Subsea production systems often need extra protection to prevent equipment damage from dropped objects or trawling. This protection can be integrated into the foundation or provided as standalone equipment. One case of standalone protection structure is a Glass Reinforced Plastic (GRP) cover, which has the advantage of being lightweight, reducing manufacturing and installation costs. Low weight can however pose risks, especially in shallow water regions where storm events have a greater impact on flow conditions near the seabed. To prevent uncontrolled displacement of the GRP covers, they must be stabilized using rockdumps. However, it may be challenging to optimize the volume of rocks needed to mitigate the risk. The ballast weight must overcome the forces from the sea current and waves. On the other hand, excess of rocks will increase installation costs. [1] offers guidelines for computing forces on subsea structures, but the drag and inertia coefficient formulas are limited to simple geometries. Therefore, accurate force computation for more complex structures requires CFD simulations or experiments.
When estimating forces, it's important to consider both drag - and inertia force generated by the oscillating velocity field from ocean waves. In fact, in storm conditions the inertia force is dominant. Although modern CFD tools typically have little trouble estimating drag force for similar geometries, inertia force may present a greater challenge in generating simulation inputs, solving, or post-processing stages. A simple procedure has been developed to calculate the maximum horizontal force that a GRP cover may experience during extreme weather conditions. Using the Morison equation and a simple curve-fitting procedure, it is also possible to estimate both drag and inertia coefficients for a specific geometry.

References:
[1] DNVGL-RP-N103 "Modelling and analysis of marine operations"