Bore design
A bore design is four numbers and a drawing: how deep, how tight it turns, how big the hole is, and how hard the pipe gets pulled. Get any one of them wrong and you find out during the pullback, which is the worst possible time.
The design basis we work from
These are the operating parameters this practice used across its own drilling record. They are a starting point calibrated on several thousand bores in western Canadian ground, not a code minimum, and every one of them moves when the project warrants it.
| Parameter | Working value | What moves it |
|---|---|---|
| Design radius | 400 m | Pipe diameter, wall and grade; the elastic bend the product will take without over-stress |
| Minimum depth of cover | 2,000 mm | Rises to 4,000 mm under high-value and archaeologically sensitive ground; permit conditions; scour depth at a watercourse |
| Entry and exit grade | +0.5% to +2.0% at tie points | Rig-side working room; the angle a given rig class can set; the approach to the tie-in |
| Final hole size | 1.5 × nominal pipe diameter | Formation stability; how many ream passes the ground will take; cuttings transport |
| Typical single pull | 500–750 m | Extends past 1,000 m with hole-conditioning tooling; falls sharply in rock or unstable ground |
| Safety factor on pull | 1.5× | Nothing, in our hands. If the margin is not there, the design is wrong. |
| Separation from a foreign line | 1.5 m minimum | Computed as half the final hole diameter plus 1.5 m; trenchless is prohibited inside 5 m of an existing facility without escalation |
Pull force is the number that decides it
Most lost bores are lost on the pullback. The load on the product pipe is the sum of frictional drag along the hole, the fluidic drag of the annulus, the capstan effect at every bend and the buoyant weight of the string — and it has to be checked segment by segment along the profile, not as a single lumped figure, because the governing case is usually not at the end.
We run that analysis on the industry pullback model and check the result against the pipe's own allowables. For reference, the allowables this practice designed steel to:
| Pipe | Outside diameter × wall | Allowable pull |
|---|---|---|
| NPS 6 | 168.3 × 4.8 mm | 121,000 lb (55,000 kg) |
| NPS 8 | 219.1 × 5.6 mm | 204,000 lb (92,727 kg) |
| NPS 10 | 273.1 × 6.35 mm | 302,000 lb (137,272 kg) |
| NPS 12 | 323.9 × 7.92 mm | 325,000 lb (147,727 kg) |
Grade 359 Category II sour-service steel, the figures carried on a recent northeast British Columbia system. Your pipe is not this pipe — the allowables are recomputed for the actual diameter, wall, grade and service every time.
The rig's pull rating and the pipe's allowable pull are different numbers, and the rig's is usually bigger. A three-hundred-tonne machine will happily pull an NPS 6 line in half. Somebody has to own the limit that matters, write it on the drill plan, and tell the driller what it is.
What you get
- A drawn profile: plan and elevation, stationed, with entry, exit, radius, cover and the tie-in geometry.
- The pull-force analysis, segment by segment, with the governing case and the margin at it.
- Hole-size and ream-pass schedule for the formation.
- The downhole assembly by pipe size — bit, motor or jetting assembly, reamer and swivel.
- The design limits written where the driller will actually see them.