Session: 04-01-05 Flexible Pipes and Umbilicals V

Submission Number: 157398

Slugging Induced Fatigue for Flexible Pipe 

Slugging is a common multi-phase flow pattern for oil and gas pipelines driven by terrain denivelations in static flowlines, hydrodynamic instability in dynamic risers or jumpers floating above seabed. Slugging manifests as pressure and flow-density fluctuations inside the pipe bore, which triggers structural pipe vibrations and can result in large-amplitude oscillations of lazy wave risers. These slugging induced vibrations can cause fatigue damage that is similar or greater than the fatigue damage caused by waves or 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 SCRs or 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 FPSOs are typically operated in lazy wave riser configuration. The complex transition of slug flow from the touchdown point trough the hog bend, sag bend and then along the upper catenary can alter the slugging pattern, pressure and flow density fluctuations, increase the oscillations in the touch down point, sag bend and hog bend areas and even reach the riser hang-off area. Hence it is heavily needed to accurately assess the slugging induced fatigue damage of flexible risers in lazy wave configuration.

This paper presents a time domain approach to assess the slugging induced fatigue of a flexible lazy wave during the detailed design phase of an actual project development. The representative slugging flow data are computed by commercial multi-phase flow software and introduced as flow density fluctuations which is varying in both time and pipe arc length. The density variation inside the pipe bore is entered as time-series input along the lazy wave riser arclength to run dynamic simulations using a commercially available global analysis software tool supplemented with Baker Hughes’ local analysis tool tailored to flexible pipes. The irregular tensile-wire stress time histories calculated from these slugging time series are then rain flow counted to calculate the slugging induced fatigue damage using appropriate SN curves derived by testing. A study case is presented and discussed using this approach.

The study is also presenting an example or 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. A brief comparison is made with ExxonMobil slugging-induced fatigue method and recommendations made for future developments and benchmarking

Presenting Author: Yucheng Hou Baker Hughes

Presenting Author Biography: Dr. Hou has been working for flexible pipe industries for over 15 years in Baker Hughes and focusing on the riser system design and analysis, including strength, fatigue, OBS, VIV and slugging, especially in the riser configuration, system design. Then he transferred the main role into engineering management.