Offshore Pipeline Drying With Dry Air and Pigs

Abstract

A properl cleaned and dried pipeline, offshore or onshore, can be commissioned and operated with dehydrated gas without encountering any internal problems caused by water or solids.... The validity of this statement is slowly being acknowledged by natural gas transmission and distribution companies, primarily those companies that have had serious problems with wet gas, hydrates, freeze ups or equipment damage. The ability to start up a new or recently hydro-tested dry gas pipeline without encountering any problems caused by the pipeline's internal condition is a compelling reason for cleaning and drying. It completely eliminates any possibility of regulator icing, hydrate formation and damage to delicate seats, seals and meters. Plus, a potential cause of corrosion is removed, the flow efficiency is improved and there will be no dissatisfied customers downstream. There is no magical cure for a pipeline full of wet gas. Once a pipeline has been commissioned there is no way to avoid the embarrassment of having to spend unbudgeted funds to correct problems. It is very difficult to dry a pipeline after it has been brought to operating pressure, because the affinity of natural gas for water is reduced drastically as the pressure is increased. At atmospheric pressure natural gas can hold approximately 30 times more water than it can at 600 psig. That means that water left in the pipeline after commissioning can be evaporated at a rate about 30 times slower than at atmospheric pressure. That rate is further reduced by the presence of some 1 to 7 pounds of water per mmscf already in the gas. Some natural gas transmission companies have begun specifying that their onshore pipelines be dried to a dew point temperature of -38.5 degree F. which corresponds to 7 pounds of water per mmscf. A few offshore gas producers are also using that specification on offshore pipelines built to carry dehydrated natural gas. There continues to be more companies specifying dryness requirements every year and in a few years it will very probably be a standard requirement. These reasons for cleaning and drying onshore natural gas pipelines also apply offshore where hydrate and internal corrosion problems perhaps cause the greatest concern. The conditions under which hydrates will form are well established and basically involve temperatures, pressures, gas composition and the presence of water. The conditions under which internal corrosion can begin are also well understood and include the presence of water. If liquid water and water bearing rust and mill scale are removed from the pipeline, then the potential for internal problems caused by those items does not exit. It is possible to remove the water, rust and mill scale from a pipeline but it is not as easy as running a few pigs. First, the hydrostatic test water is removed with a displacement pig, also called a dewatering pig (of which there are many types). But, the worlds most efficient dewatering pig is not going to dry the wall of the pipe.