Engineering application of offshore wind power scour protection based on cemented riprap

Scour around pile foundations, which are induced by marine currents and waves, greatly threatens the structural integrity of offshore wind turbines and submarine cables. While various scour protection measures have been employed to mitigate these effects, conventional methods such as riprap and sandbags present poor durability under complex hydrodynamic conditions, necessitating frequent maintenance and incurring high long-term costs. Consequently, there is a pressing demand for the development of more efficient and durable scour protection technologies.

This study proposes a scour protection technology using a gridded cemented riprap system for offshore wind power projects, detailing its protective mechanism, material selection, and structural design. Large-scale tests were performed to explore the protective effectiveness and scour resistance of both gridded cemented and traditional riprap systems. To further confirm its engineering applicability, pre-construction casting tests were performed to determine the optimal mix ratio and flowability of self-protecting underwater concrete. In parallel, an investigation was conducted to determine the effects of different pouring parameters on concrete diffusion. Additionally, a case study was performed to compare the performances of the gridded cemented and traditional riprap systems using a newly developed evaluation method for construction quality and long-term scour protection durability. First, the annual protective structure around the foundation was divided into grid units. Then, key indicators such as average elevation, mean elevation difference, and elevation standard deviation within each grid were calculated and analyzed using field monitoring data. By comparing pre- and post-construction (or pre- and post-operation) monitoring data, these parameters were assessed to confirm compliance with protection specifications, with a focus on examining riprap geometry, thickness distribution, and structural uniformity.

(1) The large-scale flume tests revealed that the cemented riprap system presented excellent anti-scour performance. Compared with the unprotected condition, the maximum scour depth decreased from pile diameter (D) to 0.18D, and the maximum scour range decreased from 2.00D to 0.42D. By contrast, the traditional riprap system showed that more than 50% of the stone particles were displaced by water flow, leading to structural instability and damage. (2) Pre-construction casting tests demonstrated that the diffusion range of underwater concrete (D_s) on the riprap surface was closely related to the pouring pipe diameter (D_t) and volume (V_m). The ratio D_s/D_t exhibited an inverse proportionality to pipe diameter until reaching a minimum value of 4.5 while demonstrating a linear correlation with pouring volume. (3) Construction quality assessment by the new evaluation method, which was based on average elevation, elevation difference, and standard deviation within grid units, showed that the scour protection for the offshore foundation met design specifications. Specifically, 88% of grids achieved a backfill thickness exceeding three times the median particle size, and post-pouring elevation standard deviations remained within 0.63, satisfying uniformity requirements. (4) Long-term performance comparisons indicated that the gridded cemented riprap system significantly outperformed the traditional system in extreme scour resistance and durability. Under combined wave-current conditions, the gridded cemented riprap system exhibited no structural erosion (local or global) over a one-year monitoring period, with a maximum elevation loss of only 0.30 m and a total scour volume of 18 m³, which was significantly lower than that of the conventional riprap system.

This study comprehensively investigates the application of gridded cemented riprap technology in offshore wind turbine scour protection through experimental investigations, construction methodology, and performance evaluation. The results demonstrate its superior erosion resistance and long-term durability compared with the traditional riprap system, offering valuable insights for future engineering applications.