A Fully Nonlinear RANS-VOF Numerical Wavetank Applied in the Analysis of Green Water on FPSO in Waves

Abstract

This paper presents numerical simulations of Green Water events and wave impact on a FPSO. The simulations are performed at model scale and the results are compared against experimental model test results. The commercial Star-CCM+ CFD software is used in the simulations. The incoming waves are modeled using 5th order Stokes theory, as implemented in the CFD software. Both fixed and free floating FPSO are considered. The moving FPSO are modeled using Chimera overset mesh technology. The vessels is free to move in heave and pitch at 180 (head sea), roll and heave at 270 (beam sea), while roll, pitch and heave is released at 225 (quartering sea). The computed water height on the deck and the relative wave height in vicinity the vessel are compared against model test results at several positions. Also the impact force on load cells blocks located at the deck of the vessel is computed and compared against model test results. The comparison of the time histories of the water elevation and load histories are in reasonable agreement with the measured time series. The number of grid cells range from 7M for the simulations at head sea, where flow is assumed to be symmetric, to 21M for the simulations at quartering sea. Total wall clock simulation time was about 10days for the most computationally demanding cases, which are the quartering sea simulations. This includes simulation of 12 wave periods with the ship fixed, and thereafter 8 wave periods of the free floating vessel. The computations show that CFD tools can be used as a research tool when studying the physics of green water and wave impact events. However, due to time CPU demanding simulations, this type of CFD analysis are not yet a practical tool for parametric design studies and deck structure optimizations. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

On the Evolution of Different Types of Green Water Events

“…In the latter, if unsteady and broken flow can be observed on the deck (see the snapshots in Figure 16 and in [4]), then the application of numerical models, rather than analytical models, could be used for detailed research. Although mesh-based Computational Fluid Dynamics (CFD) approaches are popular tools for fluid-structure interaction simulations, including green water (e.g., [38,57,[75][76][77][78][79]), progress in CFD methods based on particles (e.g., [37,51,[80][81][82][83]) could be a useful alternative, due to their ability to simulate large deformations and breaking flows [84]. A comprehensive review of these methods can be found in [85].…”

Overset meshing coupled with hybridizable discontinuous Galerkin finite elements

“…These features can be added later once we start addressing problems of interest; for now, we only wish to demonstrate the concept of the coupled system. Further information on current commercial overset methods can be found in [14][15][16][17][18][19].…”

“…The convection-diffusion problem over a single mesh in the overset system is stated as follows. The only substantive change from (7) to (19) is in (19c): We have added in terms over the overset portion of the boundary. The values ofȗ h ,q h , andn come from some T h,j ≠ T h,i that is yet to be specified.…”

3D Numerical Simulations of Green Water Impact on Forward-Speed Wigley Hull Using Open Source Codes

“…With the successful development of high performance computing, CFD simulations using the volume of fluid (VOF) method may be considered as a valuable alternative [2,[21][22][23][24][25][26][27][28]. However, to capture the nonlinear evolution of wave groups and wave-structure interactions, high mesh resolution is required in such simulations.…”