At present, the complexities of distributive characteristics in temporal and spatial domain of ocean environmental loading contribute to the difficulties of the fatigue life estimations of marine structures.
In shallow water, soft yoke mooring system (SYMS) is considered to be the best mooring system, and has been widely used in oil development in the Bohai Bay and the Gulf of Mexico. Soft yoke mooring system establishes the mooring functions via the multi-dynamic mechanism of thirteen hinge joints. The accuracy of fatigue life of the hinge joints is important to ensure the safety of mooring system. The damage failure of hinge joints would cause great financial loss.
In 2012, Dalian University of Technology set up a full coupled proto-type monitoring system which consisted of the four sub-monitoring systems, that is, ocean environmental parameters sub-system including wind, current and wave factor, motions and attitudes of the FPSO including six degree freedom of vessel motions, motions and the mooring force monitoring system of the mooring leg. The massive monitoring information is obtained by the integrated software with continuous. The present paper proposes a real-time fatigue life prediction method of upper hinge joint of SYMS based on the prototype monitoring technique.
The friction parameter of hinge joints contact surface is increased in long-term service and reduced by adding lubricant. In the SYMS design phase, there is no effective analysis of the repeated friction parameter changes. The variations of friction coefficients caused by long-term cycle stress and maintenance are considered in the fatigue calculation. The stress distribution of hinge joints under design parameter is carried out by using ABAQUS. Through calculation and comparison, the equivalent stress and fatigue damage variable of KPA (Key Process Area, large deformation units and easy wear area) units in the condition of the friction coefficient is 0.15 (design parameter) and 0.95. We found that the friction coefficient change due to long-term service will speed up the fatigue failure of the hinge joints.
The relationship between friction coefficients and KPA regional stress of mooring legs swinging angle are established through the finite element simulation. Through prototype monitoring software analysis the marine environment loading, structural response and KPA regional stress information, the abrasion of the hinge node and fatigue damage variable Dθμ can be real-time predicted.
The present fatigue life analysis method based on monitoring technique exhibits good advantages and research value for the fatigue life estimation of offshore structure subject to wave induced motions.