Calculation of Wave Forces and Internal Loads on a Semi-Submersible at Shallow Draft Using an iVOF Method

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When semi-submersibles are floating at shallow draft, only a relatively thin layer of water may be present above the floaters. Model tests and full scale observations have shown that in such cases, even in low waves, non-linear effects significantly influence the wave pattern around the floaters. These non-linear effects make conventional methods based on linear diffraction theory less reliable for the calculation of wave forces and internal loads on a semi-submersible at shallow draft. This paper describes and analyzes the non-linear hydrodynamics affecting the wave loads and internal loads at shallow draft. The feasibility of both ComFLOW and linear diffraction method for the calculation of these loads are assessed. CFD simulations were performed using ComFLOW, a program based on the incompressible Navier-Stokes equations and the improved Volume Of Fluid (iVOF) method. First, the wave loads acting on a fixed semi-submersible in regular waves were calculated with ComFLOW and compared with linear diffraction theory and model tests. Secondly, internal loads were calculated for a moving semi-submersible in regular waves using both ComFLOW and linear diffraction theory. In the ComFLOW simulations, the motions of the semi-submersible were prescribed instead of solved by the method itself. Calculations and comparisons were performed for deep draft and shallow draft conditions. The wave loads on the semi-submersible for shallow draft conditions derived with ComFLOW were reasonably close to the results from model testing, while the results from the linear diffraction method showed significant deviations from the model tests results. The internal loads calculated with ComFLOW were quite close to the results from the linear method, even for shallow draft conditions. Additional model testing is required for validation of the internal loads.