Experiment-Scale Multi-Vessel Dynamic Positioning System for the Twin-Lift Decommissioning Operation

In general, the lifetime of the offshore oil and gas platform is about 20–30 years, and a large number of oil and gas platforms is approaching the end of their service. So, how to decommission these oil and gas platforms safely and cleanly becomes an urgent problem for the oil and gas industry and related organizations. And the costs for decommissioning equipment and services are expected to be as low as possible. It is important to develop the innovative removal techniques to reduce the overall cost of decommissioning. In this paper, we propose a new concept to remove large and heavy structures with a single lift, utilizing three semi-submerged vessels. The main process of the twin-lift solution for decommissioning includes two semi-submerged vessels lifting the topside together, and then carrying the topside to the third semi-submerged vessel. This method is simple in principle and does not require the manufacture of new special vessels, only needs to make simple modifications to existing semi-submerged vessels. It requires high positioning accuracy especially under environmental disturbances to ensure the safety of transportation. To ensure efficiency and safety, we develop a dynamic positioning (DP) system and 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 system are described. It includes the topology of the experiment and details of the deck mating, DP and monitoring system.