Proceedings Fall 2015
technology. As a result, typical fuel consumption has been
reduced by an impressive 50 percent, and vessel capacities
have also increased, resulting in a much lower unit freight
cost and overall environmental footprint.
LPG carriers typ-
ically have a service life of 25 years.
The U.S. Coast Guard administers a wide range of mari-
time safety laws designed to protect merchant seamen and
the environment. These laws enforce safety standards for
navigation, lifesaving, fre extinguishing equipment, and
limited construction standards.
Title 46 U.S.C. 3711 requires foreign vessels carrying cargoes
regulated in 46 CFR Subchapter O to have a certifcate of
compliance authorizing the carriage of those cargoes in U.S.
waters. The offcer in charge, marine inspections determines
if the vessel's required certifcates are valid, and examines
and assesses the vessel's relevant components, certifcates,
documents, and safety systems.
After 50 years of LNG shipping activities and more than
79,000 cargoes successfully delivered,
liquefed natural gas
shipping has proven itself to be safe and reliable. Its export
from the U.S. will increase dramatically in the short term,
and Coast Guard involvement will be critical.
About the authors:
Mr. Michael Davison is the project development manager for BG Group's
Ship Design and Construction Team based in Houston, Texas. Mr. Davison
has been involved in LNG carrier design, construction, inspection, mainte-
nance, and repair for more than 15 years.
LT Dallas Smith is a liquefed gas carrier subject matter expert at the U.S.
Coast Guard Liquefed Gas Carrier National Center of Expertise. He has
served in the Coast Guard for 15 years, and is a fully qualifed marine inspec-
tor and casualty investigator with an extensive background in commercial
vessel safety and liquefed gas.
International Gas Union, World LNG Report 2015, http://igu.org/publications, and
U.S. Department of Energy, http://energy.gov/sites/prod/fles/2013/04/f0/LNG_
primerupd.pdf, and World LPG Association, www.wlpga.org/about-wlpga.
Poten monthly report: "LPG in World Markets," Feb. 2015, www.poten.com/wp-
GIIGNL (International Group of Liquefed Natural Gas Importers) website: www.
International Gas Union, World LNG Report 2015, http://igu.org/publications.
This reduction in effciency is a result of the move from steam turbine propul-
sion systems, to diesel-electric, to 2-stroke direct-drive over the past 15 years. For
further information on propulsion systems, please refer to the ABS (American
Bureau of Shipping) guide on Propulsion Systems for LNG Carriers: https://
SIGTTO (Society of International Gas Tanker and Terminal Operators) wesbite:
tank pairs normally share a vapor space and venting system.
LGCs have a separate cargo machinery room which is nor-
mally located on the uppermost deck. This room houses
gas handling equipment, including liquefaction equipment,
vaporizers, and compressors.
Since LNG carriers can burn boil-off gas as fuel, they also
incorporate gas-handling equipment in independent spaces
within the main machinery spaces, including the gas valve
room that controls fuel gas supply to the propulsion system
and the gas combustion unit that can burn excess BOG when
propulsion demand for gas is low.
Liquefed gas carriers operate on a closed concept. The cargo
tanks are initially flled with either vapor, a mix of liquid
and vapor, or an inert gas. During cargo loading, liquid gas
displaces the original cargo vapor, which the marine facil-
ity collects. The reverse occurs during cargo discharge. LPG
carriers can operate without vapor return by using an on-
board reliquefaction plant.
Crewmembers must control cargo tank pressures during
loading and discharging to prevent oxygen being drawn
into the tanks or cargo vapor being vented to the atmo-
sphere. Typically the venting system is a set of dual-redun-
dant, pilot-operated pressure vacuum (P/V) relief valves
that protect each cargo tank from damage due to under- or
over-pressure. These P/V valves are situated between the
vapor space and a mast riser for each tank on the deck of the
vessel. The tanks also feature multiple-level alarms. Auto-
mated control systems will shut down cargo pumps or close
valves to prevent overfow, as required.
LNG propulsion systems include steam-turbine, diesel-
electric, and two-stroke direct-drive systems that operate on
heavy fuel oil (HFO), marine diesel oil (MDO), marine gas
oil (MGO), and BOG fuel. Modern, highly effcient liquefed
natural gas carriers feature two-stroke gas injection engines
(4,400 p.s.i. gas injection pressure) or lower-pressure two-
stroke gas induction engines. Liquefed petroleum gas carri-
ers are typically ftted with two-stroke direct-drive engines
or diesel-electric systems operating on HFO, MDO, or MGO
fuels, but not gas fuels, at present.
The accepted industry service life is 40 years for an LNG car-
rier, and in some cases, 40-year-old vessels are still in good
condition and are converted to foating storage. Other older
vessels have recently been sold for conversion to regasifca-
tion vessels or foating LNG production and storage vessels.
There has been a rapid development in LNG carrier design,
particularly in the areas of boil-off rate and propulsion