A moon pool is an opening built into the hull of a ship that gives direct access to the water below. It’s essentially a hole in the bottom of the vessel, typically rectangular or circular, that allows crews to lower equipment, launch submarines, or perform drilling operations from inside the ship rather than over the side. The water fills the opening from below, creating an internal well that rises and falls with the sea.
How a Moon Pool Works
The concept is simpler than it sounds. A vertical shaft runs through the hull of the ship, open at the bottom to the ocean. Water fills the shaft to roughly the same level as the surrounding sea surface, kept in equilibrium by the weight of the water column and air pressure above it. The ship doesn’t flood because the opening is contained within a reinforced well structure, and the vessel’s buoyancy is designed to account for the added water weight inside.
As waves pass beneath the ship, the water inside the moon pool rises and falls. The air trapped above the water column compresses and decompresses with each cycle. This interaction between the internal water surface and the vessel’s motion is one of the central engineering challenges of moon pool design.
Where Moon Pools Are Used
Moon pools appear on a specific class of vessels: drilling ships, dive support boats, fall-pipe vessels, and scientific research ships. Any vessel that needs to lower something heavy or delicate straight down through its hull into the ocean is a candidate.
On drilling ships, the moon pool sits directly beneath a derrick, allowing the drill string to pass through the hull and reach the seafloor. The entire drilling operation happens from the center of the vessel, where motion is minimized. Fall-pipe vessels use the opening to deposit rock through a long pipe onto the seabed for projects like land reclamation, port construction, and reinforcing foundations for offshore wind farms. Research ships use moon pools to launch submarines and scientific instruments into hard-to-reach waters, including under Arctic ice, where over-the-side deployment would be impractical or dangerous.
Why Not Just Lower Equipment Over the Side?
The short answer: safety and weather. When you lower a remotely operated vehicle (ROV) or piece of heavy equipment over the side of a ship, it has to pass through the splash zone, the chaotic boundary between air and water where waves and wind can slam gear into the vessel’s hull or thrusters. Large waves cause suspended equipment to swing unpredictably, risking damage to both the hardware and the ship itself.
A moon pool eliminates most of that risk. By moving the deployment point to the center of the vessel’s hull, the pivot point for any suspended load shifts from a crane on deck to the bottom of the ship, drastically reducing horizontal swing. The hull itself shields equipment from wind and wave action during the critical transition into the water. Oceaneering, one of the largest subsea services companies, equips its intervention-class vessels with dedicated ROV moon pools specifically because they allow operations in sea conditions that would shut down conventional over-the-side launches. In practical terms, a moon pool means fewer days sitting idle waiting for calmer weather.
The Engineering Challenges
Cutting a hole in the bottom of a ship introduces problems that engineers have to solve carefully. The two biggest are piston-mode sloshing and wave resonance.
Piston-mode sloshing happens when the water column inside the moon pool starts moving up and down like a piston in a cylinder. If the frequency of the waves matches the natural frequency of the water column, the sloshing can amplify dramatically, sending water surging up through the well and onto the deck. This resonant excitation is a serious structural and safety concern. Engineers use several strategies to dampen it, including modifying the shape of the opening at the keel, adding internal baffles, and designing the geometry of the well to disrupt the oscillation. Research in fluid mechanics has shown that vortices forming at the edges of the moon pool opening actually help dissipate energy, and this natural damping effect can be enhanced through careful design of the opening’s geometry.
Classification societies like DNV, which set safety standards for offshore vessels, require that moon pool resonance be carefully evaluated during design. Structural checks must account for the possibility of waves entering the ship through the moon pool, particularly on deep-draught floaters where the distance between the moon pool and the outer wave zone is short. Every structural component around the opening must pass checks for both excessive stress and buckling, with a requirement that failure modes remain ductile rather than brittle.
The Most Famous Moon Pool in History
The most dramatic use of a moon pool was aboard the Hughes Glomar Explorer, a ship built in the early 1970s for a CIA operation called Project Azorian. The mission: secretly recover a Soviet submarine that had sunk in the Pacific Ocean three miles below the surface. The ship was disguised as a deep-sea mining vessel, but its center housed an enormous moon pool large enough to contain a section of the recovered submarine. A massive claw-like capture vehicle descended through the moon pool, grabbed the sub from the ocean floor, and hauled it back up into the well. Doors on the hull’s underside sealed the opening, keeping the entire operation hidden from Soviet ships, aircraft, and spy satellites. The ship took four years to build, and the moon pool’s “center docking well” was the key feature that made covert recovery possible.
Moon Pool Design Varies Widely
Not all moon pools look the same. Some are small openings barely wide enough for a cable or instrument package. Others, like those on large drilling vessels, are massive rectangular wells spanning several meters. The shape, depth, and internal features all depend on the mission. A vessel designed to launch manned submarines needs smooth walls, reliable centering systems, and enough clearance for safe docking. A drilling ship needs a moon pool aligned precisely with the derrick above and reinforced to handle the weight and vibration of a drill string extending thousands of meters to the seafloor.
On spar-type offshore platforms, which are large floating cylinders anchored to the seabed, the moon pool can run the entire vertical length of the structure. These platforms may consist of a single column with the moon pool at its core, and the structural design of the entire platform revolves around this central opening. The moon pool structure in these cases is integral to the hull itself, classified as a special structural category requiring enhanced materials and inspection standards.
One underappreciated benefit of moon pools is that they let crews work from inside the vessel. On a drilling ship in the North Sea or a research vessel in Antarctic waters, the ability to deploy and recover equipment without sending people onto an exposed deck in freezing spray and high winds is not just convenient. It fundamentally changes what operations are possible and in what conditions.