A numerical analysis of the response and air gap demand for semi-submersibles

This paper examines the effects of viscous damping on the prediction of air gap demand for semi-submersibles. This is illustrated by a case study of a typical six-column, double pontoon semi-submersible in irregular waves. Linear responses of the platform and free-surface elevations at selected locations are computed using WAMIT, in which viscous damping effects are included through the use of a linear damping matrix. The spectra of the platform's response and of the relative free-surface elevation at the selected locations, as well as the probability distributions of air gap minima obtained using different damping matrices are compared. It is demonstrated that correct modelling of viscous damping is important for an accurate prediction of air gap demand of a floating platform, especially if the peak frequency of the wave spectrum is close to the heave, roll, and pitch natural frequencies of the platform.