Comparison of Experiments and CFD Simulations of a Braceless Concrete Semi-submersible Platform

Abstract

A braceless concrete substructure for floating offshore wind turbines was designed and analysed in 2014 by Dr.tech. Olav Olsen AS,IFE and Acciona within an industrial project co-financed by the Research Council in Norway. A simplified model of the structure in scale 1:40 was tested in 2014 at the Hydrodynamic and Ocean Engineering tank at Ecole Centrale de Nantes (ECN) by IFE and CENER.A number of tests including regular and irregular waves for free and fixed hull, and decay tests were carried out. The present paper shows a comparison between experiments and CFD simulation for free decay tests in heave, and surge motion and test in regular waves for fixed and free hull.The commercial CFD solver STARCCM+ was used for the CFD simulations. A numerical wave tank was modelled with using a trimmed mesh with surface refinement. The surface refinement was varied depending on the wave length and height in order to ensure at least 20 cells per wave height and 100 cells per wave length. The time step was chosen in order to ensure a Courant number around 0.5. A volume of fluid method was used and the k-ω turbulence model was used to solve the turbulent part of the averaged Navier-Stokes equations. For the free hull cases the overset meshing technique with the DFBI model included in StarCCM+ was used. The mooring lines were simplified and equivalent elastic springs were used instead of the catenary system used in the experiments. The results show a good comparison between the experiments and the CFD simulations for both regular waves and decay tests.