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October 2011 www.sname.org/sname/mt The role of the U.S. Coast Guard (USCG) in the design of LNG carriers in the last few years has been minimal. However, in the U.S., the role of the USCG in the operation of all LNG carriers entering U.S. ports is considerable. Each U.S. port has captain of the port (COTP) LNG operational rules that have been in place for many years. e outstanding safety record of the LNG eet, overall, and of cargo operations in U.S. ports indicates how well these regulations have worked. The USCG, under Annex ll of Resolution MSC.285(86), adopted on June 1, 2009, published Interim Guidelines on Safety for Natural Gas Fueled Engine Installations in Ships .Our purpose here is not to chronicle all aspects of these guidelines, but to use the LNG carrier COTP regulations to show how operational aspects of fuel- ing a vessel with LNG may aect the design. Properties related to design LNG is carried at cryogenic temperatures and any spillage onto steel structure will fracture the struc- ture. As an illustration, a foreign LNG carrier, discharging in the U.S., had had problems with low spots in LNG piping and condensation. To ensure the lines were dry, the crew would open small ¾-in. drain lines in the LNG transfer piping before entering the discharge terminal, and blow nitrogen through the empty lines. To maximize LNG tank capacity, while keeping the deck height low, the vessel was constructed with the deck plating against the LNG tank insulation with the transverse and longitudinal stieners above the deck plating, forming an egg crate structure. One drain was inadvertently left open and upon the start of discharge, the ¾-in. stream spewed out for some 3 minutes before it could be secured. For a 30-ft. radius from the drain, the deck plating and sti- eners were fractured, requiring replacement of all of the aected deck structure. LNG is carried in single- or double-walled cryo- genic tanks and, as a light liquid, its density and viscosity properties make it susceptible to sloshing. All liquids slosh in tanks at sea to some extent; how- ever, the degree of slosh in LNG tanks is greater than most other liquids in that ba es are not installed in LNG tanks. Sloshing imparts energy to the stored LNG. Imparting energy to any liquid may cause a slight rise in liquid temperature; however, in LNG such a rise can lead to the formation of more gas from the stored LNG than might be desired. A poorly-configured rectangular storage tank, mounted well above the roll and pitch center of a research vessel, could provide more gas than the vessel can use if the research vessel is towing an array, at slow speeds, in heavy weather. e excess gas can be vented at sea; however, in the age of saving every btu possi- ble, venting non-usable gas is not economic. Excess gas can be reliqueed, but in small quantities the energy expended approaches the energy saved. Small relique- faction plants exist, but they are quite expensive. Mission pro?le e time a vessel spends at sea and in port is impor- tant in any design, but it takes on an added dimension in the LNG-fueled vessel. USCG LNG carrier COTP regulations do not allow LNG carriers to vent in port, so it is presumed LNG-fueled vessels will also not be allowed to vent in port. ere are many dierent congurations of LNG containment tanks, some with much better insulation FUEL TYPESO x (g/kWh)NOx (g/kWh)PM (g/kWh)CO2 (g/kWh)Residual oil 3.5% sulphur. Marine diesel oil, 0.5%S.. Gasoil, 0.1% sulpher... Lique?ed natural gas (LNG) TABLE . LNG EMISSION COMPARISON FUELING with LNG