Kawasaki Heavy Industries’ hyper-elliptical, Moss-type tank has been developed for use in 180,000m3-class, neo-Panamax LNG carriers
Against the backcloth of the growing volume and increasing diversity of LNG trade by sea, and the bid by shipowners and charterers for greater operating flexibility and unit cost efficiency, development and refinement of cargo containment systems continues apace.
Refined, enhanced and adapted with recourse to the latest technological tools for design, analysis and construction, the cargo containment solutions chosen for the preponderance of recent and current newbuild LNG carriers are based on thoroughly proven, longstanding systems. The sector’s circumspect approach is vindicated by an unrivalled safety record over its 50-year history.
The bid by various groups to become players in the challenging field of LNGC tank system technology acts as a competitive spur to the dominant names in LNG cargo containment design. But the fundamental importance and value placed by owners and charterers on system reliability and performance in service will have a signal bearing on the future uptake of new and untried solutions.
Meanwhile, the drive to reduce cargo evaporation rates, or ‘boil-off’, is unremitting, and the best-in-class boil-off rate (BOR) level of 0.08% achieved in recent years could be overtaken soon.
Membrane-type tanks are employed in about three-quarters of the active LNGC fleet, and predominate in 90% of more of current newbuild projects, with spherical tanks and their derivatives nominated for the balance.
Mark V debut
Market leader Gaz Transport & Technigaz (GTT) has not rested on its laurels, ploughing earnings from construction license fees back into R&D to continually improve and expand its offering of membrane solutions. GTT’s new Mark V containment system will have its first application in a 180,000m3-capacity LNGC to be constructed by Samsung for the expanding GasLog fleet, and due to be delivered during the second half of 2019.
The Mark V solution builds on extensive in-service experience with the Mark III design, using the same 304L stainless steel, corrugated primary membrane and reinforced polyurethane foam (R-PUF) insulation, but with a new nickel-steel alloy (Fe-36%Ni) corrugated secondary membrane. It promises a significantly improved, guaranteed BOR of 0.07% per day, which would set a new standard for the industry.
The French cryogenic engineering company’s membrane portfolio has also been augmented by the NO96 MAX system, a derivation of the NO96 GW solution, nominated for more than 50% of current LNGC and ‘floater’ newbuilds with GTT containment technology.
While embodying the Invar nickel-steel primary and secondary membranes and the same insulation as the NO96 GW system, in which the plywood boxes are filled with glass wool, the MAX variant has a new, pillar-type bearing box structure to enhance thermal and mechanical performance. The pillar configuration and non-structural insulation are designed to withstand loads induced under certain sloshing conditions. Compared with the 0.125%-0.13% BOR for the NO96 GW, the MAX version is proposed at a BOR of 0.09% per day.
The improved box reinforcement is also claimed to extend the market reach of the system to multi-gas carriers, small-scale LNGCs, and bunker tanks in LNG-fuelled vessels.
Easing filling restrictions
The filling levels for a membrane LNGC of the 174,000m3 class under North Atlantic conditions are typically limited to above 70% or below 10% of tank height. By virtue of its specially reinforced boxes, the NO96 MAX system will enable the filling limits to be eased to above 50% and below 15% of tank height, conferring greater ship operating flexibility. Modification of Mark III and Mark V solutions by adopting an optimised, high density (HD) foam insulation will also allow the same relaxation in filling limits. Furthermore, GTT is ready to offer a design for smaller vessels, up to 35,000m3, with no filling restrictions.
Joint development projects were initiated in 2016 with both Daewoo Shipbuilding & Marine Engineering (DSME) and Hudong Zhonghua for the industrialisation, validation and preparation of a first ship application. However, a further evolution of NO96 technology, designated the NO96-L03+ system, has recently scored a breakthough deal encompassing four newbuilds of 174,000m3 contracted from Hudong Zhonghua to the account of Mitsui OSK Lines.
Within the standard NO96 thickness, the NO96-L03+ version embodies three layers of boxes, filled with glass wool, and employing a third layer of insulation made from R-PUF foam panel. The assured BOR of the arrangements is 0.105% per day.
The ship operating flexibility offered by the robust Moss spherical tank containment technology, renowned for its ability to accept any filling level without sloshing problems, was fundamental to its selection for MISC’s incoming Seri C generation of LNG carriers (see page 37). The tanks have a full length covering in keeping with the integrated hull structure (IHS) concept, bolstering longitudinal hull strength while reducing overall weight.
The Moss system has provided the foundation for a new tank design developed by Kawasaki Heavy Industries (KHI) and granted General Approval by ClassNK in January 2017. Approval in Principle (AiP) has also recently been forthcoming from DNV GL and ABS.
Hyper-elliptical tank
KHI’s solution is focused on optimising neo-Panamax LNG carrier design through the adoption of a hyper-elliptical form of cargo tank to make greater utilisation of space within the hull envelope. The new, IMO Type B independent tank concept allows a 15% increase in cargo capacity to 180,000m3 compared to using the latest ‘stretched’ Moss spherical tank system in such vessels. Sloshing and buckling analyses indicated the same reliability and freedom from filling constraints as spherical tanks.
Under a joint technical project, DSME and DNV GL have formulated a design proposal for a membrane-type, neo-Panamax LNGC with a cargo intake of 198,000m3. A significant facet is the planned incorporation of a dedicated, manganese-steel tank to collect boil-off for use as propulsion fuel.
South Korea is the world’s pre-eminent force in LNG carrier and ‘floater’ construction, and the industry is keen to reduce its reliance on foreign containment systems with the associated expenditure on licence fees. In addition to designs that are being formulated by individual yards, home-grown technology in the shape of the KC-1 membrane containment system is about to make its sea-going debut.
Developed by natural gas importer, distributor and public utility KOGAS in collaboration with the country’s three leading shipbuilders, KC-1 has been chosen for two 174,000m3 newbuilds, the first of which is due out of Samsung’s Geoje Island yard later this year. Destined to haul shale gas from the USA to Korea, the LNGCs were contracted by SK Shipping on the back of 20-year assignments by KOGAS, which also provided charters for GTT membrane-type newbuilds of the same size booked from DSME for other Korean owner-operators.
KC-1 incorporates 1.5mm corrugated stainless steel membrane as the primary and secondary barriers, and polyurethane foam of 115kg/m3 density as the insulation material, an arrangement used in shore-based storage. 50 patents relating to the system have been registered in Korea and overseas.
The technology has a further commercial reference, having been chosen this year for a coastal LNG transport scheme. Signifying the country’s first ‘small-scale’ LNG carrier initiative, the project entails two 7,500m3-capacity vessels ordered from Samsung. As with SK Shipping’s KC-1 newbuild scheme, employment guarantees have been provided by KOGAS.
Small-scale duo
Commanding a total contract value of about US$100m, the gas tankers will shuttle LNG from the KOGAS import, storage and regasification complex at Tongyeong, on the south coast, to Jeju Island. One of the ships will have a dual capability as an LNG bunker tanker, in anticipation of the future uptake of LNG fuel by vessels trading coastwise and in intra-regional traffic. Deliveries are expected in May and December 2019.
Samsung’s will to embrace the ‘small-scale’ segment is also expressed in a recently-sealed joint technical project with DNV GL. Under the pact, two 30,000m3 designs will be developed. One will incorporate membrane-type containment and the other will use Type C, pressurised tanks. The shipbuilder decided on the parallel design approach so as to give shipowners more options, and believes that some may decide on Type C tanks for smaller tonnage, as commonly favoured in LNG bunker tankers.
Indigenous Japanese technology in the form of the IHI SPB, self supporting prismatic tank system has been specified for a series of 165,000m3 LNGCs ordered by Japanese owners on the back of employment commitments by the Tokyo Gas affiliate Tokyo LNG Tanker Co. Only two deepsea LNGCs, completed in 1993, have previously used the SPB solution.
The four newbuilds have been entrusted to Japan Marine United. It was originally anticipated that three of the quartet would be delivered in 2017, although industry sources report delays in the construction programme.
LNG New Technologies of Singapore and Norway was established several years ago to bring forward a novel solution known as the LNT A-BOX, conceived as a simple and efficient system that could enable more shipyards to build LNG carriers. Applying proven technologies in a new configuration, it comprises an IMO type A independent, prismatic tank type as the primary barrier, a conventional cargo tank support system and thermal insulation attached to the hull compartment acting as a full secondary barrier.
A newbuild of 45,000m3, developed with FKAB Marine Design of Sweden, will provide the initial reference for the LNT A-BOX concept. Powered by a dual-fuel four-stroke engine, the vessel concerned is expected to be handed over to Saga LNG Shipping in early 2018 by China Merchants Heavy Industry at Jiangsu.
Anti-ageing strategies
Notwithstanding the economic merits of using naturally-occurring boil-off to fuel ship’s propulsion machinery, as far as practicalities and power demand requires, the consequent reduction in the delivered volume of cargo is an issue for charterers. In addition, the more volatile components (nitrogen and methane) vaporise first, changing the composition and quality of the LNG over time. This is known as ageing, and is an important commercial consideration since LNG is sold and priced according to its energy density and heat value.
An increase in BOG raises the pressure in the cargo tanks. To maintain the tank pressure within the safe range, BOG should be continuously eliminated, be it through use as fuel in the main and auxiliary machinery, through reliquefaction and return to the cargo tanks, or through burning in a gas combustion unit.
Striving for a lower achievable BOR is also salient to the increased issue of excess BOG resulting from the introduction of more efficient, dual-fuel two-stroke propulsion engines.
Although BOR is primarily governed by the design of the cargo section and its insulation properties, operational conditions also have an important bearing on actual, in-service performance, such that the phenomenon is to some extent operationally controllable. Sloshing also has a direct impact on BOG. According to GTT’s wholly-owned affiliate Cryometrics, BOG is the second largest operational cost for an LNGC charterer, representing about one-third of the overall shipping costs.
GTT contended in April 2017 that, on the basis of an LNG price of US$7 per MMBtu, a 1% reduction in boil-off equated to approximately $100,000 per-ship per annum in savings for the charterer. Notwithstanding the fact that the energy price has subsequently weakened, judicious intervention clearly offers the scope for long-term savings.
Therefore, GTT and Cryometrics have together devised a decision support tool to assist both crew and shore-based personnel to monitor and manage BOG during a voyage. Based on a vessel’s schedule and referencing metocean forecasts, prediction models built into the LNG Advisor Premium software continually calculate the optimum route, speed and tank pressure, using advanced algorithms to account for complex influences on BOG generation such as sea states and LNG cargo ‘ageing’.
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Source: The Motorship
