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Naval architects extend LNG carrier design envelope in 2017

Tue 12 Dec 2017 by Mike Corkhill

Naval architects extend LNG carrier design envelope in 2017

One measure of the extent to which the LNG supply chain is being broadened to meet the growing demand for clean-burning natural gas is the frenetic activity of ship designers over the past 12 months.

Quite aside from the strengthening interest in floating storage and regasification units (FSRUs), floating LNG production (FLNG) vessels, bunker tankers and icebreaking LNGCs, naval architects continue to introduce new design concepts for large and small conventional LNG carriers.

The goal with all these initiatives is the ongoing drive for ships of increased efficiency and flexibility. Class societies, shipbuilders and gas engineers are the prime movers behind the new design offerings tabled in 2017 but shipowners and charterers are equally committed in the quest to deliver the greatest quantity of cargo in the safest, most-efficient way possible to a widening array of receiving facilities.

Designs for the times

In April Daewoo Shipbuilding and Marine Engineering (DSME) of Korea and the class society DNV GL unveiled an LNG carrier design that not only utilises today’s technology but also takes into account market trends and a regulatory regime laying down tighter restrictions on ship atmospheric emissions.

Created under a joint development programme, the design features an optimal cargo-carrying capacity, hull form and machinery and electrical systems. The availability of new US LNG export volumes and the enlarged Panama Canal locks mean that ships larger than 175,000 m3 sailing on transpacific voyages to Asian markets will feature more prominently in future.         

The DSME/DNV GL post-Panamax concept combines a higher cargo-carrying capacity with a hull form and propulsion system geared to lower, more energy-efficient transit speeds. The introduction in recent years of improved tank insulation for all types of LNGCs has yielded reduced cargo boil-off gas (BOG) rates, and this accords well with the lower fuel consumption of the powerful two-stroke dual-fuel (DF) main engines now commonly specified for these ships.

The drive to cut fuel consumption helps make lower ship speeds attractive. In addition, the more relaxed destination clauses in new charter contracts and the growing popularity of spot cargoes increase the chances of voyage diversions to take advantage of arbitrage opportunities and ease the pressure on the need for just-in-time deliveries. 

DSME and DNV GL state that their hull shape and propulsion system have been optimised for three ship-speed operating profiles on a standard transpacific route – 19.5, 16 and 12 knots. Their concept results in efficiency gains of 6, 2 and 5% over the reference design at the three operating profiles when sailing in calm water conditions.

The LNGC will utilise direct-coupled, two-stroke DF main engines and DF auxiliary engines, with LNG as the primary fuel. A combined gas turbine, electric and steam (COGES) propulsion system has been chosen for the optimised machinery.

LNGreen and variations 

DNV GL has also developed an LNGC design project with another Korean shipyard, Hyundai Heavy Industries. Launched in 2015 and including Gaz Transport & Technigaz (GTT) and GasLog as participants, LNGreen has made use of the class society’s COSSMOS integrated systems engineering approach, computational fluid dynamics calculations and a containment system design tailored to specific operational profiles and anticipated trades.

LNGreen features closely integrated subsystems and components, including the BOG compression trains, gas management system, reliquefaction (if any), propulsion and/or generating engines, exhaust gas economisers and boilers. Project development work has reached a second phase and LNGreen II is now underway. 

Although the DSME/DNV GL and LNGreen design projects have the same high cargo-carrying capacity and propulsive efficiency end-goals in mind, Daewoo wanted to extend the scope of the initiative by incorporating additional technologies the yard has been working on. These include its partial reliquefaction system and the introduction of an IMO Type B high-manganese steel tank of prismatic shape to hold the cargo BOG to be used as LNG fuel.

The cryogenic manganese steel was developed by Posco and in May 2017 the Korean steelmaker’s new material was registered as standard technology with American Society for Testing and Materials. The steel, which has a 20% manganese content, has been specified for the 500 m3 fuel tank installed on the 50,000 DWT, LNG-powered bulk carrier that the Hyundai Mipo yard is building for Ilshin Shipping, but in this application the tank has been constructed as an IMO Type C cylindrical pressure vessel.

Daewoo is promoting its concept for a 200,000 m3, membrane-tank LNGC as the ‘DSME Post-Panamax 200K LNG Carrier’. The design also incorporates the shipyard’s SloT® (ship Internet of Things) technology and wireless computer network and integration system Smartship 4.0 as part of an effort to put data to work in maintaining more robust maintenance and cyber-security regimes.

Kawasaki Heavy Industries (KHI) remains the staunchest supporter of Moss Type B spherical LNG tanks. The Japanese shipbuilder, which has only ever built LNGCs to this design, received AiPs this year from both DNV GL and the American Bureau of Shipping for a new, non-spherical version of the Moss tank.

KHI had previously introduced cylindrical mid-sections to the tank to increase capacity beyond a simple sphere of the same diameter. The latest version builds on this extension principle and yields a four-tank ship capable of carrying 180,000 m3 of LNG.

Marintec announcements

Dual-fuel engine developments over the past 15 years have rendered the traditional steam turbine the least efficient of LNGC propulsion systems. Yet 270 steam turbine-powered LNGCs remain in service, or 53% of the operational fleet.

Earlier this month, at the Marintec event in Shanghai, GE’s Marine Solutions and Dalian Shipbuilding Industry (DSIC) of China revealed details of a preliminary design for the conversion of an LNGC’s steam turbine arrangement into a gas turbine-based propulsion system. The aim is to provide a new lease of life to ships with low fuel efficiency which are not yet ready to be retired from service.

The new design will feature GE’s COGES system, and the gas turbine’s smaller footprint is being touted as an advantage that will help minimise the amount of conversion work required. GE and DSIC report that such a propulsion system conversion on a 138,000 m3 ship will result in a 30% improvement in fuel efficiency, thus increasing the attractiveness of the vessel to charterers.

DSIC announced a second LNGC design breakthrough at Marintec – the award of an AiP from Lloyd’s Register for 30,000 m3, ballast-free vessel that the yard developed in tandem with GTT. Named B-Free, the design enables savings in construction costs by obviating the need to fit a ballast water treatment system (BWTS) and not having to comply with the Performance Standard for Protective Coatings requirements for ballast tanks, ballast piping, pumps and valves.

GTT pointed out that the use of its membrane tanks will result in lower cargo BOG rates than those experienced on vessels with Type C tanks while the concept also offers lower running costs. These will stem from not having to, for example, operate a BWTS and associated ballast systems or maintain ballast tank coatings.

Another Chinese shipbuilder used the opportunity of Marintec to disclose a class society AiP for its LNGC design. Back in April, Wison Offshore & Marine unveiled a design for a 10,000 m3 vessel which it calls its LNG Distributor (LNGD), and DNV GL’s approval for the concept was announced at the Shanghai meeting. 

The Wison LNGD vessel is designed to deliver small LNG parcels to multiple locations. The gas carrier is geared for quick turnarounds at terminals, as its high degree of manoeuvrability eliminates the need for tug assistance, while the shallow-draft design enables small-scale shore stations, including at riverside and estuarine locations, to be served.    

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