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THE SHIPS All LNG ships are double hulled and rely on insulation to keep LNG liquid at -260°F (-162.2°C). The cargo is carried at atmospheric pressure in specially insulated tanks, referred to as the cargo containment system, inside the inner hull. The tanks can be either of the Moss spherical type, or the prismatic, membrane type which conforms to the shape of the ship's hull. Or again, in a development, that was announced after our print issue hit the street, they could be of a new pyramid type (see illustration) from ConocoPhillips, which has just received Approval in Principle from ABS The Moss type is the design emblematic of the LNG ship in that the tops of the spheres protrude above the hull making the ships instantly recognizable. Pioneered by Norway's Moss Rosenberg in the 1970's, the design is now owned by Moss Maritime a.s., Lysaker, Norway, a unit of Italy's ENI SAIPEM. There are three types of membrane containment systems, the Gaz Transport and Technigaz systems and a self-supporting prismatic membrane design from IHI, Japan. Gaz Transport and Technigaz are now one company, whose latest containment system, CS1, incorporates features from the existing Gaztransport No 96 and Technigaz Mk III systems. CS1 uses reinforced polyurethane foam insulation and two membranes, the first one 0.7 mm thick made of Invar (low thermal contraction coefficient metal and high nickel content), the second made of a composite aluminum-glass fiber called triplex. The system has been rationalized to make assembly easier and is prefabricated allowing quick assembly on board. THE MARKET WHERE TIME STOOD STILL To marine industry veterans, today's LNG boom is deja vu all over again. There was supposed to be this sort of LNG boom some 30 years ago. But in 1979, things soured when pricing disputes between U.S. buyers and Sonatrach of Algeria eventually led to the termination of contracts, the laying up of six LNG ships (three of which were later scrapped) and the mothballing of two out of four LNG terminals.
There was still growing LNG demand in other areas, but international LNG ship construction slowed until it got a further impetus in the 1990's. What's different this time around is that the newbuilding market is dominated by South Korea, with China already entering the market. And the technology is advancing. For the first time, some ships are being specified with diesel, rather than steam turbine, propulsion, there are improvements in insulation of cargo containment systems, established containment systems are being twitched and new containment systems are being offered. TALLER SPHERES When is a sphere not a sphere? When it's a vertically stretched sphere. Kawasaki Kisen Kaisha, Ltd. ("K" Line), in partnership with Osaka Gas International Transport Inc. ) and Nippon Yusen Kaisha (NYK), has concluded a contract for a new 153,000 m3 LNG carrier with Kawasaki Shipbuilding Corporation. The ship will be mainly used to carry LNG for Osaka Gas from the Qalhat LNG Project in Oman from 2009. It will have four spherical LNG tanks three of which are being given a 2 m vertical stretch. This advantage enables tank capacity to be increased by about 5.5 percent but within almost the same ship dimensions and with the same fuel oil consumption. THE SHAPE OF THINGS TO COME
A new entrant in the containment system market is Ocean LNG, Inc., Houston. It has developed a "more construction friendly LNG carrier" that can be built in "non-traditional LNG shipyards." ABS has issued Approval In Principle (AIP) to Ocean LNG's tank containment system and carrier design. The tank design is cylindrical with spherical dished ends. The 180,000 m3 version of Ocean LNG's ship design features five of these large 36 m diameter and 40 m high cylinder tanks each holding a volume of 36,000 m3 of LNG. Compared to the spherical containment system, the Ocean system is expected to increase cargo capacity within the same main hull dimensions by an estimated 25 percent, while still providing full access for inspections of both the insulation and tank structure. A key aspect of transporting LNG is the ability of the containment system to withstand dynamic sloshing loads of the LNG cargo when a tank is partially filled. Membrane systems are particularly sensitive to sloshing loads.< The Ocean LNG tank design is fashioned according to IMO Type B independent tank design and based on the "leak before failure" principle.
The tanks are designed to minimize filling restrictions due to sloshing effects. A detailed sloshing analysis of the tank and pump tower designs was recently completed by ABS. TO BOIL OFF ... OR NOT? Since LNG tankers rely on insulation rather than refrigeration to keep their cargo refrigerated, a small percentage will "boil off." Traditionally, LNG tankers have utilized this boil off as fuel in steam turbine based propulsion plant. That's changing. One way or another, the diesel is coming on board. One approach is to reliquefy the boil off gas, so that it remains as valuable cargo. Shipboard reliquefaction technology has matured considerably in recent years, making use of experience from land-based and LPG carrier installations. Among others Hamworthy KSE's patented Moss RS closed nitrogen cycle system for reliquefying boil-off gas offers a solution for pumping LNG back to the cargo tanks and hence the opportunity to deliver more cargo to the buyers. MAN B&W Diesel has produced a paper that thoroughly examines operating costs and additional income from the transport and sale of LNG. The paper, "LNG Carrier Propulsion by ME Engines and Reliquefaction," analyzes fuel oil, lube oil and maintenance costs for both propulsion and electrical power generation under various operating conditions on a comparison basis for a diesel-based solution against a steam plant. The evaluation shows that substantial economic benefits can be gained from diesel propulsion in conjunction with gas reliquefaction over the steam turbine-powered option, simply because of the big difference in thermal efficiencies while the first cost of the various propulsion systems is virtually the same. Overseas Shipholding Group (OSG) has ordered four 216,000 m3 LNG ships from Hyundai and Samsung. Each vessel utilizes two MAN B&W ME engines for main propulsion. Each yard will deliver two vessels for the Rasgas II project for shipping Qatari gas to the U.K. Main propulsion will be by two 6S70ME-C engines in each vessel. The engines for the Hyundai vessels will be built by Hyundai Heavy Industries, and for the Samsung vessels by HSD. The engines will be heavy fuel burning, and the boil off gas will be returned to the tanks via onboard reliquefaction plants. Apart from the reliquefaction solution, another means of utilizing the diesel is by using the boil off gas as fuel. MAN B&W's ME-GI engine is a gas injection, dual fuel low speed diesel engine, which can burn any ratio of fuel and gas desired. Wärtsilä, meanwhile, has had considerable success with its DF dual-fuel engines. The first dual fuel electric LNG carrier, the 75,000 m3 Gaz de France Energy, was recently completed by Chantiers de l'Atlantique of France and is in service with Gaz de France. Furthermore, Wärtsilä has now made a breakthrough into the Korean market with an order from Hyundai Heavy Industries Co. Ltd. to supply four sets of Wartsila 50DF dual-fuel engines to power a series of 155,000 m3 dual-fuel-electric LNG carriers, with an option on four more sets. The ships were ordered by BP Shipping of the U.K. and each ship will be equipped with two 12-cylinder and two nine-cylinder Wärtsilä 50DF dual-fuel engines with an aggregate power of 39.9 MW, as prime movers in a dual-fuel/electric machinery arrangement. ML |
