The present application is related generally to the field of underground directional drilling and, more particularly, to an advanced underground homing system, apparatus and method for directing a drill head to a homing target.
A boring tool is well-known as a steerable drill head that can carry sensors, transmitters and associated electronics. The boring tool is usually controlled through a drill string that is extendable from a drill rig. The drill string is most often formed of drill pipe sections, which may be referred to hereinafter as drill rods, that are selectively attachable with one another for purposes of advancing and retracting the drill string. Steering is often accomplished using a beveled face on the drill head. Advancing the drill string while rotating should result in the boring tool traveling straight forward, whereas advancing the drill string with the bevel oriented at some fixed angle will result in deflecting the boring tool in some direction. A number of approaches have been seen in the prior art for purposes of attempting to guide the boring tool to a desired location, a few of which will be discussed immediately hereinafter.
In one approach, the boring tool transmits an electromagnetic locating signal. Above ground, a portable detection device, known as a walkover detector, is movable so as to characterize the positional relationship between the walkover detector and the boring tool at a given time. The boring tool can be located, for example, by moving the walkover detector to a position that is directly overhead of the boring tool or at least to some unique point in the field of the electromagnetic locating signal. In some cases, however, a walkover locator is not particularly practical when drilling beneath some sort of obstacle such as, for example, a river, freeway or building. In such cases, other approaches may be more practical.
Another approach that has been taken by the prior art, which may be better adapted for coping with obstacles which prevent access to the surface of the ground above the boring tool, resides in what is commonly referred to as a “steering tool.” This term has come to describe an overall system which essentially predicts the position of the boring tool, as it is advanced through the ground using a drill string, such that the boring tool can be steered from a starting location while the location of the boring tool is tracked in an appropriate coordinate system relative to the starting position. Arrival at a target location is generally determined by comparing the determined position of the boring tool with the position of the desired target in the coordinate system.
Steering tool systems are considered as being distinct from other types of locating systems used in horizontal directional drilling at least for the reason that the position of the boring tool is determined in a step-wise fashion as it progresses through the ground. Generally, in a traditional steering tool system, pitch and yaw angles of the drill-head are measured in coordination with extension of the drill string. From this, the drill-head position coordinates are obtained by numerical integration step-by-step from one location to the next. Nominal or measured drill rod lengths can serve as a step size during integration. One concern with respect to conventional steering tools is a tendency for positional error to accumulate with increasing progress through the ground up to unacceptable levels. This accumulation of positional error is attributable to measurement error in determining the pitch and yaw angles at each measurement location. One technique in the prior art in attempting to cope with the accumulation of positional error resides in attempting to measure the pitch and yaw parameters with the highest possible precision, for example, using an optical gyroscope in an inertial guidance system. Unfortunately, such gyroscopes are generally expensive.
Another approach that has been taken by the prior art, which is also able to cope with drilling beneath obstacles, is a homing type system. In traditional homing systems, the boring tool includes a homing transmitter that transmits an electromagnetic signal. A homing receiver is positioned at a target location or at least proximate to a target location such as, for example, directly above the target location. The homing receiver is used to receive the electromagnetic signal and to generate homing commands based on characteristics of the electromagnetic signal which indicate whether the boring tool is on a course that would ultimately cause it to be directed to the target location. Generally, identifying the particular location of the boring tool is not of interest since the boring tool will ultimately arrive at the target location if the operator follows the homing commands as they are issued by the system. Applicants recognize, however, that such traditional homing systems are problematic with respect to use at relatively long ranges between the homing receiver and the boring tool, as will be discussed in detail below.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.