Fatigue Life Analysis Of Joint Structure Of Soft Yoke Mooring System Based On Physical Experiments

FPSO(Floating Production Shortage and Offloading)is widely used in the Bohai Bay and the South China Sea.Without its own dynamical system,the FPSO needs to use the mooring system to realize the offshore positioning of the platform to guarantee the safety of operation.As a kind of single-point mooring system,Soft Yoke Mooring System(SYMS)is widely applied to the oil and gas exploitation in Bohai.The soft yoke mooring system consists of a mooring jacket,a YOKE(mooring rigid arm and ballast tank),a mooring leg and a mooring bracket.The parts are connected by a rotary joint and a slewing ring.The ballast fluid in the carrier provides a mooring recovery force that limits the oil and gas development platform to a relatively fixed area.The soft yoke mooring system composed of multiple joints is a typical multi-body dynamic system.With the increase of the service life of FPSO,the joint structure of the soft yoke mooring system is prone to fatigue damage caused by complex and difficult sea conditions.In this paper,the long-term in-service behavior of the soft yoke mooring system joint structure subjected to the marine environmental load is studied.A fatigue life assessment method based on model experiment and on-site monitoring is proposed.Firstly,the structural form of the hinge joint was analyzed.Considering the gap between the joints,the motion constraint model with the gap joint structure was established.The contact stress distribution of the joint structure was analyzed by elastic mechanics,and the maximum contact stress was calculated.At the same time,the frictional shear stress of the joint structure was analyzed considering the friction behavior between the joints.The frictional shear stress and the maximum contact stress were used to synthesize the contact area stress of the joint structure.The maximum principal stress of the plane stress state of the joint point of the hinge structure was taken as the hot spot stress of the contact area.Aiming at the difficulty of on-site measurement of the friction coefficient of the joint structure,the physical experiment of the joint structure reduction model was carried out.The normal contact force model and the modified Coulomb friction model suitable for the hinge structure were determined,and the contact characteristics under reciprocating motion were analyzed.According to the experimental results,the friction coefficient of the joint structure was calculated.Furthermore,the on-site monitoring scheme of the joint structure was designed.The joint structure on-site monitoring system was established in a FPSO in Bohai Bay.The joint structure swing angle,slot displacement,stress and strain information were monitored in real time,and the mooring leg axial force was calculated by the strain data of the axial test point.The friction coefficient and the on-site monitoring strain data were measured by physical experiments,and the hot spot stress in the contact area of the platform joint structure was calculated.The rain cycle counting method was used to extract the stress cycle(35)?_i-n_i,and the fatigue fracture limit state equation was substituted to evaluate the fatigue life of the joint structure.The results showed that the fatigue life of the joint structure was lower than the design index,and the long-term service state of the joint structure should be closely monitored,and preventive maintenance should be strengthened to ensure the safe production operation of the offshore platform.