Thursday, July 18, 2024

Rock Tooling Options For HDD

Arguably, bit selection to obtain optimum drilling efficiency and economy for given ground condition is the most critical issue for HDD in rock

Arguably, bit selection to obtain optimum drilling efficiency and economy for given ground condition is the most critical issue for HDD in rock. Pilot holes can be drilled using rotary, roller cone or percussive techniques dependant on the mechanical properties of the material to be drilled.

Rock “hardness” measured by compressive strength in Psi or MPa, is a major consideration. This is defined as the maximum compressive stress that, under a gradually applied load, a given solid material will sustain without fracture.

Rotary Drilling – PDC Bits

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Rocks of low to medium hardness (up to 25,000 Psi compressive strength) are usually suitable for rotary drilling; these will include sand, shale, mudstone, sandstone and limestone.

A modern example of rotary cutting tools are PDC bits. They use a synthetic diamond cutter to shear the surface of the rock. PDC bits have several advantages which include:

  • They offer a higher rate of penetration, longer life and increased steerability compared to conventional methods
  • No moving parts
  • Less stress on the drill rig
  • Lower torque required
  • Less vibration on the drill rods
  • Producing clean holes with a consistent size
  • Repairable
  • Diameters from Diameters from 17⁄8 in to 18 in (48 mm to 458 mm) typically

Drilling with Roller Cones – Tricones

Tricones have a range of designs and specifications making them suitable for a wide variety of rock formations. Mill Tooth tricones are designed for drilling rock up to approximately 6,000 Psi compressive strength, whereas TCI tricones have designs available that are suitable from 5,000 Psi to over 50,000 Psi. Rock types include soft sandstone and limestone through to Basalt.

The roller cones on a tricone bit rotate together with the drill rod and cut the rock without percussion under pressure provided by the drill rod from the surface. Inserts situated on the surface of the roller cones penetrate the rock under pressure and crush it.

Tricones have a number of advantages and include:

  • Versatility – Option for nearly every type of rock
  • Low in initial investment
  • Excellent impact resistance which makes them suitable when the rock is fractured.
  • Diameters from 215/16 in to 26 in (75 mm to 660 mm) typically

Percussive Drilling

Down the hole (DTH) Hammers are excellent when ground conditions are very hard, especially above 30,000 psi compressive strength. Rock types include hard limestone, granite and basalt.

DTH drilling uses a pneumatic hammer directly behind a carbide impregnated drill bit. The bit is held against the rock surface under load and the impact from the hammer shatters the rock.

DTH Hammers and bits have several advantages and include:

  • Faster penetration rates in very hard rock
  • Directional hammer bits are usually supplied as either Convex or Slant face bits depending on the ground conditions
  • Lower wear rates in abrasive ground conditions, especially above 30,000 psi
  • Suitable for solid rock
    Diameters from 4 in to 71⁄2 in (100 mm to 190 mm) typically

Hole Opening / Reaming

As well as the above pilot hole techniques, rotary drilling can be used for hole opening where larger bore diameters are required.

PDC Reamers

PDC reamers work on the same principle as PDC bits with a synthetic diamond insert shearing the rock.

  • Reaming up to 12 in (300 mm) extra diameter in 1 pass (e.g. 6 in (150 mm) pilot to 18 in (450 mm))
  • Less stress on the drill rig – lower torque required and less vibration on the rods
  • No moving parts for increased reliability
  • Clean holes with a consistent size
  • Repairable and replaceable cutters
  • Diameters from 4 in to 71⁄2 in (100 mm to 190 mm) typically

Hole Openers

Hole Openers use similar cones to tricones arranged around a central body to open the pilot hole up to the required finished size.

• Cost-effective solution

• Enable larger holes to be drilled with smaller rigs by drilling in stages.

• Usually up to 4 in (100 mm) diameter hole opening, depending on ground conditions

• 8 in to 60 in (200 mm to 1,524 mm) diameter

• Mill Tooth or TCI

• Standard 5 cone – other options available

• Diameters from 8 in to 48 in (200 mm to 1,200 mm) typically

Infinity has several resources available online to help their customers choose the best bit –

Since 2009, the name Infinity Tool Manufacturing has been synonymous with manufacturing and supplying market leading rock tools to the HDD industry. Products are manufactured in a world class facility to ISO 9001 international quality management standards. The manufacturing facility is located in located in Benton, Illinois, USA with distributors and agents located across the globe.

Infinity offers advice to support you with your rock boring challenges. To discuss operating procedures when using rock tools or which products are best suited to your specific ground conditions, please get in touch with Infinity’s customer service team.

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Trenchless Works bringing you balanced journalism, accuracy, news and features for all involved in the business of trenchless and no-dig from around the world

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