By Jim Shaw 

Gas Carriers: They've Come a Long Way

 

A pioneer in the gas trades the 24,608-dwt Methane Princess was built by Great Britain's Vickers shipyard for British Gas in 1964, classed by ABS, and not scrapped until 1998. Photo courtesy of T. Jones.

The use of natural (LNG) and petroleum (LPG) gases as clean energy sources has been increasing dramatically, particularly with the advent of shale gas extraction, and more ships are being built to carry the commodity as well as operate on it. The transport of gas in liquid form is a rather new industry, although LNG was first produced in 1912 and an LNG-carrying barge was patented as early as 1914. It was not until 1959, however, that the world's first sea-going LNG carrier, Methane Pioneer, was converted out of a dry cargo ship, the 1945-built Marline Hitch, and used to carry liquid gas from Louisiana to the United Kingdom.

Five years later the 621-foot by 82-foot twins Methane Princess and Methane Progress were completed by the Harland & Wolff and Vickers-Armstrong yards in the UK to transport gas from Algeria, each having a capacity of 27,400 cubic meters in nine free-standing Conch-type tanks. By 1969 Gaz Transport of France had developed its membrane containment system, which featured membrane-style tanks built into the hull structure of the ship. Two years later Norway's Kvaerner introduced the Moss spherical containment system consisting of domed tanks that sat in the ship independent of the hull. In 1993, the twin LNG carriers Polar Eagle and Arctic Sun were completed using a new prismatic containment system developed by Japan's IHI for transporting gas across the Pacific from Alaska. Today, there are some 400 LNG carriers in operation, 75 percent of which use membrane tanks, and total tank capacity has increased steadily from the earliest gas ships to the latest Qatari Q-Max class which can accommodate 266,000 cubic meters.

Gas Handling

The loading and discharge of an LNG carrier is accomplished with considerable caution. Before the start of loading operations, the lines and tanks involved must be chilled so there is no thermal stress because of the sudden temperature change. The loading manifolds at this point will begin to go white with frost. As considerable boil-off gas is generated during loading, the ship's heavy-duty compressor will be used to return the boil-off gas ashore to keep the internal pressure of the tanks at an allowable level. During the sea voyage, natural evaporation of the LNG serves to absorb any heat, thus maintaining the required low temperature.

The evaporated gas can be directed from the tanks to a ship's boilers by a low-duty compressor for use as fuel. This has made steam turbines the traditional propulsion of choice for LNG tankers, although this is starting to change, with some 25 percent of the world fleet now using dual-fuel diesel-electric or tri-fuel diesel-electric systems.

At the discharge port, cargo pumps are used to move the LNG ashore. To prevent a pressure decrease within the ship's tanks as they are emptied, gas is pumped in from the shore facility through a vapor line. A little LNG, called "heel" is always left on the vessel to make sure all tanks are kept cold during the ballast voyage. The heel is sprayed within the tanks to force evaporation, thus keeping the tanks cool and producing enough gas to fuel the ship's boilers if required.

Alternative Propulsion

The use of natural gas for propulsion makes the LNG carrier one of the most environmentally friendly vessels afloat but the use of boiler-fed steam turbines is being challenged by the new dual-fuel diesel/electric systems, which were inaugurated in the trade by the small gas carrier Pioneer Knutsen completed in 2004 to operate along the Norwegian coast.

The Dutch-built ship utilizes two sets of twin diesels, one set capable of running on boil-off gas and the other set burning conventional diesel fuel. Each set is coupled to alternators that provide power to twin Schottel azimuthing thrusters to give a high degree of maneuverability and a service speed of 12 knots.

In the same year that Pioneer Knutsen was completed Gaz de France introduced its much larger 34,800-dwt GDF SUEZ Global Energy, a 70,000 cubic meter capacity membrane-type LNG carrier powered by four Wärtsilä 6-cylinder 50DF low-pressure dual-fuel engines with an aggregate output of 22.8 MW. This was the first major breakaway in the LNG transporting sector from steam turbines.

The LPG Carrier

In the liquefied gas carrying trades, LNG carriers have become the dominant ships capacity-wise but Liquefied Petroleum Gas (LPG) tankers are also efficient transporters of energy. The latter differ from the LNG carriers in several respects. While natural gas cannot be liquefied by applying high pressure alone, but must also be brought to a very low temperature, usually -163 degrees Celsius, petroleum gas can be liquefied by either high pressure or low temperature. Because of this, LPG carriers can be of the refrigerated type or the pressure type, although most ships employed on long ocean passages are of the refrigerated type. This requires tanks that can withstand a temperature of at least -46º C. These tanks, normally made of special steel, are insulated and supported within a double-hull while vertical and horizontal motion dampers are fitted to the sides and tops. Spherical tanks, such as used on LNG carriers, are not used on LPG carriers as owners have found cubic-style tanks to be more economical.

LPG carriers also do not use boil-off gas as fuel, as the LNG carriers do, but instead run it back though an on-board reliquefaction plant to return it to a liquid state. LPG carriers can load and discharge as a normal liquid-carrying tanker does, except that the entire loading/discharge system must first be cooled to match the temperature of the liquefied gas being carried. This guards against any sudden structural stress caused by metal contraction. LPG carriers also require a separate vapor line to control the rise or fall of pressure inside the cargo tanks during loading/discharge, again to guard against structural stress.

Propane, Butane and Ethylene

The main liquefied gases carried by LPG ships are propane and butane. These two gases are not allowed to mix, and if on the same ship, they are carried in separate tanks. In addition, ships built to carry both propane and butane are equipped with two separate pipelines so the two gases can be loaded or discharged simultaneously. If propane is going to be carried in a tank that has previously carried butane, vaporized propane is first pumped into the tank from the top to push the remaining butane residue out through the bottom and vice versa. This operation is called a "tank change".

More sophisticated in this sector are ethylene carriers, which have the ability to carry not only most other liquefied gas cargoes but also ethylene at its atmospheric boiling point of −104 °C (−155 °F). These ships usually feature cylindrical, insulated, stainless steel tanks able to accommodate cargoes at temperatures ranging from a minimum of −104 °C to a maximum of +80 °C and at a maximum tank pressure of 4 bar.

Ethane, used as a feedstock to produce ethylene, is also carried by specialized ships and some of the latest have been built to use the gas as fuel, with the world's first ethane-powered tanker, the 20,400-dwt JS Ineos Insight, having entered service in 2015. As the first of a series of eight similar-sized vessels, the Chinese-built ship has a capacity of 27,500 cubic meters in tanks kept at -90°C.

Although originally designed to operate on LNG and diesel, with two 1,000 cubic meter capacity LNG storage tanks mounted on deck, an ability to burn ethane was added to allow the ships to use a portion of their cargo as fuel. This required extra engine room ventilation and additional gas detection equipment, plus modifications to the Wärtsilä 6L20 DF main engines that included a lower compression ratio, different turbocharger nozzles, and de-rating to cope with the lower knocking resistance of ethane.

New Ships

Just coming into the gas transportation sector is the Compressed Natural Gas (CNG) carrier, with the first CNG vessel, Jayanti Baruna, completed last year by China's Jiangsu Hantong shipyard for Pelayaran Bahtera Adhiguna, a subsidiary of Indonesia's state-owned power company Perusahaan Listrik Negara.

The new 2,200 cubic meter-capacity ship carries compressed rather than liquefied gas, and at a very high pressure of 200 bar (2,900 psi). It is also powered by gas, using a 9-cylinder Wärtsilä 34DF dual-fuel main engine driving a Wärtsilä controllable-pitch propeller to give a service speed of 14 knots.

Seen nearing completion with a number of her sister ships at South Korea's Samsung Heavy Industries the 266,000-m3 capacity Mozah became the class leader for 14 Q-max LNG carriers owned and operated by Qatargas II. Photo courtesy of Samsung.

CNG is considered a more economical alternative to LNG when short distances are involved and the new ship will trade mainly between the Indonesian islands.

Still on the horizon is a ship that will transport something that is not even recognized as a cargo yet: the Natural Gas Hydrate (NGH) carrier. Japan's Mitsui Engineering & Shipbuilding has drawn up plans for a 300-meter by 46-meter vessel that would incorporate independent storage tanks of a combined 155,000 cubic meter-capacity to carry frozen hydrate pellets. The ship would operate something like a dry bulk carrier in that frozen NGH pellets would be loaded using a horizontal conveyor and off-loaded using a vertical conveyor that would feed pellets to a horizontal discharger mounted on deck, an arrangement very similar to loading, transporting and unloading bulk ice cubes but at a very low temperature. The handling technology for this unique cargo as well as the ship itself are still under development.

 
 

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