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/578020
42 Proceedings Fall 2015 www.uscg.mil/proceedings The frst step in this process is for the operator of a foreign- flagged gas carrier to submit an SOE application to the Coast Guard Marine Safety Center. The application must include a copy of the vessel's fag state or classifcation soci- ety International Maritime Organization Certifcate of Fit- ness (IMO COF), as well as the additional engineering plans and documents listed in 46 CFR 154.22. These documents allow the Marine Safety Center to verify compliance with the standards that are not found in the IGC and ensure that Coast Guard responders have the necessary drawings on fle to respond to a potential gas carrier incident in a U.S. port. SOE Issuance After Marine Safety Center personnel verify standards com- pliance, they generate the Subchapter O endorsement and forward it to the offcer in charge, marine inspection at the More Stringent U.S. Standards The U.S. standards were originally intended to mirror those found in the International Gas Code (IGC). However, there are four areas in which the Coast Guard utilized the regulatory process to establish additional requirements designed to increase liquefed gas carrier safety. These standards focus on areas of the vessel's containment system and hull construc- tion, such as: • hull material, • cargo tank pressure and temperature control, • maximum allowable relief valve settings, and • ambient design temperatures. Hull Construction Material U.S. regulations require that enhanced grades of steel be used along the cargo area for crack-arresting purposes in the event of a cargo spill. This includes the deck stringer, the sheer strake, and the turn of the bilge. The Coast Guard Ofce of Design and Engineering Standards may approve alterna- tive materials if they are determined to provide an equivalent level of protection. Cargo Tank Pressure and Temperature Control The IGC allows a vessel to control cargo pressure and temper- ature by venting cargo vapors to the atmosphere at sea and in port, if the port's administration allows this. The U.S., however, prohibits the normal cargo vapor venting into the atmosphere in all ports. The U.S. requires that foreign vessels maintain cargo tank pressure below the design vapor pres- sure indefnitely or for a period of not less than 21 days for liquefed natural gas. Approved pressure control methods include: • refrigeration systems, • burning boil-of gas, • using boil-of gas as fuel, or • a combination of methods. Maximum Allowable Relief Valve Settings U.S. design requirements for cargo containment systems are based on the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. Allowable stresses for membrane, semi-membrane, and independent type A tanks are the same in the IGC and U.S. regulations. For independent type B and C tanks, however, the U.S. uses more conservative (higher) stress factors, which results in lower permissible pressure settings. Therefore, foreign fagged gas carriers are typically approved for two maximum allowable relief valve settings (MARVS) — one for operating in international waters and one for U.S. waters. Prior to entering U.S. waters, foreign vessel crew must set tank relief valves to the lower approved U.S. MARVS. However, there have been advancements in construction materials, manufacturing, and inspection since original Subchapter O development, so the American Society of Mechanical Engineers has lowered the required stress factors to permit higher design pressures. In 2012, the Coast Guard issued CG-ENG Policy Letter No. 04-12, which notes that international MARVS for independent type B and C tanks on vessels that have been built to IGC 1993 edition standards are acceptable. Design Temperatures The International Gas Code provides general standards to evaluate the insulation and hull steel for cargo tanks and secondary barriers for the purpose of design calculations. The ambient design temperatures are 41˚F for still air, and 32˚F for sea water. The IGC also states that each administration can dictate higher or lower ambient design temperatures. U.S. regula- tions specify the following additional ambient design temper- atures for vessels required to have a secondary barrier: • 0˚F for 5 knots air, and 32˚F for still sea water for any waters in the world, except Alaskan waters; and • -20˚F for 5 knots air, and 28˚F for still sea water for Alaskan waters.