Hydrate management in deadlegs: Limiting hydrate deposition with physical restriction

Abstract

In the oil/gas production and transportation systems, deadlegs are pipe sections filled with stagnant water saturated gas. Deadlegs are often uninsulated, resulting in cold pipe walls that can readily lead to the formation of gas hydrates. Hydrate deposits consequently can build up in the deadleg, and sometimes result in hydrate plug, which cause severe operational issues. In some instances, deadlegs are used for maintenance purposes, but they pose significant hydrate challenges at the same time. Establishing strategies for hydrate management in deadlegs is therefore important for economic and safety reasons. Here we applied a physical restriction to an 1-in. vertical pipe system at several different locations to verify that the restriction can limit hydrate deposition in water saturated gas. The motiviation of this study is that the presence of a physical restriction, which can also be called an orifice plate, can change water condensation and the following hydrate deposition phenomena. At a given header/pipe wall temperature condition, when the restriction is installed lower than the position where most hydrates are supposed to form, hydrate deposition is limited by changing the temperature profile in th pipe, and thus the water condensation and hydrate deposition. If the restriction is far enough from the header, it has negligible effect on hydrate deposition as the water vapor is sufficiently cooled down. The experimental results provide a demonstration that hydrate deposition in certain types of deadleg may be limited by applying a physical restriction (e.g. a valve) to the pipe or shortening the vertical pipe length.