Flexible Risers Lifetime Extension: Riser In-Service Monitoring and Advanced Analysis Techniques

The demand for the lifetime extension of flexible pipes is increasing due to the need to extend the lifetime of the existing production fields. There have been many challenges with the lifetime extension of flexible pipes after the end of the initial design service life due to the inherent conservatism with the common analysis approach, safety factors and operation beyond the design limits. A lifetime assessment should be performed on flexible risers for re-qualification during the original design life if the design envelope is exceeded or there is a need for lifetime extension. Hence, a systematic approach for lifetime assessment execution is established to determine the integrity level of the flexible risers and define the recommended actions required, such as mitigations, repairs or monitoring to maintain an acceptable risk for the required extended service life based on consistent methodology. The primary objective of this paper is to present a riser integrity management field-proven technology to monitor the riser’s behaviour in-service in addition to the advanced analyses guidelines to form a basis for the lifetime extension of flexible risers. The primary objective for the integrity management is to manage and control the risk of failure by detecting failure at an earlier stage when preventive action can be taken to avoid failure propagation. In addition, it is demonstrated that the primary hot-spots for the dynamic behaviour and fatigue life assessments of the flexible risers are primarily in bend stiffener regions and the touchdown zone (TDZ) due to large tension fluctuations caused by vessel motions and cyclic movement in the TDZ. Therefore, analysis techniques have been developed in two primary areas: advanced bend stiffener modelling using pipe-in-pipe (PIP) to model the sliding friction and the bend stiffener/flexible pipe’s annular space and flexible pipe–seabed interaction modelling using a non-linear seabed model. Therefore, the flexible riser’s lifetime extension assessment will be based on more reliable models that reflect the realistic and dynamic behaviour of the flexible risers. Consequently, these advanced analysis techniques can be used for new designs or lifetime extension of flexible pipes.