Numerical Simulation of Vertical Bearing Capacity of Large-Diameter Steel Pipe Pile Foundations for Offshore Wind Power Using FLAC3D

Literature Overview

This paper by Luo Guangjie, Zhou Maoqiang, Zhang Qiang, Shen Xiaolei, Zhan Yide, and Shen Jiayi (2021), published in Water Power (Vol. 47, No. 1, pp. 117-121), investigates the vertical bearing capacity of large-diameter steel pipe pile foundations used in offshore wind power installations. The research is conducted by the East China Survey and Design Institute of China Power Construction Group, Zhejiang Huadong Engineering Consulting Co., Ltd., and Zhejiang University, representing a strong collaboration between design institutes and academia.

Core Technical Content

The authors employ the FLAC3D numerical analysis software to establish a large-diameter steel pipe pile numerical model and conduct simulation studies on the vertical bearing characteristics. The model is validated against on-site static load test results, demonstrating that the numerical simulation yields an ultimate bearing capacity of 10,450 kN, which is in close agreement with the field test results.

Key Numerical Simulation Results

Load Transfer Mechanism

The numerical simulation reveals that the pile axial force decreases with increasing soil depth, indicating that the axial bearing capacity is primarily provided by the shaft friction resistance. The pile tip axial force approaches zero at ultimate load, suggesting that the pile tip resistance contribution is minimal relative to the shaft resistance. When the pile embedment depth exceeds 30 meters, the ultimate bearing capacity increases rapidly, indicating that stratum ⑥-1 serves as an effective bearing layer.

Technical Interpretation and Engineering Relevance

The use of FLAC3D for pile foundation analysis is a well-established practice in geotechnical engineering. FLAC3D employs the finite difference method with explicit time-stepping, which is particularly well-suited for modeling soil-pile interaction problems involving large deformations and complex stress paths. The software's ability to handle plastic deformation and stress-dependent stiffness makes it an appropriate tool for analyzing the nonlinear behavior of soil-pile systems under load.

The finding that shaft friction dominates the vertical bearing capacity is consistent with the general behavior of long piles in cohesive soils. For large-diameter steel pipe piles, the surface area-to-volume ratio is relatively high, which favors the development of shaft resistance. The rapid increase in bearing capacity beyond 30 meters embedment depth suggests a significant improvement in soil properties at that depth, likely corresponding to a transition from softer upper layers to a denser, stronger stratum.

Design Implications for Steel Pipe Pile Foundations

Connection with Pipe Manufacturing Practice

For steel pipe manufacturers supplying large-diameter piles for offshore wind foundations, the following considerations are critical:

• The pipe diameter and wall thickness must satisfy both structural requirements (bending, torsion, fatigue) and geotechnical requirements (bearing capacity, driving resistance).
• The welding quality of the longitudinal seam (for HFW or LSAW pipes) and the circumferential welds (for spliced piles) is paramount, as any weld defect could compromise the structural integrity under cyclic wind loading.
• The steel grade selection must balance strength, toughness, and fatigue resistance. API 5L grades (X65, X70, X80) are commonly used, with X80 increasingly adopted for deeper installations requiring higher capacity.
• The surface condition of the pipe affects the shaft friction capacity. Smooth surfaces may provide less friction than roughened or textured surfaces, although this effect is typically modest compared to soil property variations.
• Corrosion protection systems (coatings, cathodic protection) must be designed for the design life of the wind farm (typically 25-30 years), with allowances for inspection and maintenance access.

Study Insights and Implications

The validation of the numerical model against field test results is a crucial step that enhances the credibility of the simulation findings. The close agreement between the simulated and tested ultimate bearing capacities (both approximately 10,450 kN) provides confidence in the model's ability to predict the behavior of similar pile foundations under different conditions.

The identification of stratum ⑥-1 as a suitable bearing layer offers practical guidance for foundation design in the coastal area of the Radiating Sand Spit. This finding is particularly valuable for the planning and design of offshore wind farms in this region, as it provides a clear target for pile driving operations.

The observation that the pile tip resistance is negligible compared to shaft resistance has important implications for pile length optimization. Extending the pile beyond the 30-meter depth where bearing capacity increases rapidly may not provide proportional benefits, and the design should focus on ensuring adequate engagement with the competent stratum rather than maximizing pile length.

In conclusion, this paper provides a rigorous numerical investigation of large-diameter steel pipe pile foundations for offshore wind power, with validated results that offer practical guidance for foundation design. The findings emphasize the importance of shaft friction resistance and the critical role of embedment depth in achieving adequate bearing capacity, providing a solid technical basis for the design and construction of offshore wind foundations in the study area.