Acceptable fatigue crack occurrence rate

摘要

In fatigue management, it is essential to know the fatigue crack growth potential. The lessons learned from use of refined fatigue analyses, fracture mechanics and probabilistic methods for platforms in-service are presented. For ageing offshore units of semi-submersible design, the inspection history of more than 20 000 NDT inspections and detection of close to 1000 fatigue cracks, are used in this study. These experience data are used to assess the fatigue failure potential. The fatigue crack growth capacities are influenced by the extent of gross errors in the design, construction and operation phase in addition to the normal uncertainties of design, construction and inspection. Corrosion in combination with not detected weakening of the capacity against fatigue crack growth is one of the main challenges. Assessments based on collected in-service observations are mandatory.