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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The uncertainties are represented in terms of statistical distributions. Not all distributions are well-defined, especially when one considers the tails of the distributions which may be important in some cases even if some guidance can be found in literature. Thus, some engineering assessment is needed for example for uncertainty for the load effect at hot spot when different analysis methodologies are used for different types of structural details and positions in the structure.

4.3. Probability of detecting fatigue cracks

The inspection quality is quantified by probability of detection (PoD) curves. POD curves are presented on the following form in DNV-RP-C210 (2019) ( ) = 1 − 1+( 1 0 ⁄ ) (3) is the crack depth and 0 and are distribution parameters. Parameters for the most frequently inspection methods are presented in DNV-RP-C210. 4.4. Target reliability level The recommended target reliability values depend on the failure consequence of the considered detail. This is normally assessed also at the design stage through a selected Design Fatigue Factor (DFF) for the considered hot spot. This requirement to DFF may also be used as basis for back calculation of required reliability during the in service life of the structure; see e.g. Moan et al. (1993). Target reliability levels for different DFF’s are presented in DNV-RP-C210. 5. Probabilistic inspection planning for FPSO’s An FPSO is considered to be a redundant structure without immediate consequence of a crack finding. Typically, most areas of the vessel are designed with a Design Fatigue Factor (DFF) of 2 for structural details that could lead to through thickness cracks, causing leakage if left undetected. Internal details are designed with a DFF of 1. However, there are special areas where higher DFF values, up to 10, are employed due to the high consequence of failure or challenging access for inspection and repair. In inspection planning projects for FPSOs, the first step is to gather all available fatigue analysis results from various analysis reports. These data are then processed, and an initial screening is conducted to identify structural details that may require inspection during service. Probabilistic fracture mechanics analyses are subsequently performed to assess the inspection plan. Fig. 4 provides an example of the results from a probabilistic fracture mechanics analysis for a butt weld connection with a calculated fatigue life of 30 years, according to DNV-RP-C210 (2019). With a target reliability of 0.01, it is seen that NDT inspections must be carried out twice within the vessel's 30-year service life. 5.1. Inspection plan for NDT based on probabilistic analysis (RBI)