First liquid hydrogen carrier
Dag Pike looks at a new tanker concept design from Japan, developed in response to an expected demand for transportation of liquid hydrogen.
Much of the focus in the development of liquefied hydrogen (LH2) has been on the containment system that will be used to carry the liquid gas. Kawasaki has used its extensive experience with the design and construction of liquefied natural gas (LNG) carrier ships as the basis for developing the liquid hydrogen containment system. Liquid hydrogen is a more volatile that LNG and so a modified containment system is required. Now Kawasaki has been granted approval by Class NK for the design and fabrication of a containment system suitable for installation on the liquefied hydrogen carrier.
This classification approval is based on complying with the IGC Code and ship classification rules as well as being based on the risk evaluation results from the hazard identification safety analysis (HAZID).
The containment tanks developed for liquid hydrogen are accumulation type cargo containment vessels with a capacity of 1,250m³. Liquid hydrogen has to be transported at a very low temperature - below -252°C - so the containment vessel will be cylindrical in shape and carried horizontally. It will be installed entirely free from the ship’s structure.
The boil off gas that is generated due to external heat penetration will be tightly contained within the pressure resistant cargo containment vessel. This will allow the liquid hydrogen to be discharged either by means of a pump built into the containment vessel itself or simply by means of the pressure build up.
In order to minimise the boil off gas rate the cargo containment vessel will be of the double shell type for which a vacuum heat insulation system has been developed similar to that used in a ‘Thermos’ flask. The support structures for the containment vessel will be constructed from a newly developed composite material that has very low heat conduction combined with adequate structural strength. A dome chamber has been added to the containment vessel that will provide a manhole to permit inspection of the interior of the vessel and this will have a similar double skin insulation system.
Two alternative ship designs have been developed: one having two of the containment vessels and the other a larger ship with units having a more spherical shape. The two-container ship will have a capacity of 2,500m³ and the hull will have both double side shells and a double bottom to minimise risks in the event of grounding or collision. The cargo hold will be covered to protect the containment vessels from external damage and from the open air.
At this stage the main engine is proposed to be a normal diesel engine without any facility to use the boil off gas. However it is planned to equip the vessel with fuel cell electrical generators so that trials can be conducted in using any boil off gas for power generation on board for future designs.
What is not yet clear from this development are the routes on which such a ship might operate. There does not seem to be a logical demand for the ocean transport of liquid hydrogen as hydrogen gas can be made in most areas where there is an adequate power supply. This suggests that the ships will be used on coastal trades to link the liquid hydrogen production sites to urban areas where the gas would be used for fuel cell-powered vehicles.
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