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Marine & Station Keeping
Moonpool Trolleys and Carts
Moonpool Trolleys and Carts carry the BOP and subsea trees on rails between their storage areas and the well centre over the moonpool. The Category covers BOP carriers and transporters, tree carts, skid carts and test stump carts, with their rails, rack and pinion or skidding drives, locks, hydraulic power and controls.
What trolleys and carts do
A drillship or deepwater semi stores its BOP on a set-back area beside the moonpool, often on a test stump for pressure testing between wells. A BOP carrier runs on rails from that area to the well centre and holds the stack while the crew connects it to the riser. Once the BOP is hanging on the riser, the carrier moves aside. Tree carts do the same for subsea trees, often on the opposite side of the moonpool so trees can be handled while the BOP stays in place.
Configurations
| Type | Load | Drive |
|---|---|---|
| BOP carrier or transporter | BOP stack, sometimes with lower riser package | Rack and pinion or hydraulic skidding |
| Tree cart | Subsea tree and running tool | Rack and pinion, winch or skidding |
| Skid cart | General subsea equipment | Hydraulic gripper jacks on skid beams |
| Test stump cart | BOP on a test stump | Moves stump and stack together |
NOV supplies BOP and tree handling equipment on many drilling units.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| SWL | Must cover the heaviest load with frames | tonnes (kips) |
| Rail gauge and rail type | Fit to existing rails | mm (in), rail profile |
| Platform and footprint | Matches BOP or tree frame | m (ft) |
| Drive and speed | Positioning over the well centre | Rack and pinion, skidding, m/min |
| Sea-fastening | Holds the cart and load while the rig moves | Locks, rated for motion loads |
Standards and certification
Most class societies and flag states treat carts that carry a BOP or tree as handling or lifting equipment. That means design approval, a proof load test above SWL after installation or major repair, and periodic thorough examination under rules such as DNV-ST-0378, the ABS lifting appliance guide or NORSOK R-002. Sea-fastening and locks must hold the load under the rig's motion design accelerations. The rails and the deck under them are hull structure, approved and surveyed by class.
Failure modes and inspection
- Wheels, bearings and rack and pinion teeth wear. Measure tooth profiles and wheel diameters against OEM limits.
- Rails wear, bend and lose fastenings. Survey straightness and gauge along the full run.
- Cart structure cracks at axle mounts, jacking points and the load platform. Use magnetic particle testing on welds in high-stress areas.
- Hydraulic motors, cylinders and hoses leak. Check rods and take an oil sample.
- Locks, brakes and end stops wear or are bypassed. Function test each one under load.
- Positioning sensors and interlocks drift. Check them against the control logic.
Documentation
- Load test certificate with the test load and date.
- Lifting or handling equipment certificate and thorough examination record.
- OEM service history and modification records.
- General arrangement drawing with rail gauge, wheelbase and footprint.
- Hydraulic and electrical schematics.
- Sea-fastening design and certificate.
Integration
Start with weight. A modern 15,000 psi BOP with six or more rams can be much heavier than the stack an older cart was built for, so compare the certified SWL with your heaviest load, including frames and test stumps. Then match rail gauge, wheelbase and footprint to your rails and set-back area. Check the drive's hydraulic or electric supply and how the cart interlocks with the doors, the BOP handling crane and the drill floor controls.
Refurbishment and the used, surplus or refurbished choice
A refurbishment renews wheels, bearings, seals and hoses, inspects the structure and rack, and ends with a proof load test and thorough examination. A used cart from a sister rig is the most likely fit. Unused surplus carts from cancelled drillships often match hulls still working. The doors these carts pass over are in Moonpool Doors and Gates, and the cranes that lift the BOP are in Gantry and Overhead Cranes.
FAQ
Frequently asked questions
What is a BOP carrier on a drillship?
A BOP carrier is a rail-mounted cart that moves the subsea BOP from its set-back area to the well centre over the moonpool. It holds the stack while the riser is connected and moves aside once the BOP hangs on the riser. Some carriers also move the test stump with the stack.
Can a used BOP cart handle a heavier BOP?
Only if its certified safe working load covers the new BOP with its frames and test stump. Check the load test certificate and the OEM design rating. Upgrading an under-rated cart needs engineering, structural work and a new proof load test accepted by class.
What should come with a used moonpool tree cart?
Ask for the load test certificate, the thorough examination record, the OEM service history and the general arrangement drawing with rail gauge and wheelbase. Hydraulic schematics and the sea-fastening certificate complete the package. Recent photos of wheels, rails and the rack show wear before an inspection.
How is a moonpool cart secured while the rig is in transit?
A moonpool cart is held by sea-fastening locks rated for the rig's motion design loads, not by its drive brakes. Check the lock design and certificate, and function test every lock. Weak sea-fastening is a common gap on carts from rigs that worked mostly in calm water.
Looking for moonpool trolleys and carts?
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