Fatigue and Fracture Degradation Inspection of Offshore Structures and Mechanical Items: The State of the Art

A significant number of offshore structures and mechanical items installed in production systems on the Norwegian Continental Shelf (NCS) are either approaching or have exceeded their intended design life. However, with the help of the advancement of technology and analysis approaches, most of the offshore production facilities are being considered for life extension. This requires regular inspection, fitness for service (FFS) assessment, remnant life assessment, maintenance and repair (or modification). In this context, fatigue and fracture related degradation play a vital role. Hence, this paper discusses the state of the art as well as two major methodologies used for fatigue life prediction of structures and mechanical items. The first (S-N approach) is based on experimentally derived S-N curves and linear damage rule (LDR). Since LDR does not take sequence effect of loading into account the S-N approach often leads to overestimation / underestimation of fatigue life. Hence, this paper also takes into simultaneous consideration the second approach, which relies on the principles of fracture mechanics (FM) and crack growth analysis. Furthermore, the paper discusses damage tolerance analysis (DTA) and the role of Risk Based Inspection (RBI) to detect cracks before they grow to a critical level and cause catastrophic failure of the component. Thereafter, the paper discusses the reliability of Non-Destructive Evaluation (NDE) methods quantified in terms of Probability of Detection (PoD), to identify the flaw size and location. Finally, probabilistic crack growth (PCG) models used for remaining useful life estimation (RULE) and for planning inspection regimes of structural and mechanical items are discussed briefly.