Numerical Simulation of Free-end Effects on Local Scour Around a Full-scale Subsea Hydrogen Storage Accumulator

Abstract Green hydrogen production is an emerging pathway aimed at substituting the exploitation of offshore renewable energies and ultimately resulting in the decarbonization of global economies. Large-scale hydrogen storage is a critical link in achieving this. Subsea hydrogen storage provides several advantages over conventional storage, such as near-isothermal and isobaric conditions. However, local scour around the subsea hydrogen storage accumulator may degrade the stability of anchoring. In this study, the local scour characteristics around low-aspect-ratio subsea hydrogen storage accumulators with different free ends, namely flat tip (FT), tip with filleted edges (FET), and approximate hemispherical tip (AHT), are investigated. The numerical simulation of the interaction between accumulators, ocean currents, and the seabed is conducted using the large eddy simulation (LES) model and the sediment scour model. The shapes of scour pits around the three accumulators, the scour time history in different directions, and the spatial distribution of the scour depth are compared and analyzed. The equilibrium scour depth and characteristic time scale are quantified. The results indicate that the shapes of the scour pits around the three accumulators exhibit a horseshoe distribution. The scour processes and spatial distributions of sediment in different directions revealed certain differences between the sediment scour and accumulation processes, but the scour change trends are relatively consistent. The flow diagram of the transient scour shows that the horseshoe vortex has the smallest influence range upstream of the AHT accumulator, but the downstream downwash flow is highly significant. The equilibrium scour depths of the three accumulators are −0.49 (FT), −0.41 (FET) and −0.35 (AHT), respectively, and the scour characteristic time scale gradually increases. These results illustrate that the design of the AHT accumulator has an inhibitory effect on local scour. The scour depth corresponding to the characteristic time scales of the three accumulators can reach 60%–65% of the equilibrium scour depth.