Natural gas is normally shipped by pipeline, but it is impractical to build a pipeline from the Middle East or Africa to the United States and other locations. This logistical challenge has led to the creation of special ships capable of carrying the liquid form of natural gas — LNG.
LNG carriers are “tank ships” — merchant vessels designed to transport liquids in bulk.
The first LNG carrier was launched from the Calcasieu River on the Louisiana Gulf coast in January 1959 with the world’s first ocean cargo of LNG and it sailed to the UK where the cargo was delivered. The expansion of the LNG trade has led to a large expansion of the fleet.
Hundreds of vessels have been built and today, giant LNG ships are sailing worldwide. Every single LNG ship that is seaworthy is active. There is not much spare capacity anywhere in the world.
Early LNG ships were made with independent aluminum cargo tanks, with a capacity of 27,000 cubic meters and were used in the Algerian LNG trade in the late 1960s.
Today, the International Maritime Organization (IMO) rules specify that all LNG ships must be one of three types. Type A are those built according to standard oil tank design. Type C refers to those that have a pressure vessel design. Type B refers to tanks that are neither of the first two types. In the eyes of the Coast Guard, all LNG tanks are Type B because Type B tanks must be designed without any of the 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.
layer of insulation. These layers are then attached to the walls of the hold. In the case of the first design, the primary and secondary barriers are sheets of nickel steel. Unlike regular steel, this nickel steel barely contracts upon cooling.
All membranes are built up from the surface of a hold using units of insulation, called panels, that are anchored to it. Special insulation is inserted around the anchors. A membrane design is very complex with many design elements.
The year was 1969 when Phillips Petroleum and Marathon Oil began shipping natural gas from Cook Inlet to Japan. The Polar Alaska and the Arctic Tokyo, identical LNG carriers, were specially designed pressure vessels just for this purpose. The tanks on these LNG ships were of the membrane design.
The alternative to a membrane tank is a self-supporting tank. The most well known is the spherical tank that most people equate with the appearance of an LNG carrier. The large spherical tanks, almost half of which appear to be above a ship’s deck, is often what people visualize when they hear “LNG carrier.” The early sphere designs were made with nickel steel. Aluminum is now used in place of nickel steel. The sphere is installed in its own hold of a double-hulled ship and it is supported by a steel cylinder called a skirt.
The second type of self-supporting tank is the prismatic tank. These tanks are similar to the tanks on old oil tankers; the framing is internal to the tank. The material for tank construction can be aluminum, nickel steel, or stainless steel. Only ships with aluminum tanks have been trading to U.S. ports. The tanks are installed in the hold of a double hull ship and are insulated with covered polyurethane foam. LNG 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. Because of the equipment needed to keep the cargo at subzero temperatures, LNG tankers are more expensive to build than oil tankers. LNG tankers are usually ordered for specific liquefaction plants to carry the LNG on a specified route.
A current inventory chart of all LNG carriers with 10,000+ cubic meter capacity shows more than 400 ships that are either in service or under construction. The chart is dominated by China, Japan and Korea.
Finland, France, Germany, Italy, Norway, Poland, Spain, Sweden and the US each have small listings. Korea shows the largest number of ships under construction. According to industry data, more than 700 LNG tankers will be needed to satisfy the global market by 2030, almost twice the current fleet of LNG ships in operation.
In our next issue we will look at the details of an LNG regasification facility. This is the fourth in a 10-part series produced by the Alaska Support Industry Alliance to educate the public about liquefied natural gas.