An artist’s impression shows one of the two 145 000 m3 shuttle and regasification vessels for the Neptune deepwater port terminal in Massachusetts Bay. The vessels are to be powered by Wärtsilä 50DF dual-fuel engines.

Known as SRVs, the craft will transport and regasify LNG onboard
Two advanced vessels are currently under construction (with an option for a third) at Samsung Heavy Industries’ yard in Korea on behalf of a joint venture between Norwegian company, Höegh LNG and Suez LNG,a United States subsidiary of French global energy company, Suez. The first of these vessels is due for delivery toward the end of 2009 and will serve the Neptune LNG deepwater port terminal in Massachusetts Bay being constructed to deliver imports of natural gas to the U.S. mainland.

The proposed terminal will receive natural gas vaporized onboard the purpose-built LNG carriers based on the concept developed by Höegh in 2001. Known as SRVs (shuttle and regasification vessels),the craft are said to be the most advanced of their kind and will be used both to transport and regasify the LNG onboard, thus enabling the gas to be piped ashore and fed straight into the existing subsea gas pipeline.

The Neptune deepwater port terminal will be located some 16 km south of the city of Gloucester, Massachusetts, U.S.A., and 35 km northeast of Boston, Massachusetts, in 76 m of water. Suez LNG sees the proposed terminal as a supplement to its existing onshore terminal at Everett, Massachusetts, and the most efficient solution to meeting New England’s growing natural gas needs.

The Neptune Project,which has recently received full licensing from both the Commonwealth of Massachusetts and the U.S. Maritime Administration, will be able to supply an average of 11 million m3 of gas per day — enough to provide the daily requirement for 1.5 million homes. On very cold days or other periods of high demand, Neptune can increase the delivery rate to 21 million m3 of gas per day.

Höegh points out that the vessels are being built to Det Norske Veritas classification and will be able to meet the most stringent environmental conditions in the world. Each craft will be 280 m long and displace 71 000 tonnes. When in transportation mode, they will have a service speed of 19.5 knots. On each craft, four gas tanks will carry 145 130 m3 of natural gas.

Main propulsion aboard each vessel is provided by three Wärtsilä 12-cylinder Vee-configuration 50DF dualfuel engines each rated at 11 400 kW, plus one five-cylinder, inline,50DF dual-fuel engine developing 5700 kW, giving the vessel a total power capacity of just under 40 MW. The main engines drive a single fixed-pitch propeller through an electric propulsion system. All four power units are designed to run on natural gas, heavy fuel oil or marine diesel oil.

Developed from Wärtsilä’s highly successful type 46 diesel engines, the 50DF models have cylinder dimensions of 550 mm bore by 580 mm stroke. Available in configurations with six-, eight- and nine-cylinders inline, and 12-, 16- and 18-cylinder Vee configurations, the 50DF units develop 950 kW per cylinder. In gas mode, the engines operate according to the Otto lean-burn process, gas being admitted into the air inlet channels of individual cylinders during the intake stroke. Reliable ignition is obtained by injecting a small quantity of diesel oil directly into the combustion chambers as pilot fuel. This ignites by compression ignition as in a conventional diesel engine.

These 50DF engines are currently being applied increasingly to drive LNG carriers in place of the old steam turbine drive system. To meet the current demand, Wärtsilä has recently entered a joint venture with Hyundai Heavy Industries to manufacture 50DF units at Yeongsam in South Korea. During a ceremony to announce this venture, Wärtsilä vice president Erik Pettersson pointed out that, to date, the company had received orders to build 206 engines for 52 LNG carriers. Four of these ships are now in service, including the British Emerald, delivered by HHI in July 2007 and currently the largest of its kind. Of the Wärtsilä orders for 50DF engines for the 52 ships, all but five were ordered at Korean shipyards.

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A diagram showing the setup on theNeptune project. The regas ships will moor at unloading buoys connected to a subsea pipeline, which will feed gas directly into the mainland natural gas network.

The SRV vessels are modified versions of standard LNG carriers with a cylindrical trunk forward of tank number one to accommodate the submerged turret mooring buoy and swivel system. Additional equipment includes skidmounted regasification units on deck; bow and stern thrusters; supplementary electrical power supply; and supplementary steam production for regasification.

The ships will moor at one of two unloading buoys connected to a subsea pipeline by a flexible riser, ensuring that the gas supply is not interrupted when one ship replaces another. An SRV will typically moor for four to eight days, depending on the shore demand. The shipboard unloading system through the submerged turret buoy arrangement is provided by Advanced Production Loading (APL) of Norway.

Three vaporizer units for regasification are supplied by Hamworthy Norway. These are skid mounted on deck and have the send-out capacity of about 21 million m3 per day. Hamworthy’s regasification unit is designed for marine applications and cryogenic working conditions. However, since it is a novel design, a small-scale test plant has been constructed as a joint industry project. It is anticipated that future offshore terminals will be sited close to market areas where significant variations in seawater temperature may prevail.

This is taken into consideration in the design. To avoid freezing of the unit, LNG is heated by propane in a closed loop with the propane being heated by seawater. In situations where the seawater is too cold to supply the necessary heat energy, additional heat can be introduced. It is possible to supply all the heat required through, for example, steam directly or in a closed loop with fresh water under very cold winter conditions, or where seawater is not allowed to be used.

At the time of writing, Hamworthy had announced another important regasification order with Golar LNG for three units to be retrofitted to the 138 000 m3 tanker, Golar Winter. This vessel is being chartered by Petrobras as a floating storage regasification unit (FSRU) in an LNG import project located in Guanabara Bay, Brazil. Hamworthy said this application will use seawater heating rather than the steam heating featured in the first systems ordered.

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