What hook load actually includes.
Hook load is the total force the traveling block, hook and drawworks see when pulling the string. It is not just the tubing weight. Four components matter:
- String weight in air — tubing or drill pipe plus collars, tools and BHA.
- Buoyancy factor — the string is immersed in well fluid, so its apparent weight drops by the buoyancy factor of the fluid.
- Traveling block and hook weight — the block, hook and elevator themselves hang from the same line.
- Dynamic and friction allowance — jarring, stuck-pipe overpull and borehole friction add temporary load above the static figure.
The calculation.
The static hook load is:
Hook load = (Wstring air × BF) + Wblock + Wtools
Where:
- Wstring air = weight of tubing/DP in air (kg/m × length, in kN)
- BF = buoyancy factor = (65.5 - mud weight in ppg) / 65.5 for steel; or (7.85 - fluid SG) / 7.85
- Wblock = traveling block + hook + elevator weight (typically 20-60 kN depending on rig class)
- Wtools = BHA, jars, accelerators, landing string
Then apply a safety margin: the expected maximum hook load should sit below the rig's nominal hook load, and the rig's maximum hook load is the structural ceiling. A practical target is 20-30% headroom between the calculated load and the nominal rating.
Worked example: 4,000 m workover with 2 7/8" tubing.
Take a 4,000 m well with 2 7/8" EU tubing (nominal weight 9.5 lb/ft = 14.1 kg/m), BHA of about 1,200 kg, and 1.15 SG brine.
| String weight in air | 4,000 m × 14.1 kg/m × 9.81 / 1000 | = 553 kN |
| Buoyancy factor (1.15 SG) | (7.85 - 1.15) / 7.85 | = 0.854 |
| Buoyed string weight | 553 × 0.854 | = 472 kN |
| Block + hook + elevator | XJ350 class | ~ 35 kN |
| BHA + tools | 1,200 kg × 9.81 / 1000 | = 12 kN |
| Static hook load | 472 + 35 + 12 | = 519 kN |
Now compare against the rig ratings. The 350HP XJ350 has a nominal hook load of 650 kN and a maximum of 900 kN. The calculated 519 kN sits below the 650 kN nominal — about 20% margin — which is the correct envelope. The 250HP XJ250 at 400 kN nominal would be undersized for this well: the calculated load exceeds the nominal rating.
How the ZYT range maps to this calculation.
| Model | Nominal hook load | Maximum hook load | Suitable string envelope |
| 150HP | 300 kN | 585 kN | Shallow, 2,600 m |
| 250HP XJ250 | 400 kN | 675 kN | Medium, 3,200 m |
| 350HP XJ350 | 650 kN | 900 kN | 4,000 m class |
| 450HP XJ450 | — | 1,125 kN | 5,500 m tubing |
| 550HP XJ550 | — | 1,350 kN | 7,000 m tubing |
| 650HP XJ650 | — | 1,470 kN | Deep / drilling |
| 750HP | — | 1,800 kN | 8,000 m tubing |
| 850HP | — | 2,250 kN | 9,000 m tubing |
Common mistakes to avoid.
- Using air weight without buoyancy. This overestimates the load and leads to over-specifying the rig, but it is safer than the reverse. Always apply the buoyancy factor for the actual well fluid.
- Comparing calculated load to maximum instead of nominal. The maximum rating is a structural ceiling for short-duration loads, not a working load. If the expected load is close to maximum, step up a rig size.
- Forgetting the block and tools. The 30-60 kN of block, hook and elevator is not optional — it hangs from the same line the drawworks is pulling.
- Ignoring stuck-pipe overpull. Jarring and free-point pulls can add 100-300 kN above the static load. The margin between calculated load and nominal rating is where that transient load lives.
What to send in the inquiry.
When you ask a supplier for a rig recommendation, include the calculated hook load, not just the depth. State: the tubing/DP size and weight, the well depth, the fluid density, the expected BHA, and the overpull you expect for stuck pipe. That produces a rig selection matched to the actual load rather than to a generic horsepower number. The workover rig selection guide walks through the rest of the selection process, and the workover vs. drilling rig comparison explains where each class applies.