Study On The Composite Mooring Positioning System Of Ultra Deepwater Semi-submersible Platform

Nowadays,the oil and gas on the land is exhausting day by day with the increasement of exploitation quantity.Many contries have to turn their attention about the exploration and production of oil and gas to ocean and even deep sea.In order to adapt the harsh condition of deep sea,various floating platforms,especially semi-submersible platforms,have been developed and widely used in actual production.However,the ultra-deep water archoring is one of the most important technology problems during the operation of semi-submersible platforms.In this paper,with the help of the Det Norske Veritas SESAM software,the composite anchoring position system of ultra-deep-water semi-submersible platforms was investigated.First of all,semi-submersible platform model was established using Genie module of SESAM,and then the dynamic response of semi-submersible platform was analysised using SESAM/Hydrod modules,with the first order value of moving response transfer function(RAO)being calculated under different frequencies and different wave direction.Results show that the sensitive period of wave for surging and swaying of platforms is about 10s-35 s,and the maximum response amplitude is less than 1.0.Both surging and swaying values are less than 0.1,which basically meets the requirements of safety in production.Secondly,based on the analysis of dynamic characteristics,the hydrodynamic performance of semi-submersible platforms and their composite archoring system were analysed using SESAM/DeepC under the condition of marine wind and current flow environment.Results show that the maxmum values of surging,swaying and heaving of the composite anchoring system are all less than 3% of the water depth(3000 m),which meet the requirements of the safety in production.Thirdly,the fatigue damage of semi-submersible platform under random wave was analysed.The stress amplitude of stress time history curve from transient analysis has been counted using rain flow statistic method,referencing the theory of miner linear cumulative,using T type S-N curve of node in air and seawater with cathodic protection,which is recommend by specification.The fatigue damage of each representative points has been calculated under random wave environment,and the fatigue damage and fatigue life of each representative points were obtained by weighting method.Results show that the fatigue life of all representative points are greater than 200,and each one of the nodes can satisfy the requirements of the fatigue life.Finally,this paper designed a drift compensation system based on the semi-submersible platform.It can compensate the displacement in the horizontal plane of platforms caused by environmental factors.Meanwhile,the working principle of the compensation system was presented,and the functions of the various components in the system was described as well.It provides a new way to effectively control the motion response of semi-submersible platforms.