|The BASF/SHI anti-sloshing solution consists of a blanket of blocks of Basotect foam with buoys, which are stitched into Vectran textile covers and connected with Vectran belts. Click to enlarge.|
BASF and the South Korean company Samsung Heavy Industries (SHI) have developed a new concept to prevent the sloshing of liquefied natural gas (LNG) during its transport in tankers. The anti-sloshing solution is a kind of blanket consisting of cubes with a volume of one cubic meter, made of the BASF foam Basotect. The open-cell foam made from melamine resin stays flexible even under cryogenic conditions; the ship’s steel tanks must remain cooled to -162 ° C to keep the gas liquid.
More than a quarter of the global production of natural gas in 2011—nearly 331 billion cubic meters—was liquefied and shipped throughout the world in ocean-going tankers. In preparation for transport, the gas is cleaned, liquefied at -162 °C, and then loaded onto tankers that carry liquid cargo. This process reduces 600 cubic meters of gas to one cubic meter of LNG (liquefied natural gas).
LNG tankers contain 3 to 5 tanks, with a maximum tank volume of 40,000 m3 of LNG. However, tankers only travel when full (> 80%) or empty (< 10%); no flexible load levels are possible, because they would otherwise be put at risk by the sloshing of the liquefied gas, which in severe cases might even cause them to capsize.
Moreover, sloshing in closed containers with the dimensions of tanks used for transporting LNG can generate extremely high-impact loads, which can damage the tank walls. Usually, these LNG tanks of the membrane type cargo containment system are built with an octagonal cross-section as an initial means of reducing sloshing and therefore also the impact load against their walls. However, this measure is not sufficient when the tanks are only half full.
Working together with BASF, the South Korean company Samsung Heavy Industries has developed a new anti-sloshing solution made of the BASF foam Basotect. The anti-sloshing blanket is lightweight and able to float, but sufficiently heavy to prevent sloshing. It is also soft enough to not damage the walls of the tank, and is resistant to extremely low temperatures.
Basotect, an open-cell foam made from melamine resin, has very high acoustic and good heat-insulation properties and is both flame-retardant (DIN 4102, B1) and resistant to many chemicals. Due to its favorable set of properties, this BASF foam is used for sound absorption in buildings and heat insulation purposes in automotive and rail engineering as well as in solar thermal and air-conditioning systems.
Basotect’s new application in LNG tankers capitalizes on an advantage that had thus far not attracted much attention—the retention of the material’s flexural properties under cryogenic conditions, i.e. down to temperatures of below -200 °C.
The anti-sloshing concept developed by BASF and Samsung consists of a blanket made of Basotect cubes. Each cube, with sides of one meter, is made of two blocks of Basotect. A buoyant entity, or type of buoy, is placed between the two blocks so that the Basotect cube does not submerge more than 80% even after becoming fully soaked with liquefied gas.
The individual cubes are stitched into Vectran textile covers and secured to one another with Vectran belts. Vectran, which is made from polyarylate fiber, is produced by the Japanese company Kuraray, and is also suitable for use under cryogenic conditions. In addition, Vectran is extremely durable and abrasion-resistant.
The anti-sloshing blanket consists of many individual elements because it has to be flexible and mobile in order to best accommodate and reduce wave action. These individual elements should be secured together at the shipyard, in fact directly inside the tanks, before the tanker is filled with LNG for the first time. The blankets can be placed in the tanks before they are loaded, and remain there after they are emptied.
Tests at BASF’s technical laboratory have shown that when the containers are emptied, Basotect releases nearly all of the LNG it has absorbed.
The principle of using floating blankets to calm the rough surface of a body of liquid is well known, and is used in aviation and aeronautics as well as in transporting liquid cargo in trucks. It has not seen use in LNG tankers so far because the extremely low temperatures in their tanks place correspondingly extreme demands on the contact materials.
Another challenge is posed by apparently contradictory physical requirements: on the one hand an anti-sloshing blanket has to be lightweight in order to float at every level of load, yet it must also be sufficiently heavy to have an anti-sloshing effect. In addition, the material has to be soft in order to withstand repeated severe impact against the walls of the tank while also not damaging the walls.
The newly developed anti-sloshing blanket system made of Basotect meets these requirements, according to the partners. The melamine resin foam’s open-celled structure enables it to absorb a certain amount of LNG, which increases the blanket’s overall inertia. Above all, however, Basotect differs from other foams because it retains its properties even at extremely low temperatures.
Using a wave simulator, the design of the Basotect blanket has been tested in South Korea by the departments of Naval Architecture & Ocean Engineering at Seoul National University and Pusan National University. The correctly dimensioned, transparent container used to simulate the sloshing conditions has the octagonal shape of an LNG tank and is equipped with pressure sensors in its walls.
Tests showed that the Basotect blanket was able to considerably reduce sloshing and reduce peak pressures against the walls by about one-fifth. Because there is less sloshing, shipbuilders can also eliminate two corners of the octagonal shape and thus have right angles at the top of the tank. This means that about five percent more LNG can be stored and the production cost can be saved in the simplified tank shape.
Also, the anti-sloshing system considerably reduces undesired abrupt vaporization in operation which could lead to a sudden increase of internal pressure. The vaporization is measured with the so called boil-off rate (BOR). The BOR is mainly determined by how well the insulation succeeds in preventing heat transfer from the LNG to its surroundings. The reduction of BOR is an additional benefit of the anti-sloshing blanket solution that some ship owners and operators are asking for: it improves safety and increases the amount of liquefied gas that arrives at the harbor. Because the blanket not only protects against sloshing but also lowers the boil-off rate, the concept is known as ABAS (= Anti-Boil-Off Anti-Sloshing).
Use of anti-sloshing blankets made of Basotect changes both economic and environmental aspects of LNG transport. Because LNG tankers will be able to take different load levels, it will be possible for them to call at several harbors in succession and to unload their contents based on need. This will reduce the number of no-load journeys and therefore also costs while at the same time making a contribution to resource efficiency.
In 2012, Samsung received “general approval” for the ABAS concept from two leading classification companies for shipping worldwide, the American Bureau of Shipping in Houston, Texas, USA, and the Bureau Veritas in Paris, France.