The presence of gas hydrates in subsea production is identified as a major flow assurance challenge due to the rapid formation of hydrates and their risk of causing flowline blockages. The risk of hydrate-associated blockages could be higher for transient (shut-in/restart) operations compared to continuous production. Often in offshore production, there are unplanned shut-ins that could lead to increased operational hazards. Despite the practical importance, few studies have addressed the hydrate plugging mechanisms during transient operation.
To investigate hydrate transportability in transient operation, tests were performed using an industrial-scale flowloop (3.8 inch in internal diameter and 295 feet in length) with crude oil for a range of water fractions (30–90 vol.%), at 5 wt.% salinity, and across a range of mixture velocities (2.4 -9.4 ft/s). For the tests performed at 50 vol.% water cut (WC) at 5.7 ft/s, the pressure drop and mass flow rate measurements suggest that the transient (or restart, RS) test could result in an earlier onset of hydrate bedding, occurring approximately twice as fast, and may lead to a higher relative pressure drop when compared to the continuous pumping (CP) test. These flowloop experiments suggest that for the RS tests, rapid hydrate formation and agglomeration followed by the water-in-oil (W/O) emulsion destabilization could cause an early increase in pressure drop (and higher operational risks) compared with the CP tests. The tests using an anti-agglomerant (AA) additive (at 1 and 2 vol.%) were performed at 50 vol.% water fraction for all mixture velocities. The tests with an AA indicate that 2 vol. % AA (at all velocities) could prevent hydrate plugging and maintain a slurry flow at 50 vol. % WC for both RS and CP tests. The tests with 1 vol.% AA (at 5.7 ft/s for 50 vol.% WC) show larger fluctuations in pressure drop in the RS test compared to CP tests performed under similar conditions. It is suggested that an improved understanding of hydrate plugging mechanisms during transient operations could help in the development of advanced strategies to manage hydrate transportation in typical subsea operations, particularly involving unplanned shut-ins and subsequent restarts.