DNV DNVGL-RP-C210 PDF

This RP is assumed to be used together with other DNV Offshore Standards (OS) or NORSOK N standards as listed in /1/ to /6/ or other recognised standards. It is assumed that this RP will be used together with DNVGL-RP-0005 Fatigue Design of Offshore Structures. This RP is intended to give sufficient guidance to the user on how to establish a sound basis for probabilistic in service inspection planning for fatigue cracks. This basis should as a minimum include advice on:

— Fatigue analysis methods for jacket structures, semisubmersibles 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. Required theory to explain the methodology should be provided in order that users of the document can easier understand the use of the standard and significance of input parameters to the analysis.

The document may be used for planning inspection for fatigue cracks in new built and existing structures and also for analysis of lifetime extension of platforms.

Purpose

The purpose of this recommended practice (RP) is to provide guidelines for use of probabilistic methods for inspection planning of fatigue cracks in jacket structures, semisubmersibles and floating production vessels. This also includes guidelines for fatigue analysis of these structures as required for probabilistic analysis. The presented analysis methodology is rather general and it may be used also for inspection planning of other structures subjected to significant dynamic loading such as jackups.

Optimal allocation of inspection effort for the considered offshore structures with respect to fatigue cracks is aimed for.

Due to the nature of the fatigue phenomena minor changes in basic assumptions can have significant influence on the predicted crack growth rates. Calculated fatigue lives are sensitive to input parameters using standard design analysis procedures. Calculated probabilities of fatigue failure using probabilistic methods are even more sensitive to the methodology and to the input parameters to the analyses. It is thus important to provide as accurate fatigue analysis of the considered structures as possible before the probabilistic analyses are performed.

For design purpose it is appropriate to use conservative values for parameters required for the analyses. However, for planning inspection for fatigue cracks in offshore structures it is important to use relevant (expected or best estimate) values and associated uncertainties in order to predict accurate results that allow the inspections to be directed to hot spot areas where the fatigue cracks are most likely to occur first. Thus, it is important to base planning of in-service inspection of offshore structures on fatigue analysis that has been performed in a consistent way. By “consistent” is understood that all joints or potential hot spots are analysed based on a similar methodology such that any inherent “conservatism” in the analysis methodology is similar for the different hot spots. Use of inconsistent assumptions in analyses may direct inspection to areas with long fatigue lives and one might thus get a false impression of the reliability of the structure with respect to fatigue.

Design of offshore structures with respect to fatigue is normally based on S-N data (test data) derived from constant amplitude testing. In-service inspection for fatigue is normally performed in order to assure that possible cracks in the structure, which may have been present from the initial delivery or have arisen at a later stage during the service life, do not exceed a critical size.

For the S-N fatigue approach, the inspection results cannot be used directly to update the estimated fatigue reliability, as no direct relationship between the crack size and the damage accumulation in the S-N approach is available. A calibration of the S-N fatigue approach to a fracture mechanics fatigue approach is therefore required. The resulting amount of required in-service inspection is dependent on how this calibration is performed. Therefore, an analysis methodology with calibrated initial defects is presented in this document to make inspection planning less time consuming and less complex for the engineers.

The reliability of a non-destructive examination 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 crack at the considered hot spot), each inspection provides information additional to that available at the design stage. Thus, this information can be utilised to update the estimated fatigue reliability.