Fatigue Design of Flowline Systems With Slug Flow

Hydraulic sizing of multiphase flowlines generally leads to significant potential for slug formation. The dynamic loads from the passage of slugs through bends are well understood. Gravity load effects associated with changing contents density are less widely recognised, but potentially more significant. Multiphase flowlines can experience several slugs every minute, which can lead to high cycle fatigue damage. Slug-induced fatigue damage has emerged as a governing design criterion in recent projects. Spans, rather than pipe bends, have become the focus of attention for slug-induced fatigue damage. These may occur at flowline terminations and spool-pieces used to connect flowlines in deepwater developments. Spans introduced by lateral buckling mitigation features can be particularly difficult to protect against slug-induced fatigue. This paper defines the slug-induced fatigue problem, and presents techniques for predicting fatigue damage during design. It describes analysis of pipeline spans, flowline connection spools and spans at buckle mitigation sleepers. Design recommendations are proposed, with suggestions for follow-up investigations to improve understanding of this phenomenon.