Probabilistic fracture mechanics analysis for optimization of sub-sea gas pipelines inspection

The dynamic nature of marine environment is the major cause of fatigue failures in subsea gas pipeline structures. Since the severest loaded part of piping system is the free span, freespan inspection is performed periodically to ensure that no free span exceeds its critical length. The objective of this paper is to optimize free-span inspection interval by means of probabilistic fracture mechanics analysis. Simulation data is taken from previous work of Tronskar [1]. Stress intensity factors at the crack tip are calculated by crack closure technique. Fatigue crack growth is simulated by cycle-by-cycle integration technique. The fracture mechanics analysis is then expanded to probabilistic analysis to include stochastic input parameters. Probability of failure is computed by modified direct simulation method. Based on the result of direct simulation, the studied pipelines are recommended to be inspected every 3 years to make sure that no free span exceeds 30 m.