Innovations in Long Gas Pipeline Loading Procedures: Staged Pressure Method From High-Pressure Rich Natural Gas

Abstract Current trends in long gas transmission systems are to operate at high pressures in the dense phase region. When the gas is rich in composition, i.e. containing appreciable amounts of C2+, as well as relatively higher water, problems arise during loading newly constructed large diameter and long pipelines, e.g. 600+ km. In such a case, condensate dropping out and hydrates formation can occur downstream of the throttling loading valves due to Joule-Thomson (T-J) effects causing potential flooding of Hydrocarbon (HC) condensation and plugging due to hydrates formation in the pipeline near the loading location. In order to circumvent this, an innovative procedure was ideated and implemented on a newly constructed NPS 48, 640 km long gas pipeline in a cold environment where hydrocarbon condensation and hydrates formation are imminent as the supply pressure is in the dense phase and the temperature is relatively low. The new procedure is based on loading every sub-section to an intermediate pressure first, e.g. 50% of the supply pressure upstream, then the next sub-section will be loaded from the upstream sub-section now maintained at the intermediate pressure. In this case, the isenthalpic expansion of the gas from the intermediate pressure will be clear of intersecting the hydrate equilibrium line corresponding to the water content in the supply gas, and only a small amount of HC condensate could drop out. The loading process starts with partially filling of the very first upstream from the high pressure to an optimally determined intermedium pressure. Once this step is completed, the rest of the pipeline sections between Main Line Block Valves (MLBVs) can be loaded, from the now, intermediate pressure in the section upstream, and sequentially to the following section(s) between consecutive MLBVs. The third step is then to load the upstream sections (again in sequential manner) to the full supply pressure. Hence, the new concept is called “staged-pressure loading”. The paper describes the new procedure along with an example of specific loading conditions which was implemented successfully on an NPS 48, 640 km long transmission line. No appreciable hydrates or HC condensate were observed during loading based on the new procedure.