Operational experience has shown that flexible risers producing different combinations of oil, gas and water can be subjected to increased dynamic motions due to slugging — a cyclic accumulation of finite volumes of liquids at a low point of the riser (e.g. sag point of a lazy-S riser) until sufficient pressure is built up behind the slug to push the liquids up through the riser. It has been observed that the slug induced dynamic riser motions can cause riser displacements larger than those generated by moderate and some extreme waves in the absence of slugging. A major impact of the slug induced riser motions is the increased fatigue damage of the tensile wires — the cross-sectional component that most frequently defines the fatigue resistance of flexible riser systems. While international standards like ISO 13628-2 & -11 require and recommend that the effects of slug flow on riser response are considered, they provide no guidance on how to practically incorporate potential slugging effects in pipe design or analysis.
A methodology has been developed to determine the remnant fatigue life of a riser subjected to slug induced motions combined with the normally considered vessel motions and wave loading. The methodology is based on using commercially available global and local riser analysis tools. The global analysis tool is used to determine the riser response induced by continuous and regular slug loading combined with loading from different irregular waves, vessel offsets and motions. The slug loading parameters are determined through an iterative process calibrating riser displacements and frequencies with those observed in the field. The local analysis tool is used to determine wire stress transfer functions, which in turn are used to derive wire stress time series from the riser tension and curvature time histories. Stress ranges are identified through rain-flow counting applied on all the calculated stress time series and fatigue performance is estimated using the Palmgren Miner summation of damage using an appropriate wire S-N curve. In a case study, the combined slug and first order wave induced fatigue damage increased by a factor of approximately two compared to the wave induced damage alone.
This methodology can be used for: a) riser fitness for service assessments by bounding the impact of slug-induced riser motions observed in the field, and b) new riser design when slugging parameters are adequately bounded by flow assurance calculations.
