Conceptual design and parametric optimization of self-propelled semi-submersible repair ships: a novel equipment providing maintenance and repair support at sea

Safety of ships and/or offshore structures at sea is increasingly crucial as seaborne activities intensify. However, maintenance and repair of them suffering marine accidents away from shipyards are an often overlooked area, with existing maintenance and/or repair ships either being of limited workshops and facilities or incapable lifting them out of water for underwater engineering. A conceptual design of a self-propelled semi-submersible repair ship is proposed for the first time to facilitate the spot maintenance and repair of damaged ships and/or offshore structures at sea, which is obviously able to reduce the time, costs and risks of transporting them from accident scenes to shipyards onshore. Furthermore, this paper focuses on parametric design and multi-objective optimization problem of this novel equipment. The ratio of deadweight to principal dimensions, working deck area and average daily cost considering marine emissions trading scheme are simultaneously chosen as objectives of this problem. Both the weighted ideal point method and the NSGA-II algorithm are used to obtain the optimization results of a 50 thousand dwt self-propelled semi-submersible repair ship and the relations and differences between the optimization results of two methods are analyzed. The research results indicate that the parametric design and multi-objective optimization method can provide theoretical support for the preliminary design.