Hydrate Formation in Small Bore Dead Legs in Subsea Processing Systems

Abstract

Natural gas hydrates, a crystalline compound forming at high pressures and low temperatures, pose flow assurance issues in natural gas processing systems. Small bore dead legs, piping containing stagnant process fluid, are prone to blocking by hydrate formation. This thesis aims to provide design guidelines and best practices for small bore dead leg design in natural gas subsea processing systems as well as providing a method for predicting hydrate thickness using computational fluid dynamics (CFD). An experiment which measured the final thickness of a hydrate in controlled conditions is recreated in CFD using three different approaches. One approach, volume of fluid (VOF) melting-solidification, shows promise with similar results to the experiment but with varying accuracy. Recommendations for future work on steady-state hydrate modelling in CFD are given. The small bore dead leg designs are evaluated from a conservative standpoint using CFD with the aim to provide general guidelines for a wide range of use cases in terms of process fluid composition, operating conditions and dead leg bore size. Design guidelines are given for common small bore dead leg functions, namely hydrocarbon displacement and mono-ethylene glycol injection. Guidelines for maximum and minimum small bore dead leg lengths are given for simple design geometries, such as straight pieces of piping. More geometrically complex designs are evaluated with respect to hydrate blocking from a thermal point of view. The guidelines and evaluated designs provide constraints and pre-verified solutions for future small bore dead leg designs regarding hydrate blocking mitigation.