The design of the ship and its safety systems need to take into account the possibility of an air leak or catastrophic failure of the airlock. To keep the chamber airtight, access from the chamber to the rest of the ship is via an airlock with airtight doors. Air pressure inside the chamber prevents water rising in the moon pool up to sea level. This arrangement is shown in part C of the diagram. It is possible to have a moon pool below the waterline and to keep water out of the chamber above it, if the chamber is airtight rather than open to the atmosphere above in any way. The sides of the moon pool are quite deep as they need to be greater than the draft of the ship by a margin of safety. Doors would be used to close the bottom of the moon pool when the ship is moving, or in rough weather. This kind of moon pool is also open to the air above the ship. Water will not enter the hull and sink the ship provided the sides of the moon pool extend up inside the hull well above the waterline, as shown in part B of the diagram. In a monohull ship the bottom of the hull is below sea level and the water rises inside the opening of the moon pool, so that from inside the hull, the moon pool looks like a swimming pool in the floor. The chamber above the moon pool is also connected to the open air via staircasewells and passages.
The research vessel Western Flyer (pictured) also has a moon pool above the waterline, which its SWATH (twin-hull) design allows. In a drilling platform, the moon pool is usually above sea level, and is open to the air above and below.
Drill pipes need to run vertically through the structure or hull and the moon pool provides the means to do this. Moon pools originated in the oil drilling industry, which uses them in drilling at sea or in lakes, to pass drilling equipment into the water from a platform or drillship.
Pressure considerations in below waterline moon pools.A moon pool also allows divers or small submersible craft to enter or leave the water easily and in a more protected environment. It provides shelter and protection so that even if the ship is in high seas or surrounded by ice, researchers can work in comfort rather than on a deck exposed to the elements. It is an opening in the floor or base of the hull, platform, or chamber giving access to the water below, allowing technicians or researchers to lower tools and instruments into the sea. A moon pool is a feature of marine drilling platforms, drillships and diving support vessels, some marine research and underwater exploration or research vessels, and underwater habitats, in which it is also known as a wet porch.
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