Two of the designs are of the self-supporting type, while the other two are of the membrane type and today the patents are owned by Gaztransport & Technigaz (GTT). There is a trend towards the use of the two different membrane types instead of the self-supporting storage systems. This is most likely because prismatic membrane tanks utilize the hull shape more efficiently and thus have less void space between the cargo-tanks and ballast tanks. As a result of this, Moss-type design compared to a membrane design of equal capacity will be far more expensive to transit the Suez Canal. However, self-supporting tanks are more robust and have greater resistance to sloshing forces, and will possibly be considered in the future for offshore storage where bad weather will be a significant factor. Moss tanks (Spherical IMO type B LNG tanks) 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 four or five tanks. The outside of the tanks have 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 three-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 ship's 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 . Because of this expansion and contraction all piping into the tank comes in the top and is connected to the ship's 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 , but this can be raised for