Free water film left in hydrostatic tested pipelines can cause formation of hydrates.
Hydrocarbon gas hydrate build-up in natural gas pipelines narrows the pipe profile and can cause a considerable loss of efficiency due to the increased friction load. Narrowing of the internal pipeline diameter starts slowly, but as it changes flow and pressure conditions it accelerates to build-up hydrates. At a certain point there is a danger of the pipeline being completely plugged.
Conditions which cause formation of hydrates in natural gas pipelines exist mainly during or immediately after commissioning of new lines which have been hydrostatically tested.
This is due to residual free water or moisture remaining in the line as a film 0,1 to 0,15 mm thick, after normal dewatering with bi-directional pigs.
In existing natural gas pipelines which already are in service, free water is separated from the gas in drying- and separation plants prior to transportation.
Hydrocarbon gas hydrates belong of complex chemical compounds which are caused by free water being available in the presence of hydrocarbon gases. They are of crystalline structure and form a firm granular substance which can be compared to ice or snow.
The initial hydrate crystals thus appear on the pipe wall and in particular where there is any unevenness such as weld seams or areas of corrosion. However, crystals will also form on valves and fittings.
It is necessary, before commissioning of gas pipelines to ensure that there is no residual moisture and in particular, no free water over its complete length of the pipeline. This means that before introduction of gas, the line must be dried to prevent hydrocarbon gas hydrate to build-up.
Another aspect is that the line should be dried immediately after bulk dewatering, to provide corrosion protection, as even with the best performance, residual water will remain in the line after dewatering, and cause corrosion.
Research has shown that by using the air-drying method of gas pipelines, which are not put into immediate operation, will protect the pipeline against corrosion. It is a known fact that corrosion is slowed considerably or even halted at dew-points below -10 ºC at atmospheric pressure.
Pipelines that have been dried by using the air-drying method after hydro-testing and have been left packed with dry air, for a period of 6 month or more showed no sign of corrosion. In cases where corrosion existed prior drying, no further spreading of corrosion could be detected.
Therefore it is also recommended that oil pipelines which are not put into immediate operation after hydrostatic test also should be dried to protect against corrosion.