Predicting Hydrate-Plug Formation in A Subsea Tieback

Summary Field data from StatoilHydro on hydrate-plug formation in the Tommeliten gas/condensate field are compared to predictions of the hydrate-growth model (CSMHyK-OLGA) for four typical operating scenarios: steady-state operation with failure of inhibitor injection, restart of an uninhibited line, restart of an underinhibited line, and restart of a depressurized line. Although the CSMHyK model was designed for oil flowlines, the model is able to predict the correct time scale for hydrate-plug formation in this gas/condensate tieback. The predicted locations of the plugs are often farther upstream than observed in the field trials. This is mainly because of the assumption of a "hydrate/oil slip factor" of zero, which forces the hydrate to accumulate where it initially formed. In reality, hydrate agglomerates would be carried further downstream before eventually jamming in dips. Predicting where and when hydrate plugs will form in subsea tiebacks is of increasing importance as the industry strives to manage the risk of plugging in oil and gas flowlines while minimizing the use of costly and environmentally harmful chemicals for hydrate inhibition. The Colorado School of Mines has been developing the CSMHyK model for the past 5 years, in collaboration with the SPT Group and several leading energy companies.