While concrete structures have been used extensively to develop large oil and gas fields in the North Sea, they might now serve new markets.
Concrete is a perfect material for marine applications, even under severe metocean conditions, with a typical design life of 50-plus years, said Rolf Eie, senior adviser for Aker Kværner Technology, Marine Structures.
The material, says Eie, is extremely tolerant of the cold - down to minus 160°C - and it is therefore impervious to liquefied natural gas contained within concrete at that temperature. "Unlike carbon steels, which are subject to embrittlement."
While Aker Kværner and Linde of Germany have been responsible for front end engineering and design for the liquefied natural gas (LNG) terminal that will serve Statoil's Snøhvit project, the former company believes the same concept could work elsewhere, too.
The Snøhvit design entails construction of a steel barge, which will carry the processing topsides for this Barents Sea project.
Once the gas processing topsides are constructed for Snøhvit - they are being built by Dragados Offshore in southern Spain - the barge will be emplaced or sunk into a prepared dock to provide a permanent LNG complex at Hammerfest in northern Norway. Ship-builder Izar is currently building the steel barge in northern Spain at its Ferrol shipyard.
More gas barges, serving either as permanent processing plants or for re-gasification roles, could be on their way to meet growing world demand for gas.
"This is a concept where we intend to place these structures close to the gas reservoirs for liquefaction - avoiding the need for long and expensive pipelines to shore - and some distance from shore for re-gasification and sending gas out," said Knut Sandvik, vice president of business development, Aker Kværner Technology.
Sandvik says these offshore liquefaction facilities could either be gravity base structures or floating barges. "We are proposing them for possible LNG import terminals in Southern Europe and in the US."
The Norwegian company has carried out an extensive design engineering program to mature the concept. "We are pretty far down the line," Sandvik said. "We have bid two concepts, one in 1997, for the Timor Sea." In this instance, Aker Kværner's bid was for a gravity-based structure designed to provide a gas liquefaction plant sitting on an offshore reef.
A second bid recently has been made for another offshore application, in this instance to Italy's Edison Gas for a shallowwater plant approximately 9 miles (15 km) from Venice in a water depth less than 100 ft (30 m) in the Adriatic Sea.
This unit is designed to be bottom fixed, with LNG tanks within the hull and re-gasification equipment on deck.
But the project is on hold at the moment; Edison is in talks with possible equity partners and is considering alternatives to reduce the cost of the total project. A decision to move the project forward should be made by the end of the year. "We are still hopeful that this will be based on our offshore terminal concept, Sandvik said.
His company is convinced that the offshore industry will see several such facilities being utilized at different locations worldwide in the not-to-distant future.
LNG markets
Success of any gas barge, either concrete or steel, depends on a range of factors, one being the development of a gas supply chain.
Without a local market for a gas resource, a pipeline to get gas to market is said to be the most economical solution up to a distance of 1,250 miles (2,000 km).
Beyond that distance, LNG projects become more appealing. This is where concepts like the concrete barge could succeed, particularly as a re-gasification facility within a receiving terminal nearing entry into the market.
According to a US Energy Information Administration (EIA) report in 2002, there is reason to believe that the market for gas transport and processing systems will grow, certainly in the United States. The EIA has predicted US natural gas consumption in 2020 will be 33.8 Tcf. That same US report, "Natural Gas Markets: Mid-Term Prospects for Natural Gas Supply" points out only three LNG terminals are currently operating in the United States: Everett, Mass.; Lake Charles, La.; and Elba Island, Ga. A fourth, at Cove Point, Md., which is due to receive imports of LNG from Statoil's Snøhvit project, is due to become operational again by 2004. Even so, these four facilities, plus predicted capacity increases of about 0.4 Tcf/year, are predicted capable of supplying only 3.3% of projected total demand in 2020, said the EIA. Another 13 LNG terminals have been proposed for the United States in the years ahead.
Therefore, the concrete barge concept being promoted by the Norwegian group could have a lot of mileage in the US market.
Sandvik said his barge could be served by LNG tankers with storage capacity typically of 8.8 million cu ft (250,000 cu m). "That is a fairly typical figure," Sandvik said. "This is governed by the frequency of the LNG carrier." Smaller carriers could be designed to carry 4.9 million cu ft (140,000 cu m) of LNG.
Looking at potential construction sites for this barge concept, Aker Kværner has its own construction facilities at Stord, Rosenberg (Stavanger) and Egersund in Norway. But that is not necessarily where a barge would be built.
"We will build it where we find it to be most effective," said Sandvik. "Some may be built in Norway, but we are also looking at other areas. If you are going to build one for the Gulf of Mexico, you have to think of ways of building it over there."
Concrete is the preferred construction material for an LNG barge designed as a receiving terminal. Although much heavier than a steel barge, this higher weight would be better for siting on the seabed.
Concrete would be better able to withstand a breach in the LNG container tanks, too, because steel would be embrittled and cracked by the frozen LNG, which is usually kept at minus 160°C. Concrete is not affected by this temperature.
"We are also considering steel structures," Sandvik said.
Several facilities, including Far East shipyards such as Keppel and Hyundai, would be capable of building them to an Aker Kværner design.
Construction of a concrete barge would present less of skills problem, too. "Concrete structures are built everywhere. You can more easily do this work locally where you do not have a very skilled labor force. Steel requires more welders."
Compared on a cost-per-tonne basis, the concrete concept works out much cheaper. "You can say that for a gravity base structure, concrete is more advantageous. For a floating structure, steel has some advantages."
Looking at weight, Sandvik suggests a concrete barge with storage for 3.53 million cu ft of gas (100,000 cu m) could work out at a 270,000-tonne structure.
"Some of these larger offshore gas fields, they are located in deep water, which lack infrastructure, and without a local gas market, you need to export the gas," Sandvik said.
It is for this reason that he is convinced either steel or concrete gas barges could play a larger role in future development of gas resources worldwide.
"I think we will see a few of these in the market in the next few years," he said. "I hope we can have a contract on this within a couple of years."
Referring to Aker Kværner's previous experience in developing onshore and offshore gas processing plants and its concrete know-how, Sandvik, says, "Our installations have operated safely, some since the1970s, in full compliance with safety and environmental regulations and despite the harsh environment."
His company has carried out more than 20 studies for design, construction and installation of LNG storage systems and liquefaction or re-gasification plants either on, near or offshore.
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