Offshore operations face a tricky challenge: the vessel is on the surface while the equipment is underwater, separated by a water column tens or even hundreds of meters deep. How do you transmit oil, electricity, and signals down there? Rigid cabling? It would snap the moment it twisted. Wireless? Signal attenuation underwater is too severe to be reliable.
The solution is the hose reel.
A large reel is mounted on the vessel, wound with hoses; one end connects to the ship's systems, while the other is lowered underwater to the equipment. The reel can pay out and retract the hose: as the vessel moves forward, the hose is deployed; when the vessel needs to leave, the reel winds the hose back in.
Here lies the problem: the hose is wrapped in multiple layers around the reel. Whenever the vessel moves, the reel rotates. This rotation causes the internal oil lines, power cables, and signal lines to twist. A few turns might be manageable, but excessive twisting leads to breakage.
The rotary joint solves this issue. Mounted on the reel's central axis, it ensures that internal channels remain open regardless of how the reel spins. Hydraulic oil flows from the vessel, through the rotary joint, and into the hose for delivery underwater; power and data signals follow the same path, passing through the rotary joint and traveling along the hose to the underwater equipment.
Simply put, the rotary joint acts as a "transfer station" for the reel-connecting to the vessel's systems at one end and the underwater hose at the other, while maintaining continuous pathways for oil, electricity, and signals despite the rotation.
At sea, this component is most commonly used to power ROVs (Remotely Operated Vehicles). These underwater robots are controlled by operators on the vessel to perform tasks like cutting, welding, and inspection. When an operator pushes a joystick, commands are converted into electrical signals and transmitted through the rotary joint to the ROV, triggering movement. Simultaneously, the rotary joint supplies hydraulic oil-essential for the ROV's robotic arms to grasp objects or turn screws via hydraulic cylinders-with pressurized oil flowing from the vessel through the joint to the underwater unit. Power delivery is also critical, as the ROV's thrusters, cameras, and lights all rely on electricity. Three elements-hydraulic fluid, electrical power, and data signals-must be delivered simultaneously; the ROV cannot operate if even one is missing. These elements must pass through a constantly rotating winch drum, a task handled entirely by the rotary joint.
The marine environment places extreme demands on rotary joints. Standard seals simply cannot withstand the combination of saltwater corrosion, high pressure, and the low temperatures found in deep water. Modern rotary joints often integrate electrical slip rings, combining hydraulic and electrical channels into a compact unit that simultaneously transmits fluid, power, and signals. Some deep-sea models operate at pressures exceeding 40 MPa and are designed for a service life of ten years without replacement.
As the ship's winch repeatedly pays out and retrieves the cable, the rotary joint spins continuously within the drum. It operates silently and unobtrusively, yet without it, the ROV could not be deployed, and no underwater work could be performed.