The short answer
The difference is how engine power reaches the drawworks, mud pumps and rotary. A mechanical rig sends diesel engine power through a compound of chains, shafts and clutches. An SCR rig turns diesel power into electricity and drives DC motors through silicon-controlled rectifiers. An AC VFD rig does the same with AC motors controlled by variable frequency drives, which gives the finest speed and torque control of the three.
Side-by-side comparison
| Mechanical | SCR (DC electric) | AC VFD | |
|---|---|---|---|
| Power path | Diesel engines → torque converters → chain compound → machinery | Diesel generators → SCR system → DC traction motors | Diesel generators (or grid) → VFD → AC induction motors |
| Control | Manual, clutch and throttle | Good low-speed torque, electrical control of each motor | Precise speed and torque, suited to auto-drillers and pipe-handling automation |
| Braking | Band brake with auxiliary brake | Mechanical brake plus auxiliary brake | Motor (dynamic) braking in addition to the mechanical brake |
| Fuel use | Engines stay loaded to the machinery they drive | Generators can be brought on and off line with demand | Generators matched to demand; can run on grid power where available |
| Maintenance skills | Mechanics; parts widely available | Electricians familiar with DC motors, brushes and commutators | Electrical and drives specialists; motors need less routine work |
| Typical buyer | Workover, shallow and mid-depth programmes, remote sites with limited electrical support | Mid to deep wells where a proven electric rig is wanted at a lower price than VFD | Deep, long-lateral or high-performance programmes, and operators that require automation |
Heat and desert operation
All three types work in Gulf conditions, but electric rigs depend on their power control house staying cool. On SCR and VFD rigs, check the air-conditioning capacity of the control house and generator enclosures for summer ambient temperatures above 50 °C. Mechanical rigs avoid that dependency, which is one reason they remain common on remote desert and workover work.
Mechanical rigs in the RIGSDOM fleet
| Rig | Power | Hook load | Depth rating | Drive |
|---|---|---|---|---|
| Dreco Precision Super Single (RIG-3) | 400 HP | 250,000 lb | 7,200 ft | Mechanical |
| NOV 5C Millennium (RIG-1) | 525 HP | 250,000 lb | 20,000 ft | Mechanical |
| Cooper 750 (RIG-2) | 750 HP | 365,000 lb | 7,800 ft | Mechanical |
| Ideco Rambler 1000 (RIG-6) | 1,000 HP | 367,400 lb | 14,700 ft | Mechanical |
| ZJ50L (ZJ-50) | 1,500 HP | 708,000 lb | 16,400 ft | Mechanical |
SCR electric rigs in the fleet
| Rig | Power | Hook load | Depth rating | Drive |
|---|---|---|---|---|
| Dreco 1000 UE Desert Mobile (RIG-8) | 1,000 HP | 500,600 lb | 12,000 ft | SCR electric |
| Cabot Desert Mobile 1500 (RIG-4) | 1,500 HP | 715,000 lb | 14,500 ft | SCR electric |
AC VFD rigs in the fleet
| Rig | Power | Hook load | Depth rating | Drive |
|---|---|---|---|---|
| ZJ70DB (ZJ-70) | 2,000 HP | 1,011,600 lb | 23,000 ft | AC VFD electric |
| ZJ80DB (ZJ-80) | 3,000 HP | 1,315,000 lb | 26,250 ft | AC VFD electric |
How to choose
- Start from the well: hook load and depth rating must cover the heaviest casing string with a margin (see sizing a rig by hook load, horsepower and depth).
- Choose VFD if the operator's contract calls for an auto-driller, rig-floor automation or detailed drilling-parameter control.
- Choose mechanical if the work is workover, shallow wells or sites where electrical support is hard to staff.
- Choose SCR as the middle ground: electric control and flexible power at a lower purchase price than a new VFD rig.
- Whatever the drive, inspect before you buy. Our inspection and certification guide lists what to check.
Related pages
Frequently asked questions
What does SCR mean on a drilling rig?
SCR stands for silicon-controlled rectifier. On an SCR rig, diesel generators produce AC power, and the SCR system converts it to controlled DC power for the DC motors that drive the drawworks, mud pumps and rotary table.
What is a VFD drilling rig?
A VFD rig uses variable frequency drives to control AC induction motors on the drawworks, mud pumps and top drive or rotary. It gives precise speed and torque control and supports automated drilling systems.
Is a mechanical rig cheaper than an electric rig?
Generally yes. Mechanical rigs are simpler and usually cost less to buy and to maintain in the field, while electric rigs cost more but offer finer control and more flexible use of generator power.
Can an SCR rig be converted to AC VFD?
Conversions are carried out in the industry by replacing the DC motors and SCR system with AC motors and drives, but they are major projects. Compare the conversion cost with buying a VFD rig before committing.
Which RIGSDOM rigs are AC VFD?
The ZJ70DB 2,000 HP and the ZJ80DB 3,000 HP are AC VFD rigs. The Cabot Desert Mobile 1500 and the Dreco 1000 UE are SCR electric, and the other units are mechanical.