The three numbers that decide whether the bore gets completed
Every HDD pipe datasheet carries three figures that matter more than any other, and they are almost always read in the wrong order:
- Bend radius — the tightest curve the pipe body can be swept through without exceeding its allowable deformation. This is a property of the pipe, not of the job.
- Max bend angle — how much of a turn a single length of pipe can absorb. This is a property of the pipe length.
- Max inclination — the steepest percent grade that arc corresponds to. This is what your bore profile actually has to respect.
Get the first one wrong and nothing downstream saves you. The rig will pull, the pipe will take the set, and the failure will show up at the tool joint rather than the middle of the tube — which is exactly where you do not want it.
What bend radius actually is
Bend radius is the minimum radius to which the tube can be bent elastically. Below that radius the outer fibre goes past yield, the wall thins, and the pipe takes a permanent set even if it does not immediately break. A pipe that has taken set on one bore is a pipe that will fatigue early on the next.
The critical point: bend radius is fixed by the tube — its outside diameter, wall thickness and steel grade. It does not change with pipe length. This is stated plainly on the KISN catalogue, and it is the single most common thing buyers get wrong.
The relationship nobody prints on the datasheet
If bend radius is fixed by the tube, why does the same 52 mm tube show a 3.0° maximum angle at 1,800 mm and 5.1° at 3,048 mm? Because the angle a pipe can absorb is pure arc geometry:
max bend angle = pipe length ÷ bend radius (in radians)
Check it against the table below. The 52 mm tube has a 34.3 m bend radius:
- 1,800 mm ÷ 34,300 mm = 0.0525 rad = 3.0°
- 3,048 mm ÷ 34,300 mm = 0.0889 rad = 5.1°
This is not a coincidence and it is not a safety margin — it is the arc the pipe sweeps. A longer pipe lays over more of the circle, so it absorbs more degrees of turn at the same radius. This means:
- Longer pipe = more degrees of turn available, not a tighter radius. Buying longer pipe does not let you drill a sharper curve.
- To tighten the actual radius on the bore, you must change the tube size or the grade — not the length.
Max inclination is the same arc expressed as a percent grade: it is roughly the tangent of the bend angle, so 3.0° reads as about 5.1%. Treat it as the steepest sustained grade that pipe length can hold without exceeding the radius limit.
The KISN HDD drill pipe table
The complete catalogue range, 42 mm to 168 mm. Read the bend radius column first, then the angle column for the length you actually run:
| Tube OD mm (in) | Thread | Length mm (ft) | Tool joint OD mm (in) | Max torque N·m | Makeup torque N·m | Bend radius m (ft) | Max bend angle | Max inclination |
|---|---|---|---|---|---|---|---|---|
| 42 (1.66) | ZX40 | 1,828 (6.00) | 48 (1.89) | 2,040 | 1,224 | 30.2 (99.1) | 3.5° | 5.9% |
| 52 (2.05) | ZX50 | 1,800 (5.91) | 57.4 (2.26) | 3,525 | 2,115 | 34.3 (112.5) | 3.0° | 5.1% |
| 52 (2.05) | ZX50 | 3,048 (10.00) | 57.4 (2.26) | 3,525 | 2,115 | 34.3 (112.5) | 5.1° | 8.7% |
| 60 (2.36) | ZX60 | 3,000 (9.84) | 68 (2.68) | 8,294 | 4,976 | 39.6 (129.9) | 4.3° | 7.4% |
| 60 (2.36) | ZX60 | 3,048 (10.00) | 68 (2.68) | 8,294 | 4,976 | 39.6 (129.9) | 4.4° | 7.5% |
| 73 (2.87) | NC23 | 3,000 (9.84) | 82 (3.23) | 13,978 | 8,387 | 52 (170.6) | 3.3° | 5.6% |
| 76 (2.99) | NC26 | 3,000 (9.84) | 88 (3.46) | 16,560 | 9,936 | 54 (177.2) | 3.2° | 5.4% |
| 83 (3.27) | ZX80 | 3,000 (9.84) | 92 (3.62) | 20,372 | 12,223 | 59 (193.6) | 2.9° | 5.0% |
| 83 (3.27) | ZX80 | 4,500 (14.76) | 92 (3.62) | 20,372 | 12,223 | 59 (193.6) | 4.4° | 7.4% |
| 89 (3.50) | NC31 | 3,000 (9.84) | 105 (4.13) | 30,867 | 18,520 | 64 (210.0) | 2.7° | 4.6% |
| 89 (3.50) | NC31 | 4,500 (14.76) | 105 (4.13) | 30,867 | 18,520 | 64 (210.0) | 4.0° | 6.9% |
| 102 (4.02) | NC38 | 4,500 (14.76) | 127 (5.00) | 39,515 | 23,709 | 73 (239.5) | 3.5° | 6.0% |
| 102 (4.02) | NC38 | 6,000 (19.69) | 127 (5.00) | 39,515 | 23,709 | 73 (239.5) | 4.7° | 8.0% |
| 114 (4.49) | NC38 | 6,000 (19.69) | 127 (5.00) | 59,972 | 35,983 | 82 (269.0) | 4.2° | 7.1% |
| 114 (4.49) | NC46 | 6,000 (19.69) | 157 (6.18) | 59,972 | 35,983 | 82 (269.0) | 4.2° | 7.1% |
| 127 (5.00) | NC50 | 6,000 (19.69) | 168.3 (6.63) | 108,759 | 65,255 | 91 (298.6) | 3.8° | 6.4% |
| 127 (5.00) | NC50 | 9,600 (31.50) | 168.3 (6.63) | 108,759 | 65,255 | 91 (298.6) | 6.0° | 10.3% |
| 140 (5.50) | 5½ FH | 9,600 (31.50) | 190.5 (7.50) | 117,340 | 70,404 | 100 (328.1) | 5.5° | 9.4% |
| 168 (6.63) | 6⅝ FH | 9,600 (31.50) | 215.9 (8.50) | 160,000 | 96,000 | 121 (397.0) | 4.5° | 7.7% |
How to match the pipe to your rig and bore profile
- Pull your tightest planned radius from the bore profile, not from the as-built. Design intent is always smoother than what the locator actually achieves.
- Convert it to a percent grade over the pipe length you intend to run, and compare against the max inclination column.
- Add margin for locator error and soil movement. If the profile calls for 5.0%, a pipe rated 5.1% has no margin at all. Aim for at least 20% headroom.
- Check the rig's pullback against the string, not the pipe. Pullback capacity is the limit on how hard you can drag the string around the arc — the bend radius is the limit on whether the pipe survives being dragged.
- Confirm the thread and makeup torque for the size you land on. The recommended makeup torque is listed per row and must be achieved on site; a loose joint on a curved bore is a fatigue initiator.
Worked example: 102 mm versus 114 mm
A 400 m river crossing is planned with a 6% entry grade and a 5% grade through the main curve. Both sizes are in stock.
- 102 mm / NC38, 6,000 mm: bend radius 73 m, max angle 4.7°, max inclination 8.0%. Headroom over the 6% profile is comfortable.
- 114 mm / NC38, 6,000 mm: bend radius 82 m, max angle 4.2°, max inclination 7.1%. Still above 6%, but the heavier tube needs more pullback for the same arc.
The 102 mm string is the right call here: it clears the grade with more margin, and the rig does not have to work as hard. The 114 mm only earns its place if the crossing gets longer or the formation demands the extra wall — not as a default upgrade.
- Choosing pipe by maximum torque alone. Torque is rarely the failure mode on a curved bore; bending is.
- Assuming longer pipe tolerates a tighter curve. It tolerates more degrees, at the same radius.
- Reading the bend radius of the tool joint as the bend radius of the pipe. The joint is stiffer and is where the set concentrates.
- Sizing from the catalogue illustration instead of the table. The drawing shows the product form; the table carries the limits.
Take the tightest radius on your profile, add 25%, then look up the smallest tube that clears it with margin. If two sizes both clear it, take the lighter one — your rig's pullback budget is finite and the bore does not get easier.
Frequently asked questions
What is the minimum bend radius for HDD drill pipe?
It depends on tube size and grade, not on a single industry figure. In the KISN friction-welded HDD range it runs from 30.2 m (99.1 ft) on the 42 mm tube up to 121 m (397.0 ft) on the 168 mm 6⅝ FH size. Look up the specific row for the size you are running.
Does a longer drill pipe have a smaller bend radius?
No. Bend radius is fixed by the tube — OD, wall thickness and steel grade. What changes with length is the maximum bend angle the pipe can absorb, and it scales directly with length: angle = length ÷ bend radius. A 3,048 mm pipe absorbs about 1.7 times the angle of a 1,800 mm pipe at the same radius.
What does max inclination mean on a drill pipe datasheet?
It is the steepest percent grade the pipe can hold over its own length without exceeding the bend radius limit. It is approximately the tangent of the maximum bend angle, so a 3.0° limit reads as about 5.1%. It is the figure to compare directly against your bore profile.
Can I use a tighter bend radius if I reduce pullback?
Reducing pullback reduces the load that tries to straighten the string, but it does not change the pipe's elastic limit. If the bore geometry demands a radius tighter than the tube allows, the only fixes are a smaller or higher-grade tube, a shorter joint spacing, or re-planning the profile.
How do I confirm the bend radius for a non-standard size?
Send the tube OD, wall thickness, steel grade and the tool joint thread you need, and we will confirm the bend radius, maximum angle and recommended makeup torque for that exact configuration before quoting.
Send us your bore profile
Tell us the rig model, the tightest planned radius and the string length. We will confirm the pipe size, thread and makeup torque before quoting.
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