Development of an experimental system for the twin-lift decommissioning operation

Abstract

Hundreds of offshore platforms are approaching the end of their productive life and will require decommissioning within the next decade. Removal of a large offshore structure is a highly complex exercise that poses significant safety challenges. In this paper, we propose a new concept to remove large and heavy structures with a single lift, utilizing three semi-submerged vessels. Such a twin-lift solution for decommissioning involves an operation of lifting of a topside structure, carried on two semi-submerged vessels, to the third semi-submerged vessel. Although the method appears simple, it requires adequate stability using ballast tank, and synchronized motion of moving vessels, especially under environmental disturbances. In order to ensure efficiency and safety, we perform a wave tank test for such a twin-lift decommissioning solution. Some aspects of an experimental facility set-up for scaled model test with dynamic positioning (DP) system are described. It includes the layout of the experiment and details of the deck mating, DP and monitoring system. Preliminary experimental results are presented, to reveal that a simple controller combined with DP system is able to produce a robust and precise performance.