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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36 Proceedings Fall 2015 www.uscg.mil/proceedings LNG CARRIER CASE STUDY Let's picture a voyage for a new 174,000 m 3 TFDE (tri-fuel diesel electric) lique- fed natural gas (LNG) carrier 1 that plans to take on cargo in a U.S. Gulf Coast load port and subsequently discharge in Tokyo Bay. The Loading Operation Once the vessel arrives alongside the LNG terminal, the vessel crew connects shore communication systems and emergency shutdown systems to the terminal. Then the crew replaces all of the inert gas in the cargo tanks with cargo vapor from the terminal. This is supplied to the vessel via shore-based hard-arms as LNG, which is vaporized on board. Once sensors detect methane leaving the cargo tank, the vapor is directed back to the terminal via the vapor return line for reliquefaction. Throughout the loading operation, all vapor displaced from the cargo tanks is also returned ashore to the shore tank. O n ce th e g assin g - up p r o cess is completed, the crew cools the cargo tanks before receiving LNG cargo at -163°C. The cool-down process and the loading rates must be carefully controlled, typically lowering system temperatures at 10°C per hour, to avoid excessively rapid cool-down that could damage vessel systems. The vessel is deemed ready to load when the lower part of the cargo tank pipe column reads -130°C. The crew then carefully loads while slowly ramping up the fow rate. This complete process takes around 14 hours. Conical, f ine -mesh stainless steel strainers ftted to the ship's manifolds assure onboard system cleanliness, which is important, because it requires considerable time, efort, and cost to enter a cargo tank for cleaning once it is under gas vapor. The Laden Voyage Once the cargo loading process is complete, the vessel departs on a three- to four-week voyage to deliver LNG to a customer in Tokyo Bay. From the moment the shore vapor line is closed at the loading terminal, the methane cargo starts to boil of in the tank, and pressure increases. A fully laden 174,000 m 3 TFDE vessel with an average boil-of rate of 0.09 percent per laden day will generate approximately 60 to 70 tonnes of boil-of gas per day that needs to be managed to avoid the risk of venting gas to the atmosphere. This is done by sending boil-of gas to the engines as fuel, reliquefying the gas, or consuming gas in the vessel's gas combustion unit. LNG carriers rarely sit at anchor when laden because of boil-of gas manage- ment requirements. As vessel speed and tank pressures are so closely linked, voyage management is critical to maximize ship performance and avoid unnecessary delays. In the days directly preceding arrival at the terminal, the crew checks the cargo system by operating valves and other safety systems to ensure that every- thing is in working order. Many LNG terminals see different vessels arrive and depart every day, which requires smooth operations and very high ship reliability so that the end user (such as a power utility) can maintain operating margins on the inventory. Cargo Discharge Once the vessel arrives at Tokyo Bay and is alongside the buyer's terminal, quali- fed cargo surveyors verify the custody transfer and measurement system, and independent authorities certif y the characteristics of the cargo, including volume, vapor pressure, and temper- ature for customs and contractual purposes before discharge commences. The cargo is discharged via cargo pumps in each tank in about 14 hours. The Ballast Voyage Most LNG carriers retain "heel," a small amount of LNG left over from cargo discharge, on board for use as fuel and to keep the tanks cool and ready for loading the next cargo shipment. This is usually about three percent of the vessel's cargo capacity on a long voyage, but varies depending on the fuel strategy employed. The boil-of rate in the heel voyage is much lower than for the laden voyage. This means that the vessel can operate at a lower speed, if the schedule demands, without creating excess boil-off gas. This is more applicable for spot trading than for liner trades. On liner trades, the voyage schedule is often known a year in advance. If the vessel arrives at the next loading port with LNG heel, the crew may start to cool down the cargo tanks and handling system prior to arrival. Upon arrival alongside, the cargo surveyor makes custody transfer checks again and the vessel repeats the loading operation. For spot trading, depending on gas and alternative fuel prices, the strategy for each voyage may be diferent. For liner trades with long-term sales and purchase agreements, the strategy varies little. Endnote: 1. The 174,000 m 3 capacity vessels are expected to frequent U.S. ports in the future because their design is optimized for facilities like the Lake Charles LNG terminal. This design also accommo- dates many other LNG ports in the world and has become popular within the industry.