Drilling Operations
Drawworks
Drawworks are the hoisting winches that spool the drilling line and raise or lower the traveling block. This Category covers AC drawworks with motor braking, older DC and mechanical units with band brakes and auxiliary brakes, active heave drawworks for floating rigs, and spare drums, gearboxes, motors and brake parts.
Operating principle
The drawworks winds the fast line onto a grooved drum. Electric motors drive the drum through a gearbox or directly. Fast line pull multiplied by the number of lines, less reeving losses, gives the load at the hook. On lowering, the drawworks has to absorb the energy of the falling string, so braking capacity matters as much as hoisting power.
Configurations
- AC drawworks run induction motors from variable frequency drives (VFDs). The motors do most of the braking and send energy back to the drives, which dump it into braking resistors. Spring-applied, hydraulically released disc brakes hold the load when parked and in an emergency.
- DC drawworks run from SCR drives. They usually have band brakes or disc brakes, with an eddy current or hydrodynamic auxiliary brake to absorb lowering energy.
- Mechanical drawworks take power through chains and clutches from engines. They are now rare offshore.
- Active heave drawworks (AHD) run the motors to follow vessel heave, which replaces a separate compensator on some floating rigs.
- Gear-driven units use single or multi-speed gearboxes. Direct-drive units use low-speed motors with no gearbox.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Input power | Sets hoisting speed at a given load | hp or kW, continuous and intermittent |
| Maximum fast line pull | With line count, sets hook load | lbf or kN |
| Drum diameter and grooving | Must match your line size | in or mm, groove for one line diameter |
| Drum capacity | Layers and length of line spooled | ft or m of line |
| Brake capacity | Static holding and dynamic lowering | ft-lb or kN·m |
| Drive and motors | Must match your power plant and drives | AC VFD, DC SCR, mechanical |
Standards and class
API 7K covers the design, manufacture and testing of drawworks components. API RP 9B gives the recommended drum diameter and grooving for drilling line, and reeving efficiency factors. On a classed rig, DNV-OS-E101 and the ABS drilling systems guide require class approval of the hoisting system, with attention to brake redundancy and the crown and floor protection functions.
Failure modes and inspection
- Drum groove wear. Worn or wrong-size grooves crush the line and cause poor spooling at layer changes.
- Brake disc heat checking and thickness loss, caliper leaks and spring fatigue.
- Band brake and rim wear on older units, measured against OEM limits.
- Gear tooth pitting and bearing wear, tracked by oil analysis and vibration readings.
- Motor insulation breakdown, checked with insulation resistance and polarisation index tests.
- Cracks in the drum shaft and flanges, found by MPI.
A static brake test at rated load proves the holding brakes. A run test at load through the drives shows motor braking and the block position control.
Documentation
- OEM service history, overhaul records and product bulletins applied.
- Gearbox oil analysis trend and vibration history.
- Motor test reports and VFD or SCR drive records.
- Brake test results and caliper or band replacement dates.
- Class certificates and the last annual survey on a classed unit.
- Records of overloads, dropped-block or crown strike events.
Integration and compatibility
An AC drawworks needs drives sized for its motors and a power plant that can absorb or dump regenerative energy. The control system holds the crown saver, floor saver and block position limits, so swapping drawworks usually means reprogramming them. Check the drum grooving against your line diameter, the fleet angle to the crown fast line sheave, and the foundation bolt pattern. Converting from DC with band brakes to AC with motor braking brings new drives, cabling and control logic, not just a new winch.
Refurbishment and recertification
An OEM or licensed overhaul usually includes gearbox teardown, new bearings and seals, motor reconditioning, new brake discs or pads, NDT of the drum and shaft, and regrooving or new groove sleeves. A load test and a brake test close it out.
A used drawworks is most practical when it is the same model as yours or a sister rig's, so drives, foundations and controls carry over. Unused surplus units from cancelled newbuilds can be close to new, but check motor insulation and gearbox internals after long storage.
Common offshore OEMs include NOV, HMH and Bentec. Match the drum to the drilling line and rigging you run, and compare active heave drawworks with motion compensators.
FAQ
Frequently asked questions
What should I check before buying a used drawworks?
Check the drum grooves, brake condition, gearbox and motor condition, and whether the drives and controls it needs come with it. Ask for oil analysis trends, motor test reports, brake test results and the overhaul history. Confirm the drum is grooved for your line diameter.
Can I replace a DC drawworks with an AC drawworks?
Yes, but it is a power and controls project as well as a mechanical swap. An AC drawworks needs matched variable frequency drives, braking resistors or energy recovery, new cabling and new control logic for the crown and floor savers. Foundations and drill floor layout often change too.
How does an AC drawworks brake without band brakes?
An AC drawworks brakes mainly through its motors, which act as generators when lowering and feed energy to the drives and braking resistors. Spring-applied disc brakes hold the load when parked and stop it if power or the drive fails. That makes the drive and resistor system part of the braking function.
Does the drawworks drum have to match the drilling line size?
Yes, the drum grooves are cut for one line diameter. A line that is too small sits loose in the grooves and spools badly, while one that is too large gets pinched and crushed. Changing line size usually means regrooving the drum or fitting new groove sleeves.
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