Fatigue Performance of Friction Welded Studs

Early in the life of the Genesis spar, cracking developed at the welded connections between the riser guide supports and the hull wall plate. The cracking was caused by the movements of the top-tensioned risers within the steel guide frames in the moon pool of the structure. The remedial action taken to minimize the riser movements and its effects on the hull involved the use of novel rubber bumpers, which were installed in lieu of the steel guides. The bumpers around the periphery of the moon pool were fastened to the hull wall via threaded studs that were friction welded to the hull wall plate underwater. This paper describes a testing program specifically designed to qualify the fatigue performance of the stud-plate friction welds. Results verify the use of the F2 S-N curve from British Standard 7608 with a single slope for the design of the friction-welded connections subjected to axial load. It was also found that the fatigue performance of friction welds is sensitive to the stud preload. One unique feature of the fatigue failure mode of the connection, when the load is transferred through the stud into the plate, is that cracking takes place along the semi-circular heat-affected zone (HAZ) of the bond-line between the stud and the plate, and not through the hull plate thickness. As a result, failure of a stud connection does not compromise the structural integrity of the spar hull.