Truncation Optimization Design Of Deepwater Mooring System

With oil and gas exploration expanding to deep sea,deepwater and ultra-deepwater platforms are gradually used for actual production.Based on the test pool available size limit,it is difficult to use conventional scale ratio do full depth model test.Hybrid model test method is currently the most widely used and most effective way to resolve this problem.How to design an equivalent truncated system to well substitute the full depth system is the key.In the paper,an equivalent truncated design program is developed based on the static characteristics similar critetia,more in-depth research on truncated design is made,and the reliability is checked by comparison with the results of the full-depth static and dynamic characteristics and by model test verification.The following aspects are mainly studied.Firstly,mooring system static analysis theory is the theoretical basis.A mooring system static calculation program is developed based on the multi-component catenary theory and static calculation principle.Accuracy of the program is verified by the examples results.The effects of some mooring line parameters such as wet weight per unit length/axial stiffness/length on the system static characteristics are studied.At the same time,the mooring system dynamic analysis theory is briefly discussed,in order to make theoretical basis for time domain coupled dynamic response analysis in subsequent chapters.Secondly,taking deepwater FPSO and its mooring systems for example,frequency domain hydrodynamic analysis and time domain coupled analysis are made.Based on frequency domain analysis theory,some FPSO linear wave hydrodynamic parameters such as RAOs,wave loads,additional quality factors,additional damping factors are calculated.Based on time domain coupled calculation theory,FPSO and its mooring systems coupled dynamic responses are calculated.Considering regular wave or irregular wave condition,135 degree or 180 degree loads direction,catenary or tension mooring mode,FPSO sway,surge,yaw and other motion responses and each mooring line force are respectively calculated.Thirdly,based on static characteristics similar criteria,ensuring truncated system equivalent as full depth system in total restoring force characteristics and typical mooring line top tension-displacement charateristics,the equivalent truncated program is developed by utilizing genetic optimization algorithm and MATLAB.Equivalent truncated design of both deepwater catenary mooring systems and tension mooring systems can be achieved.Taking deepwater FPSO mooring systems for example,firstly,aquivalent truncated designs of 1500 m depth catenary mooring system are made by only the middle rope truncated or both the bottom chain and the middile rope truncated.Secondly,truncated designs of 1500 m and 3000 m depth tension mooring systems are made by only the middle rope truncated.Lastly,the static characteristic differences between full depth system and truncation system are contrasted.The results show that optimized errors are all within the reasonable range,the designed truncated systems meet the requirements,and the equivalent truncated design program is reliable.Lastly,truncated systems time domain coupled analysis is respectively calculated under the same environmental conditions as the full depth system.The time domain variation differences of FPSO sway,surge,yaw and other motion responses and the representative mooring line top tension between the full depth system and truncated systems are respectively compared.The comparison charts display that there are some differences between the two systems,but the differences are not great.Then based on the existing model parameters and test data,460 m depth truncated system equivalent as 914 m full-depth system is designed.The differences of full-depth system model test results and numerical results and truncated system numerical results are contrasted,in order to check the results accuracy calculated by the numerical calculation software and the reliability of designed truncated system.The results show static charateristics are in good agreement,but dynamic charateristics have some differences.So numerical reconstruction and numerical extrapolation are still needed.In summary,an equivalent truncated design program is developed and verified,the static and dynamic charateristics of full depth system and truncated system are compared,and results accuracy is tested and verified by model test.The reliability of the developed programs is ensured and some references are provided for the study of truncated design.