Sensitivity Study of Slug Induced Fatigue in Flow Line Spools

Abstract

Abstract The fatigue generated by slugs in pipelines depends on the pipe structure, slug density, velocity and length. This research project sponsored by five operators and engineering companies assesses the influence of the main parameters mentioned above on pipeline vibration levels and predicts associated fatigue. A 3D Finite Element Model of an existing pipe system is used to simulate the force generated by the passage of slugs. Gravity variations and impact forces at elbows are adjusted based on specific formulas and experimental results for a range of slug sizes and velocities. Resonance sources and effects are analyzed. Fatigue damage is then computed using time-domain Finite Element software and a rainflow-counting algorithm. Finally, a detailed sensitivity study is performed to quantify the influence of cell length and velocity. Results show that above a minimal slug length, a static equilibrium is reached. For slugs longer than this minimal length, the piping oscillates around a new equilibrium point until the liquid fills the pipe. For any given spool, the maximum fatigue generated by a slug is encountered at a slug velocity and length specific to the spool. The most significant new finding shows that this maximum fatigue non-linearly varies with the slug velocity, length and the pipe structure (elbow location, length of unsupported pipe, 3D routing). This is mainly due to the fact that the force generated by the liquid after the slug passage is in phase with an existing pipe oscillation generated at the entrance of the slug. This is important as until now, slug fatigue computation was based on average slug parameters, now recognized as not conservative. The novelty of this new computation method is its ability to accurately predict the fatigue levels in a subsea spool. It also shows that the existing method taking into account mean slugs does not necessarily generate the worst-case scenario and that all the slugs expected to exist throughout the lifetime of the pipe system have to be considered.