Study On Mooring System Design And Optimization Of A Semi-submersible Crane Vessel

With the increasing exploration of offshore oil and gas resource, semi-submersible crane vessels capable of performing heavy lifting operation have failed to meet the rapidly rising market demand. As the exploration moves forward to deeper waters, the survival ability of semi-submersible crane vessels has been challenged due to the harsh environment in deepwater, which highlights urgent need for mooring system with high security and reliability.Considering that the oilfield the crane vessel serves in may be various, the mooring system is expected to provide the vessel a convincing guarantee to adapt the operation environment in the world’s major offshore oil fields. The uplift loads from the mooring lines should be strictly controlled due to its adverse effect upon the holding capacity of drag embedment anchors. As such,the mooring system design and optimization is always regarded as one of the focused issues and key technologies in the design of a semi-submersible crane vessel. Considering the interaction between the vessel and the mooring system, a fully coupled dynamic analysis has been performed for an accurate prediction of the dynamic response to design and optimize the mooring system of a semi-submersible crane vessel. The contents are summarized as follows:1. Hydrodynamic analysis and environment load calculation for the semi-submersible crane vessel. Numerical simulation technique was applied to evaluate the hydrodynamic performance of the semi-submersible crane vessel, and contrast verification was presented by comparing the results of numerical calculation with those of model test. Numerical simulation technique and model tests were carried out for an accurate calculation of the environment load.2. Investigation on the design and optimization of the mooring system. The study was conducted in the industrial design process, including three steps: preliminary design, scheme optimization and safety and economy evaluation. A coupled dynamic analysis was performed in the time domain to optimize the preliminary design, in which the effects of basic parameters of mooring system on its dynamic response characteristics were evaluated.Considering both reliability and economy, the mooring system optimized was evaluated.3. Study on the ultimate bearing capacity of mooring system. Based on the preliminary design, the extreme environment condition the semi-submersible crane vessel can survive was predicted by ascending, and the survival capacity of the crane vessel was verified in both intact and damaged conditions respectively.4. Study on the performance of the mooring system employing constant tension winches.A numerical model of the automatic mooring winches was built to achieve the simulation of the constant tension control of cables. Particular attention was placed on the effects constant cable tension on the dynamic response characteristics of the mooring system and the vessel,based on that, the constant tension in the cables was optimized.