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Marine & Station Keeping
Thrusters and Propellers
Thrusters and Propellers are the propulsion units that produce station-keeping and transit thrust on a DP drilling unit. The Category covers underwater-mountable and retractable azimuth thrusters, fixed azimuth thrusters, tunnel thrusters, their propellers and nozzles, steering gear and the thruster motors that come with them. Drillships and deepwater semis typically run six or eight azimuth units.
How an azimuth thruster works
An electric motor drives the propeller through a right-angle gearbox in the lower unit, or through two gear stages in a Z-drive. A slewing bearing and steering motors turn the whole lower unit through 360 degrees, so the thruster can push in any direction. On most DP drilling units the propeller is fixed pitch and the variable frequency drive sets thrust by changing speed and direction. A ducted nozzle raises bollard pull at low speed. Some designs tilt the propeller axis a few degrees downward to reduce thrust lost against the hull or the pontoons.
Configurations
| Type | How it is mounted | Where you see it |
|---|---|---|
| Underwater-mountable azimuth | Lower unit bolts into a hull canister from below | Drillships, deepwater semis |
| Retractable azimuth | Lifts into a trunk inside the hull | Semis and vessels that also transit |
| Fixed azimuth | Mounted from inside the hull in drydock | Older or shallower-water units |
| Tunnel | Transverse tunnel, side force only | Support vessels, bow thrust on some drillships |
Underwater-mountable designs let a crew change a thruster at sea with a crane or the derrick and divers or an ROV, without drydock. Kongsberg (including the former Rolls-Royce Marine range), Wärtsilä and Thrustmaster have built units of this type.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Input power | Sets thrust and loads the switchboard | kW or MW (hp) |
| Bollard pull | Thrust the FMEA counts on | kN or tonnes-force |
| Mounting interface | Must match the hull canister or foundation | OEM canister type, flange drawing |
| Propeller | Diameter, pitch type, blade count | m (ft), fixed or controllable pitch |
| Motor and drive | Voltage, speed range, cooling | kV, rpm, air or water cooled |
| Steering | Slew rate and motor type | degrees per second, electric or hydraulic |
Standards and class
Thrusters on a classed rig need a class certificate from manufacture, issued under the society's machinery rules for propulsion and steering. Gears, shafts and slewing bearings get material certificates and type approval. In service, class surveys thrusters through the continuous machinery survey and in-water or drydock surveys, with condition monitoring often accepted in place of opening up. A replacement unit must also pass the DP system's FMEA and trials, because its thrust feeds the worst-case failure calculation.
Failure modes and inspection
- Shaft seal failure lets seawater into gear oil, or gear oil out to sea. Check seal history and leak records.
- Gear and bearing wear shows in oil analysis as rising iron, copper and particle counts. Ask for the full trend, not one sample.
- Slewing bearing wear shows as backlash and noise in steering. Measure it against the OEM limit.
- Propeller blades suffer cavitation erosion, edge damage and bending. Check for weld repairs and pitch records.
- Motor insulation degrades with damp storage. Ask for insulation resistance and polarization index results.
- Vibration surveys and borescope images of gears give the clearest view inside a sealed lower unit.
Documentation
- Class certificate and material certificates for the unit.
- Running hours, and hours since the last overhaul.
- Oil analysis history and vibration trend reports.
- OEM overhaul report listing replaced bearings, seals and gears.
- Preservation log for stored units, with oil changes and periodic rotation.
Integration
Check the mounting interface first, because an underwater-mountable lower unit fits only its own canister type. Then match motor voltage and power to the variable frequency drive and switchboard, and the steering and feedback signals to the DP control system. Cooling water, lube oil systems and seal oil header tanks must suit the existing piping.
Overhaul and the used, surplus or refurbished choice
A full overhaul strips the lower unit, renews bearings and seals, inspects gears, checks the slewing bearing and load tests the steering. An OEM or OEM-approved workshop then issues a new service report for class. Unused surplus thrusters from cancelled drillships often match sister hulls exactly. A used unit with a recent overhaul suits an urgent spare, while a unit without records is a core for overhaul.
FAQ
Frequently asked questions
Can an azimuth thruster on a drillship be changed without drydock?
Yes, an underwater-mountable thruster can be changed with the rig afloat. The lower unit is lowered on wires and bolted into its hull canister with diver or ROV support. Only a unit designed for that canister type fits this way.
What should I check before buying a used azimuth thruster?
Start with the oil analysis trend, shaft seal history, running hours and the last OEM overhaul report. Then check slewing bearing backlash, propeller blade condition and motor insulation results. A vibration survey or gear borescope adds evidence a sealed lower unit cannot give by eye.
Are surplus drillship thrusters interchangeable between rigs?
Only when the model, mounting interface and power match. Sister drillships from one yard often share thruster models, so surplus units from cancelled hulls can serve as spares. A different model usually means canister work, an FMEA update and class approval.
Does a used thruster need a new class certificate?
It keeps its original certificate, but class must accept it on your rig through a survey after installation. An overhaul by the OEM or an approved workshop, with a service report, makes that acceptance simpler. The DP trials then confirm its thrust in the redundancy groups.
Why do some drillship thrusters have tilted propellers?
Tilting the propeller axis a few degrees downward reduces the thrust lost when the jet hits the hull or another thruster. It helps most on drillships, where several thrusters work close to a flat bottom. Match the tilt to the original design, because the FMEA thrust figures assume it.
Looking for thrusters and propellers?
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