Horizontal directional drilling (HDD) is a useful trenchless method of installing new pipelines. It's the go-to method for oil drilling companies due to its ability to go further and, depending on the drill head and other factors, drill through different types of soil.
Controlling the drill head is a challenge for all operators. While technology is ever-changing, the two most common ways of steering the drill head through the ground are through the use of either gyro or magnetic wireline steering tools. (Read A Quick Guide to Underground HDD Steering and Tools.)
Gyro steering is one of the two main choices in steering HDD. It relays accurate navigation information from the drill head to the operator in real-time without the effect of external influences.
Gyro steering works through a tool which is mounted behind the jetting assembly. The tool then takes measurements of the position of the steering tool along with data about the drill length. This information is combined and calculated using an algorithm to steer the head accurately.
When is Gyro Steering Necessary?
While both wireline steering techniques work in most situations, there are times when gyro steering is a better fit than magnetic steering. One such instance is when there is a casing involved in the drilling process.
Some HDD projects require the use of casing when completing the initial drill or a portion of the drill. In those instances, the casing may interfere with a magnetic steering tool. Gyro steering tools are insensitive to magnetic disturbances and, therefore, better suited for jobs requiring casings.
Other times when a gyro steering tool includes times when the drilling site runs under structures that may obstruct the magnetic coil on the surface, it may include movement beneath high voltage cables or near moving traffic. Creating a magnetic field above lakes and rivers can be difficult. In these cases, project managers may opt for gyro steering instead of magnetic steering.
Gyro steering is also used in areas considered environmentally sensitive where a magnetic field may disturb the surrounding environment or habitats.
What Equipment Necessary for Gyro Steering?
Gyro steering is not the same as the toys with which most kids played. Instead, the gyro steering equipment contains an optical gyro that resides in a solid-state device. The tool fires photons simultaneously and transmits through an optical fiber to the operator. There are three fiber-optic gyroscopes in the tool that help to measure pitch, yaw, and roll.
There are some benefits to using gyro steering instead of the more popular magnetic steering option. Jobs, where magnetic interference can occur, do not have to worry about interference with the wireline steering tool. Gyro steering does not need or use magnetic fields for orientation and therefore are unaffected by potential interference from the surrounding area.
As it is not affected by magnetic interference, there is no need for a non-magnetic spacing collar. The gyro steering tool can run closer to the bit and is placed behind the jetting assembly to provide accurate real-time readings and shorter setup times.
While newer gyro steering tools are sophisticated enough to navigate the drilling tool, there are some drawbacks to using them. There are indications that gyros, while accurate and responsive, can deviate from the track easily.
When Crossing the IJ River in the spring of 2006, engineers needed to cross with a strict bending requirements at depth of 50 meters. Due to the bending requirements there was no way to make corrections at the end of the directory. While the gyro steering tool did stay within the gas pipe limits, there were deviations while boring. The deviations were between 1.5 and -2-degrees variation which equated to less than 0.5 a meter right/left deviation.
The lack of external sensors makes it difficult to ensure that the drill is on track to make corrections. That is why many drillers prefer magnetic wireline steering is better to complete the project.
Magnetic Wireline Steering
Magnetic wireline steering is one of the more popular choices when it comes to steering in HDD. The tool uses an inline tool which communicates a surface tracking system to help verify where the precise location of the tool while it is underground.
There are two main ways for the magnetic steering either through direct current (DC) or alternating current (AC) tracking. The DC method is considered the more accurate of the two. With the DC method, a wire is looped in a rectangular shape and placed on the ground surface. The wire starts and ends at the power source to create a magnetic field on each side of the loop.
The AC method is like DC tracking but only uses one side of the wire instead of either side. However, if the return wire is not placed far enough away, it can interfere with the signal.
When is Magnetic Steering Necessary?
Project managers use magnetic steering in most HDD projects. It is accurate and easy to control once the pre-drilling survey is complete. The utilization of this method is available in most areas. Only when it is impractical do these managers turn to gyro steering methods to compensate.
What Equipment Necessary for Magnetic Steering?
There are several components used for magnetic steering in HDD. For magnetic wireline steering, in addition to the in-line sensor located behind the drilling head, there is a tracking grid system on the surface. This tracing system uses micro-coils place in decongested areas.
The tool and tracking system relay information to the driller’s console, and the interface unit allows the operator to control the drill accurately.
The magnetic steering technique is an accurate method of steering HDD. Unlike gyros, magnetic is not as sensitive to shock or vibration that can occur when breaking up rocks, which may be in the way.
Because there is an on-surface tracking tool, magnetic steering is often referred to as more accurate than its gyro steering counterpart. Some magnetic steering tools do not require surface tracking coils, but they make them more accurate.
The main disadvantage of using magnetic steering in HDD is the coil-based guidance system. Older magnetic steering tools used a walkover system where a technician walked over the path, caring for a receiver that monitored the probe. However, with the new systems using a coil generating the magnetic field on the surface and the sensor on the drilling apparatus, there are times when there is no place to put the coils.
Congested areas, such as a refinery, may not have space to place the coils on the surface. Going under lakes and rivers also proves problematic for the same reason.
Additionally, there are areas that generate magnetic fields that may interfere with the on-surface coils. A strong enough magnetic field from says a power station makes it difficult for the sensor in the ground to accurately find the coils sitting on the ground surface. The steering tool cannot read through casings, which are often used in gas and oilfield drillings.
The environmental impacts of HDD come less from the use of magnetic wireline steering or gyro steering and more from the drilling fluids used. There is some concern that the creation of artificial magnetic fields may affect habitats in the area where the drilling occurs. In these areas, project managers may opt to use gyro steering instead of magnetic wireline steering.
Both gyro steering and mechanical wireline steering tools are effective ways to control HDD bits. While mechanical wireline steering is the go-to choice for most project managers, gyro steering is gaining wider acceptance as it works well in areas in which magnetic steering does not.