Horizontal Directional Drilling

Horizontal Directional Drilling is used for installing underground infrastructure such as telecommunications and power cable conduits; water, gas and oil mains, as well as sewers and environmental remediation casings. It is mainly used for crossing waterways, roadways, shore approaches, congested areas, environmentally sensitive areas, and areas where other methods are costlier or not practical.

When taking into account the carbon footprint involved with conventional open-cut methods, Directional Drilling is an often overlooked alternative that can greatly increase sustainability by massively reducing the volume of plant required, use of imported aggregates and minimising local traffic disruption.

Equipment

The equipment used in a directional drill operation depends on the diameter of the pipe to be installed, length of the installation, and the ground conditions through which the drilling will be carried out.

AE Yates Trenchless Solutions Ltd currently operate a fleet of 7No Directional Drilling Rigs ranging from 14-70 ton pullback capacities. This gives us the capability to install a large range of lengths, and diameters from 30mm - 750mm. All our drilling systems come complete with drill-fluid mixing units and where necessary we can also provide Drill-Fluid Recycling Systems allowing us to clean and re-use drilling fluid to cut down on the quantity of water needed and also minimise the volume of used fluid being sent to tip.

In terms of dealing with varying ground conditions we have a range of drilling heads that can deal with soft clay to more difficult conditions such as weak rock and dense sand. Solid rock can be dealt with by using one of our Down the Hole (DTH) Air Impact Hammers that use a percussive action to crush and hammer through rock. This method is suitable for intact rock for lengths of approximately 100m. Alternatively a Mud Motor can also be used to penetrate the rock using a rotary action generated by pumping high pressure mud to a tri-cone roller bit which cuts its way through the ground. This method is sometimes also used for long drills in softer ground where thrust alone would not be sufficient to progress the drilling head.

Process

The process begins with the excavation of launch and reception pits at either end of the length to be drilled. These pits allow the drilling fluid to be collected throughout the process and either re-cycled during the job or removed for disposal upon final completion. The first stage of the process involves drilling a small diameter pilot bore along a prescribed route from launch to reception pits. The second stage of the process enlarges the pilot bore by pulling a larger cutting tool known as the back reamer from the reception pit to the launch pit connected to the drill rods installed during pilot boring. The back reamer is also rotated during this stage cutting the surrounding ground to create a passage for product. The third stage of the process is to install the product or casing pipe by pulling it back through the hole enlarged by the reaming process. If required the reaming stage may actually be carried out in a number of successively larger stages until the final bore size is between 25 and 50% larger than the product to be installed.

Throughout all stages of the Horizontal Directional Drilling (HDD) process a drilling fluid is pumped down the bore to the drill head or reaming tool. This fluid is used to facilitate the removal of cuttings, stabilise the bore hole, cool the cutting head + transmitter, and also to lubricate the passage of the product pipe. The constituents of the drilling fluid are chosen to match the properties of the ground being drilled. In sandy conditions a bentonite fluid is used to gel the walls of the bore stabilising the ground and preventing fluid loss. In clay conditions polymer additives are used to break down the clay, stop it taking on water and also to prevent it sticking to the tooling. In environmentally sensitive areas such as watercourse crossings or areas of special scientific interest, alternative biodegradable drilling additives can be used to further minimise the impact of construction works.

A E Yates Trenchless Solution Ltd have a number of Horizontal Directional Drilling case studies

Guidance (Locating) methods in Horizontal Directional Drilling

There are two main types of drill locating systems utilised with Horizontal Directional Drilling Techniques. The basic principles of both are described below.

Walk Over System.

1. Walk Over System.

This method of drill locating is the most commonly used system particularly when using small rigs for short shallow crossings/installations. The method utilises an electromagnetic signal from a transmitter located directly behind the drilling head. The signal is received by an antenna (handheld locator) on the surface. As the name suggest this method relies on the tracker (handheld locator) being able to Walk Over the top of the drill head to accurately monitor its progress. The information received on the surface details the drill's lateral position, depth, pitch and roll. This data allows the drilling team to steer the drill along the desired path/route.

Wire-Line System

2. Wire-Line System

This method of drill locating differs from the above in that rather than locating the drill directly overhead, information on position, inclination and roll is relayed from the steering probe (tool) back to the drill rig via a continuous wire connection installed inside each individual drill rod, allowing accurate steering where overhead locating is not possible. The steering tool uses magnetometers and inclinometers to orientate itself to the Earths Magnetic Field; this system is very sensitive and is prone to disturbance from other magnetic sources such as metallic objects, electric currents etc. To overcome this and increase overall positional accuracy a surface cable loop is laid above the axis of the bore and connected to a power source to generate an artificial magnetic field. The measurement probe can now be referenced to this known field eliminating outside interference.

Guide Probe (Tool)

Guide Probe (Tool)

Sensors located within the downhole Probe provide constant updates of inclination, azimuth and tool face orientation to the surface via a 'Wireline'.

The data is processed utilising surface equipment (interface unit, laptop computer and printer) and specialised steering software to provide information on depth, course length (away) and distance off the predetermined azimuth (right/left).

Surface Monitoring
Surface Monitoring System

Verification of the location of the Probe and the accuracy of the survey can be improved using the Surface Monitoring System, an independent secondary monitoring that incorporates an energized grid of wire laid on the surface to compensate for any magnetic interference.

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