Experimental investigation of CTOD fracture toughness in FCAW-GS welds of offshore jacket structure

Offshore jackets are essential structures in the energy sector, including wind farms and oil and gas activities. The experiment involves welding a 72 mm K-bevel S355 KT-40 plate utilizing gas-shielded flux-cored arc welding (FCAW-GS) in a horizontal position. Welds are evaluated using microhardness, tensile, Charpy impact toughness at -40°C, crack tip opening displacement (CTOD) at 0°C, and microscopy, subsequently accompanied by statistical analysis and numerical modelling. The highest microhardness is 246 Hv5, meeting the maximum requisite of 325 Hv5, while the yield stress and tensile strength of the welds exceed those of the base metal, measured at 565 and 602 MPa, respectively. The minimum Charpy single value is 73.9 J, and the average value is 108 J, both exceeding the 40 J industry norm. Weld metals (WMs) at the weld centerline and the grain-coarsened heat-affected zones (GCHAZs) have minimal CTOD values of 0.28 mm and 0.36 mm, respectively, surpassing the requisite threshold of 0.15 mm. The microstructure of the GCHAZs has a grain size between 31.8 and 63.5 µm, hence improving corrosion resistance. A one-way ANOVA of the Charpy and microhardness has significant differences corresponding to measurement regions. Mesh and contour plots demonstrate a robust positive correlation among CTOD, load, and displacement. The quadratic equation modelling fracture lengths demonstrates a significant correlation coefficient of 74.7%, producing a parabola with its vertex at x, y (35.435385, 40.2809) mm, which defines optimal crack length. The welds surpass AWS D1.1 criteria, enhancing performance and prolonging structural integrity.