in Water Power in 2021 (Vol. 47, No. 1, pp. 117-121), presents a numerical investigation of the vertical bearing capacity of large-diameter steel pipe pile foundations used in offshore wind power installations. The research team includes engineers from China Power Engineering Consulting Group's East China Institute of Survey and Design, Zhejiang East China Engineering Consulting Co., Ltd., and Zhejiang University. The study addresses the practical challenges of foundation design for offshore wind farms in the waters of the offshore radiating sandbank area (岸外辐射沙洲海域), which is a significant region for China's offshore wind energy development.

Numerical Model and Methodology

The authors employ FLAC3D, a three-dimensional finite difference code widely used in geotechnical engineering, to establish a numerical model of the large-diameter steel pipe pile foundation. FLAC3D is particularly well-suited for this application because it can handle large displacements, complex constitutive behavior, and soil-structure interaction through interface elements.

The numerical model incorporates multiple soil layers with depth-varying mechanical properties, which is essential for accurately capturing the load transfer mechanism along the pile shaft. The pile-soil interaction is modeled using interface elements that define the frictional resistance and adhesion at the pile surface.

Key Findings and Technical Analysis

The study reports a predicted ultimate vertical bearing capacity of 10,450 kN, which is in close agreement with the results obtained from on-site static load testing. This validation is crucial for establishing confidence in the numerical model and its predictive capability for design applications.

The analysis reveals several important engineering insights:

1. Axial force distribution: The pile axial force decreases with increasing depth, indicating that the majority of the vertical load is transferred to the soil through shaft friction (side resistance). The pile tip axial force approaches zero at depth, confirming that tip resistance contributes minimally to the total bearing capacity in this soil profile.
2. Critical embedment depth: When the pile embedment depth exceeds 30 m, the ultimate bearing capacity increases rapidly. This identifies soil layer ⑥-1 as a suitable bearing layer (持力层), providing clear guidance for pile length optimization.
3. Load transfer mechanism: The progressive load transfer from the pile head to the surrounding soil, with increasing depth, demonstrates the importance of adequate pile-soil contact along the full pile length.

Engineering Practice Implications for Offshore Wind Foundation Design

For engineers designing offshore wind turbine foundations, this study provides several practical insights:

• Pile length optimization: The identification of a critical depth beyond which bearing capacity increases rapidly allows for cost-effective pile length selection. Extending the pile beyond the identified bearing layer provides diminishing returns and increases driving costs.
• Steel pipe specification: Large-diameter steel pipe piles for offshore wind foundations typically conform to API 5L or ISO 3183 specifications. Common grades include X65, X70, and X80, with wall thicknesses ranging from 20 to 50 mm depending on the diameter and design loads. The steel grade selection must account for the driving stresses, operational loads (including fatigue from wind and wave loading), and potential corrosion in the marine environment.
• Driving process considerations: The numerical model should ideally incorporate the effects of pile driving, including soil disturbance, strain hardening, and setup of shaft resistance. The static load test results used for validation were likely conducted after a sufficient setup period (typically 7-28 days), which allows the soil to recover from driving-induced disturbance.
• Corrosion allowance: In marine environments, steel pipe piles are subject to corrosion, particularly in the splash zone and tidal zone. The design must include appropriate corrosion allowances (typically 1.5-3.0 mm per side over the design life) and may incorporate cathodic protection systems.

Welding and Fabrication Considerations

Large-diameter steel pipe piles for offshore wind foundations are typically manufactured as spiral-welded or longitudinally submerged arc welded (LSAW/UOE) pipes. The welding quality is critical because any weld defects can serve as initiation sites for fatigue cracking under cyclic wind and wave loading. Key quality control measures include:

The longitudinal weld seam in LSAW pipes is particularly critical for offshore wind applications because it is subjected to combined bending, axial, and torsional stresses from wind and wave loading. The weld procedure specification (WPS) must be qualified for the specific steel grade, wall thickness, and joint geometry, with appropriate preheat and interpass temperature controls to minimize hydrogen-induced cracking and ensure adequate toughness in the heat-affected zone (HAZ).

Critical Reflections

The study is commendable in its practical orientation and validation against field data. However, several aspects could be further explored:

• Dynamic loading effects: Offshore wind turbines are subjected to dynamic loads from wind turbulence, wave action, and rotor dynamics. The static analysis presented does not capture the fatigue behavior of the pile-soil system under cyclic loading.
• Lateral and overturning loads: Wind turbine foundations are primarily designed for lateral and overturning loads, not just vertical loads. The study focuses exclusively on vertical capacity, which represents only a fraction of the design challenge.
• Soil-pile interaction nonlinearity: FLAC3D can model nonlinear soil behavior, but the accuracy of the constitutive model parameters (Mohr-Coulomb, Hardening Soil, or more advanced models) significantly affects the predicted load transfer and ultimate capacity.
• Uncertainty and safety factors: The agreement between predicted and measured capacity is encouraging, but the inherent uncertainties in soil properties, installation effects, and long-term degradation should be addressed through reliability-based design approaches.

Summary

This study demonstrates the effectiveness of FLAC3D numerical modeling in predicting the vertical bearing capacity of large-diameter steel pipe pile foundations for offshore wind power applications. The close agreement between numerical predictions and field static load test results validates the model and provides confidence for its application in design. The identification of soil layer ⑥-1 as a suitable bearing layer at depths exceeding 30 m offers clear, actionable guidance for foundation design in the offshore radiating sandbank area. For steel pipe manufacturers and welding engineers, the study reinforces the importance of high-quality pipe fabrication and welding, as the structural integrity of the pile foundation ultimately depends on the quality of the steel pipe and its welds.