Directional drilling in oil and gas is the practice of steering a wellbore along a planned non-vertical path so it reaches a hydrocarbon target that does not sit directly beneath the rig. The bore may run vertically, curve through a build section, then travel horizontally through the reservoir for hundreds or thousands of metres. The finished hole is the asset. It gets cased, completed and produced from.
Horizontal directional drilling on a UK utility scheme borrows the same physical idea and almost none of the same engineering. Directional drilling as a utility civils technique is a trenchless installation method: a guided pilot bore from an entry pit to an exit pit, reamed out to size, with a product pipe or duct pulled back through it. The hole is temporary. The pipe is the asset.
The two share a name, a vocabulary and a family tree. They share very little else. If you are specifying, programming or procuring HDD on a water, power or telecoms scheme, the differences below are the ones that change what you buy and what it costs.
What directional drilling means in oil and gas
Oilfield directional drilling exists to put a wellbore somewhere a vertical hole cannot reach. That might mean maximising contact with a reservoir, drilling several wells from one surface pad, sidestepping a fault or a salt dome, or intersecting another well as a relief bore.
The practice is old. SLB’s technical overview of directional drilling traces it to the 1920s, when the first basic wellbore surveying methods made it possible to know where a hole had actually gone. Modern steering uses either a bent-housing mud motor, oriented and pushed without rotating the string, or a rotary steerable system that deflects the bit while the whole drillstring keeps turning. Measurement while drilling tools sit in the bottom-hole assembly and telemeter direction, inclination and formation data back to surface.
Depths run to several kilometres. Drilling fluid is doing pressure control work as well as carrying cuttings, because the formation is pushing back.
What horizontal directional drilling means in UK utility civils
Utility HDD exists to get a pipe or duct under something without opening it up. Roads, railways, watercourses, protected verges, mature trees, listed frontages, live carriageways where a lane closure would cost more than the drill.
The scale is a different world. UKSTT’s technical guidance on horizontal directional drilling puts bore lengths anywhere from a few metres to over 1 km, and diameters from 50 mm up to several hundred millimetres. Rigs are classed by maximum pulling force rather than depth capability: mini rigs up to 150 kN, midi rigs above 150 kN to 400 kN, and maxi rigs above 400 kN to 2,500 kN. The final reamed bore is normally 30 to 50% larger than the product pipe being installed.
The sequence is pilot bore, one or more reaming passes, then pullback of the product. Nobody produces anything from the hole afterwards.
The distinction that explains all the others
An oilfield well is a blind bore. It starts at surface and terminates underground, at the target. There is no far end to work from.
A utility bore has two ends. It launches from an entry pit, breaks out at a reception pit, and that changes everything downstream. The reamer is attached at the exit and pulled back towards the rig. The product pipe is attached behind the reamer and pulled through from the far side, running on a swivel so the reamer’s rotation is not transmitted into the pipe. There is no casing programme, no cementing, no completion string.
Once you hold that difference in your head, the rest of the divergence stops being a list of trivia and becomes predictable. Two ends means two excavations, two sets of surface consents, two reinstatements and an as-built that has to tie into a live network at both ends.
Oil and gas directional drilling and utility HDD side by side
Oil and gas directional drilling compared with UK utility HDD
| Aspect | Oil and gas directional drilling | UK utility HDD |
|---|---|---|
| Purpose of the bore | The bore is the asset, cased and produced from | The bore is temporary construction space |
| Geometry | Blind wellbore, terminating at a subsurface target | Entry pit to reception pit, breaking out at surface |
| Typical depth | Kilometres | Metres, occasionally tens of metres |
| Typical length | Kilometres of measured depth | A few metres to over 1 km |
| Steering mechanism | Bent-housing mud motor or rotary steerable system | Asymmetric slant-face head, or a mud motor in rock |
| Navigation | Downhole MWD, telemetered to surface | Walkover locator reading a sonde, or wireline guidance on longer crossings |
| Role of drilling fluid | Cuttings transport, cooling, and formation pressure control | Cuttings transport, cooling, bore stability, lubrication of the pullback |
| Principal ground risk | Formation pressure, wellbore stability, well collision | Other people's buried apparatus, and inadvertent fluid returns at surface |
| Finishing | Casing, cement, completion | Product pipe pulled in, pits backfilled and reinstated |
Steering and navigation are not the same thing
Utility HDD gets described as the crude cousin of oilfield drilling, with the shorthand that a driller “just follows a bloke with a locator”. That is a misreading of what the locator does.
The walkover locator is a navigation instrument. It reads the electromagnetic field from a battery-powered sonde carried in or immediately behind the drill head, and reports depth, pitch and roll back to the rig operator. It tells you where the head is and which way it is pointing. It does not move it.
Steering is mechanical. In soft ground the drill head has an asymmetric slant face. Push it without rotating and the face bites to one side, curving the bore. Rotate the string continuously and the bias averages out, so the head runs straight. Alternating between the two is how a pilot bore is walked onto line. In rock, a mud motor or a mechanically driven rock bit does the work instead.
Functionally, that is much closer to sliding a bent-housing motor in an oilfield well than it is to a rotary steerable system. The physics is the same family. The difference is depth, telemetry and how much money is riding on each metre.
On long crossings, under water, or where structures and traffic block walkover access, the locator stops being viable and wireline magnetic guidance takes over. That decision is usually the first sign of whether a crossing is routine or genuinely difficult.
The regulatory regime is entirely different
This is where the two disciplines separate completely, and where the oilfield answer to “what is directional drilling” becomes actively misleading for a UK utilities reader.
Utility HDD is construction work, so it sits inside the construction safety regime. The HSE’s HSG47, Avoiding danger from underground services, third edition 2014, is the principal guidance, and paragraphs 85 and 86 deal with trenchless methods by name, including directional drilling. The requirement is not softened because the work is trenchless: records, locating devices and trial excavations are all expected before a bore route is set. Underneath that sits the legal duty in regulation 25(4) of the Construction (Design and Management) Regulations 2015, which prohibits work presenting a risk from an underground service until suitable and sufficient preventive steps have been taken.
Locating those services properly is specified separately again, in PAS 128, the specification for underground utility detection, verification and location. It runs from quality level D, meaning records only, up to quality level A, meaning physically verified position. A significant HDD route usually warrants a QL-B detection survey with physical verification at the crossings, the pit positions and any high-risk asset.
Then there is the street. If the bore, the entry pit or the reception pit touches the public highway, the works fall under the New Roads and Street Works Act regime, which means permitting, noticing and a reinstatement specification. Our guides to obtaining a street works licence and to what the NRSWA Red Book actually requires set out how that works in practice.
None of this has an oilfield equivalent. An offshore or onshore well is consented through an entirely separate route, and the hazard it is engineered against is formation pressure, not a 1960s ductile iron main that is 400 mm off where the record says it is.
Where the shared vocabulary trips people up
What this changes when you are procuring HDD in the UK
If you are pricing or programming a crossing, the questions that matter are not the ones the oilfield literature prepares you for.
- Ask what the reamed diameter will be, not the pilot diameter. With a final bore normally 30 to 50% larger than the product, the clearance you agree with each asset owner needs to be measured against the reamer envelope, not the pilot centreline.
- Ask what utility survey the bore plan is based on, and at what PAS 128 quality level. A plan drawn from statutory records alone is a plan drawn from indicative information.
- Ask where the entry and reception pits go, how much working space they need, and who is reinstating them. On a constrained urban site the pits, not the bore, are frequently the binding constraint.
- Ask what happens if drilling fluid surfaces where it should not. An inadvertent returns plan is cheap to write beforehand and expensive to improvise on a live carriageway.
- And ask whether HDD is the right technique at all. For short, shallow crossings under a driveway or a verge, an impact mole is often faster and less disruptive. Our comparison of moling and directional drilling walks through where the crossover sits, and the explainer on what moling is covers the technique on its own terms.
The reason the oil and gas answer keeps surfacing when people search for directional drilling is that it is the older, larger, better documented discipline. It is also the wrong one for anyone laying a water main under a Hertfordshire A-road. Same steering principle, different depth, different fluid, different hazard, different law, different asset at the end of it. Our directional drilling capability page sets out how the utility version of the technique is planned and delivered on live UK schemes, and what a workable bore plan looks like before anyone breaks ground.
Sources and further reading
- SLB, The Defining Series: Directional Drilling Practices
- UKSTT, Horizontal Directional Drilling
- HSE, HSG47 Avoiding danger from underground services
- Construction (Design and Management) Regulations 2015, regulation 25
- BSI, PAS 128 Underground utility detection, verification and location
Questions on this topic
It is the practice of steering a wellbore along a planned non-vertical path to reach a hydrocarbon target that is not directly beneath the rig. The bore is steered using a bent-housing mud motor or a rotary steerable system, with measurement while drilling tools reporting direction and inclination back to surface. The finished wellbore is the asset: it is cased, completed and produced from.
No. Directional drilling is how the wellbore is steered and positioned. Hydraulic fracturing is a separate stimulation process carried out after the well is drilled and completed, to increase flow from tight formations. The two are often used together in shale development, which is why they get conflated, but one is a drilling technique and the other is a completion technique. Neither has anything to do with utility HDD.
In oil and gas it is also called deviated drilling or slant drilling. In UK utility civils the same broad family of work is called horizontal directional drilling, HDD, directional boring or guided boring. The terms are not fully interchangeable across the two industries, so it is worth confirming which discipline someone means before you price against it.
UKSTT gives an overall HDD range from a few metres to over 1 km, with diameters from 50 mm upwards. The practical limit on any given crossing depends on the rig’s maximum pulling force, the ground conditions, the product diameter and whether the guidance system can hold position over the full length. Rigs are classed by pulling force: mini up to 150 kN, midi above 150 kN to 400 kN, and maxi above 400 kN to 2,500 kN.
No. Moling uses a pneumatic percussive tool that displaces soil to form a small, essentially unsteered bore over short distances. HDD uses a rig, a steerable drill head, drilling fluid and a reaming and pullback sequence, and can be guided along a designed profile over much longer runs. Our comparison of moling and directional drilling covers where the crossover point sits on a typical scheme.
Yes, if the bore, the entry pit or the reception pit affects the public highway. The works fall under the New Roads and Street Works Act regime, which means permitting or noticing with the street authority and reinstatement to the required specification. Being trenchless does not remove the obligation, because the pits still break the surface. Separately, HSG47 and CDM 2015 regulation 25(4) apply to the buried services risk regardless of technique.
