Case Study 1 Hydrate Prevention in a Deepwater Gas Pipeline

1 Case Study 1 Hydrate Prevention in a Deepwater Gas Pipeline 

The ocean cools the fluids as they flow, including both produced water (here assumed to be salt-free) and condensed water that is always salt-free. At about 9 miles the flowing hydrocarbons and water enter the hydrate region (to the left of the line marked hydrate formation curve ), remaining in the uninhibited hydrate envelope until mile 45. Such a distance may represent several days of residence time for the water phase (which flows slower than the hydrocarbon phases) so that hydrates would undoubtedly form, were no inhibition steps taken. 

Hydrate inhibition occurs in the aqueous liquid, rather than in the vapor or hydrocarbon liquid phases. While a significant portion of the methanol partitions into the water phase, a significant amount of methanol either remains with the vapor or partitions into any liquid hydrocarbon phase. Although the methanol mole fraction in the vapor or liquid hydrocarbon may be low relative to the water phase, the large amounts (phase fractions) of vapor and liquid phases will cause a substantial amount of inhibitor loss. 

Note that regular methanol (or monoethylene glycol) injection is used only with gas-dominated systems. In oil-dominated systems the higher liquid heat capacity allows the system to retain reservoir heat, so that insulation maintains sufficient temperatures to prevent hydrate formation. Thermodynamic inhibitor is normally only injected for planned shutdowns in oil-dominated systems. 

2 Case Study 2 Hydrates Prevention via Combination of Methods 

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