Conceptual study on dynamic responses of semi-submersible platforms

Abstract

Hydrodynamic analysis of semi-submersible offshore structures has been the topic of several numerical and experimental research studies conducted in the past. In this work, the effect of structure alignment and mooring lines on the dynamic behaviour of semi-submersible platforms was taken into account. The contribution of pitch and roll into heave motion of side-points was also investigated. Mooring lines are connected to semi-submersible platforms at the sides; therefore, the total displacement of side-points is related to the total displacement of mooring lines and total tension. A three-dimensional boundary element model of Amirkabir platform in the Caspian Sea was created using the boundary element method and the resulting response amplitude operators (RAOs) were obtained. A conceptual study on RAOs was conducted, which showed that structure alignment with respect to wave direction is highly important for a range of wave periods. In addition, in surge and sway, the effect of mooring lines is significant in particular ranges of wave periods. The contributions of roll and pitch to the side-points of semi-submersible platforms are also considerable in a range of periods.