Viscous Drift Forces and Responses on a Semisubmersible Platform in High Waves

Abstract

In this paper the significance of viscous effects on a moored semisubmersible platform in various sea state conditions is explored. Experimental data from a 1:50 model scale tests in a 50m × 80m wave basin are compared with numerical time domain simulations. Both regular and irregular waves are included, and some tests are also run with current. This paper presents results from a horizontally moored semisubmersible, where we focus on the hull hydrodynamics alone. The emphasis here is low frequency surge responses. Use of conventional potential theory shows large discrepancies when compared with experimental results in high sea states. For surge motion, they are believed to be due to viscous forces in the wave zone. Viscous forces and damping in the numerical model are included by the drag term of Morison’s equation using a total relative velocity approach, which is integrated up to the instantaneous free surface elevation. A common challenge is to choose a suitable Cd coefficient which provides for sufficient excitation force without introducing excessive motion damping. It is found that using a larger Cd coefficient in the wave zone gives larger excitation without influencing the total damping significantly. This way, applying the drag term of Morison’s equation can give results that compare well with measurements. Also, the coefficient is found to be lower in waves with current than in waves only.