Numerical study on diffraction problem for a truncated cylinder with dual dampers

Abstract

In the last decades, offshore wind energy industries have moved to deep sea area, since 80% of the offshore wind resources are located in deep water region (over 60 meters). As a result, various floating offshore wind turbine (FOWT) structures based on the semi-submersible, TLP or spar platforms have been widely proposed. However, accompanied with abundant wind resources, harsher environmental loads in deep sea area may also cause damage to offshore structures. In general, the semi-submersible platform of FOWT are mainly composed of large truncated circular columns which tend to generate significant diffraction effects under the incident waves. These diffraction effects are usually associated with significant rapid amplification of local free-surface elevation surrounding these cylindrical structures. Therefore, single dampers with arbitrary shapes are commonly attached at the bottom of FOWT to improve the motion performance of the FOWT system. The present research aims to investigate the effects of the separating distance, the porosity and the thickness of the damping plate on wave run-up around the truncated cylinder with dual dampers under the regular wave conditions. The target model is an offset column of OC4-DeepCWind FOWT. To evaluate the three-dimensional (3-D) wave run-up around the truncated cylinder with dual dampers in a finite water depth, a matched eigen-function expansion method (MEEM) based on linear potential theory was developed. In this study, fluid around the truncated cylinder was divided into multiple regions based on different boundary conditions. The velocity potential in each region is analytically derived by using the Helmhotz Equation and eigen-function expansion. Darcys law is applied to satisfy the boundary condition on the porous damping plate. The velocity and pressure of fluid across the adjacent sub-regions should satisfy the continuous conditions (mass conservation) on the boundary. Finally, the unknown complex coefficients matrix could be derived and solved by using the matching conditions along the boundaries of regions. The present MEEM solutions for the truncated cylinder with various types of damping plate are validated by comparing with BEM solutions. Furthermore, with regard to the other simple porous structures such as horizontal porous membrane, the complex wave number, wave run-up and hydrodynamic loads are also compared to previous computational results. The results of present study clearly indicate that compared with the impermeable damping plate, porous damper could significantly reduce the heave wave exciting force and wave run-up around the truncated cylinder. Impermeable dual damping plates with large separating distance could cause significant wave run-up and hydrodynamic loads acting on truncated cylinder due to the shallow water effect in the local flow. It is expected that present porous dual damping plate model can be applied to the offshore structures such like FOWT.