Global motion and airgap computations for semi-submersible floating production unit in waves

We study the global hydrodynamic performance of a semi-submersible floating platform unit in order to optimize the hull form in the future. The hydrodynamic problem is solved by employing potential flow theory and Morison equation for modelling of the viscous effects. The added mass and damping coefficients, as well as the first-order motion responses, second-order mean drift forces, diffracted and radiated wave field, and airgap are computed to examine the hydrodynamic behavior of the floating production unit. The computational results show that the motion responses in short-crested waves are mostly smaller than those in long-crested waves. The maximum wave elevation occurs at WP45 in 45°45° wave heading in long-crested waves. In addition, the minimum airgap occurs at AG45 in 45°45° wave heading in linear waves, while the worst airgap point in nonlinear waves is AG0 in 0°0° wave heading. Extensive parametric studies have been performed to examine the dependence of the motion responses and the other key design criteria on the principal dimensions including hull draft, column width, column spacing, column corner radius, pontoon height, pontoon width, and the size of cakepiece. By comprehensive and systematic hydrodynamic computations and analyses, it is revealed that the combined vertical motion at the worst airgap location is almost in phase with the wave elevation in extreme wave condition with a peak wave period around 14–15 s. Moreover, it is found that the most efficient way to reduce the motion is to increase the hull draft, though the airgap may also decrease. Besides, reducing the pontoon height can achieve better motion performance and larger airgap simultaneously. This paper aims to provide a benchmark for future studies on automatic hull form optimization