Hydrate Formation Characteristics of Natural Gas During Transient Operation of a Flow Line

Abstract

Hydrate management during transient operation of subsea flow lines has been identified as one of the greatest challenges for deep water O&G production. Experiments using natural gas and de-ionized water were conducted in a laboratory flow loop (1" diameter and 40 m length) to investigate the dynamics of hydrate formation during simulated shut-in and restart conditions. Variables studied include different operational pressures and sub-coolings.This work will provide valuable information for the establishment of cost-effective strategies to guarantee the continuous flow of fluids in subsea production systems, particularly during transient operations such as scheduled maintenance or unplanned shut-downs.During the shut-in period hydrate appears to nucleate along the gas-water interface and grow as a film that spreads across the entire interface. At a pressure of 1,500 psi and sub-cooling of 15°C, dendrite crystals start to appear at the film and grow into the water phase until a thickness is reached where further grow stops. It is noted that, although hydrate is observed as a thin film during the shut-in period, most of the water is converted to hydrate upon restart. At every pressure condition evaluated in this study, slurry like hydrate flow has been observed upon restart, being flushed along the pipe-line by the gas flow.Transient operations of subsea flow lines require strict adherence to operational procedures which include depressurization and flushing of the lines with thermodynamic hydrate inhibitors. The results of our work suggest that the blockage is more likely to occur during restart so operating procedures must be designed to minimize the risk of hydrate blockage based on specific factors such as driving force, water content, and gas flow rate. It is considered that the flow loop experiments could provide valuable insight into the dynamics of hydrate blockages formed under flow conditions in gas dominant flows.