Est. 2005 · UK-wide trenchless installations07920 553 578·chris@swdd.co.uk
Explainer
The whole point of directional drilling is that the bore is steered along a planned path, not just pushed straight ahead. A guidance system follows the drill head underground in real time so the operator can hold the design and bring the bore out exactly where it needs to be. The system used depends on how deep and long the crossing is, what sits on the surface above it, and how accurate the bore has to be.
The driller cannot see the drill head once it is in the ground, so a guidance system stands in for their eyes. It reports the head's position, depth, angle and direction back to the rig continuously. The operator compares that against the designed profile and makes small corrections, nudging the head up, down, left or right, so the bore holds its line, level and grade all the way across.
On a pressure pipe or duct that mostly means getting safely from one side to the other. On gravity drainage it matters far more, because the pipe has to fall at a precise gradient to flow, so the bore must be steered to an exact level the whole way. The harder the accuracy and the less you can see from the surface, the more capable the guidance system has to be.
The systems
There are three main guidance systems, from the simple and everyday to the precise and specialist. The right one comes from the crossing, not the other way round.
| Walkover guidance | A handheld locator at the surface follows a transmitter (sonde) in the drill head, reading its position, depth and angle. Simple, quick and accurate enough for most crossings, and workable even at 15 to 20m deep, where the accuracy eases off but is usually still fine. With drill-to mode it can guide crossings well beyond normal walking distance. |
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| Wireline guidance | A probe in the drill string sends data up a wire to the surface, refined by a coil laid around the bore. It needs no access above the bore and has no real depth limit, so it suits deep, long crossings under rivers, roads and railways, but it is a specialist system with a daily cost. |
| Gyro guidance | Inertial gyroscopes fix direction from the earth's rotation, so they are immune to magnetic interference from steel, rebar and power cables. The most accurate option, used on the longest crossings and near heavy interference. |
We match the guidance system to the crossing. Tell us the depth, the length and what's on the surface, and we'll guide it the right way.
The biggest single factor is whether we can safely get to the surface directly above the drill head. That is what separates an everyday crossing from a major one.
It is a good rule of thumb in its own right. If you cannot safely stand above the line, the bore is guided from inside the drill string rather than from the surface.
Walkover is often assumed to be only for short, shallow work, but good equipment takes it a long way. We run a DigiTrak Falcon F5+, one of the best locating systems available, with a sonde detection range of up to 30m. That range is mostly about distance, how far the locator can sit from the drill head and still read it, which is what makes its drill-to mode so useful. The locator is set ahead at the target and the head is steered toward it, so we can guide a crossing without walking directly over every metre of it.
In practice that 30m reach can fully guide a crossing of around 60m, picking up the head from each side with information the whole way across. On rare occasions we have pushed it further. We guided a 120m river crossing this way, with a short section in the middle un-guided, at the client's request to avoid the cost of a downhole system. We are straight about when that trade-off is sensible and when it is not.
The reason it is worth pushing walkover as far as it sensibly goes is cost. Wireline and gyro guidance are excellent, but they are specialist systems that typically run to £3,000 to £4,000 a day for the guidance alone. Where walkover will do the job safely and accurately, using it keeps that cost off your bill.
That sum even changes how we approach major crossings. On a National Highways or Network Rail job it can be far cheaper to put in a limited amount of traffic management, so we can safely use walkover or drill-to from the surface, than to hire a downhole system. A couple of thousand pounds of traffic management can avoid a wireline or gyro spread that would otherwise add many times that over the job. We cost the crossing both ways and tell you which gives the better value, rather than defaulting to the expensive system out of habit.
Wireline and gyro steering are usually associated with deep, long crossings under rivers and motorways, and that is their main home. But the same accuracy is worth having on a short job when the tolerance is tight. A gravity drainage run drilled to falls has to hold an exact gradient over its whole length to drain properly, so even a short run can justify a more capable guidance system than its length alone would suggest. We pick the system that hits the accuracy the job needs, not just the one that suits the distance.
Questions answered
The drill head carries a guidance system that reports its position, depth, angle and direction back to the operator in real time. The driller compares that against the designed bore profile and makes small steering corrections as they go, so the bore follows the planned line and level from entry to exit even though the head is metres underground. The system used to guide it depends on how deep and long the crossing is, what is on the surface above it, and how accurate the bore has to be.
Walkover guidance is the simplest and most common method. A small transmitter, called a sonde, sits just behind the drill head and gives off a magnetic field. An operator at the surface holds a handheld locator that reads that field to fix the head's position, depth and angle, and relays it to the rig. It is quick and accurate enough for most work, and it copes with more than people expect. It stays workable even at 15 to 20m deep, where the locating accuracy eases off but is usually still fine. With a good locator and its drill-to mode it can also guide crossings well beyond walking distance, not just short, shallow ones.
Both are downhole systems that guide the bore from inside the drill string rather than from the surface, so nobody has to walk above the drill head. Wireline (downhole magnetic) steering runs a probe that sends readings up a wire, often refined by a coil laid around the bore for accuracy. Gyro steering uses inertial gyroscopes that are immune to magnetic interference from steel, rebar and power cables. We move to these systems when a crossing is too deep or too long for walkover, when there is no safe surface access above the line, such as under a live motorway, railway or river, or when the bore has to be exceptionally accurate. These downhole systems are more expensive to run, often three to four thousand pounds a day for the guidance alone, so we use walkover wherever a crossing safely allows it.
Differently to a minor road. On a small local-authority road we can usually follow the head from the surface or the verge with a walkover locator. On a National Highways motorway, major A road or trunk road, or a Network Rail crossing, there is not usually safe access to follow the head from the surface across a live carriageway or railway, and there is often magnetic interference from rebar, traffic-signal loops and buried cables, so those crossings are commonly guided with a downhole wireline or gyro system, or by setting a guidance coil on each side with the bore steered beneath in between. Sometimes, though, it works out cheaper to put limited traffic management in place so we can use walkover or drill-to from the surface instead. A couple of thousand pounds of traffic management can save far more than hiring a downhole system at three to four thousand pounds a day. We cost it both ways and advise on whichever is better value. Rivers and runways are handled with downhole guidance.
Very accurate when the right system is used. Entry, exit and depth are controlled to the design and recorded as an as-built. A guided downhole system can bring a long crossing out within a few tenths of a metre of its target. That accuracy is not only for big crossings. A short gravity drainage run that has to hold an exact fall to flow can also justify wireline or gyro steering, because the bore has to keep a precise gradient the whole way rather than just get from one side to the other.
Related
Whether it is a short drainage run to falls or a deep crossing under a motorway or river, send us the detail and we'll tell you how we'd guide it and what it should cost.