Hydrate/Wax/Asphalt in Subsea Pipeline

Gas hydrates are solid crystalline compounds which have a structure wherein guest molecules are entrapped in a cage-like framework of host molecules without forming a chemical bond. It is water’s hydrogen bond that allows formation of hydrates. The hydrogen bond causes the water molecules to align in regular orientations. The presence of certain compounds causes the aligned molecules to stabilize and a solid mixture precipitates. The water molecules are referred to as the host molecules and the other compounds, which stabilize the crystal, are called the guest molecules. The hydrate crystals have complex, three dimensional structures in which the water molecules form a cage and the guest molecules are entrapped in the cages. 

The stabilization resulting from the guest molecule is postulated to be caused by Van der Waal’s forces, which is the attraction between molecules that is not a result of electrostatic attraction. The hydrogen bond is different from the Van der Waal’s force because it is due to strong electrostatic attraction, although some classify the hydrogen bond as a Van der Waals force. The formation of a hydrate requires the following three conditions:

1.      Low temperature and high pressure

2.      Presence of hydrate formers such as CH4, C2H4, CO2 and H2S

3.      Sufficient amount of water.

Hydrate’s plugs and disturbed the normal flow of natural gas and other reservoir fluids in the  production and transportation lines and have claimed lives of personnel and resulted loss of  properties in the oil and gas industries (Sloan, 2008). Hydrate propagation tends to gradually form a plug that separates the pipe into two pressure sections: a high pressure section  between the well or high pressure gas source and the plug and a second section at low  pressure between the plug and the gas recovery division. In the upstream section, a pipe blast can occur due to pressure rise. 

The plug can also behave as a projectile that destroys the pipe when the pressure difference between the upstream and downstream sections increases. Both events can endanger personnel safety and damage production equipment. (Sloan, 2008).

Source : https://www.academia.edu/10081327/EFFECTS_OF_HYDRATES_FORMATION_ON_GAS_PIPELINES_A_CASE_STUDY_ON_ERIEMU_FIELD_