Reducing conservatism in free spanning pipeline vortex-induced vibration fatigue analysis
This paper is concerned with the problem encountered in submarine pipeline designand maintenance where a loss of contact with the sea-bed has occurred. This iscommonly referred to as a free span, and is where susceptibility to Vortex-InducedVibration (VIV) in long unsupported sections may ultimately lead to fatigue failure ofthe pipeline. Free span assessment has traditionally been performed in deterministicform, where uncertainties are accounted for in partial safety factors. However,traditional assessment is believed to be overly conservative (Hagen, Mork, Sigurdssonand Nielsen, 2003) due to the generalised and wide range of design conditionsaccounted for in the calibration of the safety factors. Excessive conservatismover-estimates the threat to pipeline integrity and leads to unnecessary capital andoperating expenditure in free span control and intervention work. ProbabilisticAnalysis of a typical free spanning pipeline, within a Norwegian Deep-waterdevelopment area, is presented in this paper. The computational model employedutilised a Monte Carlo approach based on recommended practice DNV-RP-F105 (DNV, 2006) to ensure a conceptually sound analysis. Thedeterministic treatment of a bi-linear S-N fatigue curve, which retains theconservatism inherent in representation of the experimental data, differentiates thisprobabilistic analysis from those undertaken previously. In addition, an alternativelimit state for Cross-Flow VIV (CF VIV) is proposed where the focus is on themaximum response amplitude. The study produced evidence consistent with theproposition that traditional free span assessments are overly conservative.Specifically, in comparison, the probabilistic analysis allowed the free span length toincrease by approximately 5-6m. Acceptance of the methodology presented in thispaper has potential benefit regarding free span intervention and the associatedsignificant costs that can be avoided in cases that would otherwise proceed under thetraditional deterministic form of assessment.