This article concludes a series that began in the Aug. 12 issue.
A host of national and international codes and standards usually comprises a list in a shipbuilder's contract specification for a new LNG carrier. Prospective owners must review this list carefully, if the vessel is to be built in full compliance with all applicable requirements.
Because LNG carriers are expected to have a service life of at least 40 years, the list of applicable requirements must cover not only geographic regions and port states associated with the first charter period but also must provide for as much flexibility as possible for future trades.
IMO Gas Code
Many codes, standards, and guidelines deal with specific aspects of the trading LNG carrier. The most comprehensive criteria are in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk or the "IMO Gas Code," as it is commonly called.
The International Maritime Organization, London, is the marine branch of the UN. Flag states that are signatory to an IMO instrument agree to implement the requirements in their national laws and apply them to vessels flying their flags. The code was adopted by IMO as Resolution A.328 (IX) in November 1975. A working group of the International Association of Classification Societies (IACS) developed a working paper the IMO used as a base document from which the Gas Code took shape.
The code deals with the unique hazards associated with carrying a flammable cryogenic liquid at sea. It places no upper limit on the cargo-carrying capacity of the vessel but specifies design loads, material, welding, and non-destructive inspection requirements.
All LNG carriers building today, currently about 50 on order, incorporate either membrane-type tanks or independent tanks. The requirements in Chapter 4 of the Gas Code, written almost 30 years ago, are still fully applicable.
Minimum requirements provide for the vessel's survival under assumed damage conditions and for arrangements of spaces to protect the cargo-containment systems and reduce the risk of fire and explosion from escaped vapor migrating into areas where a source of ignition is normally present.
Specific requirements are laid down for design, construction, and testing of the cargo containment, cargo handling, and control systems. Special fire-fighting requirements are provided and the code addresses special personnel-protection equipment.
Code interpretation
The US is represented at IMO by a delegation headed by the US Coast Guard. The team is often supplemented with US industry experts on particular subject matters. At the time the IMO Gas Code was developed, the US, UK, France, and Norway had the most experience with LNG technology.
At that time, for example, the US was both importing LNG (into Boston) and exporting LNG (from Alaska to Japan). Consequently, the US took a very active role in development of the requirements.
ABS played a dual role in development of the Gas Code. Having experience with LNG technology going back to 1952 and with 12 LNG carriers built to its class requirements by 1975 incorporating both independent and membrane tanks, ABS was a leader in the development of the IACS working paper.
As could be expected with any document that must meet the agreement of more than 100 individual nations, the IMO Gas Code is very general in many areas, leaving interpretation up to the flag states. The class societies that, in many cases, act on behalf of the flag states have developed a set of Unified Interpretations of the IMO Gas Code.
ABS has included all these in its Rules for Building and Classing Steel Vessels, Part 5 Section 4, "Vessels Intended to Carry Liquefied Gases in Bulk." Also included in this section are additional interpretations that have evolved in more than 40 years of involvement with the LNG shipping industry.
In 1975, ABS was also one of the original eight members of the US Chemical Transportation Industry Advisory Committee and has maintained this position to the present. In this capacity, ABS served in a technical advisory capacity to the US Coast Guard during the several years of deliberations at IMO while the code was being finalized.
Liquefaction, storage
Because methane cannot exist as a liquid at temperatures warmer than -82° C., it is always carried as a cryogenic liquid at a temperature of -160° C. and a pressure of 0.25 bar.
Even with the most effective insulation systems, there will always be heat leaking into the cryogenic cargo. Keeping the cargo at a very low pressure means extracting the resultant boil-off from the tanks.
Because warm LNG vapor is lighter than air, the boil-off is permitted, by the code, to be led in double wall piping to the engine room for combustion as fuel in the vessel's propulsion plant.
Although the IMO Gas Code permits combustion on boil-off gas in gas turbines or internal combustion engines, all LNG carriers until this year had been propelled by steam plants. The flag states and the class societies have long addressed all technical issues associated with bringing the gas to the firing platform of the boiler in compliance with all Gas Code requirements.
There is now a strong drive in the industry, however, toward more efficient propulsion plants. Other sectors of the shipping industry favor diesel propulsion and, in the case of some passenger vessels, diesel electric plants have been used. This change requires a new look at the prescriptive requirements in Chapter 16 of the Gas Code for burning boil-off vapor.
For maximum efficiency in a slow-speed diesel engine, the gas should be compressed before it is brought into the engine room. This introduces new hazards not contemplated in the original requirements.
Another propulsion scenario calls for as many as four electric generators, driven by four medium speed dual-fuel engines with twin electric motors driving a single propeller. Running double-wall gas piping to four engines in such a way as not to inhibit engine maintenance creates another challenge.
ABS and the other class societies are looking at these proposals and determining how these new arrangements may be permitted without compromising the minimum level of safety established in the Gas Code that has served the industry so well for so many years.
Other issues
In addition to the IMO Gas Code, there are a number of other important documents normally referred to in a shipbuilding contract that not only deal with the design and construction of the vessel but also address vessel habitability and operability. Still others speak to concerns that may be unique to a particular flag or coastal state.
Other IMO codes often referred to address noise and vibration, alarms and indicators, marine pollution, load lines, performance standards for navigation equipment, tonnage measurements, telecommunications and radio regulations, and regulations for prevention of collisions at sea.
The International Labour Organization's (ILO) regulations concerning crew accommodations aboard ship and codes of practice, safety, and health in dockwork are also normally referred to.
These codes and standards are not unique to LNG carriers. Several sets of requirements prepared by the Oil Companies International Marine Forum (OCIMF) and the Society of International Gas Tanker & Terminal Operators (SIGTTO), London, deal with such issues unique to LNG carriers as the following:
- Standardization of manifolds.
- Safe mooring.
- Ship-to-ship transfer.
- Linked ship-shore emergency shutdown of LNG transfer.
- Installation of cargo strainers.
These requirements not only deal with safety issues but also allow the vessel to trade into as many LNG terminals as possible. This is becoming more of an issue as we are seeing an increase in the spot trade of LNG, a commodity previously traded almost exclusively under long-term sales and purchase agreements.
The US-based Society of Naval Architects and Marine Engineers (SNAME) has developed a standard for carrying out gas trials prior to delivery. This has served as the industry standard since the mid 1970s and is employed today in vessels building worldwide.
Nations signing IMO instruments agree to implement the requirements of that instrument in its entirety. But they are also at liberty to implement additional requirements that, in their view, provide an increased level of safety or address conditions or issues applicable in their jurisdictions, which may be as a port state or as a flag state or as both.
The most notable such requirements for LNG carriers are those of the US Coast Guard. The USCG has included in the US Code of Federal Regulations (CFR), Title 46, Parts 153 and 154, "Special Requirements for US Flag and Non-U S Flag Vessels Entering US Waters" that exceed the requirements of the IMO Gas Code. For example, the USCG requires that the vessel be constructed with crack-arresting steel in three locations: the deck stringer, the sheer strake, and the turn of the bilge.
The USCG also requires that in the selection of material for the inner hull, the following minimum ambient temperatures be assumed for vessels trading to the Lower 48 states:
- Air temperature: -18° C.
- Water temperature: 0° C.
and for vessels trading to Alaska:
- Air temperature: -29° C.
- Water temperature: -2° C.
The IMO Gas Code specifies minimum ambient temperatures of +5° C. for air and 0° for water. This means that in some cases, vessels designed to trade to the US may need to have a grade of steel in the inner hull with better toughness characteristics.
Applicable class society requirements are published in the society's rules. The applicable edition of the rules is tied to the date of contract signing. All major class societies have incorporated the requirements of the IMO Gas Code into their rules.
The most significant area addressed in neither the IMO Gas Code nor the requirements of flag states, such as the US, is hull structure. This is an area considered best left to the experienced class society.
Dynamic loads for carriers
The IMO Code identifies dynamic loads that must be applied to the cargo-containment system. But how those loads are balanced in the hull structure, together with the loads due to hull bending and wave interaction, must be accounted for in the requirements of the classification society.
ABS has developed tools for carrying out sea-keeping studies, sloshing analysis, structural analysis, and fatigue evaluations. In the ABS criteria, the vessel is assessed against the North Atlantic wave environment with fatigue lives of a minimum 20 years and, in most cases, 40 years.
During 2001, ABS developed ABS SafeHull for membrane-type LNG carriers to simplify the application of the dynamic loading approach to the design evaluation. ABS SafeHull for LNG is currently being applied to LNG carriers building in South Korea for Pacific Rim and European owners.
During the period of contract negotiations between builder and ship owners, the exact list of applicable standards, guidelines, and codes that will be applied to the vessel design and construction must be agreed to. Likewise, the criteria for hull structure design and fatigue life that the class society will require the yard to meet are of utmost importance if reliable, safe marine transportation of LNG over a long service life is to be attained.
The author Jim Gaughan ([email protected]) is a senior staff consultant for ABS after serving as director of business development in ABS Consulting 1994-99 and director of a strategic business development focus group specializing in LNG and LPG projects 1990-94. He was appointed a chief engineer in 1987 with responsibility for the ABS Engineering Systems Division and nine satellite technical offices around the world. He joined ABS in 1972. Gaughan holds a mechanical engineering degree from the Cooper Union, New York, and a master of engineering from Stevens Institute, Hoboken, N J. He has been a licensed professional engineer in New York since 1977.
