Study On The Effect Of Residual Stress On Fatigue Strength Of Typical Welded Joints Of FPSO

FPSO bears constantly alternating load caused by waves and other load for a long timeduring service, and the welded joints prone to fatigue damage. FPSO is mostly built with highstrength steel, and it is more sensitive on the welding residual stress. Residual stress will bereleased by external loads, so that the hot spot stress changes, which ultimately affects the fatiguelife of the welded joints. Therefore, the analysis of the effects of the welding residual stress and itsrelease on the fatigue strength for the typical welded joints of FPSO is particularly important.In this paper, the three-cargo finite element model of a FPSO is established based on the JTPrules firstly, and hot spot stress and fatigue life of typical welded joints are calculated; Then theinitial residual stress and released residual stress of the typical welded joints are studied by finiteelement method and experiment method; Finally, the influence of the residual stress on fatigue lifeof the typical welded joints is studied. The main research work and important conclusions arelisted as follows:(1) The overall strength of a three-cargo finite element model of a FPSO is calculated basedon the JTP rules firstly, and the FPSO typical joints location are determined based on the results:the connection of the bilge and the bottom of the FPSO (typical joint1), and the connection of theribs and the longitudinal bulkhead bracket toe(typical joint2). Then the FEM of fine grid isestablished for local stress analysis. Finally, fatigue lives of the two typical joints are calculated,and the results meet the life requirement of the owner for50years.(2) The simulation of the welding residual stress for typical welded joint2is done by thefinite element method with APDL of ANSYS, and the results show that the residual stress at theweld toe area is larger and it is tensile stress. Also the effects of the geometry and processparameters on the initial residual stress of the typical welded joints are studied, and the resultsshow that the maximum welding residual stress increases with the increase of the thickness ofwelding panels and welding maximum temperature, and it also increases with the decrease of webthickness.(3) The release of the residual stress is simulated by the finite element method after the initialresidual stress being calculated. The effects of the external load on the release of the residualstress is studied. It shows that the value of the released residual stress becomes larger with theexternal load increasing. The release of the residual stress is not affected by the number of cyclesof the external load, and it is only related to the amplitude of the external load. (4) The initial residual stress and the released residual stress are measured with X-raynondestructive detection, and it show that the simulated values and the measured values areconsistent, which proves the reliability of the finite element simulation of the welding initialresidual stress and the release of residual stress.(5) The effects of welding residual stress and its release on the fatigue strength for the typicalwelded joints of the FPSO is studied based on the S-N curve method. The study shows that thewelding residual stress will decrease fatigue strength of the typical welded joints. As the preloadon the typical welded joints is greater, then the released residual stress is larger and the fatigue lifeis longer.