Walking Analysis, Submarine Pipeline Susceptibility Analysis

2. Group of Hydrology and Water Resources, Faculty of Engineering, Agostinho Neto University, Angola

 Abstract: As offshore oil and gas fields in Angola are being exploited in deeper water, subsea pipelines are required to operate at high temperatures and pressures, leading to a major subjection to movements in axial and lateral directions caused by the loads generated by high pressures and temperatures. Some high-temperature pipelines have experienced the cumulative axial displacement of an overall pipeline length over several start-up/shut-in cycles, commonly known as “Pipeline walking”. The research presents walking analyses for water injection (WI-9) rigid pipeline for AGOGO Phase 2 Oil field located in Block 15/06 offshore entailing Cyclic Warm-up/ Shut-down events, relevant transient temperature and pressure profiles, and Seabed slopes. The analyses were performed following the methodology in the flowlines design premises aiming at the evaluation through FEA tools of the susceptibility to pipe walking and the walking rate and the identification of possible philosophy mitigation measures if applicable, to limit the maximum cumulated axial displacements within acceptance (+/-2 m for flexible jumper). Furthermore, two alternative warm-up scenarios were simulated namely, Case A: WI-9 warm-up A (WI-2 and FJ-104 were warmed-up before WI-9 warm-up) and Case B: WI-9 warm-up B (WI-9 is warmed up together with WI-1, WI-2, and FJ-104) respectively; the evaluation of the WI-9 flowlines axial displacements potentially induced by the cyclic variation of operating conditions (shut down/restart cycles) during the pipeline design life of twenty years and due to seabed slope was carried out in this research; by considering warm restart and shut-down for WI lines transient scenarios with the nonlinear finite element analysis software ABAQUS another assessment was carried; a detailed FE analysis was lastly performed with an LBMS at KP = 6000 m enabling the evaluation of the axial walking susceptibility. The minimum LE axial residual friction between undrained and drained conditions and the maximum BE lateral residual friction were also considered to evaluate the walking.