LNG CARRIER CONTAINMENT SYSTEMS
Unstinting R&D endeavours by membrane tank technology specialist GTT have brought down LNG cargo evaporation rates.
French cryogenic engineering specialist Gaz Transport & Technigaz (GTT) consolidated its position as the industry’s pre-eminent force in LNG cargo containment technology by attracting orders for 50 membrane systems in 2018, to be fitted in 48 newbuild LNG carriers and two LNG floating storage and regasification units (FSRUs).
One-quarter of all GTT employees are engaged in R&D, mainly at the company’s laboratories in Saint-Remy-les-Chevreuse. Around half the R&D commitment is devoted to thermal performance of GTT technologies, resulting in continual refinement of, and new choices in, the product offering to meet evolving market demands.
Seemingly minuscule gains in insulation performance that reduce cargo evaporation rates, or ‘boil off’, can have a considerable cumulative, beneficial effect, by maximising delivered cargo volume. Furthermore, the introduction of more efficient propulsion systems in LNG tankers has meant that the engines require less boil-off gas (BOG) as fuel, as has long been employed in LNGCs, which has consequently placed increased onus on the efficacy of the cargo containment installation.
At lower ship speeds, in particular, established containment solutions may yield more gas than can be used. The latest versions of GTT’s Mark III and NO96 systems obviate lost boil-off at lower speeds, increasing the vessel’s operational flexibility.
Having ramped-up research into cargo vaporisation, GTT’s work is bearing fruit in a number of areas. Its latest membrane tank solution devised by GTT, designated the Mark III Flex+, has set a new benchmark for the industry by guaranteeing a BOR of 0.07% of cargo volume per day. The previous best had been the 0.085% achieved with the Mark III Flex system that made its seagoing debut in 2014, based on the original Mark III technology with its corrugated stainless steel primary barrier.
The lower BOR achieved by the Mark III Flex+ has been obtained by increasing overall insulation thickness from 400mm to 480mm, and enhancing the secondary barrier arrangement. While the 100mm-thick primary space configuration has been retained, the thickness of the secondary space has been increased from 300m to 380mm. Moreover, the assembly of the secondary barrier below the top bridge pads has been further development in the latest iteration, through the addition of a supplementary layer of rigid triplex. This enhances the strength of the barrier against thermal and mechanical loads.
Early market endorsement of the class-approved system was signalled by the specification of the Mark III Flex+ solution for a total of five LNG carrier newbuildings last year, all at South Korean yards. The first recipient is under construction at the Koje Island complex of Samsung Heavy Industries and is scheduled to make her seagoing debut in 2019.
Moreover, just a few days into the new year, the tally of contracted installations rose to seven by way of the system’s nomination for two 180,000m3 LNGCs booked at the Samsung yard by GasLog. The pair has been ordered for completion in mid 2021 on the strength of two new seven-year charter party agreements sealed with Cheniere Marketing International for shipments from the USA. Propulsive power in each case will be delivered by a dual-fuel two-stroke engine.
The new evolution of the Mark III range has allowed GTT to re-visit, in parallel, the development project for the Mark V system, whose market release has been put back so as to allow revalidation to overcome certain technical challenges and better ensure cost competitiveness.
GTT’s commanding market lead of at least 75% of cargo containment systems for LNGCs and LNG FSRU is based wholly on membrane technology, and is split fairly evenly between the Mark III and NO96 system families. With the distinctive Invar 36% nickel steel primary and secondary barriers retained throughout its evolution, the original NO96 solution has been continually refined and the offering expanded over the years, including NO96 GW, NO96MAX, NO96 L03 and NO96 L03+/NO96+ versions, embodying different insulation arrangements.
A further stage in the updating and enhancement of the system has been signalled by the development of NO96 Flex, for which GTT obtained classification society approval-in-principle last year. NO96 Flex builds on the use of foam insulation panels in conjunction with the proven NO96 technology to hold out the promise of a daily BOR of 0.07%, on a par with the new Mark III Flex+ design.
GTT has also devised a solution for cargo vapour pocket management, allowing LNGCs to operate with an increased tank filling limit(above 98%) in compliance with the revised IGC code. Under the new restrictive constraints, no isolated vapour pocket can appear and remain out of pressure control in the event of excessive list or trim condition following a grounding, collision or allision.
To meet the requirements, GTT has developed a gas crossing at the ceiling of membrane tanks. These crossings of the containment and trunk spaces effectively connect the tank to the vapour header and pressure safety valves. For a typical vessel configuration, only two crossings are necessary to deal with all trim and list scenarios, each located close to the middle of the upper tank dihedrons, and sufficiently recessed from cargo sloshing areas.
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