Ultimate strength and fatigue of stiffened welded tubular joints in floating energy production structures

•Monotonic and cyclic mechanical experiments on stiffened welded tubular joints.•Effect of HFMI post-weld treatment on the structural performance of the joints.•Fractographic examination of two fractured specimens.•Identification of fatigue crack initiation locations and fracture path.•Static strength of fatigue-cracked welded joint. Monotonic and cyclic loading experiments are performed on 1/6-scale stiffened steel joints representing the welded connection between the tubular braces and the vertical cylinders of a prototype offshore floating structure for combined wind-wave energy production. The structural behavior of the welded joints is investigated to determine their ultimate strength under monotonic loading and their fatigue life. The brace-to-cylinder welded connection is full penetration and semi-automatic, whereas the stiffeners are welded to the inner surface of the cylinder with fillet welds. High-frequency mechanical impact post-weld treatment is applied to five specimens. Experimental observations indicate two critical locations for fatigue crack initiation: the crown of the brace-to-cylinder weld and the fillet weld of the central stiffener. These locations are also verified by finite element simulations and fractographic examination of failed specimens. Post-weld treatment improves the fatigue resistance of the welded joints in the high-cycle fatigue domain but has negligible influence on the low-cycle fatigue response of the connection and its monotonic ultimate strength. Finally, one fatigue-cracked specimen is tested under monotonic loading exhibiting structural performance similar to the intact specimens.