With the global LNG (liquefied natural gas) market forecast to double in size over the next decade, industry is readying to provide ample liquefaction capacity to meet demand. In the United States alone, LNG imports reached
4.9 million tons in 2000, about 5 percent of the global LNG trade, and up from 3.4 million tons in 1999. Drivers for the rapid LNG growth include continued demand for a cleaner-burning fuel and the need to bring "stranded" gas from deepwater and remote areas to market. Concern over long-term U.S. gas supply; a larger number of new supply projects; and lower LNG infrastructure costs from a decade ago also are contributors to the LNG growth spurt in recent years.
But perhaps the reality of Sept. 11 is probably the biggest motivator for considering, for the first time, development of offshore LNG terminals in an environment remote from high-risk populated areas. The prospect of designing, building and installing LNG terminals offshore finds ABS readying to meet industry requirements for growth and regulatory assistance, says William J. Sember, ABS vice president of offshore development.
"There's a lot of public and government concern about storing quantities of LNG on land. The more isolated offshore environment, however, provides less exposure to people and cities from the import, storage and pipeline transmission of LNG to shore facilities," said Sember.
The ABS "Guidance Notes for Building and Classing Offshore LNG Terminals," says Sember, is intended to facilitate industry development of both gravity-based and floating terminals contemplated for offshore installation.
Industry forecasts, he adds, estimate that anywhere from five to 20 offshore terminals could be built for U.S. offshore installation in the next 10 years. Target locations include the East and West Coasts as well as the Gulf of Mexico.
Overseas locations where offshore LNG terminals are under consideration include Australia and West Africa. Offshore LNG terminals will be designed for site-specific environmental conditions and are expected to be purpose-built, rather than converted existing LNG carriers.
Global sites for offshore LNG terminals, adds Sember, will facilitate continued growth among emerging countries eager to find new and accessible markets for gas production.
"Industry will be able to safely and efficiently transport and offload LNG to many regions of the world, thus spurring development of burgeoning markets," said Sember.
There are two major types of offshore LNG terminals: loading terminals and discharge terminals. Loading terminals receive gas directly from one or more wells or from another offshore facility where it may or may not have
been processed. The gas is liquefied in an onboard liquefaction facility and stored for offloading as LNG to a trading LNG carrier.
Discharge terminals receive LNG from trading LNG carriers and store the LNG until it is vaporized in a re-gasification facility for pipeline transport to shore. However, offloading LNG in a lightering operation also is envisioned.
ABS brings substantial experience to support the offshore industry in its potential development of offshore LNG terminals, says Sember: experience with both LNG carriers and site-specific offshore structures designed to operate for as much as 30 years in harsh environment and intrinsic knowledge of the U.S. regulatory arena.
"These particular attributes will contribute to the value of the Guidance Notes to the U.S. industry," said Sember. He adds that the Guidance Notes will provide industry with a comprehensive document that outlines ABS procedures for LNG terminal classification. A significant part of that process includes cross-references with ABS Rules and other relevant industry documents.
Within the Guidance Notes, class notations are clearly defined to effectively describe the function of each LNG configuration. The class notations include:
· F Floating
· GB Gravity Base
· P Gas Processing Facility
· L Liquefaction Facility
· S Storage Facility
· O LNG Offloading
· R Re-gasification Facility
Areas of emphasis within the Guidance Notes include the terminal structure; the hull design; the mooring system; the offloading system; LNG containment systems; process facilities; and support and safety systems.
The document also addresses LNG terminal installation, hook-up and commissioning; and surveys during construction and operation. Designers and operators also may consider risk-based alternatives to strict compliance with prescriptive rules.
Safety is a critical issue here, adds Sember, as industry consider taking facilities that are historically on land and putting these gas processing and cryogenic storage facilities offshore.
"Although industry has successfully dealt with oil and gas processing facilities on Floating Production Storage Units for a number of years, the liquefaction and cryogenic storage of LNG offshore is a whole new arena. This transition requires a comprehensive approach toward design and review of all the elements and criteria involved in an offshore LNG terminal," said Sember.
A key issue for the floating terminal concepts, adds Sember, is designing for the relative motion between the terminal and LNG carrier during offloading operations. Whether offloading through a loading arm or some other special system for the transfer of cryogenic liquid between the terminals and the trading LNG carrier, the stresses on the transfer system are significant.
"Transfer of LNG, at subzero temperatures, through a loading hose presents industry with a technological challenge for managing system stress. Industry is presently evaluating appropriate technologies, possibly even a cryogenic floating hose, to create more reliability and flexibility for the LNG transfer in a totally mobile environment," said Sember.
The technology surrounding containment systems also is part of the critical evolution of offshore LNG terminals, says Todd Grove, ABS director of Offshore Project Development
. Grove advises that the shipping industry has enhanced the basis of design for membrane systems, thus strengthening containment systems to handle partial loading and to sustain sloshing load once a tanker has offloaded.
"An industry 'by-product' of this technology improvement," said Grove, "is advancement toward development of offshore LNG terminals, which also will require partial-loading capability in a marine environment."
Traditional LNG ship-shaped containment systems are highly specialized with an elaborate aluminum insulation system, says Grove. The containment system on LNG carriers is required to minimize the heat leaking into the cargo from the surroundings and protect the ship's hull from brittle fracture, which would result if the cryogenic cargo came in contact with the steel hull. There are four types of containment systems that can be incorporated in LNG carriers: two membrane types, an independent spherical design and an independent prismatic tank design.
"To date, 43 LNG carriers have been built to ABS class and have incorporated every type of containment system available," said Grove.
The membrane tanks incorporate a primary and secondary membrane, separated by an insulation system and from a vessel's hull. In the event that the primary membrane fails, the secondary membrane must be able to keep the cargo away from the vessel's hull for at least 15 days to allow for emergency offloading.
Independent tanks are specially designed with stress analysis and fracture mechanics studies to assure that, should a crack develop, the crack will grow so slowly that any escaped liquid will vaporize or be deflected by a spray shield to drip pans below the tank.
LNG carriers building
today have cargo containment systems ranging in sizes up to 145,000 cubic meters, with future designs expected to increase to 200,000 cubic meters. Designs for offshore LNG terminals currently under development are typically at least twice that size.
Further, industry envisions that on some gravity-based offshore LNG terminals the containment system may be very similar to that typically employed on shore side LNG storage tanks
. The ABS Guidance Notes makes appropriate reference to the relevant industry standard published by the National Fire Protection Association.
Design components of an LNG containment system, says Grove, incorporate the following features:
· Primary and secondary containment systems
· At least two methods to determine liquid level
· Flexibility to fill the tank from both the top and bottom to avoid
· At least one pressure gauge connected to the vapor space
· Two independent overpressure protection devices
· Devices for measuring liquid temperature at the top and bottom of a
· Gas detection system
Prospects on when the industry's first offshore LNG terminal will be built and installed are still uncertain, particularly in the United States, where regulatory issues regarding jurisdiction are presently being addressed. ABS is well-positioned with comprehensive documentation in place to assist industry once these regulatory matters are resolved, says Grove.
"ABS is ready to go. We know the technology?both in terms of LNG and marine issues?and we have a documented plan in place to meet industry requirements in an efficient manner," said Grove.