| With the development of oil and gas exploitation to the deep sea,semi-submersible platform has gradually become the main force of deep-sea floating equipment,whose main components are deck box,double buoyancy tank,column and brace.When a semisubmersible platform is working in deep water,the mooring system is used to locate the platform.Due to the uncertainty of the Marine environment,typhoon,rapid current and huge waves will threaten the safety of the platform.The semi-submersible platform is attacked by huge waves under self-storage conditions,negative air gap phenomenon is likely to occur,which will lead to the platform brace,column,bottom of deck box and other positions being hit by wave,which will damage the structure and threaten the safety of operators.Therefore,negative air gap response and the impact of wave slam should be considered in the platform design stage under adverse sea conditions.In this paper,based on the minus air gap phenomenon that is prone to occur on semisubmersible platforms,the research status of air gap response,wave slamming,coupled motion between platform and mooring system of semi-submersible platforms at home and abroad is summarized.Considering the influence of wind and wave flow,the hydrodynamic analysis of the semi-submersible platform with eight-point symmetric mooring under selfstorage condition is carried out by using the time-domain full-coupling analysis method,and the air gap value of the lower deck concern is calculated to study the impact of the wave pounding load on the structure strength of the semi-submersible platform.The air gap and wave slam distribution characteristics of the platform were studied by using CFD method.Finally,the accuracy of the method is verified by model test.The main research contents include:(1)Aiming at a typical two-floating and four-column semi-submersible platform,a set of eight-point symmetric tensioning mooring scheme is designed.Based on potential flow theory,using finite element analysis method,the finite element model is established,combining with the characteristics of the south China sea environmental conditions,air gap distribution characteristics and the motion response of the mooring cable tension is analyzed,and it is concluded that the air gap distribution,in order to further study platform minus air gap phenomenon and wave slamming problem.At the same time,the modified model is used to check the yield strength of the brace under wave load,and it is concluded that the wave crash load has a great influence on the structural strength of the brace.Therefore,the wave crash load of the brace and other slender structures should be taken into account in the design of semi-submersible offshore platforms.(2)In order to study the local negative air gap and wave slamming load of the platform under severe sea conditions more accurately,the overlapping grid technique and the fluid volume function(VOF)method are used to carry out numerical simulation of the movement of the semi-submersible platform under regular wave,and the sensitivity of the platform wave slamming is studied by changing the wave direction angle,velocity,period and high wave parameters under the full coupling effect of the platform and the anchorage system.The pressure of the platform column and deck box subjected to wave slamming was calculated,and the distribution law and load characteristics were analyzed and compared with the air gap distribution obtained above.According to the research results,the wave thump is closely related to the period,wave height,velocity and wave incident Angle,and the thump pressure is mainly distributed under and on the top of the column and on the side of the wave face at the junction of the bottom deck and the column.This result provides a reference for the structural areas that need to be strengthened in the design of semi-submersible platforms.(3)By using wave superposition method,20 waves are superposed in STARCCM+ to generate abnormal waves with prominent non-linear features at the specified position.The bump pressure peak of semi-submersible platform under the influence of abnormal waves is calculated and its distribution characteristics are analyzed.The results show that the downstream side of the bottom of the deck box has a strong wave slamming phenomenon and negative air gap phenomenon,and the bottom of the column and the pontoon also bear a large wave slamming load.The results provide a theoretical basis for the design of semisubmersible platform.(4)Model test was carried out in the pool.According to the similarity criteria of froude and strouhal,platform model and anchor system with a scale ratio of 1:100 was designed.Free attenuation experiments measured platform of longitudinal and transverse wave attenuation cycle,by changing the parameters such as wave height,period,wave incident Angle to carry on the mooring conditions of wave slamming load test platform,through the pressure sensor wave slamming pressure have been measured,and compared with the results of numerical simulation analysis to verify the accuracy and feasibility of the numerical method. |
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