Partial filling of membrane type tanks

Membrane LNG tankers can handle offshore cargo transfers and partial filling operations without any drastic changes to their design, according to a study conducted by Bureau Veritas (BV) and Gaztransport & Technigaz (GTT).

The findings were disclosed by Wong Kin Hoong, chief executive of Bureau Veritas Singapore, when he spoke at the Gas Markets ? Shipping ? FPSO 2002 Summit. The paper was prepared by Bruno Dabouis, director of sales and marketing at BV and a specialist in the LNG shipping sector.

BV and GTT recently studied the feasibility of offshore cargo transfer and partial filling operation on a 138,000m3 membrane LNG tanker. Wong says the study aimed to assess the capacity of a membrane LNG tanker to sustain sloshing loads generated during partial filling operations ? either for offshore cargo transfer or for navigation.

A membrane LNG tank is fitted with an insulation system next to the inner hull of the vessel. It is composed of two membranes acting as tightness barriers for the cargo tanks and an insulation that transfers the loads to the ship?s structures. Membrane tankers are designed to carry loads of 90% or more. For loads of less than 70%, reinforcements to the insulation and hull are necessary.

The study showed that the pressure distribution of sloshing loads was in two phases. Phase I is a “transient peak” reached in the first 0.2 milliseconds. After that, it falls dramatically to a much lower quasi-static pressure.

Wong says: “Fatigue of a ship structure is highly dependent on the loading cases considered for ship operation, but remains within a range that can be addressed in shipbuilding practice. Projects can therefore be considered where LNG vessels would have to perform safely offshore cargo transfer and partial filling operations. The insulation used in the membrane has to be strengthened at the top of the tank.”

He adds that the corresponding area of the hull would also have to be strengthened. Ultimately, this means that membrane LNG tankers can safely undertake partial loads without any radical or revolutionary changes in design or technology.

l Competing classification society, Lloyd?s Register (LR) has recently revealed that it is undertaking similar research to BV, in partnership with GTT and Marintek, the Norwegian Marine Technology Research Institute. The objective of its current research is to determine whether the dynamic pressures likely to be experienced in partially filled LNG tanks will exceed the maximum acceptable values for the containment system. This research is scheduled for completion in February 2003.

LNG shipping: Stable growth outlook

The outlook for the LNG shipping market is for continued and stable demand growth, believes John C Harris, director of specialist shipping at Drewry Shipping Consultants.

Driving demand is the growth in gas-fired power stations as well as the liberalisation in key LNG markets. Harris says that an expansion in US LNG demand is on the cards: “The key US market is expected to be the most important source of demand for LNG producers in the coming years.”

He adds that the “commodisation” of gas prices is unlikely to occur in the near future since LNG movements represent a small fraction of world gas trades.

Overall, the demand for LNG and LNG tankers are running high. Plant capacities are regularly exceeding maximum design capacity by about 10%.

The Asia Pacific is the largest producer of LNG with an annual capacity of 64.4 million tonnes. Another 14 million tonnes are under construction there. Africa is number two with a capacity of 29.7 million tonnes plus 3 million being constructed. The Middle East is becoming a growing source for LNG. The region has a capacity of 26.4 million tonnes with another 21.4 million tonnes being built.

On the LNG shipping sector, Harris says: “Moves by major players to secure uncommitted shipping capacity reflect the widely held belief that a lack of shipping surplus, which has largely been controlled by the sellers of LNG, has held back the growth of the short-term market thus far.”

He concludes that despite the costs and risks present in the LNG market, the number of new projects and fleet development underline the importance of the industry.