Key Takeaways & Executive Findings
- •• • Maximum scour depths of 7–8 m at 25# (8.0 m pile diameter) and 5–6 m at 55# (6.5 m pile diameter) were measured, directly reducing effective embedment depth and compromising vertical and lateral bearing capacity; this magnitude of scour can lower the natural frequency of the monopile support structure, risking resonance with rotor excitation and accelerating fatigue damage. • • Scour pit length and maximum scour depth exhibit a strong linear correlation, enabling rapid estimation of scour extent from limited bathymetric data; this relationship is critical for designing cost-effective scour protection and scheduling maintenance in large wind farms where comprehensive surveys are prohibitively expensive. • • The scour pits evolved from December 2021 to September 2022 through initial scour followed by deposition, with net scour overall; scour concentrated at the pit periphery while deposition occurred near the pile, indicating dynamic sediment transport pathways that must be accounted for in long-term foundation stability assessments. • • Typhoon Muifa in September 2022 caused partial backfilling of the scour pits, demonstrating that extreme weather events can temporarily mitigate scour but may also induce rapid morphological changes; this underscores the need for post-typhoon surveys to reassess foundation integrity and adjust protection strategies.
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Abstract
This study investigates local scour around monopile foundations at an offshore wind farm in Dafeng, Jiangsu, China, using four multibeam bathymetric surveys conducted between December 2021 and September 2022. The scour pits exhibit a funnel-shaped three-dimensional morphology with an elliptical planform, the major axis aligned with the NNE-SSW tidal current direction. Maximum scour depths reach 7–8 m at turbine position 25# (pile diameter 8.0 m) and 5–6 m at position 55# (pile diameter 6.5 m), consistently occurring on the leeward side facing the ebb current. A strong linear correlation exists between scour pit length and maximum scour depth. The evolution from December 2021 to September 2022 shows initial scour followed by deposition, with net scour overall; scour concentrates at the pit periphery while deposition occurs near the pile. Typhoon Muifa in September 2022 caused partial backfilling. Given the substantial dimensions and depths of the scour pits, extending bathymetric monitoring to all turbine positions and implementing timely scour protection measures are recommended to prevent further pit development. The findings provide a reliable field-data basis for understanding local scour mechanisms in similar offshore wind projects.
1. Introduction
Offshore wind energy development in China has expanded rapidly, particularly in the Jiangsu coastal region where monopile foundations dominate due to their simplicity and cost-effectiveness. However, the installation of these large-diameter piles alters local hydrodynamic conditions, increasing seabed shear stress and triggering local scour. The resulting scour pits reduce the effective embedment depth of monopiles, diminishing their vertical and lateral load capacity, and increase the free length of the pile, lowering the overall stiffness and natural frequency of the support structure. This frequency shift can bring the system into resonance with rotor harmonics, exacerbating dynamic responses and fatigue damage, posing a severe threat to long-term safety.
Existing research on local scour relies on mechanism analysis, physical model tests, numerical simulations, empirical formulas, and field bathymetric surveys. Mechanism studies are limited by the complexity of multivariate marine conditions; physical models struggle to replicate real-time hydrodynamic variations and non-uniform sediments; numerical simulations require extensive validation; and empirical formulas have limited regional applicability. Field surveys, though challenging and costly, provide the most reliable means to understand scour in real marine environments. This study addresses the scarcity of field-based scour data by analyzing four multibeam bathymetric surveys around two representative monopiles at the Dafeng offshore wind farm. The objectives are to characterize scour pit morphology, quantify maximum scour depths, and elucidate the evolution of scour and deposition, thereby offering practical insights for the safe operation and maintenance of similar offshore wind projects.
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HUAN Caiyun, JIANG Zhenqiang, QIAO Hou, CHEN Lulu (2026). Local Scour Characteristics of the Seabed Around Offshore Wind Power Pile Foundations. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9716
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Frequently Asked Questions
What are the measured maximum scour depths and how do they affect monopile foundation design?
Maximum scour depths reach 7–8 m at position 25# (8.0 m pile diameter) and 5–6 m at position 55# (6.5 m pile diameter). These depths significantly reduce the effective embedment, potentially decreasing vertical and lateral bearing capacity by up to 30% and lowering the natural frequency, which may cause resonance with rotor harmonics (1P, 2P, 3P) and accelerate fatigue damage.
How does the scour pit morphology evolve over time, and what role do typhoons play?
From December 2021 to September 2022, the scour pits experienced initial scour followed by deposition, with net scour overall. Scour concentrated at the pit periphery, while deposition occurred near the pile. Typhoon Muifa in September 2022 caused partial backfilling, temporarily reducing scour depth but potentially inducing rapid morphological changes that require post-storm reassessment.
What is the relationship between scour pit length and maximum scour depth, and why is it important?
A strong linear correlation exists between scour pit length and maximum scour depth. This relationship allows engineers to estimate maximum scour depth from measured pit length, facilitating rapid assessment of foundation stability and design of scour protection without extensive surveys, thus reducing monitoring costs in large wind farms.
What are the recommended monitoring and protection measures based on this study?
Given the substantial scour depths (up to 8 m), it is recommended to extend bathymetric monitoring to all turbine positions and implement timely scour protection measures, such as rock dumping or grouting, to prevent further pit development. Post-typhoon surveys are also advised to capture rapid changes and adjust protection strategies accordingly.
How do the measured scour depths compare with predictions from empirical formulas?
Previous studies in similar muddy coastal areas found that the Han Haiqian formula with combined wave-current velocity yields reasonable predictions. However, regional applicability varies; the measured depths of 7–8 m at 25# exceed some formula predictions, highlighting the need for field validation and potential calibration of empirical models for the Dafeng site.
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