Flexible Risers Fatigue Analysis Based on Vessel Motion Sensor Monitoring

Abstract

This paper aims to present the use and advantages of the implementation of motion sensor measurements in a flexible riser fatigue analysis. A practical offshore application in Brazil is discussed in terms of extrapolation of the results to the life of field, gains in accuracy when disregarding uncertainties related with vessel Response Amplitude Operator (RAO) and weather description, lifetime assessment and challenges of implementation. A global model is developed based on OrcaFlex, a commercial software developed by Orcina dedicated to marine dynamics, to represent the FPSO and flexible riser system with motion data directly applied to the FPSO within the model. The convergence of the fatigue life calculation is then assessed to ensure that individual sea states have small impact on the long-term calculations. Finally, the operational history is classified into representative operational conditions and the calculated damage is combined to estimate the consumed life and calculate the remaining life based on the expected future operational condition.

When comparing motion statistics from the OrcaFlex model with the measured response, Heave and Pitch showed good agreement, however Roll was inaccurate. Roll motions are in general important for riser fatigue on ship-shaped FPSOs. This mismatch likely stems from uncertainties in displacement RAOs and the simplified single-partition wave modelling. The Roll RAO curve is sharply peaked, causing alternating under- and overpredictions, and linear RAOs struggle with non-linear roll effects. Roll is also sensitive to the vessel loading condition. To achieve a more precise analysis, the most effective approach was to superimpose motion data from sensors directly into the OrcaFlex model. Wave loads were excluded from the model, as direct wave loading onto the riser has minimal impact on fatigue damage in deep-water locations: vessel excitation is governing.

The convergence of the fatigue life is observed after 15 months of monitoring and the calculated damage can be extrapolated to estimate annual fatigue damage based on the monitoring period. The resulting fatigue life predictions are considered more accurate and based on actual operational conditions. This paper presents another success case of using measurement data to replicate actual operational conditions on the performance analysis of flexible risers. The use of sensors has been crucial to understand key aspects of riser systems and predict the remaining life of subsea assets. The authors expect that this paper motivates others to use sensor-aided approaches to ensure safe life of field decisions.

Authors

Daniel de Moraes Lobo
Team Lead at 4Subsea

Bjørn Wilhelm Jæger
Senior Engineer at 4Subsea

Håvard Skjerve
Lead Engineer at 4Subsea

Luana Ferreira Serrão
Project Manager at 4Subsea