DNV GL has launched a recommended practice providing the first international guidance on how to establish a sound basis for in-service inspection planning for fatigue cracks in marine structures.
The low oil price and a reduction in newbuilds have driven the integrity of existing assets up the agenda for the oil & gas industry. However, the cost of inspection programmes can be significant, adding to the operational costs. The guidance will save operational costs by using a risk-based approach.
Reducing the Overall Inspection Scope
The cost reduction is obtained by reducing the overall inspection and maintenance scope, still ensuring continued cost-efficient safe operation of the asset. This is done by using risk based methods in identifying the critical areas, so-called "hot spots", that need to be inspected and non-critical areas that do not need further assessment.
Fatigue Inspection
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:
- Fatigue analysis methods for jacket structures, semi-submersibles and floating production vessels (FPSOs)
- Effect of methodology/refinement used in fatigue analysis with respect to calculated fatigue life
- Basic distributions of parameters required for calculation of stochastic properties for load effects and capacity
- Derivation of target reliability level in relation to consequence of a fatigue failure
- Methodology for probabilistic analyses for planning inspection for fatigue cracks
DNVGL-RP-C210 is based on the findings of a DNV GL led joint industry project (JIP) completed in 2013.
Picture: Survey of a brace system (by DNV GL).
