Setdown of a catenary-moored gravity platform

Abstract

The setdown phase of the installation of a gravity platform requires a careful analysis of each step involved to prevent any damage to the structure. To do this, it is important to include all the significant forces, including the tensions in the mooring lines, in a setdown simulation. In this article, the setdown of a conical gravity platform is simulated when temporary mooring lines that assist the setdown are fully accounted for. The setdown is achieved by ballasting the tanks in the platform according to a sequence controlled basically by the stability of the platform, as well as by its final location. A theory is developed to account for all the exact hydrostatic and ballasting forces, as well as the tensions in the mooring lines, which are catenary. Based on this theory, an interactive microcomputer program is developed to instantly monitor, report, and take action to correct any undesirable movements of the platform that may lead to negative metacentric height. The numerical scheme devised is applied to a conical platform to analyze its real-time behavior during the setdown process. The effect of catenary mooring lines on the setdown process is shown by comparing the stability parameters with and without the mooring lines. Also, different ballasting sequences are shown to have a significant effect on the safety of the platform.