Fatigue behaviour of high-strength steel-welded joints in offshore and marine systems (FATHOMS)

Recently the technological evolution in structural steels, particularly in the area of offshore and marine applications, points towards an increasing use of high-strength weldable steels with the aim of guaranteeing high-quality/high-strength welded joints for lightweight structures, reducing both weight and cost. The need to develop specific knowledge on the fatigue behaviour of high-grade steel-welded joints, considering the consistent improvement in mechanical properties of consumables and in welding technologies, is becoming a more crucial issue. The FATHOMS project was aimed at improving the knowledge on this matter, contributing to potential upgrades of the design criteria. The activity carried out highlights the conservativeness of the current design standards in force in the offshore area in predicting the fatigue performances of welded joints when high-steel grades, together with improved and innovative welding techniques, are used, in particular for the girth welds on pipes made from one side usually penalised by the codes. The conservativeness of the standards was highlighted both in terms of S-N design curves and in terms of limit values for the accumulated fatigue damage in conditions similar to the in-service ones. The beneficial effect in fatigue performances of the use of high-strength steels, new welding technologies and post-welding treatments was outlined. In particular promising behaviour was shown in terms of fatigue strength of laser hybrid welding technology when applied in the root pass in single-side welds, which allows fatigue performances to be reached that are comparable with double-side good quality welds.