The resilience of coastal and offshore infrastructure against wave-induced scouring is a critical concern. Cemented granular materials (CGMs), which strengthen mass stability through interparticle bonds, present a potential solution. However, their design is limited by a lack of understanding of microscale fluid–solid interactions under dynamic hydraulic loading. This study aims to create a high-fidelity numerical framework to investigate these anti-scouring mechanisms, focusing on cementation distribution and two-phase flow.

A coupled computational fluid dynamics and discrete element method (CFD-DEM) model was developed to simulate the complexities of two-phase flow (water and air) and irregular particle shapes. The model was based on a validated virtual domain/immersed boundary method. Realistic rubble shapes were modeled using clumped particles, where each irregular grain was represented by a rigid cluster of sub-spheres. The pore structure was analyzed via Voronoi tessellation, and a mesoscale bond model was implemented to define the spatial distribution of cementation. Bonds, generated as cylindrical elements connecting particle centers, were created if the Voronoi faces met a criterion based on a critical interparticle distance related to the cementation ratio (CR), which was the volume of cement over the total volume. These bonds followed a linear elastic model with tensile and shear strength limits; they broke once these limits were exceeded. To account for flow obstruction caused by cement within pores, cemented zones were modeled as an isotropic homogeneous porous medium using a Darcy–Forchheimer approach, with momentum sink terms added to the fluid equations. The DEM module (using LIGGGHTS) solved the translational and rotational motions of each clumped particle, considering contact, bond, and fluid forces. The CFD module (using OpenFOAM) solved the Navier–Stokes equations for the two-phase fluid. Coupling was achieved through momentum exchange, where fluid forces acted on particles, and particle motion constrained the fluid velocity via porosity-weighted correction. A dam-break wave scenario was simulated in a 3D numerical flume to scour a triangular rockfill mound composed of 292 clumped particles, representing a transient high-impact loading condition. Simulations were carried out for noncemented and cemented cases with CR values of 10%.

The model successfully replicated the fluid and contact force dynamics observed in noncemented rockfill. For the cemented cases, key findings included: the following: (1) Permeability control: cementation significantly reduced water infiltration. The noncemented mound exhibited deep seepage, while at CR = 5%, penetration was greatly decreased. At CR = 10% and 20%, water was mainly confined to surface flow, with minimal internal seepage. The effect saturated, with little difference between CR = 10% and 20%. (2) Micromechanical response: cementation bonds changed how internal forces were transmitted. They supported substantial tangential forces during wave action, and their progressive failure (via tensile/shear rupture) dissipated energy. This led to a decrease in the average normal contact force and an increase in tangential force, indicating dynamic rearrangement of force chains. (3) Structural integrity: despite similar peak fluid impact forces, cemented structures retained greater stability by stabilizing the granular skeleton and redistributing internal stresses through the bond network.

An advanced CFD-DEM model is developed to reveal the micromechanisms behind CGMs' resistance to scour. Cementation improves performance not only by strengthening contacts but also by reducing permeability and enabling energy dissipation through bond failure. The effect of cement content is nonlinear; beyond around 10% CR, additional cement shows diminishing returns, which is important for cost-effective design. Future work will include more complex material models and realistic wave conditions to support engineering applications.

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.