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1. Introduction

The underground infrastructure is not yet well mapped. The underground infrastructure gets more and more crowded with gas pipeline networks, separated waste and rain water lines, pipes for chemical substances, electricity cables, communication lines etc. etc..

Construction companies provide drawings to the drilling contractor with precise way-points in local grid co-ordinates. These way-points should be followed by the bore-hole, taking into account the minimum allowed curvature.
In urbanised areas or when crossing railways or rivers the present day navigation systems can not always be used due to inaccessibility of the surface. This results in using magnetic steering tools, which use the Earth magnetic field as orientation reference. However, in crowded areas this Earth magnetic field is disturbed by the presence of magnetised materials, electric cables or even cars or ships that are crossing at the surface. This results in inaccurate guidance during the drilling operation. Eventually the way-points can not be precisely followed. The various underground structures as a consequence must be planned of safe distances, which will result in a couple of decades in overcrowded underground infrastructures.
Nowadays a drill-path is surveyed after the drilling, but the trajectory has already been laid and nothing can be corrected.

In order to get the bore-hole trajectory precisely laid during the (pilot) drilling, a precise navigation system is required, which is not dependent on the accessibility of the surface directly above the drill-head. In the following sections such a steering tool will be described.

2. Current systems

A navigation system measures the position, orientation and distance travelled of the drill-head, whereas the guidance system directs the drilling operator along the desired path by comparing this desired trajectory with the actual trajectory determined by the navigation system. The guidance instructions are given via the graphical user interface (human machine interface (HMI)).

It indicates the pitch and azimuth angles to be steered by the drilling operator. When the pitch and azimuth bar-graphs indicate zero, the actual trajectory corresponds with the desired trajectory. The HMI also gives additional information important for the drilling operator. Nowadays basically three navigation methods are applied for the horizontal directional drilling operations :
Magnetometer based systems with or without an artificial magnetic field. Walkover locators. Mechanical gyroscopic systems.

The magnetometer-based systems are sensitive to the disturbance of the Earth magnetic field. To improve this artificial magnetic fields are created at the surface. This requires laying electric cables, generating this artificial magnetic field at the surface all along the drilling trajectory. Besides the time it consumes to lay these electric cables, it also not always is possible to lay these cables. In urbanised areas no access exists. For railway and river crossings also problems exist. Another problem is that the accuracy of the drill-head position measurement decreases with increasing drill-path depths. In most circumstances the walkover locator is used. At the surface the relative location (position with respect to where the locator is placed) of the beacon is measured. The beacon is placed directly behind the drill-head. Here again accessibility at the surface is required. Another limiting factor is the depth of the drill-path.

Some manufacturers recently introduced mechanical gyroscopic navigation tools. These systems are derived from the surveying systems. As gyro a dynamically tuned gyro is used. A small mass spins at a very high rotational speed. In this way the geographical Earth North can be found. The disadvantage of these mechanical gyro systems is that the azimuth angle only can be measured, when the drilling is stopped. In practical terms this means that a measurement only can be done, when a drill-pipe is added to the drill-string. To do a measurement up to 15 minutes are required, while no drilling can happen.

3. Steering tools

GST

DrillGuide® is a new navigation and guidance system, based an inertial navigation system. The core of this system is the gyroscope. Optical gyros are applied for this new guidance tool.
The DRILLGUIDE GST has the best accuracy of all present day drilling navigation and guidance systems. The DRILLGUIDE GST gives the drilling contractor the highest possible certainty to reach the bore-hole exit in the shortest possible time within the required accuracy.

The GST directional drilling guidance system measures the azimuth angle with an accuracy of 0.04 degrees and the pitch angle is measured with an accuracy of better than 0.02 degrees. Especially for larger drilling depths, this makes the GST the best possible choice among the present day drilling guidance systems. The GST also measures with the above accuracy, while drilling. This allows the drilling engineer to supervise the drilling process at all times, even when the drill-string rotates, and to determine corrective steering actions when needed.
The GST with the guidance software enable the drilling contractors to more precisely follow the desired way-points within the allowed bore-hole curvatures in urbanised and other locations, where presently difficulties exist in achieving the required accuracy Brownline is doing continuous development. Present and future work will be on the drill-path planning. Presently the drill-path is derived from an AUTOCAD drawing with local grid co-ordinates. The minimum bending radius is the main constraint, while making these drawings.
The present day practice with the GST is that precise calculations can be made, prior to the actual drilling, such that no further preparations at the drill-site are needed. This saves time for the drilling contractor.

WDS

Brownline started early year 2000 with a magnetic steering tool (MST) with a total wireless signal transmission, making Measurement While Drilling (MWD) a reality. The Brownline MWD wireless solution does not need the time consuming connection for every drill-pipe of the wireline. It simply does not exist.
The Drillguide WDS wireless signal transmission uses the drill-string as positive signal path and the soil as negative signal path. At the surface the Drillguide receiver station translates the received signal back to normal language which is made visible at the display. The display update rate is with three times per second near real-time.Downhole electric power is provided by high power density rechargeable batteries.

The unique feature of the Drillguide WDS system is its real wireless high speed signal transmission.

Based on the big succes of the MST tool, the market asked for a wireless signal transmissison in combination with a standard walkover system. Operators will now be able to drill, even without using a locator. Pitch, Roll, Temperature beacon and battery status will be shown on a special designed remote dispay.
For actual depth the locator has to be used.

This Drillguide WDS includes:
- Drillhead with batteries
- Remote Display
- Battery loader

Fully loaded batteries will allow for 10 to 12 drilling hours.

MST

The DrillGuide MST gives you the advance of wireless drilling and actual information about your drillhead’s position.
The operator uses the laptop for the guidance advice. Basically three pages are available: preparation of the job, job guidance in easily understandable mimics, and reporting. Each page gives important warnings and messages, such as a warning for too much disturbance of the geomagnetic field due to obstacles.The resultant deviation of the magnetic field cannot be compensated for in the DrillGuide MST programme. The wireless operation provides reliable control over longer distances. This achievement results from using the new DrillGuide MST guidance tool. Should the signal/noise ratio become too low, a warning is given. The operator can then simply take alternative actions such as placing the surface pick-up antenna at another location or possibly adding a signal repeater in the drillstring.

The new DrillGuide MST guidance tool provides precise feedback to the HDD operator and to the engineer operating the drilling machine, as is shown on the drilling engineer’s indication display (DRI).
This DRI shows the tool-face position, the actual azimuth, and the pitch angle. It also advises the drilling engineer to steer left or right, up or down, as indicated by the bargraphs above and right of the tool-face location. The engineer should steer until no deflection on either bargraph is apparent. In this case, the actual drilltrack is exactly following the planned drilltrack.

The DrillGuide MST guidance tool offers drilltrack planning, drilltrack guidance and track position reporting.
The downhole probe consists of magnetometers, accelerometers, mud pressure sensor, signal processors, electric power management and a transmitter. The data signal is transmitted via the drillstring, such that no electric wiring is required. The pressure tube contains the downhole electronics together with the non-magnetic carriers for rigid and shock-resistant installation in a non-magnetic drill collar. Various new features have been added, such as the connection of a (D)GPS signal for quick and accurate measurement of the entry and end point of the drill string, both during the preparation and the reporting of the drill job. This method enables an improved planning by using absolute (true North) coordinates. Also walkover data can be used in cases where the operator gets too many warnings about geomagnetic disturbances. This data can be introduced in the reporting page at the laptop and is clearly marked as being 'plugged in'.
At the laptop, a programme estimates the precise location of the drillstring in left/right (metres) and in depth (metres). These estimates are put in a report, which can be hardcopied. The estimates of the actual position also are compared with the preparation data. Any difference will result in advice to the operator and drilling engineer for steering.


The operator of the drilling machine has real-time data available on the Drillguide WDS display.

The well-known walk-over locator can not always be used at the railtracks or in busy traffic. The Drillguide WDS solves this by a signal transmission system from downhole, which is without any wireline.
The display offers all relevant information in a very comprehensive interface.
The Drillguide WDS display shows the drillhead’s pitch and roll, the downhole beacon temperature and the state of the batteries.