ADVANCED TECHNOLOGY

Leonard LeBlanc Houston Gas-in-ice process is used to convert natural gas into pelletized hydrates for easy handling and transport. Table The transport of natural gas as a frozen hydrate appears to be viable process, and developers of the method say it is less expensive and safer than LNG transport. The process involves creation, storage, and transport of frozen hydrate pellets with only refrigerated containment.

Leonard LeBlanc
Houston

Natural gas transport as hydrates 25% below cost of LNG

The transport of natural gas as a frozen hydrate appears to be viable process, and developers of the method say it is less expensive and safer than LNG transport. The process involves creation, storage, and transport of frozen hydrate pellets with only refrigerated containment.

Frozen hydrates can be refrigerated to -15° C at which point they become stable enough for long-distance transport. At this temperature and at atmospheric conditions, the hydrates remain in a state where there is no expansion or contraction.

In order for the hydrates to decompose into gas and water, heat is required. Since the thermal conductivity of hydrates is so low, very little of a cargo of hydrates will decompose during transport. As long as the cargo is below 0° C, thawing hydrates nearest the hull members will re-freeze as solid ice, protecting the remainder of the carbon.

In order to render the hydrates into a condition where they may be easily loaded, stored, and unloaded, the Norwegian Institute of Technology has developed a gas-in-ice process where natural gas is injected into an ice and water slurry. Simple, unjacketed tank reactors continuously stir the forming hydrates as the ice melts. In the end, the water slurry is separated from the hydrates with decanters or cyclone separators, and further dried before storage as pellets. The resulting pelletized hydrates can be stored in any insulated containment until time for vessel loading.

At destination, unloading of the vessel and storage of the hydrates is the reverse of loading. Regasification is simply a matter of pouring warm water (warmed by power plant effluents) directly onto the pellet cargo and compressing the escaping gas. The water (same volume as hydrates) is re-loaded into the ship and transported back to the hydrate formation facility, which means that a constant supply of new water does not have to be provided.

Gas hydrates contain 15% gas and 85% water. There are 150 cu meters of gas in one cu meter of hydrates. This means that in order to carry about the same amount of natural gas, a cargo vessel would have to transport 250,000 cu meters, instead of a 125,000 cu meter vessel for LNG. However, the cost of the large vessel would actually be much lower, since specialized pressurization and refrigeration would not have to be provided. Where a 125,000 cu meter LNG vessel would cost $250 million, a 250,000 cu meter gas hydrate bulk carrier with only insulation would cost $60-70 million.

The loading and unloading times (with regasification) for the gas hydrate carrier is estimated to be about the same as LNG transport. Production costs for gas hydrates would be about 40% less than LNG. Regasification costs would be about 10% higher than LNG. Transport costs would be about 25% less than LNG shipment.

Reference:

Gudmundsson, J., Hveding, F., Bdashrrehaug, A., "Transport of Natural Gas as Frozen Hydrate," ISOPE Conference, Proceedings - Vol. 1, The Hague, The Netherlands, June, 1995.

Roll-on, roll-off semisubmersible will launch satellites

The idea developed 10 years ago to launch rockets bearing satellites from jackups or semisubmersibles stationed far offshore has re-surfaced - this time, in contract form. Originally, Rowan Companies, in response to a query from the US National Aeronautics and Space Administration, designed newbuilt and converted drilling units for rocket launching. Although vetoed as unnecessary during a period of US government budgetary constraints, the program was held on abeyance.

There are four good reasons for satellite rocket launches at sea. The launch vessel can be positioned nearest the geostationary or orbit location in order to minimize the size and cost of the rocket and simplify satellite-launch separation processes. The launch platform can be moved to clear weather launching conditions. Land sites of sufficient size in clear weather areas are hard to find. Errant rockets can be easily destroyed when they are over the ocean.

Now, the ten-year-old idea has moved to contract, minus NASA. Boeing Aircraft, Kvaerner Energy, RSC Energia (Russian space company), NPO Yazhnoye (Ukrainian rocket company), and others have issued contracts to two Kvaerner yards for construction of launch and support vessels. Kvaerner Rosenberg of Norway received a $50 million contract to convert a semisubmersible drilling unit into a mobile rocket launching pad. Kvaerner Govan Shipyard of Scotland will build a $60 million roll-on, roll-off ship that will provide support for the launch. Kvaerner is responsible for all marine equipment and marine logistics.

Firing of the rockets from the semisubmersible will be controlled from the support vessel.

The initial plan is to build rockets in Russia, satellites in the Ukraine, and load the two at St. Petersburg, Russia. From there, the rockets will be transported to positions off the US West Coast, where the launches will take place. The fabrication contracts contain cancellation clauses, in the event of insufficiency in launch contracts or financing. First launch is scheduled for 1998.

PVDF lined pipes may accommodate higher temperatures

An increasing number of fields in the North Sea with H2S, CO2, and high temperatures are being developed. In order to accommodate these corrosive environments, the production stream must be dried or corrosion resistant alloys used to contain and transport production. Both solutions are expensive.

British Gas is investigating the use of swaged polyvinylidene fluoride (PVDF) liners for pipelines, risers, wellheads, and other production flow equipment. PVDF offers temperature protection up to 150° C and resists most production chemicals.

Currently, polyethylene lined pipe is used in low pressure distribution and water injection offshore. The liners were installed mechanically and not swaged, meaning that joining crevices offered chemical access to steel surfaces. Further, polyethylene is not effective in temperatures over 90° C.

British Gas feels that swaging PVDF onto pipes and other equipment will solve most of the chemical and temperature problems and enable designers to use a less expensive alloy or standard carbon steel pipe grades.

Copyright 1995 Offshore. All Rights Reserved.

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