Zhuojin Pipe Fitting Co., LtdNumerical Analysis of Horizontal Bearing Characteristics of Steel Pipe Piles with Constraint Discs for Offshore Wind Turbine Foundations
Literature Overview
This paper by Li Jiale, Zhang Yong, Wang Xuefei, and Wang Chen from Hebei University of Technology and Tongji University, published in Journal of Tongji University (Natural Science) in 2021 (Vol. 49, No. 11, pp. 1556-1564), investigates a novel single-pile foundation design for offshore wind turbines that incorporates internal constraint discs within the steel pipe pile. The study was supported by the National Natural Science Foundation of China (Grant No. 51908185). The research combines centrifuge model testing with ABAQUS finite element analysis to characterize the horizontal bearing performance of the modified pile system.
Innovation and Design Concept
Constraint Disc Configuration
The novel foundation concept involves installing internal constraint discs within the traditional open or closed steel pipe pile. Two configurations were investigated:
| Configuration | Description | Mechanism |
|---|---|---|
| Single-hole disc | One constraint disc with single aperture | Restricts soil flow through pile |
| Four-hole disc | One constraint disc with four apertures | Partial restriction with controlled soil flow |
| Traditional open pile | No constraint disc | Free soil flow through pile |
| Traditional closed pile | Solid pile base | Complete soil flow prevention |
Centrifuge Model Testing
A series of centrifuge model tests were conducted using a custom-designed horizontal loading apparatus. The tests simulated geostatic stress conditions in the soil surrounding the pile models and applied horizontal loads to determine the ultimate bearing capacity and failure modes.
Numerical Modeling with ABAQUS
Finite Element Model Setup
The ABAQUS finite element models were developed to complement the centrifuge test results and provide a more detailed understanding of the failure mechanisms. Key modeling aspects included:
- Soil constitutive model: Appropriate soil model (e.g., Mohr-Coulomb or Cam-Clay) calibrated from centrifuge test data.
- Pile-soil interaction: Contact elements with appropriate friction parameters to capture the complex interaction.
- Constraint disc modeling: The internal discs were modeled as rigid or semi-rigid elements that restrict lateral soil displacement within the pile.
- Mesh sensitivity: Fine mesh near the pile-soil interface to capture stress concentrations.
Failure Mode Analysis
The numerical analysis revealed distinct failure modes for different pile configurations:
- Traditional open pile: Soil flows freely through the pile, resulting in lower lateral resistance and earlier failure.
- Traditional closed pile: Soil arching develops at the pile base, providing additional resistance but potentially causing uplift issues.
- Constraint disc pile: The discs create partial soil confinement within the pile, generating additional passive resistance and significantly improving horizontal capacity.
Key Results
Horizontal Ultimate Load Comparison
| Pile Type | Relative Horizontal Ultimate Load | Stiffness Characteristic |
|---|---|---|
| Open pile (baseline) | 1.0 | Lowest initial stiffness |
| Single-hole disc | Significant improvement | Moderate stiffness increase |
| Four-hole disc | Significant improvement | Moderate stiffness increase |
| Closed pile | Moderate improvement | Higher initial stiffness |
Rotation Center Behavior
A critical finding is the evolution of the pile rotation center under increasing horizontal load:
- Elastic stage: Rotation center is located at the pile head or near the ground surface.
- Plastic stage: Rotation center migrates downward as plastic hinges develop.
- Ultimate stage: The rotation center stabilizes at a depth related to the pile embedment and soil properties.
The constraint disc configuration influences the rotation center depth and the overall failure mechanism, providing a more predictable and stable failure mode.
Engineering Practice Implications
Steel Pipe Pile Manufacturing Requirements
For the constraint disc pile system to function effectively, the following manufacturing and installation quality aspects are critical:
- Pile straightness: Must meet API 5L or ISO 13671 tolerances to ensure proper installation and soil-pile contact.
- Internal disc welding: The constraint discs must be securely welded to the pile wall with full-penetration welds capable of withstanding soil pressures during service.
- Disc positioning accuracy: The axial position of the discs within the pile must be controlled to within specified tolerances for optimal performance.
- Coating compatibility: Internal coatings must be compatible with the constraint disc welding process and must not compromise weld integrity.
Design Parameters for Engineering Application
| Design Parameter | Recommended Approach |
|---|---|
| Disc position | Optimize based on soil stratification and expected load levels |
| Disc diameter | Match pile internal diameter with clearance for installation |
| Number of discs | Based on required capacity improvement and cost optimization |
| Disc material grade | Match or exceed pile steel grade for weld compatibility |
| Weld type | Full-penetration butt weld with NDE verification |
Study Insights and Reflections
This research represents a significant advancement in offshore wind turbine foundation design. The constraint disc concept is elegant in its simplicity—by partially restricting soil flow within the pile, additional passive resistance is mobilized without substantially increasing the pile diameter or wall thickness. This approach offers a cost-effective solution for improving horizontal capacity in the challenging conditions of offshore wind farm foundations.
The combination of centrifuge testing and numerical analysis provides a robust validation framework. The centrifuge tests confirm the basic performance trends, while the finite element analysis provides detailed insight into failure mechanisms that cannot be directly observed in physical tests. This dual approach is particularly valuable for foundation design where field testing is limited and design margins must be carefully evaluated.
The finding that the constraint disc pile exhibits better stability in both stiffness and strength compared to traditional piles suggests that this concept could be extended to other offshore foundation applications, including oil and gas platforms and marine bridge foundations.