In the April issue of Inflection Points, we explored the use of magnetic guidance systems for locating and steering horizontal drilling systems. As we described, two fundamental types of magnetic systems exist; walkover and remote – and both are capable of high accuracy and repeatable precision. However, both types of magnetic systems have drawbacks. In most cases, these magnetic methods require access over the bore path to either track the drill head directly or to set up a coil to do so remotely. This restricts their use in steep terrain, on heavily industrialized sites, under waterways, or other areas where access is not available. Walkover and remote magnetic systems may also be affected by strong electromagnetic fields.
And both systems have depth limitations, although in most cases a remote magnetic system can achieve sufficient depth to complete the average environmental project.
To address the technical shortfalls of magnetic systems, a completely different type of locating system was developed. Originally used in the petroleum industry, inertial systems do not rely on magnetic fields to locate the drill head in three-dimensional space. These systems use fiber-optic gyroscopes, in combination with sensitive accelerometers, to precisely locate the drill head along the desired bore path. Within the industry, these systems are called Gyroscopic Steering Tools, or GSTs. Other than a wireline-supplied power source and a connection to a computer at the surface, a GST requires no external input or sensors to accurately locate the drilling head, even thousands of feet deep.
Unlike the toy gyroscopes that you may have owned as a child, or the fast-spinning mechanical gyros used for navigation in airliners for decades, an optical gyro is a completely solid state device. The system works by firing photons simultaneously in opposite directions down a wound core of optical fiber. As much as 5 kilometers of fiber may be wrapped into a core no larger than a softball! As the photons travel through the core, any rotation along that axis will cause one photon to be sped up (relativistically speaking) and the other to slow down – when the photons are detected at the end of the coil, the phase shift between them is measured and converted to pitch, roll, and yaw increments. A GST contains three fiber-optic gyroscopes, as well as additional accelerometers that measure
time and distance along a vector. When these outputs are all combined, the result is the current location and direction vector of the drill head, accurate to within 2% or less of the depth.
Diagram courtesy of Slimdrill International – Houston, TX
Photons are unaffected by the types of electromagnetic fields found in the drilling environment, making the GST highly resistant to passive or active electromagnetic interference.
The GST has several advantages over comparable remote magnetic systems. With the GST, it is unnecessary to lay out surface coils, or even to access the ground surface over the bore path. The GST will traverse beneath roads, aircraft runways, rivers, refineries, or any other surface obstacle without hindrance. Where a remote magnetic coil requires precision surveying to map out the shape and location of key points of the coil, the GST only requires a known starting point and a detailed CAD drawing of the bore path from which three dimensional coordinates can be determined for the various inflection, tangent, and other virtual points along the bore path. Since setup is minimal, with a GST the drilling can begin as soon as the rig is ready to go, saving mobilization and setup time.
The primary disadvantage of the GST is cost. The GST tooling is robust and expensive, requiring a large drill rig, beefy drill rods to carry the tools, and specialty technicians to operate it. As a result, projects suitable for GST locating tend to be one (or a combination) of three varieties:
Long (over 1,000′) and deep (over 100′) wells
Wells under areas inaccessible to surface tracking methods such as a body of water, manufacturing building, or airport runway
Wells located in areas of high electromagnetic interference such as a rail switching yard or a power generating station
Most HDD drilling contractors, DTD included, subcontract GST steering services to specialty firms like SlimDril International, from Houston, TX. These services typically add, on average, $5,000/day to the cost of the drilling program for advancing the pilot bore (no locating is done for subsequent reaming passes). However, balanced against the somewhat less expensive remote magnetic systems, plus the extra setup time they require, the cost differential may not be significant.
So what type of site may require a GST? DTD recently completed a project at a bulk fuel terminal. Close your eyes and imagine the type of interference, both active and passive that were present at the location. First think about the passive interference issues which included:
1,000,000 gallon steel fuel tanks
Underground steel pipe lines
Overhead steel pipelines
A steel chain link security fence running parallel to the bore path
If that wasn’t enough, the sources of active interference included:
Overhead electrical lines
Cathodic protection currents on the underground piping
Multiple, high capacity pumps driven by electric motors
Several large electrical switching control boxes
The combination of passive and active interference was too great for either a walkover navigation or remote system to overcome. Therefore the GST tool was utilized at this site to successfully navigate/locate the pilot bore leading to the installation of two air sparge wells. The wells were completed in blind boreholes, at a depth of about 70′ bgs with lengths of 882′ and 978′.
Locating technology makes the difference between an unguided bore that may or may not reach your target area of concern and a precisely guided well that safely transects a contaminated zone. Any of the front line systems currently in use are capable of accurate results when properly applied. The benefit of the GST system is that it allows the drill bit to be accurately tracked in areas that have high electromagnetic interference. Key to the project design process is selecting a locating system that meets the project needs, without needlessly taxing the budget. Each project location requires a detailed review before choosing a locating method.