Results of the JaCo project: fatigue strength of robot-welded tubular joints for offshore wind energy converters

Abstract

Jacket foundations are lattice-like structures, whose assembly requires the welding of a large number of tubular joints. Such foundations type is suitable to support wind energy converters in deeper water with large turbine size. In order to increase the production speed and its quality, robot systems were developed to produce tubular joints. As fatigue is dominating the design of these structures, an assessment of the performance of tubular joints produced by four different robots was performed and compared with the performance of manually welded joints. In total, 18 large-scale tests were performed on joints with dimensions representative for offshore structures, which were produced in industrial environment. Almost all breakthrough cracks occurred through the chord, with cracks initiated at the weld toe, although in some cases cracks were also initiated and propagated between weld beads. Strain measurements have demonstrated that when multiple cracks are present in one specimen, they interact only in the last phase of the fatigue life, when they are so large that they affect the stiffness of the tubular components. The measured fatigue strengths of joints produced by robot were similar or higher than the T-curve of DNV-RP-C203. Two fabricators delivered components of which the fatigue strength was more than 20% higher than the standard curve. These results emphasize that mastering the welding process with robots is necessary to achieve superior levels of fatigue strength.