High-Cycle and Low-Cycle Fatigue Resistance of Welded T-Joint Plate Connections Made of Mild and High-Strength Steel for Offshore Energy Applications

Abstract The present paper examines the fatigue performance of sixteen welded plate T-joint specimens, commonly used in offshore steel platforms. The research work is part of an extensive national research program that aims at improving the structural performance of offshore floating platforms installed in deep-water locations. It extends the work performed by the research team in previous European research projects for developing a hybrid tension-leg-platform, suitable for combined offshore wind and wave energy exploitation. The application of high-frequency mechanical impact post-weld treatment and the use of high-strength steel are considered. Full penetration welded T-joint plate specimens are tested, representing the critical locations in brace-to-cylinder connections of the prototype offshore floating structure. The specimens are made of mild steel S355 and high-strength S700 grade steel, and are subjected to four-point bending cyclic loading in high- and low-cycle regimes. Two different welding processes for the connection have been employed: (a) manual (semi-automatic) weld without post-weld treatment; (b) manual weld with high-frequency mechanical impact post-weld treatment. The experiments are supported by fractographic examination and a preliminary finite element analysis.