VT Halter Marine, the American subsidiary of Singapore-based ST Engineering, notes of several technical challenges faced during the construction of Crowley Maritime Corporation (Crowley)’s liquefied natural gas (LNG)-powered Combination Container Roll-on/Roll off (ConRo) ships.
Crowley took delivery of El Coquí, the world's first LNG-fuelled ConRo vessel in July.
An important consideration in the development of El Coquí was the requirement to use LNG fuel – a move aligned to impending International Maritime Organisation (IMO) regulations on lowering shipboard emissions, according to Buck Younger, Vice President of Engineering at VT Halter Marine.
“The slow speed main engine, which runs on a high press diesel cycle, was specially selected for its ability to offer the lowest levels of methane slip, which accounts for cleaner methane emissions,” he says.
“Care was also taken in the design of the fuel gas system to trap and reuse any methane discharge associated with system purging. The customer brief was to achieve zero methane discharge!”
Another key consideration was the requirement to locate the LNG storage tanks and LNG pump room below the main deck.
In the absence of an international safety code governing LNG-powered ship designs at the time, the team adopted the IMO Interim Guidelines MSC 285(86) and United States Coast Guard (USCG) Policy Letter 01-12 instead, requiring close collaboration with DNV GL and USCG.
Efforts were also invested to insulate the vessel’s steel plates against potential LNG spillage on board. Besides locating the LNG storage and pump room in a stainless steel room below deck, the team insisted on the use of IMO Type C double-wall vacuum-insulated LNG storage tanks, which were built for high resistance to cracks and fractures.
These tanks offered a 58-day hold-time compared to the Class requirement of 21 days, thereby prolonging the efficient storage of LNG while eliminating the risk of unlikely leaks in the inner tank.
Other green ship innovations incorporated on board the El Coquí included two ballast water treatment units that comply with regulations on the discharge of ballast water; an oily water separator that prevents the discharge of oil and gasoline overboard; and the use of double-wall vacuum-insulated stainless-steel gas pipes for the safe conveyance of LNG.
Photo credit: Crowley Maritime Corp.
Published: 10 October, 2018
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