The short answer
Size a land rig from the heaviest load it must lift, not from horsepower alone. That load is usually the longest, heaviest casing string, reduced by the buoyancy of the mud and increased by the overpull you want in reserve. Horsepower and depth rating then follow: across the drilling rigs listed on RIGSDOM, the rated depth works out at roughly 9 to 15 feet per horsepower.
Step 1: find the heaviest string
Multiply the weight per foot of each casing string by its length. The largest figure is usually the governing load, because casing is heavier per foot than drill pipe.
Step 2: apply buoyancy
Steel weighs about 65.5 pounds per gallon, so the buoyancy factor is:
Buoyancy factor = 1 − (mud weight in ppg ÷ 65.5)
Step 3: add overpull and margin
Add the overpull you want available to free stuck pipe, plus any safety margin your company standard requires. The result is the minimum static hook load the rig should be rated for.
Worked example
- 9⅝-inch casing at 47 lb/ft run to 12,000 ft: 47 × 12,000 = 564,000 lb in air
- Mud weight 10 ppg: buoyancy factor = 1 − 10 ÷ 65.5 = 0.847
- Buoyed casing weight: 564,000 × 0.847 ≈ 477,893 lb
- Add 100,000 lb overpull: ≈ 577,893 lb required
In this example the 500,600 lb and 367,400 lb rigs are too small, while the Cabot Desert Mobile 1500 (715,000 lb) and ZJ50L (708,000 lb) have capacity to spare. This is a planning illustration only; your well programme and company design factors govern.
Step 4: check depth rating and horsepower
Manufacturers quote a nominal depth rating for a specific drill pipe size, so treat it as a guide rather than a limit. The table shows how rated depth compares with power across the drilling rigs on this site.
| Rig | Power | Hook load | Depth rating | Rated ft per HP |
|---|---|---|---|---|
| Cooper 750 (RIG-2) | 750 HP | 365,000 lb | 7,800 ft | 10.4 |
| Ideco Rambler 1000 (RIG-6) | 1,000 HP | 367,400 lb | 14,700 ft | 14.7 |
| Dreco 1000 UE Desert Mobile (RIG-8) | 1,000 HP | 500,600 lb | 12,000 ft | 12.0 |
| Cabot Desert Mobile 1500 (RIG-4) | 1,500 HP | 715,000 lb | 14,500 ft | 9.7 |
| ZJ50L (ZJ-50) | 1,500 HP | 708,000 lb | 16,400 ft | 10.9 |
| ZJ70DB (ZJ-70) | 2,000 HP | 1,011,600 lb | 23,000 ft | 11.5 |
| ZJ80DB (ZJ-80) | 3,000 HP | 1,315,000 lb | 26,250 ft | 8.8 |
Step 5: check the rest of the rig
- Mast and substructure: setback capacity for the stands you will rack, and clear height for your BOP stack.
- Mud pumps: total hydraulic horsepower and pressure rating against planned flow rates.
- Well control: BOP pressure rating and whether H₂S service is needed (see H₂S sour service rigs).
- Mobility: truck, trailer, desert mobile or skid mounting against your field layout and move frequency.
Related pages
Frequently asked questions
How deep can a 1,500 HP drilling rig drill?
It depends on the rig and the drill pipe used for the rating. The two 1,500 HP rigs listed on RIGSDOM are rated to 14,500 ft and 16,400 ft.
How deep can a 1,000 HP rig drill?
The 1,000 HP drilling rigs listed on RIGSDOM are rated to 12,000 ft and 14,700 ft.
What is hook load on a drilling rig?
Hook load is the weight the rig's hoisting system can carry at the hook, from the drill string or casing plus the travelling equipment. Rigs are specified by their maximum static hook load.
How do you calculate buoyancy factor?
Buoyancy factor equals 1 minus the mud weight in pounds per gallon divided by 65.5, the approximate density of steel in pounds per gallon. At 10 ppg mud the factor is about 0.847.
Does horsepower or hook load matter more?
Hook load decides whether the rig can lift your heaviest string. Horsepower decides how fast and at what depth it can do the work. Check both, with hook load first.