Novel subsea gas dehydration process, the process plant and dehydration performance

Abstract

To ensure that offshore produced gas can be transported reliably by subsea pipelines in cold water, blockage caused by the formation and accumulation of hydrates must be avoided. The method used widely today is to inject chemicals such as glycol or methanol into the gas stream to thermodynamically move the hydrate formation conditions outside those encountered by the pipeline. An alternative, potentially much more economic approach is to dehydrate the gas stream subsea at the wellhead so that no liquid water is ever present in the pipeline and hydrates cannot form. A novel dehydration method that is applicable for subsea installation has been designed and tested in the laboratories of Clean Gas Technology Australia department of Curtin University. The method utilises the concepts of isenthalpic gas cooling through expansion and the controlled formation and management of gas hydrates to reduce the water content of a saturated gas stream to levels suitable for gas transport in subsea pipelines. A pilot plant was implemented to test the solution design at inlet pressures up to 10 MPa using natural gas from a domestic gas main at flow rates up to 35sm3/hr. The pilot plant successfully demonstrated dehydration of the natural gas stream to a water content suitable for transportation, without condensation of liquid water, at subsea pipeline temperatures of 4 °C.Although the concept has been demonstrated, the implementation of the concept in a field viable manner is pending. This paper describes the process plant utilised and presents the achieved dehydration results.