Probabilistic inspection planning of fixed offshore structures

The aim of inspections is to reveal time dependent deterioration and damages. The focus herein is on fatigue crack damages in North Sea structures. A Probabilistic Inspection Analysis tool, PIA is briefly presented. PIA is based on a probabilistic fracture mechanics model of fatigue crack growth as well as a simplified systems reliability approach. PLA provides information about where and when to perform inspections to detect fatigue crack growth and thereby initiate necessary repair actions. Data from more than 4000 NDT inspections of jackets are used to calibrate the method. For main structural and horizontal framing components the theory overpredicts the number of propagating cracks by a factor of approximately 3 while it predicts 70 to 80 percent of the propagating cracks observed. The use of PIA has generally given a 70 to 80 per cent reduction of the inspection programs. Improved fatigue analysis and use of PIA has also increased the number of fatigue cracks that are detected by a factor of 3 to 4.