Fatigue Life Assessment of an In-Service Subsea Riser Tower Subjected to Flow Induced Vibration: Part 1 of 2 – Measurement

Abstract Flow induced vibration (FIV) can go undetected in subsea pipework, potentially leading to fatigue failures. Although FIV screening methods have been developed and improved over recent years [1], these are deliberately conservative for multiphase pipe flows and do not accurately quantify the actual fatigue performance of subsea pipework already in situ. The last few years has seen significant development of reliable simulation processes to predict the effects of multiphase flow induced vibration and they have been successfully compared with experimental data for a wide range of operating conditions. However, there is limited in-situ vibration measurement data available from actual operating subsea equipment which would allow a direct verification of the analytical / numerical results. As part of a routine ROV inspection, two spools on an inservice subsea riser tower were observed to be vibrating, and a combined simulation and measurement campaign was engaged upon to quantify the risk of fatigue failure. This paper is focused on the measurement campaign. Based on initial modal Finite Element Analysis, suitable locations were selected for placement of subsea accelerometers. The extracted and processed data, covering almost 2 months of operations, highlighted that the stresses based on the measured vibration were within 35% of the simulation results, and it also showed an unexpected behaviour where the 2 risers interacted with each other; although this was unlikely to cause significant damage. Subsequently mitigations could be evaluated and implemented with confidence utilising a reasonably large safety margin.