Technology Overview For Directional Drilling
Directional drilling was invented in the 1920 and the technology was originally used in the oil field to increase oil production. In the earlier 1990's the technology was adapted for utility installations. Directional bores have been installed for pipelines carrying oil, natural gas, petrochemicals, water, sewerage and other products. Also, conduits have been installed to carry electric and fiber optic cable. Besides crossing under highways, railroads, airport runways, shore approaches, traffic islands, areas congested with buildings; installations have been made under rivers and waterways, pipeline corridors and protected wetlands.
Directional borings have the least environmental impact of any alternate method. The directional drilling technology also allows conduit placement under obstacles and provides maximum protection and minimizes maintenance costs. During installation normal business operations are not interrupted. Directional borings have a predictable and short construction schedule.
Installation of directional borings is completed in four stages that are as follows:
1st Stage - Pilot Hole
A pilot hole is drilled horizontally and continues under and across the obstacle along a predetermined design path. An electronic transmitter is placed in a housing which is directly behind the cutting head. The transmitter sends a signal to the surface which is read by the receiver, which then transmits the information back to the drill rig operator. The driller then, by using the relayed information, can steer the bore path in any desired direction.
2nd Stage - Pre-ream
Once the pilot hole is complete, the hole must be enlarged to a suitable diameter for the product pipeline. This is accomplished by "prereaming" the hole to successively
larger diameters. Generally, the reamer is attached to the drill string on the opposite end of the borehole from the drill and pulled back into the pilot hole. Slurry is pumped into the hole to maintain the integrity of the hole and to flush out cuttings.
||3rd Stage - Pullback
Once the drilled hole is enlarged, the conduit can be pulled through it. The pipeline is prefabricated at the end of the bore opposite the drill rig. A reamer is attached to the drill string, and then connected to the product by a pulling head and swivel. The swivel allows for the reamer to turn without the product turning. The drilling rig then begins the pullback operation, rotating and pulling on the drill string as well as circulating drilling fluids. The pullback continues until the reamer exits the bore path by the drill rig and the pipeline is in place.
Some of the benefits of pipe installation using directional drilling include:
- Continuation of normal operation
- Avoidance of identified infrastructure installations
- Maintain integrity of roadways, buildings, and natural features
- Little or no disturbance to wetlands or nature conservators.
- Minimum site restoration
Directionally drilled utility conduits are protected from ambient environmental conditions during installation as well as long-term use. In addition to the construction benefits, directionally installed lines have a useful life expectance and service requirement similar to that of trenched lines and superior to that of exposed lines.
The price of the project is determined by several factors, such as the length and size of the installed product, the expected ground conditions, and the ongoing operation. If the expected ground conditions are bedrock, or other hard to drill conditions, horizontal directional drilling may be cost prohibitive.
Design Rules of Thumb
Every site is unique and every installation method should be selected to draw on the experience of the engineer and contractor as well as meet the needs of the client and address the characteristics of the site. With this as a caveat, the following Rules of Thumb have been described by directional boring designers and constructors:
- Under good conditions a driller can install 200- 300 ft of pipe per day
- A setback is needed for entrance and exit points and must be included in the length of the borehole. Setbacks range from 3 to 5 feet for each foot below grade of the installation. For example, for a placement 5 feet below grade, include 15 to 25 feet of boring on each side of the installation for the set backs.
- Accuracy of monitoring and placement of bore holes less than 30 feet below grade is measured in inches.
- Mobilization and set up usually takes less than an hour
- Cobbles are the nemesis of directional drillers and can deflect drill strings. Difficult-to-penetrate conditions will increase the time needed to complete a project and increase the cost.