Model Testing Method Of Deepwater Semi-Submersible Platform By Truncation Of Mooring System Based On Statics And Low-Frequency Damping Equivalence

Nowadays, the gas & oil industry and researchers are concentrating their efforts in developing fields in deeper waters, cumulatively. Within reasonable model scale, physical model testing of floating platforms with full-depth mooring system present obstacles because no wave basin is large enough to perform model testing of floating offshore structures with complete mooring system in 1500-3000m water depths. Thus, physical model testing of floating platforms need to be conducted in truncated water-depth. Then, mooring system need to be truncated and reproduced to make sure that the motion responses of floating platforms in truncated water-depth are equivalent with those in full water-depth.Mooring system is the positioning and motion constraint system for deepwater floating platforms, which could provide static restoring forces and damping forces for floating platforms. The design of mooring system is required to provide sufficient static restoring forces and damping for maintaining the vessel within a certain radius above the wellhead. The low frequency excitation caused by the irregular wave may induce large resonant motion responses. Thus, mooring damping is crucial to estimate vessel motions. Once the truncated mooring system could be designed to reproduce both the static and damping characteristics as similar as the full-depth mooring system, there will be an opportunity that the truncated mooring system could replace the full-depth mooring system equivalently.In this dissertation, design approach of equivalent truncated mooring system is the key aspect. Focusing on deepwater semi-submersible platform, researches are conducted on model testing method by truncation of mooring system based on statics and low-frequency damping equivalence. The main works of this dissertation are as follows:1. Research on mooring statics and damping solution:First, the piecewise extrapolating method to calculate the static restoring forces of mooring system is described; Then, considering normal drag, tangential drag and seabed friction, an improved quasi-static method is presented, and the analytical results based on the improved method are compared to other quasi-static and time domain results. Finally, model tests of a 1:70 scaled catenary mooring line are conducted in the State Key Laboratory of Coastal and Offshore Engineering in Dalian University of Technology.2. Effects of static restoring force and mooring-induced damping characteristic on motion responses of deepwater semi-submersible platform:First, frequency domain analysis method based on three dimensional potential flow theory is introduced. Then, coupled analysis method in time domain based on the impulse response function is described. Finally, as control groups, considering the static restoring force and damping characteristic, four kinds of mooring systems are designed, based on an original deepwater catenary mooring system used for a semi- submersible platform of 1500m water depths. The motion responses of the platform with all the five kinds of mooring systems discussed above are compared.3. Design approach of equivalent truncated mooring system:First, an optimized program for designing truncated mooring system is developed using genetic algorithm based on only mooring static equivalence. Case study is performed using this design program. Then, an innovative approach is developed to design truncated mooring system based on both static and damping equivalence. Employing genetic algorithm, the optimized design program based on the innovative approach is developed and used to conduct case study. Finally, considering a deepwater semi-submersible platform with full-depth and corresponding two equivalent truncated mooring systems, the floater responses are compared.4. Applicability analysis of truncated mooring system based on static and damping equivalence:First, the applicability for different type of mooring system is discussed. Considering the catenary, semi-taut and taut mooring systems used for a semi-submersible platform of 1500m water depths, the motion responses of the platform with full-depth and truncated mooring systems are calculated and compared. Then, the applicability for different truncated water depths is discussed. Considering 1500m as full water depths, set the truncated water depths as 700m and 400m, respectively. The platform motion responses in full water-depth and truncated water-depth are calculated and compared.5. Experimental verification of a semi-submersible platform with truncated mooring system based on static and damping equivalence:In order to verify the feasibility of the innovative model testing method, physical model tests of a semi-submersible platform with full-depth and truncated mooring system are performed in the nonlinear wave tank of the State Key Laboratory of Coastal and Offshore Engineering in Dalian University of Technology. The physical models of the semi-submersible platform and mooring lines are processed based on Froude scale set as 1:70. In regular and irregular wave conditions, the motion responses of the semi-submersible platform are compared. Good agreements are found between the full and truncated water-depth model test results.