Liquefied Natural Gas is an attractive alternative fuel for offshore and riverboat vessels. But the use of LNG requires at the present time very costly methods of refueling. The Hamburg-based engineering firm Marine Service GmbH offers therefor a promising alternative, which relies on the use of mobile filling stations.
Ships are the main means of transport for goods worldwide. More than 55,000 merchant ships transport approx. 10 billion tons of goods per year. While the large transport volume results in a positive environmental performance per ton and kilometer compared to other means of transport, the absolute amount of CO2 emissions is significantly higher than that of other transport means. And for the pollutants sulfur oxides (SOx), nitrogen oxides (NOx) and particulate matter, the balance sheet looks even worse. Especially in maritime zones with dense traffic such as the North and the Baltic Sea and in port cities, exhaust gases of ships add significantly to the exposure to nitrogen oxides and particulate matter.
Due to the requirements of the International Maritime Organization (IMO) and the EU, the use of heavy fuel oil (particularly with high sulfur content) is no longer permitted in many sea areas unless systems for exhaust gas aftertreatment are installed. The use of cleaner fuels offers an alternative to the expensive exhaust gas treatment. Here the use of natural gas is a good choice. However, in order to be able to store a sufficient quantity on board, the gas has to be cooled down to about -160°C and liquefied, which reduces the specific volume by a factor of 600.
In 2015, the first 2 ships, which use LNG as fuel, went into service in Germany. Refueling of these ships is currently done by tank truck, with the LNG being transported by road from Rotterdam or Zeebrugge to Cuxhaven and Emden.
In order to improve the logistics for the supply of larger amounts of LNG, Marine Service GmbH has developed a mobile filling station using barge mounted LNG container tanks.
The LNG tank containers are specially designed IMO IGC code approved units, which can be filled and emptied on board of a vessel.
All devices needed for the filling and transport of the tank containers are installed on the barge. Up to 12 40‘ tank containers can be loaded. The pontoon can be built as a non-self-propelled barge (as shown in the picture), which is transported by tug or pusher tug to the desired location.
If frequent relocations are necessary, a self-propelled alternative can be employed.
The bunker barge can be filled directly from an LNG tanker equipped with a bunker port. For this the barge is brought into position alongside the tanker close to its bunker door. The barge is equipped with spud poles, which are lowered to the seabed to give a safe position holding without having to use mooring ropes. The barge transfer manifold is connected with hoses to the tank system of the LNG carrier and the LNG containers can then be filled.
The barges manifold station is located at the midship of the barge, to which the containers are connected via hoses and quick couplings. All containers can be filled simultaneously with a maximum bunker rate of 300 m³/h.
After filling the containers, the barge can be towed/pushed to the ship, which is to be bunkered with LNG. A pump at the barge‘s manifold station is used to transfer the LNG to the recipient ship with a transfer rate of approximately 25 to 100 m³/h.
Filled containers can also be loaded with a container crane onto a truck or rail car and then be transported to an end consumer. In this way, a supply of petrol stations and industrial plants on land is also possible. In addition to the filling and the transport of LNG containers, containers with liquid nitrogen can also be loaded and transported. These are required, for example, whenever LNG fuel tanks onboard a ship have to be cooled down again to -160°C after a docking time. Here, the nitrogen may be transferred to the ship either in liquid form or, by using a vapourizer installed on the barge, in gaseous form.
A problem up to now when supplying LNG has been accurately measuring the delivered quantities. A volumetric measurement does not deliver precise figures because the LNG can change its density significantly depending on composition and temperature. To solve this, the barge is equipped with a weighing cell system, which determines the exact weights when loading or discharging the container. For controlling and evaluating the weighing, a small office is provided onboard, which also serves as a common room for the crew.
For power generation, the barge is equipped with a diesel generator. Attention has, of course, also been given to safety. Gas detectors monitor all areas of the barge. A water spray system protects the containers against heating-up in case of fire. A powder extinguishing system in way of the bunker connections serves as fire-fighting system. By using the mobile LNG filling station based on the LNG container barge, the logistic gap between truck supply of small amounts of LNG and supply of larger amounts via ship-to-ship transfer is covered.
Dr.-Ing. Jochen Schmidt-Lüßmann
Since 1988, the author is Senior Mechanical and Cryogenic Engineer at Marine Service GmbH. Dr. Schmidt-Lüßmann holds a doctorate of mechanical engineering from the Technical University Hamburg-Harburg.