A walkover locator is a device used in horizontal directional drilling (HDD), also referred to as underbalanced drilling. It is used to ensure the drill bit does not hit any obstructions while drilling through the earth. This allows for continuous and easily reaccessible directional boreholes, reducing costs incurred due to expensive rotary steerable rigs and the time it takes to set up and use them.
Walkover locating systems are quite simple in their design, yet extremely effective in ensuring top quality directional drill holes.
How does it work?
Walkover locators are commonly used by an hdd company in offshore drilling and well logging activities. They are also known as magnetic field detectors. The device functions by detecting variations in Earth’s magnetic field caused by structures buried underground.
Using the device to locate underground structures is based on one’s knowledge of the Earth’s magnetic field and how it varies according to location. It will also depend on what area you are drilling in, whether or not there are metallic instruments that could interfere with your instrument readings and any other existing fault lines.
The basic principle behind walkover locators is that all materials contain iron. Since the Earth has a magnetic field, any variation in this field makes it possible to detect objects buried underground.
Knowing how the variations work will enable you to determine where structures are located and at what depth. The detection area covered by a walkover locator is affected by many factors including an object’s size, shape and its magnetism.
The walkover locator operates by using two pairs of coils which are placed horizontally and vertically on the device. The first pair is used to locate objects in the vertical plane while the second pair detects structures above or below ground level.
To detect metal buried underground, a third coil is attached to the bottom of the compass. This coil emits radio waves at a specified frequency which reacts with metal objects underground and is received by the coils on top of the device. These readings are used to create an image of where the structures are located.
Walkover Locating Systems Used in HDD Operations
A walkover locator operator accompanies crew members on site during both horizontal directional drilling and vertical directional drilling operations. They are responsible for operating and maintaining the various locating systems used by an hdd company to determine shaft alignment and depth, as well as lateral position while drilling.
The walkover locator operator is in charge of systems that include:
Magnetic Locators
Used for locating the horizontal and vertical position of a boring tool, magnetic locators are placed on either end of the drill string. They are used primarily for work in low-to-medium conductor count areas where they can be used to accurately locate a borehole within 0.5 to 1 inch.
Induction Locators
Used in areas with higher conductor counts, such as steel pipes or power cables, induction locators measure the magnetic field of a metallic surface and determine distance from the boring tool in relation to the target signal. In these situations they can be used to accurately locate a borehole within 0.125 to 3 inches.
Doppler Systems
Doppler systems are used in areas with steel pipe or power cables where magnetic and induction locators fail to achieve the required accuracy of less than 1 inch. These locating systems utilize sound waves to calculate distance from the boring tool when pointed toward the target. They can be used to accurately locate a borehole within 0.125 to 3 inches and are considered the most accurate in terms of location.
Resistivity
Used for locating targets with non-ferrous metallic conductors such as plastic pipes or water lines, resistivity measuring devices are used in conjunction with an optional ground pulse generator that emits a specific electric current.
When the target is struck, the ground pulse generator detects changes in current on the conductor and relays this information to the walkover locator operator who can then determine drill shaft depth and direction of travel.
These locating systems are used for work in areas with conductor counts ranging from 0-500 per square foot and can be used to accurately locate a borehole within 2-12 inches.
Ultrasonic Locators
Used for locating non-metallic targets such as water or gas lines, ultrasonic locators bounce high frequency sound waves off of the target. When they return, these sound waves are detected and used to determine drill shaft depth and direction of travel.
Ultrasonic locators can be used to accurately locate a borehole within 2-12 inches and are used in areas with conductor counts ranging from 0-500 per square foot.
Fixed Survey Markers
Used to help drill operators determine borehole depth, fixed survey markers are placed at set intervals along a boring route once the location of their target has been established by a magnetic or induction locator. They are used in areas with conductor counts ranging from 0-200 per square foot and can be used to accurately locate a borehole within 2-10 inches.
Shoe Trackers
Used to help drill operators determine lateral position of the boring tool while drilling, shoe trackers are placed beneath the shoe of each drill pipe in the drill string. They relay information during the drilling process and are used in areas with conductor counts ranging from 0-500 per square foot. Shoe trackers are used to accurately locate a borehole within 2-12 inches.
Takeaway
The walkover locator operator is also responsible for maintaining all of these locating systems using well maintained test sets. They must also have a thorough understanding of how to troubleshoot minor and major malfunctions, as well as thoroughly explain system functionality to any incoming drill operators or rig crew members.
For the most part, walkover locators used for horizontal directional drilling are simply advanced global positioning systems that use an array of sensors including electromagnetic induction devices, accelerometers and magnetic compasses. They are calibrated to determine drill shaft depth, heading direction and lateral position of the boring tool relative to a starting point.
Operators must know each locating system’s capabilities inside and out in order to be successful with them.
