Hydrate Deposition in a Subsea Gas Pipeline with Low Moisture Content

Abstract Gas hydrates are an operational and safety concern in the transportation of oil and gas through pipelines. This study examines the influence of the moisture level on the hydrate deposition process in gas-dominant systems. In an operation in the Marlim Sul field in Brazil, gas dehydrated with a triethylene glycol unit was transmitted through a subsea pipeline, where fluctuating, low moisture remained in the gas and resulted in hydrate buildup. The rate of hydrate deposition was slow compared with the typical conceptual picture of hydrate blockages. In this case, the pressure drop (dP) approximately doubled in the pipeline and slowly increased from hydrate deposition over 11 months. To investigate this gradual deposition behavior, flow simulations with a hydrate deposition model were conducted with varying low moisture levels that affect both hydrate equilibrium and water dew point. In the simulations, the extent of the water phase at the beginning of hydrate formation had a strong impact on the hydrate deposition process. When the water phase existed, the simulation results were comparable to typical quick hydrate deposition. The sharp dP increase occurred within weeks, at the latest. Once the hydrate formation commenced, the liquid water was quickly converted to hydrate, and this caused a significant hydrate deposition at a specific point. Where the water phase did not exist, the simulations represented the slow dP increase with mild, gradual hydrate deposition. These findings offer new insights into the relationship between the moisture level and the hydrate deposition process in a gas-dominant system.