The effect of shot peening on fatigue life of welded tubular joint in offshore structure

An offshore structure is exposed to environmental loading and cyclic stress variations throughout its life. This would result in the possibility of fatigue cracking in its members. Cracks in these structures usually initiate around welded areas, then grow progressively from the heat-affected zone to base metal through the thickness of the tubular members. Therefore, fatigue life improvement of offshore structures is of great importance. In this paper, structural analysis of an offshore platform is carried out and the critical joint is specified. The welding process is simulated and the residual stresses due to the welding are determined. Experimental residual stress measurement is carried out and is compared with the simulation result. The simulation results show that the heat affected zone (HAZ) is formed in whole thickness of the brace at weld toe location and approximately half thickness of the chord. The fatigue life of the welded tubular X-joints of offshore structure is investigated experimentally. Furthermore, some joints are shot peened and the effect of shot peening on fatigue life of these joints is explored. The results of experiment show that the fatigue life of shot peened tubular joint is about twice of the original tubular joint. Cracks are initiated in all cases at weld toe. Also a crack on chord branch of joints under fatigue test is observed.

► The critical joint of an offshore platform is specified by structural analysis.
► The welding of the joint is simulated.
► HAZ region was formed in whole thickness of the brace at weld toe
► Fatigue life of shot peened tubular joint is about twice of the original tubular joint.
► Cracks are initiated in all cases at weld toe.