Hydrates studies focus on stranded gas solution

Company, onshore pilot plant started

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After a 10-year period cooperating on the development of natural gas hydrate technology, Aker Engineering and Professor Jon S. Gudmundsson of the Norwegian University of Science and Technology (NTNU) in Trondheim are ready to bring their knowledge to the market.

This task will be undertaken through a newly established company, Natural Gas Hydrate (NGH), which is equally owned by the two parties. The first step will be to build an onshore pilot plant. This will be a prelude to the first commercial facility, for which engineering could start in 2002 at the earliest.

"We've set up NGH because the scientific and technical results we've achieved at NTNU have been so encouraging," says Gudmundsson. "The time is right to take the technology from the laboratory to commercial usage."

Research has been carried out by Gudmundsson and his team at a dedicated NTNU hydrate laboratory. Aker Engineering's role is to develop cost-effective systems on a commercial scale. "Interaction between research and design has worked very well," says Oscar Graff, Aker Engineering's Project Manager.

Over the last couple of years, the research and development work has taken the form of a joint industry project supported by oil companies Amerada Hess, Arco, Fortum, Phillips, Shell and Total, and the Research Council of Norway. The JIP is scheduled to be completed later this year.

Bad reputation

Natural gas hydrates have a bad reputation through their ability to clog up pipelines and force licensees to spend money on preventive measures. But Gudmundsson and Aker Engineering have found a number of ways

in which hydrate properties can be used to advantage. NGH's initial efforts will concentrate on providing a solution to the problem of stranded gas. "We've focused on this because it's what the oil companies say they want," says Gudmundsson.

In simple terms, the solution involves converting associated gas into frozen hydrate, which is mixed with refrigerated crude oil to make a slurry, and shipped to land in a shuttle tanker.

Hydrate formation takes place in a temperature range of 5-15°C and a pressure range of 50-100 bar. Once formed, the hydrates can be kept in a stable condition under atmospheric pressure by refrigerating them to minus10-20° C. These findings represent a breakthrough, as previous studies indicated that the hydrates would have to be kept under a significant pressure - 50 bar - to keep them stable.

Hydrates-crude mix

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Flow diagram showing the hydrate slurry process.
Click here to enlarge image

The research group has also shown that the frozen hydrates can be mixed with refrigerated crude to form a slurry, which can then be pumped into a shuttle tanker and transported to shore.

Once delivered to land, the mixture is heated and the natural gas, oil and water phases separate. The gas is then available for further processing, storage, and use. The water can be transported back to the production platform by the shuttle tanker and re-used for a further cycle of hydrate formation.

Each cu meter of hydrate contains about 160-180 cu meters of gas. The oil/hydrate slurry is typically composed by weight of 50% oil, 42.5% water, and 7.5% gas. A typical 127,200-cu meter shuttle tanker could therefore transport a cargo of about 63,600 cu meters (400,000 bbl) of oil, 48,500 cu meters of water, and 12 MMcm of gas.

Gudmundsson's group has estimated the cost of installing hydrate slurry modules on an FPSO at $160 million. The capital cost of a new reception terminal would be $60 million, or $30 million for adding the necessary facilities to an existing refinery.

The cost of a shuttle tanker equipped to transport hydrate slurry is estimated to be $110-125 million. For the total hydrate slurry chain, assuming one tanker and a new receiving terminal, the capital cost would therefore be around $345 million.

300 km distance

It is estimated that one tanker would be sufficient to serve a field producing some 80,000 b/d of oil at distances of up to 1,000 km from the terminal. Using the same cost estimates, a comparison of gas pipeline and hydrate slurry costs indicates that the hydrate slurry solution would be more economic at distances above 300 km.

The hydrate process also compares well with alternative forms of offshore gas conversion, according to Graff. "Safety is an important issue here," he says. "The hydrate solution is an inherently safe process using simple methods which operates at safe temperatures and pressures."

Aker Engineering has performed a study for Fortum on the application of the hydrate slurry process to a field in the Barents Sea with delivery to a terminal in Finland. The results were encouraging, says Graff.

Other applications of hydrate technology have been identified by the partners. For pure gas fields, it could be used to form solid hydrates for storage and transport - as with the slurry technology, this could be the solution for remote gas fields. It can also be used to form hydrates of volatile organic compounds, in which case it has the advantage of binding into the hydrates the lighter gases which in current VOC treatment processes are released to the atmosphere.

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