This page deals explicitly with gas hydrates in pipelines and the problems associated with them in operating pipelines. There are a lot more to say on the theory of hydrates and hydrates on the sea bed. But these are not my area of interst. Feel free to add this material here.

Hydrates consist of a water lattice in which light hydrocarbon molecules are embedded resembling dirty ice. Hydrates normally form when a gas stream is cooled below its hydrate formation temperature in the presence of free water i.e. the gas is colder than its water dew point temperature.

The two major conditions that promotes hydrate formation are thus:

  • High gas pressure and low gas temperature
  • The gas at or below its water dew point with “free water” present

Secondary conditions such as high gas velocity, agitation and the formation of a nucleation site may also help form hydrates.

Hydrate formation is undesirable because the crystals might cause plugging of flow lines, valves and instrumentation. This could reduce line capacity and could cause physical damage to pipe line and equipment.

Hydrate formation prevention and mitigation philosophy

The formation of hydrate should be avoided because of the fact that the hydrate does not dissociate at the same conditions at which they are created. Significantly higher temperature and/or lower pressure are required. Even at these conditions the hydrates to dissociation is a slow process. Furthermore hydrates have a strong tendency to agglomerate and to stick to the pipe wall and thereby plug-ging up the pipeline.

A hydrate prevention philosophy could typically be based on three levels of security listed in prioritised order:

  1. Avoid operational conditions that might cause formation of hydrates
  2. Temporarily change operating conditions in order to avoid hydrate formation
  3. Prevent formation of hydrates by addition of chemicals that lowers the hydrate formation conditions (inhibitors)

The actual philosophy would depend on operational circumstances such as pressure, temperature, type of flow (gas, liquid, presences of water etc.)

Hydrate inhibitors

When operating within a set of parameters where hydrates could be formed there are still ways to avoid the formation of hydrates. Altering the gas composition by adding chemicals can lower the hydrate formation temperature and / or delay the formation of hydrates. Two options generally exists:

The most common thermodynamic inhibitors are, methanol, mono ethylene glycol (MEG) and di-ethylene glycol (DEG) commonly referred to as glycol. All may be recovered and re-circulated, but the economics of methanol recovery will not be favourable in most cases.

Methanol is a colourless volatile liquid fully soluble in water. Synonyms are methyl alcohol, wood alcohol, wood spirits, or curbinol. Methanol is used primarily in anti-freeze compounds, paints, cements, inks, varnishes, shellacs, wood strippers, windshield wiper sol-vents, gasoline antifreeze and as a solvent in dyes. Methanol is highly toxic and readily absorbed from any routes of exposure. Symptoms include malaise, headache, dizziness, confusion, abdominal cramps with excruciating pain and tenderness, stupor, weakness, and acidosis. When methanol is swallowed, formaldehyde metabolically generated. This formaldehyde is more toxic than the metha-nol itself. Blindness and death may occur following ingestion.

Ethylene glycol is a colourless, odourless, involatile, hygroscopic liquid. It is characterised by two hydroxyl groups, which contribute to its high water solubility, hygroscopic and reactivity with many organic compounds. Major applications for ethylene glycol are as an intermediate for the manufac-ture of polyester resins, fibres and surface coatings, as well as antifreeze in the automotive industry. Mono-, di- and triethylene glycols (MEG, DEG and TEG) are the first three members of a homolo-gous series of dihydroxyalcohols. They are colourless, essentially odourless stable liquids with low viscosities and high boiling points that are poisonous when ingested. Ingestion may result in depres-sion followed by respiratory and cardiac failure, kidney damage and brain damage. (Mono) ethylene glycol is by far the largest volume of the glycol products and is used in a variety of applications

Methanol, mono ethylene glycol and di-ethylene glycol could all three be used as a gyrate inhibitor. Methanol would currently be the cheapest solution and the solutions preferred by most for applications where the inhibitor is not expected to be re-used.

MEG is preferred over DEG for applications where the temperature is expected to be –10ºC or lower due to high viscosity at low temperatures. TEG has to low a vapour pressure to be suited as an inhibitor injected into a gas stream.

More methanol will be lost in the gas phase when compared to MEG or DEG.

The use of kinetic inhibitors and anti-agglomerators in actual field operations is a new and evolving technology. The use requires extensive tests and optimisation to the actual system. Kinetic inhibitors might be interesting because of the lower volumes associated with the use of this type of inhibitor.