Risk of hydrate formation during the shutdown and restart of deepwater multiphase pipelines

Hydrate blocking tends to occur easily in a long-distance deepwater multiphase pipeline during its shutdown and restart. In this paper, the risk of hydrate formation during the shutdown and restart of multiphase pipelines was analyzed by PIPEPHASE and OLGA multiphase flow softwares based on the development pattern of a certain deepwater block in Angola. It is shown that after 2 hours' shutdown, hydrate formation area appears in the pipeline. In the early stage of restart after shutdown, the hydrate formation area in the subsea pipeline disappears gradually near the wellhead and enlarges rapidly in the riser section near the sea level. As the extension of restart time, the hydrate formation area diminishes gradually from both sides to the center, and finally disappears in the riser section at the depth of 700 m after it is restarted for 6 hours. Then, a qualitative risk analysis method for the formation of hydrate was proposed based on the quantitative description of the temperature and pressure sensitivity of multiphase pipelines. It is indicated that the risk of hydrate formation increases with the increase of subsea pipeline length, inner diameter and gas-oil ratio, and decreases with the increase of water content. The calculation results can provide the good guidance for the type selection, routing and blending ratio design of multiphase pipelines.