PSI - Issue 57

Arne Fjeldstad et al. / Procedia Structural Integrity 57 (2024) 692–700 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Fatigue is an important design criterion for offshore structures in harsh waters. Large uncertainties are generally associated with fatigue analyses of offshore structures. Fatigue design is normally based on S-N data established primarily by constant amplitude testing. In-service inspection to detect fatigue cracks is normally performed to assure that potential cracks in the structure do not exceed a critical size. The reliability of NDT inspection is described by the ability to detect an existing crack as a function of the crack size and by the uncertainty associated with the sizing of an identified crack. Regardless of the inspection outcome (detection or no detection of a defect), each inspection provides additional information beyond that available during the design stage. Thus, this information can be utilised to update the estimated fatigue reliability. For the S-N fatigue approach, the inspection results cannot be used directly to update the estimated fatigue reliability, as no direct relationship exists between the crack size and the damage accumulation in the S-N approach. A calibration of the fracture mechanics fatigue approach to the S-N approach is therefore required to update the fatigue reliability. The basis for this methodology, denoted Risk Based Inspection (RBI) goes back to the early eighties when DNV performed a joint industry project on cost optimal inspection and maintenance of offshore structures, Sletten et al. (1982). The concept of probability of detection curves was introduced together with crack growth analysis based on fracture mechanics. The effect of inspection was included in reliability analysis using a Bayesian updating approach, Itagaki et al. (1983). Updating of fatigue failure probabilities based on inspection events was presented by Madsen (1985), and this methodology has later been used for evaluation of inspection of fatigue cracks; see e.g. Madsen et al. (1987). Later, in the period 2011-2013 DNV performed a joint industry project on use of probabilistic methods for planning of inspection for fatigue cracks in offshore structures. This work included assessment of this method for jacket structures, semisubmersibles, and floating production ships (FPSO’s) , Lotsberg et al. (2016). The recommendations from this project are now included in DNV-RP-C210 (2019). This paper gives an introduction to the Risk Based Inspection (RBI) methodology as described in DNV-RP-C210 and then presents how the methodology is applied to establish inspection plans for FPSO’s. Unlike sailing ships where maintenance and assessment are based on periodical dry- docking and reclassification every 5 years, FPSO’s are often designed for operation at a single location throughout its entire lifetime. Consequently, it is important to have control of the integrity of the structures such that continued operation can be ensured. There is a vast number of structural details in an FPSO that may be prone to fatigue crack initiation. Many of these are located inside storage and ballast tanks where many are difficult to access. Entering such tanks also have implications on the operation of the FPSO as e.g. cargo tanks must be cleaned and gas freed prior to inspection. It is crucial to establish inspection plans that not only maintain structural integrity but also consider operational aspects related to inspection campaigns. The most practical way to meet these needs is to organize inspections in campaigns where tanks are systematically inspected in regular intervals. This paper presents the philosophy behind the assessment of such inspection plans. DNV has previously published numerous articles on the use of RBI for assessing inspection plans for offshore structures. Some of these articles include Lotsberg et al (1999), Landet et al (2000), Sigurdsson et al (2000), Sigurdsson et al (2004), Landet et al (2011), Lotsberg and Sigurdsson (2014), and Lotsberg et al. (2016).

Nomenclature CVI

Close Visual Inspection

DFF Design Fatigue Factor FPSO Floating Production Storage and Offloading unit GVI General Visual Inspection NDT Non-Destructive Testing POD Probability of Detection RBI Risk Based Inspection