Chemists declare war on 'ice-plugs' in oil pipelines

The researchers are trying to solve the mystery of what happens when hydrates are formed. These ice-like crystals can accumulate in the pipelines that carry oil and gas ashore from subsea fields on the continental shelf.

Senior scientist Sylvi Høiland of SINTEF Petroleum Research is the leader of the hydrate project called "Hyades," which is supported by the Research Council of Norway. The University of Bergen, StatoilHydro's research centre in Bergen and the US oil company Chevron are also members of the research team.

"Our final goal is that it will become possible to take a sample of oil into a laboratory, where simple analyses of the composition of the oil can produce results such as that: the hydrates found in oil A are very likely to block the pipeline, while oil B will not be a problem at all," says Sylvi Høiland.

Høiland emphasises that the Hyades project on its own will not be enough to bring us to that point, but that it should take the offshore industry an important step further towards the goal.

In any case, the international petroleum industry has long been aware of the project. Shell recently announced at a conference that the company regards the method for characterising different types of oil as promising.

Long distances

The background for this project is oil and gas recovery from subsea fields, from which the well flow is carried in pipelines either to neighbouring platforms or directly to shore, in the latter case over increasingly long distances, which is not always a simple matter.

This is because it is not often that a flow of pure oil or pure gas emerges from the well. The product flow is nearly always a mixture of oil, natural gas and water.

In the pipelines that carry mixtures of this sort across the seabed, the gas and water will form hydrates if the temperature inside the pipeline falls far enough, which is what happens when pipeline transportation takes place over long distances, because the cold seawater outside the pipe gradually cools the oil and gas on the inside.

Similarly, when maintenance work or other circumstances make it necessary to shut down production on a field for some time, the pipeline temperature may fall so low that it creates ideal conditions for the formation of hydrates.

Large plugs

The scientists' hope is due to the fact that certain types of oil possess properties that prevent hydrates from becoming sticky "snowballs." When such oils are present, the hydrates stay in the form of a fine powder that can easily be carried along the pipeline.

But which components of the "unproblematic" oil actually make it unproblematic? That is the key question being studied by the Hyades scientists.

Some types of hydrate crystals are worse than others. Certain hydrates have properties that make the crystals resemble a sticky slush. These are capable of growing into large plugs that can completely block the pipeline.

In order to prevent hydrate formation, the oil companies pump large volumes of methanol or glycol into many of their wells, pipelines and process systems.

Similarly, during both planned and unforeseen production shutdowns, large amounts of chemicals are added to production systems to keep hydrates from forming, all of which is expensive.

Not only are the chemicals themselves dear. When shutdowns are ordered, production losses are prolonged by the time needed to pump large volumes of chemicals down the pipelines, and on the shelf, lost time means lost income.

But for now, the SINTEF scientists have raised a hope that the use of chemicals in pipelines can be reduced in the future.