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

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 in-service 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 simulation approach and the comparison against measurement. The simulation approach predicts the loading of the structure based on only the geometry and the nominal operation conditions of the subsea riser tower which are both readily available. Detailed modelling using Computational Fluid Dynamics then provides a forcing function which is subsequently applied to a structural Finite Element Model to calculate stresses in the frequency domain which is then used to estimate the fatigue performance of the structure. Compared to the measurements, the simulations were shown to be conservative, but still within 35% of the measurements, which would allow mitigations to be evaluated and implemented with confidence utilising a reasonably large safety margin.