page_title: LNG carrier
section_title: Moss tanks (Spherical IMO type B LNG tanks)
hierarchical_section_title: LNG carrier / Containment systems / Moss tanks (Spherical IMO type B LNG tanks)
caption_attribution_description: Liquefied natural gas (LNG) tanker, section view from side.
context_page_description: An LNG carrier is a tank ship designed for transporting liquefied natural gas. As the LNG market grows rapidly, the fleet of LNG carriers continues to experience tremendous growth.
context_section_description: Named after the company that designed them, the Norwegian company Moss Maritime, the Spherical IMO type B LNG tanks are spherical in shape. Most Moss type vessels have 4 or 5 tanks.
  The outside of the tank has a thick layer of foam insulation that is either fitted in panels or in more modern designs wound round the tank. Over this insulation is a thin layer of "tinfoil" which allows the insulation to be kept dry with a nitrogen atmosphere. This atmosphere is constantly checked for any methane that would indicate a leak of the tank. Also the outside of the tank is checked at 3-month intervals for any cold spots that would indicate breakdown in the insulation.
  The tank is supported around its circumference by the equatorial ring, which is supported by a large circular skirt, known as a data-couple that is a unique combination of aluminum and steel, which takes the weight of the tank down to the ships structure. This skirt allows the tank to expand and contract during cool-down and warm-up operations. During cool-down or warm-up the tank can expand or contract about 60 cm (24 in). Because of this expansion and contraction all piping into the tank comes in the top and is connected to the ships lines via flexible bellows.
  Inside each tank there is a set of spray heads. These heads are mounted around the equatorial ring and are used to spray LNG onto the tank walls to reduce the temperature.
  Tanks normally have a working pressure of up to 22 kPa (3.2 psi), but this can be raised for an emergency discharge. If both main pumps fail then to remove the cargo, the tank's safety valves are adjusted to lift at 100 kPa. Then the filling line which goes to the bottom of the tank is opened along with the filling lines of the other tanks on board. The pressure is then raised in the tank with the defective pumps which pushes the cargo into the other tanks where it can be pumped out.
caption: LNG Moss-type tanker, side view
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