Coupled Analysis Of Offloading System And Study On The Model Test Method

High speed development of the world economy has becoming more and more dependent on petroleum and other nature resources,and oil has become the key factor which has great influence on national economic security and sustainable development.As the biggest developing country,also as one of the most rapidly developing countries,our country has large demand for energy,but a great part of the energy depend on imports.Therefore,the exploitation of offshore oil resources,especially deep-sea resources is the key point to support the future development of our country,besides,independent exploration and active participation in foreign offshore oil and gas exploration is one of country major economic strategies too.West Africa is one of three hotspots for offshore oil and gas resources development in the world,while as one kind of development mode that has been worldwide recognized,the offloading system has also been applied in West Africa seas.In allusion to the problems that exist in researches of offloading system,this paper takes the offloading system that serves in West Africa Sea for offshore oil and gas development as research subject,and carries out research about offloading system model test method.By using combination of numerical simulation method and model test method,some analysis about the response of offloading system under the action of typical West Africa Wind Sea and swell has also been done,and the couled effect between structures in offloading system was analyzed.Research about offloading system model test method carried out firstly.In allusion to the problem that the oversize of OOL horizontal span in offloading system model test,the horizontal truncation method of large span OOL were presented,and the truncation method was validated by specific cases.The sources of tension difference between mooring systems that before and after the vertical truancation was analyzed,base on this,the truncation method and character adjustment method were provided.Besides,in allusion to the problem that CALM buoy physical models is too light while the tanker model is too heavy,model character adjustment methods that applied to small model and the large model were provided separately.Calculation methods of hydrodynamic load acting on CALM buoy were analyzed.Accurate calculation of hydrodynamic load is the basis to predict coupled motion of offloading system,while the difficulty is the simulation of viscous load of the buoy skirt.The disc elements group were designed to calculate the viscous load of CALM buoy skirt,and Morison element were adopted to simulate viscous force acting on buoy hull.Based on these,CALM buoy numerical models that base on the slender body theory and radiation/diffraction theory were set up,and a series of model tests were designed to verify these two models.Coulpled analysis of offloading buoy system serves in West Africa Sea.Base on the verification of calculation method of CALM hydrodynamic load,offloading coupled analysis model that contains OOL and mooring system were built up.By using the method of combining numerical simulation and model test,the coupled motion characteristics of CALM,mooring system and OOL system under the action of West Africa swell and wind sea were researched.The relationship between environmental conditions and the floating body motions were analyzed,and in order to see the coupling relationship of CALM buoy between mooringline and OOL,analysis about the mooring lines and OOL top tension characteristics were also done.Horizontal motion characteristics of single point mooring tanker were analyzed.First,a fixed-point mooring tanker model was established,and corresponding model tests were carried out to validate the numerical model.Motion characteristics of single point mooring tanker in the horizontal plane were analyzed under the action of wave condition.Based on this,model test method was used to analyze the impact of hawser length and stiffness on plane motion features of a single point mooring tanker.Analysis about coulpling effect of offloading system under West Africa environmental conditions was done.On the basis of validations of CALM buoy system model and single tanker model,the whole offloading system numerical model was established.The method that combines model test and numerical simulation was used to analyze the motion and mechanics characteristic of offloading system under the swell and wind sea environmental condition of West Africa.Besides,by means of the comparisons of CALM motion characteristics when it stays with and without the tanker,and the analysis of motion characteristics difference of the fixed-point mooring tanker and the buoy mooring tanker,the motion coupling effect between CALM and tanker was analyzed.OOL fatigue damage is one of the key issues in offloading system,and unique boundary conditions of OOL make its fatigue characteristics different from conventional riser.But there are few researches that aim at OOL fatigue problems.Still take offloading system which services in the West Africa as back ground,fatigue properties analysis about the full water depth OOLs of the Offloading System was made.The content includes the analysis of different contributions of waves in West Africa to OOL fatigue damage,the sensitivity analysis of hydrodynamic and how the characters of OOL itself affect the fatigue damage.The purpose is to get the factors that mainly lead to the fatigue damage of OOLs,and obtain the different effects of different parameters for the OOL fatigue results by these analyses,and provide some reference for the design of OOL.Results of offloading system coupled analysis show that CALM motion was the principal factor which leads to the high frequency tension and fatigue damage of OOL and mooringline,so further analysis about how changes of main scale impact CALM motion characteristics was made.First,the numerical simulation method was used to analyze the impact of changes in the main hull,and then influence of the changes in OD of CALM skirt and changes in the vertical position of the skirt on motion characteristics was studied by using model test method.The influence rule which structure change of CALM acting on its motion characteristics was obtained.