Moon pool

Moon pool

Underside of the Research Vessel Western Flyer, showing its moon pool between the two hulls.
Uses	Protected access to the water for divers from a vessel, floating platform or underwater habitat

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. 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. 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. A moon pool also allows divers or small submersible craft to enter or leave the water easily and in a more protected environment.

Moon pools can be used in chambers below sea level, especially for the use of scuba divers, and their design requires more complex consideration of air and water pressure acting on the moon pool surface.

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. Drill pipes need to run vertically through the structure or hull and the moon pool provides the means to do this.

In a drilling platform, the moon pool is usually above sea level, and is open to the air above and below. The research vessel Western Flyer (pictured) also has a moon pool above the waterline, which its SWATH (twin-hull) design allows. See part A of the diagram. The chamber above the moon pool is also connected to the open air via staircasewells and passages.