These are tanks used on large LPG tankers. Semi-membrane tanks are built up with an inner tank, insulation, membrane and insulation against hull. It is the membrane that takes up the thermal expansion. The tanks are built of aluminum, ferronickel steel with 36% nickel, or built of stainless steel. The insulation is mostly perlite, but can also be polyurethane or polystyrene. The hull absorbs all dynamic loads from the cargo tank when the tank is loaded. Normal excess pressure for such cargo tanks are 0,25 bars, and there is a demand for a secondary barrier.

One can use the hull as secondary barrier for cargo temperature down to -55 deg C, but one must utilize low-temperature steel in the hull around the cargo tank. One can also place a tank into the insulation as secondary barrier. One can not utilise the hull as secondary barrier for a temperature colder than -55 deg C. A membrane inside is then built in the insulation as secondary barrier. This tank type was designed for LPG transportation, but no LPG tankers are built with this tank type. In recent years, Japanese yards have started to utilise this tank type on large LPG tankers.

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