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.
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37 Fall 2015 Proceedings www.uscg.mil/proceedings • Semi-refrigerated: The cargo is carried at an intermedi- ate pressure and temperature. • Fully pressurized: The cargo is carried at ambient tem- perature. Design Considerations The cargo's volume and critical temperature infuence cargo containment and handling system design. (Critical tempera- ture is the point above which a gas cannot be liquefed by an increase of pressure alone.) While many LPGs can be liquefed by an increase of pressure that is economical to apply on a ship, methane can only be practically liquefed by refrigeration at -260 degrees F. The majority of fully pressurized and semi-refrigerated ships are outftted to carry several types of cargoes. For example, petrochemical gases such as propylene, butadiene, vinyl chloride monomer, and anhydrous ammonia can be carried on liquefed gas carriers, as they have similar charac- teristics to LPG. However, certain ships are designed only to carry specifc cargoes due to temperature constraints (such as ethane and ethylene carriers). Small liquefed gas carriers employ fully pressurized tanks, intermediate-sized vessels are typically semi-refrigerated, and fully refrigerated ves- sels at near ambient pressure have the largest tanks. A key consideration for fully refrigerated and semi-refriger- ated LGC cargo tank and cargo handling system design is that the cargoes will boil off at varying rates during a voy- age due to heat ingress from the surrounding environment. The build-up of this boil-off gas (BOG) is handled in differ- ent ways — either by building the tank structure to man- age the increased pressure and/or by ftting reliquefaction plants onboard. Pressurized tanks inhibit cargo boil-off. Cargo Containment Design Requirements Very cold cargoes can cause brittle fracture of the ship's structure if it is directly exposed to low temperatures, and, as it is not economically feasible to build large LCGs with low temperature-resistant metals, cargo tanks do not share a common boundary with the shell plating. LGC cargo containment systems vary based on the low- temperature nature of the cargo, ranging from low-temper- ature mild steel to certain stainless steels and nickel alloys. The tanks fall into two main categories: independent tanks and membrane tanks. Due to the risk of structural damage if very low-temperature cargoes such as LNG come into con- tact with the ship's hull, ships are ftted with secondary con- tainment systems to capture leakage in the unlikely event that the primary containment system fails. For warmer car- goes, like some LPGs, the secondary barrier can be the ship's hull. Independent tanks sit within the ship's hull and do not use the ship's structure as a part of the containment system. IMO Type B tanks on LNG carriers are engineered for extremely high reliability, such that, in case of failure, fracture propa- gation is very slow and leaks are very small. These tanks only require a drip tray below the lowest point to capture any leaks, with gas detection systems ftted. Membrane containment systems are employed for LNG car- goes and use the ship's structure for support. Such vessels are ftted with two continuous membrane and insulation systems, so that any cargo leakage caused by failure of the inner (primary) membrane will be contained by the second- ary membrane. General Arrangement, Operation, and Safety Systems The requirements for ship design, construction, and testing vary depending on the product characteristics, including cargo tank type and location, monitoring systems, alarms, and other safety measures. Additionally, due to the low- temperature or high-pressure nature of the cargoes, the ves- sels are ftted with a number of redundant safety features. For LNG carriers, all vessels are double-hulled in the way of cargo tanks. LPG carriers typically have double-bottom ballast tanks and the cargo tanks are separated from the side shell. An LGC usually has one to fve cargo tanks, depend- ing on the vessel type and capacity. Some vessels may have individual tanks split into port and starboard tanks, but such An LNG carrier transits the Suez Canal at night. LNG carriers under construction.