Slimdril International

Gyro gives steer to HDD

11th July 2012

A technically challenging HDD project in North Yorkshire, England, involved heavy
interference at the job site and could only be completed by a gyro steering tool.

Drilling Contractor AMS No-Dig from Scunthorpe, North Lincolnshire, UK, was commissioned by Network Rail through Global Rail Construction to design and build a project to replace electricity cables as part of the Selby Swing Bridge Interlocking Renewal scheme.

The works consisted of an HDD installation that went under a Grade 2-listed building, as well as under the Ouse Road, the River Ouse, and the main railway line in the town centre of Selby, North Yorkshire, UK

Challenges to Overcome

Right from the start, the project had difficult challenges to overcome. Due to land access and easement issues, the requirement was to design a complicated three-radius horizontal plan; this involved a 32° left turn, followed by 6° to the right, This is the combination radius of the drill string changing direction vertically and horizontally at the same time, this project had a 475m vertical radius and 250m Horizontal radius to ensure that once installed, the cables would be located within agreed land boundaries.

In addition to the drill passing under listed buildings, a road and river parallel to a metallic railway swing bridge, it also passed beneath a sheet-piled flood defence and under the railway lines, all areas of very high magnetic interference.

Engineering Solution

AMS No-Dig worked closely with SlimDril International to provide the most cost-effective and technically sound engineering solution for steering the pilot. Given the lack of space to lay out surface coils and beacons, and given the large amount of magnetic interference present, the Brownline DrillGuide GST system was deemed the best steering tool for the job, as traditional wireline steering tools would not be suitable for such an application. The DrillGuide GST is an optical gyro tool that does not require any surface wire grids and is not affected by any external magnetic interference. The tool does not require to be housed inside a non-magnetic drill collar and therefore can be positioned nearer to the drill bit, giving accurate hole data as soon as possible. The accuracy of the GST tool is 0.04° on azimuth and 0.02° on inclination.

Horizontal directional drilling (HDD) is a specialist profession nowadays. The days when directional drilling was ruled by assumptions are far behind us. Clients are more demanding, drillings get more and more complex, and the underground infrastructure is more dense and complex. All these factors require a professional approach to complete a successful drilling project. Also, the capital-intensive investment, related to the drilling project, requires a smooth completion, resulting in a positive revenue per project.

Magnetic steering tools were the standard guidance systems of drilling companies. But the drawbacks were numerous; these included disturbance, inaccessible areas above a drill path, and a huge tolerance of the actual drilling trajectory towards the desired trajectory.

Gyro Steering Tool

The DrillGuide Gyro Steering Tool (GST) was developed to eliminate these problems and achieve greater accuracy. It was decided to combine new technologies, in which gyroscopes play an important role. This technique does not depend on magnetic North and is insensitive to magnetic disturbances. The advantages of this approach translate into a reliable system, that is able to perform under virtually all conditions. The system is resilient to vibrations and practice shows that pilot drilling, performed with a GST, is completed much sooner than with previous systems. The DrillGuide GST is not effected by external disturbance, and there is no involvement from external magnetic field interference. Railways, bridges, pipelines (with or without cathodic protection) and high voltage cables are no longer disruptive elements. Also, moving traffic, whether by road, rail or river, poses no obstacles.

Furthermore, there is no requirement to create a known magnetic field above the drill-path. This saves time, money and offers new drilling opportunities. Drillings under lakes, rivers, roads, or environmentally sensitive areas can be accurately and efficiently undertaken.


Due to the DrillGuide GST being insensitive to magnetic disturbance (therefore not requiring to be housed in a non-magnetic environment), the need for expensive non-magnetic collars is no longer present.

Due to the advanced technology of the DrillGuide GST, it is continually known where the drill-head is located. Not only is the ‘seen’ exit point exactly where it is expected, but you also have the assurance that the entire pilot hole route is being drilled as planned.

This offers new opportunities in areas where the infrastructure under the ground calls for a specific route, for example, to avoid other pipes, sewers or foundations. In addition, this technique can also be applied to specific projects, such as parallel-drills involving multiple parallel routes that must be maintained at a fixed distance apart.

Permanent communication with the drill head supplies continuous data, which provides accurate x, y and z co-ordinates of the drill head. The DrillGuide GST has no restriction on drill-depth and works on the principle of continuous measurement while drilling (CMWD). Measuring the location of the drill head can also be achieved when the drill head is rotating, as the DrillGuide GST does not require a stationary drill head to take readings. During drilling, all information is provided in real time, so the driller promptly knows exactly what is required.

Correct Measurements

There are no problems in obtaining correct measurement readings when drilling with a mud motor. Measuring the mud pressure in the borehole is increasingly important, therefore, the DrillGuide GST is equipped with a mud pressure sensor. The mud pressure is measured in the borehole. The start-up time of a pilot hole, in which the system is deployed, is significantly reduced due to the known magnetic field grid being redundant. For a drilling project, the surveying engineer arrives with the DrillGuide GST assembly. He shoots in the tool, connects it to the drill pipe and is ready to go within approximately one hour from arrival. Drilling can then begin immediately, following the already digitised crossing profile.

Network Rail

Network Rail maintained a close monitoring regime throughout and, as there was very little tolerance for SlimDril’s steering engineer to work to, this called for careful project planning and attention to detail to ensure the pilot hit the many critical tangent points along the route, including providing enough space for future planned piling works.

The project commenced on July 13, 2012, using a standard jetting assembly but unforeseen ground conditions encountered after just 20m meant that one of SlimDril’s 6¾in 7-8 Lobe three-stage mud motors had to be quickly dispatched from its Great Yarmouth base over the weekend to minimise downtime. The motor was fitted with a 9.7/8in sealed bearing, milled tooth bit (IADC 1-1- 7) which was run in hole to drill the harder formation. Once the hard section had been negotiated, the motor was tripped out and exchanged for the softer formation jetting assembly to enable steering through the remaining soft formation to the exit point.

Reaming was carried out over a five-day period using a combination of fly cutters, barrel reamers and fluted rock reamers. Initially, a 508mm fly cutter and barrel reamer combination was used to ream from chainage 300m to chainage 80m; this comprised stiff clays down to 7m below ground, followed by sand and silt down to hole bottom at -21m od. The reamer was then tripped out of the hole and exchanged for a rock reamer which was used to ream the harder rock section situated between chainage 80m and chainage 20m.

Space Restrictions

Due to the limited space available at the drilling compound and with the requirement to run 5in drill pipe to satisfy the criteria set by the steering tool insurers, AMS opted to use the Powerbore 70t. The rig is very compact at 10m x 2.2m and, when considering it offers 70t of thrust and pullback, proved to be the ideal choice for this application.

In total 100 m3 of fluid adhered cuttings was removed from the hole. This was processed using one of AMS’s bespoke mud management systems. The innovative modular design consists of an 18,000L mixing tank, a mud cleaning system and two 26,000L storage tanks. This arrangement provided optimum flexibility in terms of both site planning, as dictated by circumstances on the ground, and operational requirements, as dictated by conditions encountered under the ground.

The high performance Derrick recycling unit has been designed to process 3,000L/min of high viscosity drilling fluid, and comprises two Derrick Equipment FLC-504 shale shakers for primary screening; a Derrick FLC-504 mud cleaner, complete with 3 x 10in (254mm) de-sander cones and 20 x 4in (100mm) de-silter cones, for de-sanding and de-silting. The Derrick 504 units are mounted on a 7.3m base tank incorporating integrated centrifugal pumps to feed the de-sander and de-silter cones. Screening is carried out using the revolutionary pyramid screening system which ensures consistent cleaning down to 20 Hm.

The two 6m holding tanks include strategically-placed agitators to ensure the quality and flow of clean mud is maintained to the drilling rig, even when the 1500L mud pump is working to capacity.

Fast Mobilisation

The entire plant can be mobilised and made operational within hours of arrival on site, helping to eliminate the long lead times usually associated with installing a liquid mud plant.

AMS No-Dig is the largest independent HDD and auger boring contractor in the UK operating 15 horizontal directional drilling systems ranging from a 17t Vermeer mini rig system, ideal for small diameter applications, right up to a 250t maxi drilling system suited for large diameter and longer drilling applications.