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LNG Tankers

Liquefied Natural Gas [LNG] tank ships look different from regular tank ships carrying oil and chemicals. Most LNG tank ships have two hulls, so that, if a collision or grounding punctures the outer hull, the ship will still float and the LNG will not spill out. LNG tanks are either spherical (and the upper half of the sphere sticks out above the deck), or box-shaped. The ships tend to ride high in the water, even when loaded. A typical LNG ship is 950 feet long and 150 feet wide, and many new ships being built are even bigger.

LNG is liquefied natural gas, which is the very cold liquid form of natural gas-the fuel that's burned in gas stoves, home heaters, and electric power plants. When it warms back up, LNG becomes natural gas again. You can't liquefy natural gas without cooling it. Many countries export and many others import LNG by ship; the United States does both.

LNG is very cold natural gas that is in a liquid form rather than gas. Chemically, it's mostly methane, with small amounts of ethane, propane, and butane. LPG (liquefied petroleum gas), sometimes referred to as bottled gas, is a heavier gas that can be liquefied under pressure or by refrigeration. It is mostly propane and butane. Gasoline is heavier still and is a liquid at room temperature. Heating oil is even heavier and doesn't boil unless heated. And asphalt is so heavy that it's a solid. But in a way they are all pretty similar, because they all burn.

LNG comes from natural gas that's been cooled to below -256 degrees F, with some impurities removed. Natural gas comes from underground gas fields by itself or in oil fields, along with crude oil. There's very little difference between natural gas and vaporized LNG; mostly LNG is a little purer; before liquefying the natural gas engineers remove the pollutants, like sulfur.

The US gets liquefied natural gas from countries including Algeria, Brunei, Malaysia, Nigeria, Trinidad and Tobago, Oman, and Qatar. In the future we can expect the U.S. to get LNG from even more countries. As of 2006 there were 17 terminals worldwide where LNG is liquefied and pumped aboard LNG ships, and approximately 40 terminals where LNG is pumped off LNG ships and stored in large tanks on land and vaporized as needed by consumers.

As the United States' energy needs continue to rise, its domestic natural gas production is nearing its peak. Canada's natural gas pipelines, the proximate and primary source of imported gas, are not expected to be able to meet the growing residential, industrial, and electricitygenerating demands for natural gas. At the same time, the steady march of technology has significantly reduced the cost of natural gas liquefaction and transport, leading to a jump in the number of gas-producing countries that are eager to supply our natural gas demands. These "supply and demand" principles have united to fuel rapid growth in the international LNG market.

Currently, the United States consumes about 25 percent of the world's annual natural gas production, although over 95 percent of the entire world's proven natural gas reserves are outside of North America. Over the next 20 years, U.S. natural gas consumption is projected to increase by 40 percent, and it is doubtful that our domestic gas production will rise at the same rate. Therefore, the difference between our consumption and production will have to be made up by importing natural gas, and the most viable method of this is the seaborne importation of LNG.

In the United States, natural gas is liquefied and exported from the Gulf of Alaska; LNG is imported and vaporized into natural gas at Boston, Mass.; Cove Point, Md.; Savannah, Ga.; and Lake Charles, La. Recently, a new offshore terminal in the Gulf of Mexico opened and took its first shipload of LNG. To increase the United States' ability to import LNG to meet this rising demand, the energy industry identified several potential sites for additional LNG import terminals along our coasts. Right on the heels of this shoreside facility expansion, the engineering and technology for deepwater LNG ports matured, and a 2002 amendment to the Deepwater Port Act opened the door for LNG deepwater port applications. As of 2006 the Coast Guard and other agencies were reviewing as many as 40 more proposals for onshore and offshore LNG importation terminals; while not all of these proposed terminals will be built, many will no doubt be.

Normally natural gas is shipped by pipeline, but it is impossible to build a pipeline from the Middle East or Africa to the United States, so engineers created ships capable of carrying the liquid form of natural gas. Natural gas needs to be liquefied (cooled to below -256 degrees F), because you'd need the volume capacity of 600 ships of natural gas at ambient temperature/ pressure to equal one shipload of LNG. Since it is not affordable to build and operate that many ships to carry that amount of natural gas, shipping LNG is the only practical way to import the necessary quantities that America needs.

Gas carrier tanks, according to International Maritime Organization (IMO) rules, must be one of three types. Those are ones built according to standard oil tank design (Type A), others that are of pressure vessel design (Type C), and, finally, tanks that are neither of the first two types (Type B). All LNG tanks are Type B from the Coast Guard perspective, because Type B tanks must be designed without any general assumptions that go into designing the other tank types. There are three general Type B tank designs for LNG. The first type of design, the membrane tank, is supported by the hold it occupies. The other two designs, spherical and prismatic, are self-supporting.

Membrane tanks are composed of a layer of metal (primary barrier), a layer of insulation, another liquid-proof layer, and another layer of insulation. Those several layers are then attached to the walls of the externally framed hold. In the case of the first design, the primary and secondary barriers are sheets of Invar, an alloy of 36-percent nickel steel. Unlike regular steel, Invar hardly contracts upon cooling. The insulation layers are plywood boxes holding perlite, a glassy material. The Coast Guard, while reviewing the design, requested testing that would show the integrity of both the primary and secondary barriers. Secondary barrier testing and acceptance criteria were very hard to develop but are necessary to ensure containment integrity. It should be noted that the insulation for the Gaz Transport membranes has been discussed generally. All membranes are built up from the surface of a hold using discrete units of insulation (called panels) that are anchored to it. Special insulation is inserted around the anchors (called studs). Also, there are special methods for sealing joints between panels. A membrane design, therefore, is fairly complex, and a complete discussion of any one design's intricacies would be too lengthy to completely detail.

The alternative to a membrane tank is a self-supporting tank. The most well known is the Moss-designed spherical tank that many people equate with the appearance of an LNG carrier. The large spherical tanks, almost half of which appear to protrude above a ship's deck, is often what people visualize when someone says "LNG carrier." The early sphere designs were shells of 9-percent nickel steel. Subsequently, aluminum was used. The sphere is installed in its own hold of a double-hulled ship, so that it is supported around its equator by a steel cylinder (called a skirt). The covered insulation surrounding the sphere can channel any leakage to a drip tray located under the sphere's "south pole."

Some older 9-percent nickel steel tanks have shown significant amounts of swallow cracking after years of service. The cracks develop next to the welds due to the effect of the heat of the welding on the original material (known as the "heat-affected zone''). The cracks can be repaired by gouging them out and welding in new material. Aluminum tanks can have a different cracking problem. Attaching the aluminum tank to a steel cylinder is a difficult process, due to the metals involved, and cracks are liable to develop where those materials are joined.

The second type of self-supporting tank is the Self-supporting, Prismatic, Type B (SPB) tanks by Ishikawajima Heavy Industries (IHI). These tanks are reminiscent of the tanks on old single-skin oil tankers; the framing is internal to the tank. The material for tank construction can be aluminum, 9-percent nickel steel, or 304 stainless steel, but only ships with aluminum tanks have been trading to US ports. The tanks are installed in the hold of a double hull ship and are insulated with covered polyurethane foam that also is able to serve as channeling for any possible tank leakage to drip trays.

Beside these types of tank designs, there are several types that were proposed some years back but were never built. Both the IHI "flat top" and the Hitachi Zosen (for LPG) prismatic designs were not considered acceptable because carbon-manganese steel is not suitable for prismatic designs. Gaz Transport and Technigaz make prismatic membrane tanks, but in the early 1970s, both companies were interested in making spherical membrane tanks. The Gaz Transport design was a joint effort with Pittsburgh-Des Moines Steel Company. Mitsubishi Heavy Industries proposed a cylindrical design that was conceptually similar to the Moss sphere design. That proposal was a hemispherical base (supported equatorially by a skirt) with a short cylindrical section above the hemisphere, and all topped with a shape that was oval in cross section.

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Page last modified: 07-07-2011 12:59:39 ZULU