Hydrates in Offshore Gas Pipelines

Hydrate plugs in gas pipelines and flowlinesHydrates are crystals that formed when water and gas are combined in a low temperature and high pressure environment. During production, the oil and gas that comes from the reservoir is a mixture of oil, gas, water and sand. If the pressure is high enough and the temperature low enough, water and gas are combined and form hydrates.

Hydrates are solids with a crystal form. Once formed hydrates can reduce the diameter and ultimately block the pipeline. Hydrates can plug the downhole tubing, tree and manifold piping, flowlines and risers. The plugs can be difficult to locate and remove leading to significant losses in production and revenues

We can prevent or mitigate the formation of hydrates by:
1. Chemical injection. Depending on the reservoir fluids the injection chemical can be
- Methanol or Mono Ethylene glycol (MEG) for gas and gas/condensate wells.
- Kinetic gas hydrate inhibitors for low water production rates
2. Thermal insulation. Thermal insulation of subsea infrastructure(pipelines, risers, manifolds etc) results in maintaining the temperature of the flow above the hydrate formation temperature. Thermal insulation also acts in preventing the rapid cool-down during shut downs  providing thus time for the operator to take remedial action.
3. Lower the pressure. Operating pipelines at lower pressure especially during shut-in conditions. 
4. Removal of water. It is possible to prevent the formation of hydrates by removing the water from the flow before the the fluid reaches temperature and pressure conditions that hydrates are formed.

 hydrate-formation-in-pipelines

 

Further Reading:

http://www.ipt.ntnu.no/~jsg/undervisning/naturgass/lysark/LysarkSandengen2010.pdf

http://www.drbratland.com/free-book-pipe-flow-2-multi-phase-flow-assurance/table-of-contents/