Abstract

Natural gas hydrates occasionally plug the 58 km subsea pipeline that transports natural gas from Platform QK18-1 in southwest of Bohai Bay to the processing facility onshore in Northeast China. This is because it is a wet gas subsea pipeline that operates at high pressures and low temperatures, which are the conditions that are appropriate for hydrate formation to occur. In this study, we proposed that the best way to prevent the occasional plugging of the pipeline is to rightly evaluate the upper limit of water that can be permitted in the bulk gas and dehydrate the gas accordingly before transport. Current industrial techniques are mainly based on water dewpoint evaluations. In our recent work we have proposed another approach that considers the impact of the rust (Hematite) on the internal walls of pipelines. These two methods have been used for this study. The results of the method of adsorption of water onto rusty (Hematite) surfaces suggest that the current approach (dewpoint method) overestimates the safe-limit of water about 18 to 19 times higher. Thus, the risk of hydrate formation may still exist if the dewpoint method is used as basis for drying the gas. Sensitivity analysis shows the influence of pressure on the upper limit of water - the higher the pressure the lower the maximum concentration of water that is safe to accompany the gas. Our calculations were done using a FORTRAN code that utilize thermodynamic data from molecular dynamics simulation.