Sammendrag
The nowadays frequent use of FPSOs for offshore oil production in areas prone to green water events has increased the industrys focus on wave-induced impact loads as an important design parameter. This is a complex hydrodynamic problem where simplified engineering methods are often used in connection with model testing. Various efforts have been presented during the recent 10–15 years to establish reasonably good industry design tools, while the use of fully nonlinear methods and CFD is still in its development. The main focus of this paper is to investigate the potential of a simplified coupled method between a potential theory based Green Water engineer tool (Kinema3) and the commercial CFD tool Star-CCM+ based on its Navier-Stokes Solver (NS) and the Volume of Fluid (VOF) method. Results from a case study application on a large FPSO are validated against model test data. The case study contains analyses of the FPSO in long crested regular seas, both in fixed and in moored conditions. Three different heading directions are included. The approach for modeling green water events uses a Finite-Volume-VOF method with a complex velocity inlet boundary condition. Thus the Kinema3 engineering tool is used to generate simplified spatio-temporal inlet conditions from the relative wave elevation and wave kinematics at the bulwark, based on linear potential theory combined with nonlinear random wave kinematics. The VOF method is then used to model the detailed flow on deck, including impact forces on deck structures. Kinema3 can also generate simplified estimates for the peak water height, velocity as well as impact force values assuming an extended dam-break approach together with a simplified, local 2D deck layout, and comparisons to the CFD results show an overall fairly good agreement although flow details on deck can of course not be expected to be modeled that well. Comparisons of the above results to model test data show good agreement both for the relative wave height, water height and impact force level, in regular and irregular waves. Detailed time histories, including force rise time, from the coupled Kinema3 - Star-CCM+ CFD simulation analysis are quite similar to the measured ones. The CPU time consumption for the coupled simulation is moderate compared to a full CFD simulation of the FPSO in waves. Hence the achieved calculation time and the simplicity of the simulation setup of the numerical simulation makes this method an interesting candidate for industrial use. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.