Hydrate plants for stranded gas fields

Japan?s New Energy and Industrial Technology Development Organization (NEDO) has awarded Mitsui Engineering & Shipbuilding (MES) a development grant to assist it in the construction of a 600kg/day output natural gas hydrate continuous production plant.

The demonstrator plant is being built to verify the effectiveness of a high-speed natural gas hydrate continuous production technology developed by MES. It is scheduled for completion by the end of March 2003. MES is building the plant at its Chiba works.

If the technology proves successful MES says it could pave the way for wider use of natural gas in Japan particularly in remote areas. It could also revolutionise the shipping potential of natural gas from marginal fields.

The development follows ongoing experiments with a similar demonstration plant by the Norwegian University of Science and Technology (NTNU) in partnership with Aker Kvaerner Technology. This plant is currently producing about 2t/day of dry hydrate according to Jón Steinar Gudmunsson who heads the development work at the university on behalf of Natural Gas Hydrate, a joint venture company.

The Norwegians have designed hydrate production processes for dry hydrate blocks (for long-distance transportation) and hydrate slurry. The hydrate slurry process involves mixing frozen hydrate with refrigerated crude oil in a gas-to-oil ratio of about 100:1 and transporting it ashore at atmospheric pressure in shuttle tankers from an offshore production facil-ity (FPSO).

Both the Norwegians and the Japanese will have to increase the output of their demonstrator plants considerably if they are to produce hydrate at a rate sufficient to fill a ship in a relatively short timeframe. Detailed calculations made by the Norwegians for their hydrate slurry shuttle tanker, for example, reckon on a required hydrate production volume of 8,000m3 per day (equating to about 8,000t per day). However Gudmundsson says the upsizing of the plants doesn?t, in theory, present any specific technical challenges.

MES expects the natural gas hydrate system to have profitable applications on small gas fields, which are not economically viable at present. The Norwegians agree.

“Hydrate technology and other non-pipeline technologies based on compressed natural gas and chemical conversion to liquid hydrocarbons are considered appropriate for the conversion of stranded gas [gas found far away from a pipeline] because established LNG technology is only considered feasible in large-scale development,” says Gudmunsson in a paper written in concert with Marit Mork, a student at NTNU and Oscar Graff of Aker Kvaerner Technology and presented earlier this year at the Yokohama Hydrate Conference.

As this statement points out natural gas hydrate technology is in competition with compressed natural gas (CNG) as a potential means of profitably exploiting marginal gas fields. Gudmundsson admits to not having done detailed cost comparisons of the two technologies although he thinks the costs will be similar “depending on the situation”.

He clarifies this by explaining that in some instances the carriage of gas under pressure is useful at the receiving end – such as in a high pressure transmission system ? while in other cases ? such as at a power plant ? there is no pressure demand at the receiving end. He adds that there will also be instances where large compressor power is required for CNG shipment because there is little incoming gas pressure at the point of take-up, while in other cases the gas will come in at high pressure.

MES considers natural gas hydrate technology as “an important means of natural gas transport to substitute or supplement conventional LNG technology”. Natural Gas Hydrate sees the technology as an “attractive alternative” to pipelines and LNG technology in stranded gas situations. It is currently looking for a client that wants to use its technology.

Natural gas hydrate is made of a methane molecule surrounded by water molecules. It can contain methane of about 150-180 times its volume, according to various experts, and its production consumes only half the energy of producing liquefied natural gas (LNG). In addition, say the experts, natural gas hydrate is transported and stored under atmospheric pressure at temperatures of about -15oC making its transportation easier and less costly.

LNG contains about 600 times its volume of natural gas. A natural gas hydrate carrier would therefore have to have three to four times the capacity of a LNG carrier in order to deliver a comparable quantity of standard gas to the pipeline.