Slugging Induced Fatigue for Flexible Pipe

Abstract Slug flow is a common multi-phase flow pattern for oil and gas pipelines driven by terrain variation and/or hydrodynamic instability in static flowlines, dynamic risers, or jumpers. The loading from slug flow manifests as pressure and density fluctuations inside the pipe bore, which triggers structural vibrations and can result in large-amplitude oscillations of lazy wave risers. The fatigue damage caused by slugging flow induced vibrations, also referred as slugging induced fatigue, could be similar or greater than the fatigue damage caused by waves or vortex induced vibration (VIV), especially for steel pipes, and require careful consideration in the fatigue design of risers and floating jumpers. Slugging induced fatigue for flexible risers is relatively less studied when compared to Steel Catenary Risers (SCRs) or Steel Lower Wave Risers (SLWRs), likely because the inherent high flexibility and structural damping of flexible pipe structures tend to suppress the slugging induced flexible riser oscillation magnitude. Deepwater production flexible risers hung-off from floating production and storage offloading (FPSOs) are typically operated in lazy wave riser configuration. The complex transition of slug flow from the touchdown point (TDP) through the hog bend, sag bend and then along the upper catenary can alter the slug flow pattern in terms of pressure and density fluctuations and increase the oscillations in the touch down point, sag bend, and hog bend areas and even reach the riser hang-off area. Hence the need to accurately assess the slugging induced fatigue damage of flexible risers in lazy wave configuration is increasing. This paper presents a time domain approach to assess the slugging induced fatigue of a flexible lazy wave riser system. The actual flexible configuration is considered in the flow assurance model in a commercial multi-phase flow software and the output of the slug flow simulation are represented as flow density fluctuations inside the pipe bore, varying in both time and pipe arc length. The density variation is entered as time-series input along the lazy wave riser arclength, using the ‘tabular’ contents [2] option of the commercially available global analysis software tool-Orcaflex and dynamic simulations were run to estimate curvature and tension variations, and further supplemented with Baker Hughes’ local analysis tool tailored to flexible pipes, where the irregular tensile-wire stress time histories calculated from these slugging response are then rain-flow counted to calculate the slugging induced fatigue damage using appropriate SN curves derived by testing. A benchmarking exercise for steel riser is performed comparing the response computed by proposed approach with that obtained from propagation approach as per [3] and salient details are discussed briefly to highlight the source of differences and consequences. Finally, a case study is presented and discussed using the proposed approach. The study is also extended with an example of a riser subjected to the combined loading of slugging flow and waves. Fatigue damage differences between slugging-induced fatigue and combined-slugging-and-wave-induced fatigue are presented at hot spot locations and discussed. The results indicate this approach can conservatively capture the slugging induced fatigue in a flexible lazy wave riser.