Reducing conservatism in free spanning pipeline vortex-induced vibration fatigue analysis

Abstract

This paper is concerned with the problem encountered in submarine pipeline design and maintenance where a loss of contact with the seabed has occurred. This is commonly referred to as a free span, and is where susceptibility to vortex-induced vibration (VIV) in long unsupported sections may ultimately lead to fatigue failure of the pipeline. Free span assessment has traditionally been performed in deterministic form, where uncertainties are accounted for in partial safety factors. However, traditional assessment is believed to be overly conservative (Hagen et al, 2003) due to the generalised and wide range of design conditions accounted for in the calibration of the safety factors. Excessive conservatism overestimates the threat to pipeline integrity, and leads to unnecessary capital and operating expenditure in free span control and intervention work. Probabilistic analysis of a typical free spanning pipeline, within a Norwegian deep-water development area, is presented in this paper. The computational model employed utilised a Monte Carlo approach based on recommended practice DNV-RP-F105 (DNV, 2006) to ensure a conceptually sound analysis. The deterministic treatment of a bilinear S-N fatigue curve, which retains the conservatism inherent in representation of the experimental data, differentiates this probabilistic analysis from those undertaken previously. In addition, an alternative limit state for cross-flow VIV is proposed where the focus is on the maximum response amplitude. The study produced evidence consistent with the proposition that traditional free span assessments are overly conservative. Specifically, in comparison, the probabilistic analysis allowed the free span length to increase by approximately 5–6 m. Acceptance of the methodology presented in this paper has potential benefit regarding free span intervention and the associated significant costs that can be avoided in cases that would otherwise proceed under the traditional deterministic form of assessment.

Keywords:

• Vortex-induced vibration (VIV)
• DNV-RP-F105
• free span assessment
• free spanning pipeline
• probabilistic analysis
• submarine pipeline
• Monte Carlo simulation

Additional information

Notes on contributors

G D Esplin

Gregory Esplin is a doctoral candidate at The University of Western Australia. He holds a BEng in Mechanical Engineering and a Masters degree in Oil and Gas Engineering. His current research efforts are focused on the issues surrounding risk and reliability of offshore structures.

B Stappenbelt

Dr Brad Stappenbelt is a Lecturer with the School of Mechanical, Materials and Mechatronic Engineering at The University of Wollongong, and an adjunct Lecturer in the field of Ocean Engineering at The University of Western Australia. He holds a BEng degree in Mechanical Engineering and a PhD in the Offshore Engineering field. He is principally involved in flow structure interaction research and the development of offshore wave energy conversion technology.