Chapter 24. Reliability-Based Inspection Intervals of Offshore Jacket Fleets
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Published:2022
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The kernel underwater inspection interval of offshore steel piled jackets in API 2SIM is 5 years. Thousands of steel piled jackets worldwide use this value to set Level II underwater inspection frequencies. However, the genesis of the 5 year interval is uncertain. It could be either based on the damage found in early platform inspections or it could have more logically followed from the ship hull Drydocking survey practice. It is worth noting, however, that the later does not directly apply to jackets which are completely different structural systems. In any case, the uncertainty in the 5 year origin, plus the lack of anomalies in most underwater inspections beg the question: are we over inspecting offshore platforms? To answer this question, we commenced a quest to establish an inspection interval methodology from fundamentals of jacket response to environmental loading and resulting reliability. A reliability-based inspection interval (RLII) methodology was born. It is based on the use of platform annual reliability degradation due to corrosion, fatigue, and potential mechanical damage. First, nonlinear pushovers of platform at onset and as expected at a future point is determined; the future model captures severed joints based on fatigue analysis and any expected corrosion damage. The lateral capacities are then mapped to a platform base shear hazard curve to estimate the corresponding probability of failure. The difference in failure probability at onset and in the future, factored by platform life, determine annual reliability degradation. With probability of failure mapped to a Normal distribution, a minimum target reliability index and a corresponding factor of safety selected, a reliability-based inspection interval can then be established. Furthermore, a cap on inspection intervals can be recommended to cover general uncertainty in finding platform damage including corrosion. In this paper, we summarize and publicize the novel RLII approach, and then demonstrate its application to example jackets. This methodology has been successfully used internally to establish inspection intervals for ExxonMobil fleets of steel piled jackets.