FRACTURE MECHANICS ANALYSIS OF A HIGH STRENGTH STEEL FOR OFFSHORE APPLICATION

摘要

One of the main bearing elements of the Siri jack-up is three tubular legs; outside diameter 3.5m and an overall length of 104m. The main material grade is high strength steel with SMYS ≥ 690MPa. This strength was required due to contact forces during the jacking phase. One of the major issues concerning integrity of the platform was associated with the fatigue life of the legs. The fatigue life of the legs WAS to be 60 years for inaccessible locations, 40 years for accessible submerged locations and 20 years for accessible dry locations. To achieve the required fatigue lives, wall thickness of 65mm, 75mm and 90mm were utilized for the 690 steel. Furthermore, to meet the 40-year requirement, lives for the lower circumferential welds were designed to Class C, i.e. they needed to be improved by grinding. Based on performed research, it was aimed at minimum CTOD values of 0.20mm and 0.25mm in the PWHT and AW conditions respectively. Although the aim was achieved for the base material and sub critical HAZ, the weld metal and the coarse grained HAZ values were significantly below these values, i.e. 0.12mm and 0.02mm respectively. In order to verify the integrity of the platform a fracture mechanics analysis was performed. The assessment comprised evaluation of critical defect sizes and fatigue life calculations using PD6493. Wide plate testing, residual stress measurements and fatigue testing have been performed. The assessment showed that the integrity of the platform was maintained. In order to obtain additional safety an inspection program for the critical welds has been implemented in the operational phase.