Hydrate Plug Dissociation via Active Heating: Uniform Heating and a Simple Predictive Model

Direct electrical heating (DEH) is one class of active heating technology for subsea pipelines used to prevent hydrate blockages during transient operations. Pipeline heating technologies may also allow for remediation of existing hydrate blockages by raising the local pipeline conditions to a point outside the hydrate equilibrium region prior to restart. However, there is a concern that the gas released as a result of plug heating will increase pressure through the pipeline system and may lead to operational and safety hazards. Accurate prediction of the resulting pressure profile requires knowledge of both the initial conditions of the plug (including phase fractions) and the nature of heat and mass transfer within it. A new hydrate plug cell of fixed volume containing a well-characterized multiphase mixture of hydrate, liquid hydrocarbon, liquid water, and gas was used to measure the resulting pressure profile produced when such plugs were heated uniformly, and the results were compared to a thermodynamic model with no adjustable parameters, the hydrate plug remediation and intrinsic safety model (HyPRISM). For heating rates of 0.2-1.0 degrees C/h, HyPRISM predicted experimental pressure increases within 2.5 bar or 1.5% of the average final pressure, with deviations being primarily due to dynamic effects not considered in the model. Plugs filled with gas, water, or oil phases behaved similarly, where pressure communication was maintained through each experiment. The results indicate that, for a sufficiently permeable plug, excessive pressure rises only occur for systems with extremely low initial void (gas) fractions.