Moon Pool

What Is a Moon Pool?

Moon pool (also called a drill well or center well) is a vertically oriented, water-filled opening cut through the hull of a drillship or the deck of a semi-submersible drilling rig, through which the drill string, marine riser, blowout preventer (BOP), and all subsea equipment are deployed and recovered. The moon pool provides a protected working area that is shielded from open ocean wave action and vessel-side sea conditions, allowing continuous deepwater drilling operations even when surface conditions are rough. The surrounding hull structure dampens the hydrodynamic forces that would otherwise act directly on the drill string and riser, making controlled pipe handling possible in conditions that would be impossible from an open deck.

Key Takeaways

Moon pools are a feature of drillships and semi-submersible rigs only; jack-up rigs use a cantilevered derrick structure over the side or end of the hull and do not have moon pools.
Typical moon pool dimensions are 6-8 m wide by 10-12 m long, sized to accommodate the largest BOP stack and riser system the vessel is designed to handle.
The protected water surface inside the moon pool is significantly calmer than the open sea, but keel currents and vessel heave still transmit dynamic loads to the riser system that must be compensated by tensioners.
Riser tensioners, located directly above the moon pool, maintain constant upward tension on the marine riser to prevent buckling as the vessel heaves in ocean swells.
BOP handling, coiled tubing operations, wireline logging, and completion equipment deployment all route through the moon pool, making it the central functional hub of any deepwater drilling vessel.

How a Moon Pool Works

The moon pool is structurally a through-hull opening bounded by a reinforced moon pool trunk or "wet porch," a ring of heavy structural steel that transfers the substantial loads from riser tensioners and rotary table forces into the vessel's main hull framing. On a drillship, the moon pool is located near amidships, close to the vessel's center of pitch and roll motion, to minimize the amplitude of heave-induced riser dynamics. On a semi-submersible, the moon pool is centered in the main deck structure between the pontoons, which already provide excellent motion dampening through their deep draft and distributed buoyancy. The water inside the moon pool communicates directly with the sea through the open bottom of the hull, so sea state at depth determines the still-water level. In heavy swells, the moon pool surface can surge and swell by several meters, which must be managed to protect equipment during BOP deployment.

The drill string passes through the moon pool via the rotary table or top drive, through the marine riser (a large-diameter pipe running from the vessel to the wellhead), and down to the BOP stack sitting on the subsea wellhead. The riser tensioner system, a set of hydraulic tensioner cylinders mounted in the moon pool framework, applies upward force on the top of the riser to counteract the riser's weight and vessel heave. Tensioners must apply enough upward force to keep the riser in tension throughout the full heave cycle while also accommodating lateral drift if the vessel offsets from the wellhead due to current or weather. Modern drillships carry tensioners rated at 2,000-4,000 kips aggregate tension capacity. Passive tensioners use pressurized gas springs; active tensioners use hydraulic servos that adjust stroke velocity to compensate for measured vessel heave in real time, keeping riser load excursions within acceptable bounds.

Fast Facts: Moon Pool

Typical dimensions: 6-8 m (width) x 10-12 m (length) for a standard drillship
Location on drillship: Amidships, near roll/pitch center to minimize motion amplification
Location on semi-sub: Center deck between pontoons and columns
Riser tensioner capacity: 2,000-4,000 kips total for ultra-deepwater vessels
BOP clearance: Moon pool width must exceed BOP stack outer diameter by minimum 0.5 m on each side
Keel current issue: Vessel motion creates water currents through moon pool that induce VIV on riser; managed with strakes
Alternative for jack-ups: Cantilever deck extends derrick over the platform slot; no moon pool involved
Maximum operating sea state: Typically limited to 6-7 m significant wave height for BOP running operations

Field Tip:

During BOP deployment through the moon pool, watch for keel current velocities exceeding 0.5 knots inside the opening. Even in calm surface seas, vessel drift from current or thruster activity can create strong crossflows through the moon pool that impose unexpected side loads on the riser and BOP handling system. Check the vessel's keel current monitoring readout before and during any heavy lift through the moon pool, and coordinate with the dynamic positioning (DP) operator to minimize translational drift during the critical period when the BOP is hanging in the water column without the riser guiding it.

Moon Pool Design on Drillships vs. Semi-Submersibles

While both vessel types use moon pools, the structural integration and operational context differ. On a drillship, the moon pool is cut through a displacement hull that is otherwise a continuous watertight shell. The structural frames surrounding the moon pool are heavily reinforced because the discontinuity in the hull creates a stress concentration that must resist the combined effects of vessel bending in a seaway, tensioner loads, and rotary table reaction forces. Naval architects optimize moon pool dimensions to be just large enough to pass the largest anticipated BOP stack with required clearances, because every additional meter of moon pool width removes hull plating that provides longitudinal bending strength. Modern ultra-deepwater drillships from Samsung, Hyundai, and Daewoo shipyards typically have moon pools of approximately 8 m x 12 m, with a clear working deck around the perimeter for riser handling equipment, tensioner frames, and diverter systems.

On a semi-submersible, the moon pool is formed by the geometry of the pontoon and column arrangement rather than by cutting through a solid hull. Because the main deck is elevated high above the waterline and the structural loads are carried by the columns and pontoons, the moon pool area between the columns is a natural open space. Semi-sub moon pools are often larger than drillship moon pools, which gives more room for complex BOP handling and riser running tools but also means the vessel's wave-induced motions in the moon pool area are more sensitive to pontoon submergence depth. Semi-subs compensate with deeper draft, reducing wave-frequency excitation, and with active or passive heave compensation systems on the traveling block that decouple the hook load from vessel motion during pipe running.

Drill well - a common alternative name, particularly in older British and Norwegian industry literature, referring to the same through-hull opening used for drilling operations.
Center well - used occasionally in semi-submersible documentation to describe the open area at the center of the deck structure through which drilling operations are conducted.
Keel slot - sometimes used to describe the bottom opening of the moon pool where the riser exits the hull into open water; the keel slot clearance determines the maximum riser OD the vessel can handle.
Wet porch - the structural ring assembly lining the moon pool trunk, including the structural frames, guides, and sealing surfaces that support riser handling equipment and diverter housing.

Frequently Asked Questions About Moon Pools

Why do jack-up rigs not have moon pools?

Jack-up rigs rest their legs on the seabed, which fixes them in position and eliminates the need for a through-hull opening to accommodate a dynamic riser system. Instead, jack-up rigs use a cantilever structure, a large steel frame that slides outboard from the main hull on rails and positions the derrick directly over the platform's conductor slot or over open water beside a fixed structure. The drill string descends through the rotary table on the cantilevered derrick and enters the water (or the platform slot) without needing a protective hull enclosure. Since jack-ups operate in shallow water (typically less than 150 m), the drill string is relatively short and rigid enough to handle without the heave compensation and riser tensioning systems that moon pools support in deepwater.

How does a BOP get deployed through the moon pool?

BOP deployment is one of the most complex lifts performed through the moon pool. The BOP stack (which can weigh 300-600 tonnes for a deepwater configuration) is built up horizontally on the main deck and then upended using the derrick crane or auxiliary cranes. Once vertical, it is moved over the moon pool on trolleys or a dedicated BOP handling system, stabbed onto the top of the marine riser, and lowered by the drill string or riser tensioners through the moon pool into the water. Divers or ROVs guide the stack during the initial descent to clear the keel slot. Once the BOP is in the water column, it is run to the wellhead on the lower marine riser package (LMRP) and latched hydraulically. The entire operation is paused if significant moon pool surge or keel current develops, because a swinging BOP in restricted clearance can damage both the stack and the moon pool structure.

What is heave compensation and why is it critical for moon pool operations?

Heave compensation is a control system that isolates the drill string or riser from the vertical motion of the vessel caused by ocean swells. Without compensation, every time the vessel rises on a wave, the drill string would be jerked upward; every time the vessel falls, the string would slacken and potentially buckle. At 3,000 m water depth with a vessel heaving 3 m peak-to-peak at one wave per 12 seconds, the uncompensated cyclic load on the drill string would be enormous and would make controlled drilling impossible. Passive heave compensators use large hydraulic-pneumatic accumulators connected to the traveling block to absorb heave energy; active compensators add a servo-controlled cylinder that measures vessel motion and applies corrective force. Modern ultra-deepwater rigs achieve hook load variations of plus or minus 5-10% of the static load throughout the heave cycle, which is tight enough to permit drilling, connection making, and BOP running with acceptable fatigue and operational risk.

Why Moon Pools Matter in Oil and Gas

The moon pool is the enabling technology that makes deepwater drilling possible at all. Without a through-hull protected working area, deploying and maintaining a 5,000 m riser string connected to a subsea wellhead in water depths exceeding 3,000 m would be mechanically impossible against the forces of open-ocean wave and current action. As deepwater and ultra-deepwater reserves in the Gulf of Mexico, offshore Brazil, West Africa, and the Norwegian continental shelf become the primary frontier for new oil and gas supply, the engineering of moon pools, riser tensioner systems, and heave compensation continues to advance. Each new generation of drillships pushes operating depth deeper and operating sea states higher, placing greater structural and mechanical demands on the moon pool systems. For offshore drilling engineers, marine architects, and rig operations teams, the moon pool is the operational center of the vessel, and understanding its limitations defines the envelope of safe and productive drilling operations.