DNV GL Guidance on Inspection Planning of Offshore Structures
The low oil price and a reduction in new-builds have driven the integrity of existing assets up the agenda for the oil and gas industry. However, the cost of inspection programmes can be significant, adding to the operational costs. DNV GL has now launched a recommended practice (DNVGL-RP-C210) providing the first international guidance on how to establish a sound basis for in-service inspection planning for fatigue cracks in marine structures. The guidance will save operational costs by using a risk-based approach to identify the hot spots requiring inspection.
The cost of inspections of marine structures such as jackets, semi-submersibles and floating production vessels, can put a strain on already challenged operational budgets, driving the need to develop inspection programs only addressing critical areas. The cost reduction is obtained by diminishing the overall inspection and maintenance scope while ensuring continued cost-efficient safe operation of the asset. This is done by using risk-based methods to identify critical areas (hot spots) that need to be inspected and non-critical areas that do not need further assessment. In-service inspection for fatigue is carried out to assure that possible cracks in the structure do not exceed a critical level. The outcome of in-service inspections can then be used to update the estimated fatigue reliability of the structure.
The RP includes guidance on:
- the fatigue analysis methods for jacket structures, semi-submersibles and FPSOs,
- the effect of methodology/refinement used in fatigue analysis with respect to calculated fatigue life,
- the basic distributions of parameters required for calculation of stochastic properties for load effects and capacity,
- the derivation of target reliability level in relation to consequence of a fatigue failure, and
- the methodology for probabilistic analyses for planning inspection for fatigue cracks.