CFD Analysis of Entrapped Water Response of FPSO/FLNG Turret

Abstract

Abstract In recent years, large diameter internal turret systems are increasingly becoming a viable solution for large FPSOs and FLNGs in harsh environmental conditions, because of higher vertical load capacity and flexibility in the riser number. The large volume of water in the moonpool introduces unique design challenges in extreme operating conditions, due to complex interactions between the water and the turret structure. The water trapped inside the moonpool exerts considerable dynamic loads on the turret and moonpool structures. These loads are combined with other environmental loadings and need to be considered in the design of the system. In the present work, computational fluid dynamics (CFD) models were created for simulating complex entrapped water dynamics in a generic large turret design. The CFD model in this study used fully 3-D Reynolds-Averaged Navier-Stokes (RANS) equations and the k-ω SST turbulence model. Additionally, a multiphase flow model, using the volume of fluid (VOF) method, captured the free-surface. The analysis matrix included cases with forced free surface motions, vessel forced motions and vessel responses to regular and irregular waves. The CFD simulation started with grid and time step sensitivity studies for an open moonpool. The CFD model was further validated with free decay cases by comparing the results with empirical estimates. After that, a chain table and a turret system were added into the model to perform free decay cases and forced motion cases. That CFD model was also assessed with the vessel motions in regular and irregular waves. Simulation results revealed sloshing and piston modes and natural decay rates in the moonpool, as well as temporal and frequency characteristics of water motion in response to wave forcing. The CFD simulations clearly captured the effects of the moonpool, the chain table and the turret structures on the dynamics of entrapped water. The simulation results offered valuable insights into the flow patterns of entrapped water in the moonpool as well as the resultant dynamic loads, which are crucial to the moonpool and turret design.