CNG in shipping as an LNG alternative
Transporting natural gas has become big business for the shipping industry, says Dag Pike, who looks at the opportunities in compressing, rather than liquefying, gas for transport and fuel.
Today, a huge infrastructure has been developed to transport gas in liquid form. This has required heavy investment in both ships and shore facilities. Transporting gas in liquid form means that it has to be cooled to -163°C, and the onboard cargo tank designs have to enable the liquid to remain at this temperature. The very high investment in both ships and shore facilities means that this form of gas transport tends to be restricted to dedicated routes involving considerable quantities of gas where the infrastructure investment can be justified.
Part of the distribution chain for LNG involves its gasification for transport by road or rail in pressure vessels. Now the compressed natural gas (CNG) concept is once again being applied to sea transport as an alternative to transporting the gas in liquid form. The concept of CNG ships is not new. In the 1960s a ship was converted in the US with a series of vertical gas cylinders mounted on deck. This ship completed several voyages but the low cost of the gas at that time made the project unviable. Now a contract for the construction of the world’s first modern CNG ship has been designed by Qingdoa Heavy Industry in Northern China and this could mark a step change in the way that natural gas is transported.
This CNG ship has been ordered by Pelayaran Bahtera Adhiguna - a subsidiary of Indonesia’s state owned power company - and will transport gas from the fields in East Java to communities on the island of Lombok. This is a relatively short transport route, where the investment in a pipeline cannot be justified and where the infrastructure for liquefying the gas is too high for the quantities and distances involved. Here the compressed gas carrier ship creates a viable alternative. The ship will have a capacity of 2,230 m³ of gas, a length of 110m and a speed of 14 knots. It is a small ship by modern gas carrier standards, but it demonstrates the role that CNG ships could play in the future transport of gas.
Transportation of compressed gas is basically less efficient than carrying LNG. When the gas is liquefied its volume is reduced 600 times, whereas when the gas is compressed to safe limits its volume is only reduced by a factor of 200. Storage pressures proposed for CNG ships are in the range of 140bar to 200bar. However with CNG there is no boil off of the gas, so that the full loaded amount is delivered at the end of the voyage, and the cost of the ship is considerably less.
Various ship designs have been developed for transporting CNG. With the Coselle system the gas is stored in very long lengths of small diameter steel piping. The small diameter of the pipes means that it can withstand high pressures with less weight of steel containment, These long lengths of pipe are coiled onto carousels, each of a standard size, which are installed onboard in stacks.
The alternative is a series of pressure vessels that are mounted either horizontally or vertically within the hull. These vessels tend to be of a standard size in order to optimise construction costs and may be constructed in steel or composite materials. The Neptune CNG system uses composite-construction tanks, with a useful weight saving claimed. BMT has designed a CNG ship with vertical storage tanks mounted on deck.
The Voltrans CNG system is something of a hybrid as it combines both cooling and compression of the gas. By cooling the gas to around -30°C the same volume of gas can be transported at a lower pressure thus reducing the weight and cost of the pressure vessels.
ABS is a leader in the development and approval of the various CNG ship concepts that have been proposed. Considerable research has gone into the safety of the various systems, and to a certain extent these are based on experience with LNG ships in determining standards for collision and grounding risks. Pressure vessel technology is now well advanced for road and rail transport and there is no record of any corrosive effects from the gas. In the event of a leak developing the gas is lighter than air and it requires very specific gas/air mixtures before it becomes explosive.
With the rapid increase in the use and transport of gas, the merits of CNG could lead to the development of CNG ships as a form of feeder system to distribute natural gas in smaller quantities. Alongside the development of CNG transport by ship, the way is being opened for the use of CNG as a marine fuel. Damen Shipyards has recently received its first order for a CNG tug that will be gas powered with gas stored onboard under compression.
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