Concrete deemed best option for LNG floater hull

Growing demand for natural gas and tougher competition is forcing liquefied natural gas (LNG) producers to look for cheaper methods.

With gas developments anticipated in increasingly remote locations, one concept currently exciting interest is an extension of floating production, storage, and off- loading (FPSO) technology to LNG production.

Jean Claude, project engineering manager, LNG storage systems, SN Technigaz, Paris, told delegates at the Deep Offshore Technology conference in The Hague last week of current studies for LNG floaters involving liquefaction and storage units on a concrete barge.

"This new concept," said Claude, "will allow economical development of many offshore hydrocarbon discoveries containing large quantities of gas or associated gas. Given long distances from consuming centers, many such discoveries could not be produced today, or gas would have to be reinjected, which is usually an uneconomic solution."

LNG concrete hull

Technigaz, a unit of Bouygues Offshore SA, has designed a concrete hull to carry facilities for production, storage, and offloading of LNG.

It is also intended to serve as a floating LNG import/export terminal hull and as a concept for offshore production and export of liquefied petroleum gas (LPG).

"The FPSO for LNG consists of a turret-moored monohull vessel," said Claude, "with a gas liquefaction unit on deck and LNG, LPG, and condensate storage tanks fully integrated within the concrete hull.

"This concept can be adapted to various environments-for instance, the Gulf of Guinea or more severe environments such as Southeast Asia, and for production of 1-3 million metric tons/year of LNG and storage capacity of 140,000-280,000 cu m."

Claude said a concrete hull had been chosen because it provides an unmatched safety margin for external hazards, such as fire and collision with other boats, compared with any other LNG floater concept.

He cited concrete's good resistance to thermal shocks, in the case of LNG spills on deck; good resistance to fire; excellent resistance to external loading from collision or blasts; and a hull design that incorporates a double concrete deck to separate process areas from storage tanks.

Technigaz has studied a hypothetical LNG concrete-hulled floater with liquefaction capacity of 1.5 million tons/year and storage capacity of 140,000 cu m of LNG, located in Southeast Asia.

"This high safety level for a concrete FPSO for LNG was confirmed by a concept risk analysis performed within a development study, for which results should be published shortly," said Claude.

"Furthermore, because of these features of concrete, it is possible to adapt the membrane containment system for use in land storage tanks, instead of the more complex and costly marine containment system for steel structures.

"The cryogenic properties of concrete also have a significant impact on cost. A simple containment system can be used that makes a concrete FPSO for LNG particularly cost-effective; and, as concrete is fatigue-resistant, durable, and requires virtually no maintenance, operating costs are minimized."