Effect of roll damping tanks on the motion of offshore wind turbine platforms

Abstract

Roll Damping Tanks (RDTs) are passive roll-damping devices, employed in vessels to minimize roll motion. However, their efficacy in controlling the motion of floating offshore platforms has not been widely researched. This thesis investigates the effect of roll damping tanks on the motion response of an offshore wind turbine platform. It further analyzes whether roll-damping tanks are beneficial for a selected semi-submersible platform design. A semi-submersible Floating Offshore Wind Turbine (FOWT) platform, “SEAWORTHY” from Floating Power Plants (FPP) is selected for tank integration. Design evaluation of the RDT system for the selected platform is conducted using in-house methodologies developed at Hoppe Marine. A C-shaped box tank is designed and its placement within the General Arrangement (GA) of the platform is determined. Computational Fluid Dynamics (CFD) analysis is conducted using OpenFOAM to obtain moment-phase diagrams with respect to motion of the designed tank. A methodology for open-source CFD analysis of RDTs on clusters is developed. Benchmark studies using experimental data from Field & Martin and HERMes roll damping tanks are performed to validate the proposed CFD set-up and methodology. After validation of previous benchmarks, a parameterized model of the designed C-shaped tank is developed and analyzed. The resultant moment-phase curves are calculated for the designed tank and converted into added mass and damping coefficients for the platform. Finally, a sea-keeping analysis using the open-source tool NEMOH is performed to compare two configurations of the FOWT platform: with and without the RDT integrated. For benchmarking, the roll Response Amplitude Operator (RAO) of the platform without tank is compared with the RAO provided by FPP. Subsequently, the roll damping provided by the tank is incorporated into the platform’s RAO. The effect of variation in the mass distribution and the stiffness of the platform due to tank integration is investigated and the modified RAO is computed. The influence of viscous damping is also studied. Final conclusions are drawn by calculating the percentage reduction in peak of roll RAO, highlighting the designed RDT's effectiveness for motion control. It was concluded that the platform’s high initial viscous damping significantly limits the tank’s roll damping effectiveness. The efficacy of RDT is also reduced by increase in Vertical Center of Gravity (VCG) of platform after tank integration.