Abstract
Risk of asset failure due to planned/unplanned operational upset conditions in a high volume, high pressure gas storage injection system is mitigated through rigorous simulations. Dynamic flow assurance analysis of the process parameters along with operational constraints were performed to establish safe operating envelope during depressurization.
A detailed flow assurance simulation model representing a 24" injection trunk line and 30" gas gathering manifold (GGM) was developed to calculate pressures and temperatures during depressurization. Rigorous compositional simulations were performed considering a range of pressures from 2,600 psi(G) to 5,500 psi(G) and cooldown durations (up to 24 hours) to study important factors affecting the metal temperatures during depressurization. Single sided depressurization via 4" blowdown valves was considered with the goal to depressurize the entire system to atmospheric conditions. A safe operating envelope was developed to ensure the metal transition temperature of 68°F (95°F with margin) was not violated.
The work identified operating conditions required for a safe depressurization of high volume, high pressure gas storage/gas injection system. These simulations considered system constraints that prevented fluid temperature exceeding 160°F and results from material tests that recommended maintaining the 30" section of the GGM above 95°F to avoid transition of the material behavior from ductile to brittle. These simulations indicated the cooldown duration had a considerable impact on the Joule – Thomson expansion with the system. Further, it indicated the allowable cooldown duration reduced substantially at elevated operating pressures, greater than 5,200 psi(G), the margin between the allowable and lowest temperature is less than 2°F. An empirical correlation was developed based on regression analysis to determine the material temperature for a given initial pressure and cooldown duration. In addition, stress analysis test was performed to validate the new temperature limit.
In order to eliminate any risk of brittle fracture, the West Jefferson test and Drop Weight Tear tests results recommend maintaining the GGM temperature above 95°F which is greater than the ambient temperature. This article demonstrates the requirement for a rigorous modelling procedure considering all constraints and the need for establishing an integrated approach for the safe operation of the system. This work allows the operator to identify key parameters to consider while performing depressurization and help maintain the system integrity