Hydrodynamic loading and responses of semisubmersibles

• Main Author: Mohamed, Hassan
• Published: University of Newcastle Upon Tyne 2011
• Subjects: 623.8205
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.556026

In moderate sea states, the predictions of the wave drift forces based on the potential theory correlate well with measurements. However, in severe wave conditions model tests have shown that there is an increase in wave drift forces as the sea states increases. Such difference is explained by the viscous drift forces acting in the waterline zone of a structure. This thesis presents an experimental study of the low-frequency surge motion loading on and response of a semisubmersible model in different sea states ranging between moderate and extreme. In addition to the experimental study, the thesis includes numerical simulations. The experimental tests were conducted in MARINTEK ocean basin with a 1/50 scale model which was moored using horizontal springs and catenary mooring lines. The environmental conditions included waves, current and combined wave and current fields. The waves used included both regular and irregular waves. In addition, decay tests were conducted in still water and in current. For irregular waves, statistical analysis of the values of mean value and standard deviation of the motion responses was carried out to investigate the effect of wave-current interaction and different sea states on these values. As for the numerical simulations, a mathematical model that is based on Morison's approach was used to predict the dynamic surge motion loading on and responses of a semisubmersible platform. In addition, the steady and slowly varying wave forces are predicted using Pinkster's procedure. The second numerical simulation was carried out using three-dimensional diffraction program that estimates the six degrees of freedom loading and responses in regular waves with and without the viscous damping effects. The damping of the system stems from both the hull and the mooring system. The mooring system damping was studied using the energy dissipation method to estimate the damping of the mooring line using the amplitude of the surge motion of the vessel.