A 130 ft single-pass, gas-dominant flowloop has been constructed to study hydrate formation in an annular flow regime by exposing warm process fluids to a cold pipe wall. Hydrate was formed in six experiments from a natural gas mixture, with 6–18 °F subcooling from hydrate equilibrium. At lower subcooling values a stenosis-type hydrate film growth model without adjustable parameters was used to estimate the resulting pressure drop and yielded an average deviation of 15.8 psi from the experimental value. The accuracy of this model decreases appreciably with increasing subcooling, suggesting the occurrence of a transition after which the pressure drop becomes dominated by additional hydrate phenomena such as particle deposition or wall sloughing. For experiments with 18 °F subcooling, the pressure drop signal contained periodic peak-and-trough behavior and the primary hydrate restriction was observed to migrate downstream at a rate of 3 ft/min over the course of the experiment. Average hydrate growth rates ...