A practical approach to calculating inertial forces for nontrivial subsea structures using CFD simulation

Abstract

In the oil and gas industry, the majority of subsea structures are subjected to considerable forces from sea waves and currents. It is therefore of critical importance to be able to accurately assess these forces, both during the commissioning process, when the structure is lowered with a crane, and for stability calculations, such as those required for ballast estimation. Of particular importance in this context is the second scenario, which is particularly relevant for protection structures made with glass-reinforced plastic (GRP) in the form of large covers. Due to their relatively low weight and high surface area, these structures require ballast to ensure a fixed position. The following paper presents a set of observations and guidelines for calculating wave and current loads for non-trivial objects, such as GRP covers. The work was focused on the inertial forces, as these forces are most problematic due to their transient nature. Two approaches are presented, along with their applicability and limitations: a simplified and a detailed. Both methods are based on curve-fitting the outcome of the simulation to the Morison equation. Furthermore, a simple validation study is presented for the two methods, comparing simulation data to values obtained from the literature. The test demonstrated decent accuracy, particularly for the detailed method. A comparison between the simplified and detailed methods indicates that while the simplified method produces comparable results for thin-walled objects, such as GRP covers, it is not as effective for thick-walled structures.Finally, the influence of the seabed was investigated. The study demonstrated that the inertia forces acting on a structure increase considerably when the object is in close proximity or in contact with the seabed. As the majority of existing literature provides data for relatively simple objects set away from the boundaries, the presented methods appear to be a promising tool for the analysis of structures situated on a seabed.