Evaluating the Effects of Emergency Depressurization on a High-Pressure High-Temperature (HPHT) Gas Injection Onshore Pipeline

Abstract An operating strategy is proposed for the emergency depressurization of a NPS30 gas-gathering manifold (GGM) and its associated NPS24 trunk line transporting high-pressure, high-temperature (HPHT) gas to a storage reservoir subject to various system constraints. Abrupt pipeline depressurization containing lean gas causes cooling downstream of the blowdown valve (BDV) and at locations within the pipeline due to Joule-Thompson (J-T) expansion. A safe operating envelope for the injection system was developed through rigorous flow assurance, materials and buckling analysis. Pipeline fracture tests indicated the GGM material transition temperature (MTT) is approximately 20°C (35°C with margin) and recommended to maintain GGM above this temperature. Pressure corresponding to a metal temperature of 35°C during depressurization was calculated as an increase in the inlet gas temperature to mitigate low temperature during depressurization. This was used as an input for stress calculations to determine the propensity of fracture propagation. An increase the inlet gas temperature will mitigate low temperatures during depressurization. The risk of pipeline failure due to upheaval buckling at elevated pipeline temperatures and pressures was also considered. Upheaval buckling of pipelines can occur when axial force reaches a critical limit. This force is accentuated during episodes of large temperature differences and high internal pressures. An operating strategy presented here incorporates the impact of these limits on upheaval buckling.