Origin of the Name

The story goes that, when the moon pool was first designed, on seeing its drawings, someone asked the naval architect responsible what exactly it was. He replied that it was a moon pool - In some states of America, a 'moonpool' is a place where one goes with one's lover for fornication.

A moon pool is basically an opening located at the floor or the hull of a sailing vessel, which is used for gaining easier and safer access to the water below. It is used by sailors, divers, researchers, marine biologists, etc., for entering the water easily. The concept of a moon pool originated in the oil drilling industry, and is now widely used in all marine drilling platforms and ships. Moon pools are used to safely lower tools and instruments into the water, especially in rough seas. Along with sheltering and protecting the sailors from the weather outside, a moon pool also permits access to the water below an ice cover. Through a moon pool, divers or small submersible crafts can straight away enter the water via the ice cover broken by the ship.

To understand the working of a moon pool, we will need to examine its different types. There are mainly four types of moon pools, each of which has a unique working principle. Let's examine them one by one. This is the first and most basic type of moon pool. As the name itself suggests, this moon pool is located within a sailing vessel, inside a chamber that is built above sea level. This chamber is constructed such that it is easily accessible via hallways and stairways, inside the sailing vessel. This type of moon pool usually requires ships having a twin-hull design, and is most commonly found on oil drilling platforms. This type is found in ships having a single (mono) hull design. In these ships, the hull is well below sea level, which causes the water to rise up inside the opening of the moon pool, making it seem like there is a swimming pool present on the floor. To prevent the rising water from flooding the hull and sinking the ship, the sides of these moon pools are designed such that they rise up higher than the water level in the pool, thus containing an overflow. This type also has doors present inside the hull. When the ship is sailing or is on rough seas, the openings to these moon pools are closed by shutting the doors. This is the third type of moon pool, and as is indicated by its name, it is present below the sea water level. To make it work without flooding the interior and sinking the ship, the chamber housing the moon pool is made completely airtight. Typically, the chamber has a two-door mechanism. The first door opens to the hull while the second one opens to the sea water. When a diver wants to enter the water, he/she will enter into the middle chamber through the first door, while the second one is kept locked. Once inside the chamber, the first door is locked and sealed. Then the second door is opened and the diver is able to enter the water. To get back in, he will need to get into the chamber through the second door while the first remains locked. Once he is inside the chamber, the second door is closed, and the water in the chamber is pumped out before the first door is opened. This type comprises those moon pools that are usually found in deep underwater habitats. These habitats are made completely airtight and filled with air up to normal atmospheric pressure. The moon pools provides the only way to enter or exit from these habitats. The entry and exit mechanism is similar to the one used by the below sea level type of moon pools. These habitats are very useful for divers and researchers. They provide dry areas for recuperation and working, without the need to go back all the way up to the surface.