Proceedings magazine is a communication tool for the Coast Guard's Marine Safety & Security Council. Each quarterly magazine focuses on a specific theme of interest to the marine industry.
Issue link: http://uscgproceedings.epubxp.com/i/264352
www.uscg.mil/proceedings 54 Proceedings Winter 2013–2014 Engineers Boiler and Pressure Vessel Code or Title 46 Code of Federal Regulations Part 154, in lieu of specifc IGC Code requirements. LNG in Action Advanced insulating techniques, usually involving an evacuated outer tank shell, keep an inner pressurized fuel tank from heating up too quickly, which may be rec- ognized as a design similar to that of some high-perfor- mance consumer thermoses. The pressurized design of this tank permits liquefed natural gas to be idly stored for a longer period without risk of over-pressurization. Usually, the residual pressure within the tank forces the liquid out and through heat exchangers that convert the liquid to gas at a prescribed pressure. From there, the gas may be pressurized further with pumps or simply routed to the engines via a gas valve unit. Gas detectors and automatic isolation valves must be installed in key locations to segregate piping sections and alert operators of hazardous leaks. In the event of a gas leak, an integrated inerting system would food the buffer spaces with an inert gas, such as nitrogen, thereby pushing the remaining methane out through vent pipes terminating high above the weather deck and away from the vessel. If the engines are dual-fuel (capable of consuming either conventional liquid fuel only or a liquid-gas mixture), a seamless transition occurs from a 95 percent gas- to-5 percent liquid mixture to a liquid-only combustion cycle. The small amount of liquid fuel used in normal operating mode is chiefy used for pilot ignition and to aid in the emergency transition process. These engines are especially attractive to owners and operators who are concerned about future cost fuctua- tions for LNG and ultralow sulfur die- sel, as either may be used as the primary fuel without modifcations to shipboard equipment. Looking Ahead The policy letter and interim guidelines by which the above confgurations were designed are just that — guidelines. Pro- ponents of alternative designs are invited to present the operating principles and features that demonstrate a degree of safety at least equivalent to a design in accordance with the accepted standards. If the arrangement is found to be satisfac- tory, the Coast Guard will issue a unique Design Standards Currently, the most straightforward approach for ship- builders pursuing LNG as a fuel is to make use of the USCG Offce of Design and Engineering Standards Policy Letter Number 01-12 Equivalency Determination – Design Criteria for Natural Gas Fuel Systems, which incorporates the International Maritime Organization (IMO) Interim Guidelines on the Safety for Natural Gas‑Fuelled Engine Installations in Ships, as a baseline standard with some modifcations. For example, according to the IMO guidelines, two types of system confgurations address fuel distribu- tion within machinery spaces: gas safe and emergency shutdown. Only the "gas safe" avenue for piping system confguration is accepted under the Coast Guard policy letter. In this design, the fuel distribution system must be completely encapsulated by a secondary barrier, creat- ing a buffer zone between the fuel and possible ignition sources. This involves using gas-tight enclosures around valves and double-walled piping in machinery spaces all the way to the engine cylinder intakes. In addition to safe LNG and gas transfer within the ship, the actual storage of the cryogenic liquid presents unique engineering challenges. The IMO International Gas Carrier Code defines three types of tanks from which designers may choose. Tanks of types A and B carry LNG at or near atmospheric pressure. All design proposals for use of liquefed natural gas as fuel pre- sented to the Coast Guard to date employ tanks of type C, which contain LNG at elevated pressure and relatively "warmer" temperatures (a balmy -229°F). Further, the policy letter stipulates they are to be designed in accor- dance with either the American Society of Mechanical The importance of the OSV is readily clear to those who visit the Louisiana coast, where workboats of all sizes, design types, and capabilities abound. Winter �2013_45.indd 54 2/10/14 9:32 AM