HDD TECHNOLOGIES

DIRECTIONAL BORING TECHNOLOGY


Directional boring, commonly called horizontal directional drilling or HDD, is a method of installing underground pipes and conduits along a prescribed bore path from the surface, with minimal impact to the surrounding area. Midsized drilling rigs typically install lengths in the range of 500' to 2,000' and have the capacity to pull back conduit(s) or pipe up to 24" in diameter. Longer runs are accomplished by putting in a series of bore runs. Larger drilling rigs can install product with greater diameters for longer lengths. Accu-Bore can provide for most any configuration of length and product size.

APPLICATIONS


The directional drilling process is used for installing power cable & telecommunications conduits, water lines, sewer lines, gas lines, oil lines, product pipelines and environmental remediation casings. It is used for crossing waterways, roadways, shore approaches, congested areas, environmentally sensitive areas and any area where other methods are more expensive or not feasible
Directional boring is used in place of other techniques for the following reasons:

  • Lower cost
  • Deeper installation possible
  • Longer installation possible
  • No access pit required
  • Shorter completion times
  • Directional capabilities
  • Safer for the environment


The process begins when a directional bore machine pushes a bore head connected to hollow pipe into the ground at an angle. As each joint of drill pipe is pushed into the ground a new one is added behind.



Most directional boring machines use drilling fluid (mud). Drilling fluid is generally a mixture of bentonite clay and water, with additives used to improve performance. In softer soils the high pressure jet of fluid cuts through the soil, with the cuttings suspended in the fluid. As fluid is pumped down the hole the cuttings are carried back out to the surface where they are either allowed to settle out in a pit or removed mechanically into a vacuum tank. Drilling fluid is classified as non-toxic and is disposed accordingly.

In softer soils, an angled bit is used and the pipe string is rotated, if necessary, to bore straight. To steer, rotation is stopped, the angle of the bit is aligned to the desired direction and forward thrust is applied. The directed jet of the drilling fluid and forward thrust cuts a new path.

Upon reaching the exit point, the bit is detached and the end of the drill pipe is attached to a reamer or hole opener (for rock), if the bore hole must be enlarged. The reamer is pulled back while rotating the drill pipe to enlarge the bore hole, with as many consecutive passes as required. Drill pipe is added behind the reamer or hole opener so that there is always drill pipe in the bore hole.

When the bore hole is at least 25% larger than the pipe to be installed, the end of the drill pipe is connected to a reamer attached to a swivel connected to the product pipe. Drilling fluid is pumped downhole to provide lubrication and the product pipe is pulled in while rotating the drill pipe & reamer. The swivel prevents rotation of the product pipe.

To prevent pipe breakage or stretching a “break-away” link can be installed between the pipe and swivel. The break-away link will separate prior to the manufacturers’ recommended maximum safe pull strength.

Once the pipe is installed the exit and entry points are excavated if necessary and connections made as needed.

Directional Bore Machines


Directional bore machines are rated by thrust and pullback force and rotary pressure. Sizes range from small machines with a few thousand pounds of thrust and pullback to the largest with over a million pounds of pullback force. Rotation is measured in pressure. Most machines are track or trailer mounted with a few smaller machines designed to be used in pits.

Locating & Guidance
The most commonly used equipment for determining the location of the bore head is called a ‘walk-over’ locating system. A sonde, or transmitter, behind the bore head registers angle, rotation, direction and temperature data. The information is then encoded into an electro-magnetic signal which is transmitted through the ground to the surface. At the surface a receiver is manually positioned over the sonde and the signal decoded and steering directions relayed to the operator of the bore machine.

When conditions do not allow a receiver to be positioned over the sonde or interference causes degradation of the signal, a ‘downhole system’ is used. The most commonly used type of downhole system is called a ‘wire-line’ and uses a wire to transmit the data up the inside of the drill pipe. At the surface the data from the wire is decoded by a computer to provide depth, angle, rotation, direction and other information. Gaining in popularity are newer downhole wireless systems such as the Polaris EM System which transmits the data through the ground by an electro-magnetic signal to a stationary receiver.

Pipe Reaming
A variation of directional boring called pipe reaming can be used to replace existing clay, asbestos cement, non-reinforced concrete and PVC pipe. A reamer is pulled through the existing pipe which cuts the pipe into small pieces. The pipe pieces are flushed out the bore hole with the drilling fluid. A new HDPE or PVC pipe is pulled in behind the reamer.

Pipe reaming can often be used instead of pipe-bursting. The advantages of pipe reaming are lower cost, faster installation, no compaction of the surrounding formation and much greater upsizing capabilities.

Limitations


Directional boring can be used in a wide variety of conditions but is not the optimal method in all conditions. The most difficult ground formation for any method is un-consolidated soils (cobble). In some cases the un-consolidated soils can be grouted and then bored. Directional boring can be used for sanitary sewers but only when ground conditions permit a straight path.

Design Considerations


When designing a project for directional boring it is important to have accurate geo-technical data, sufficient space for the bore rig and support equipment and enough space for laying out the pipe on the other side. It is best to allow extended work-hours for boring operations and is essential for pull-back. Additional considerations may be required for specific projects. Pipe which can be used for directional boring installations includes HDPE, mechanical joint PVC and steel.

Costs


Directional boring has evolved steadily over the last twenty-years and is now the preferred method on many installations due to its low cost and low impact on surroundings. It is generally less expensive than other methods such as micro tunneling, jack & bore and open trenching in urban areas. In urban areas it can not only save a considerable amount on installation cost it can provide a tremendous amount of public goodwill.