Patent Description:
Directional drilling, and more particularly, horizontal directional drilling, is a well-known technology that is used to form boreholes, typically for pipeline construction, although other applications are also known. In a typical pipeline construction application the directional drilling operation may be accomplished in three main stages. The first stage involves the drilling of a relatively small diameter pilot hole in the formation so that it follows a defined directional path established for the pipeline. The second stage, commonly referred to as a reaming stage, involves the use of a reamer to enlarge the size of the pilot hole to accommodate the desired pipeline. Depending the required final size of the borehole, several reaming steps may be required, with reamers of gradually increasing diameters being used to enlarge the borehole to the desired size. After the reaming stage, the pipeline may then be pulled back into the enlarged borehole to complete the process.

As mentioned, the pilot hole drilling apparatus is steerable or directable so that the pilot hole may be formed along the planned or desired pathway. Any of a wide range of steerable or directable drill strings and surveying techniques may be used for this purpose. While the pilot hole may follow the defined path within an acceptable tolerance, the subsequent reaming and pipe pulling operations may result in significant deviations from the path defined by the pilot hole, particularly if the pilot hole extends through formations of different types and properties.

For example, if the borehole traverses a rocky formation, it is possible that during the reaming process the borehole can 'walk' up to half the diameter of the final reamed size to get around a harder section of the rocky formation. In a sand or dirt hole, it is possible that a reamer can drop more than <NUM> meters from the path of the pilot hole. Both of these occurrences not only would place the pipeline in a different location than the desired pathway, but the undetected deviation may place added stress on the pipeline, thereby increasing the possibility of an in-service failure. Moreover, increasing constraints in pipeline development and the desire or necessity to place increasing numbers of pipelines in existing rights of way means that it is more important than ever to ensure that the installed pipeline does not deviate significantly from its planned path.

<CIT> discloses a target-directed (i.e., steerable) drilling rod <NUM> for use behind a drill bit in a drill string that includes an inner rotatably mounted shaft <NUM> and an outer fixed part <NUM>. The inner shaft <NUM> defines a flushing channel <NUM> therein which may be connected to a supply of flushing fluid. The outer fixed part <NUM> is provided with a plurality of control bars <NUM> that are pivotally mounted to the head <NUM> of drilling rod <NUM>. The control bars <NUM> may be actuated by hydraulic cylinders <NUM> and <NUM> provided on the upper part <NUM> of outer part <NUM> in response to signals produced by inclinometers <NUM>.

<CIT> discloses a system and method for deploying optical fibre in a well environment. A fibre optic line protection system <NUM> includes tubes <NUM> designed to protect one or more internal optical fibres <NUM>. The fibre optic line protection system <NUM> has an entry at one end and a turn around <NUM> at an opposite end to enable fluid flow between flow passages defined by tubes <NUM> to deploy the optic fibres <NUM> therein. The fibre optic line protection system <NUM> may also include one or more splices <NUM> to splice together individual sections of the line protection system <NUM> while maintaining the pressure integrity of the tubing <NUM>.

<CIT> discloses an apparatus and method for drilling and then installing a production conduit <NUM> within an enlarged borehole D along an underground path P between two surface locations C and D. Upon completion of a pilot bore hole B, a reamer <NUM> enlarges the pilot bore hole B. A drill string <NUM> is returned to the exit opening E for connection to the production conduit <NUM> for pulling the production conduit <NUM> within the enlarged opening D. In some embodiments, electronic survey equipment 93A may be provided within a pipe section 10D having a smooth outer surface <NUM>.

According to a first aspect of the invention there is provided a borehole mapping tool as set out in Claims <NUM> to <NUM>.

According to a second aspect of the invention there is provided a method of mapping a borehole as set out in Claims <NUM> to <NUM>.

Illustrative and presently preferred exemplary embodiments of the invention are shown in the drawings in which:.

One embodiment of a borehole mapping tool <NUM> is best seen in <FIG> and <FIG> and may comprise a probe housing or casing <NUM> having first and second ends <NUM> and <NUM>. Probe casing <NUM> is sized to receive one or more location probes <NUM>. The location probe(s) <NUM> are operable, either alone or in conjunction with other equipment and devices (not shown), to determine the location of the probe(s) <NUM> with respect to any convenient coordinate or location system. Borehole mapping tool <NUM> may also comprise an outer housing or casing <NUM>. The outer casing <NUM> may be mounted to the probe casing <NUM> so that an interior space or cavity <NUM> is defined between outer casing <NUM> and probe casing <NUM>. As will be described in greater detail below, outer casing <NUM> may be sized to be closely received by the borehole <NUM> to be mapped, as best seen in <FIG>.

Borehole mapping tool <NUM> may also comprise a first end cap <NUM> mounted to a first end <NUM> of outer casing <NUM>. First end cap <NUM> may be provided with an opening <NUM> therein that is sized to receive the probe casing <NUM>. The arrangement is such that the first end <NUM> of probe casing <NUM> extends beyond the first end cap <NUM>. Similarly, borehole mapping tool <NUM> may also comprise a second end cap <NUM> mounted to a second end <NUM> of outer casing <NUM>. Second end cap <NUM> may be provided with an opening <NUM> therein that is sized to receive the probe casing <NUM>, again so that the second end <NUM> of probe casing <NUM> extends beyond the second end cap <NUM>.

In some embodiments, borehole mapping tool <NUM> may be provided with one or more nozzles <NUM> that are fluidically connected to a supply of drilling fluid <NUM> (<FIG>). Nozzles <NUM> may be mounted to the first and second end caps <NUM> and <NUM>, although other arrangements are possible. Drilling fluid <NUM> discharged from the nozzles <NUM> helps to lubricate the borehole mapping tool <NUM> as it moves within borehole <NUM>, thereby reducing the forces required to move the borehole mapping tool <NUM> through borehole <NUM>. Drilling fluid <NUM> may also assist in the dislodgement and removal of any loose or partially-excavated material that may remain in borehole <NUM>. In one embodiment, the drilling fluid <NUM> may pumped through an interior conduit <NUM> defined by probe casing <NUM>. The various nozzles <NUM> may be fluidically connected to the interior conduit <NUM> so that pressurized drilling fluid <NUM> contained therein is conducted to nozzles <NUM>.

With reference now primarily to <FIG>, the borehole mapping tool <NUM> may be used as follows to map the location of the borehole <NUM>. Assuming that the borehole <NUM> is ready to receive the pipeline, i.e., that the pilot and reaming phases have been completed, the borehole mapping tool <NUM> may be positioned within a first end <NUM> of borehole <NUM>. Thereafter, borehole mapping tool <NUM> may be attached to a drill string <NUM>. At this point, the location probe(s) <NUM> provided within the borehole mapping tool <NUM> may be activated or otherwise energized so that they can determine the position of the borehole mapping tool <NUM> with respect to a suitable coordinate or location system. The borehole mapping tool <NUM> may then be moved through the borehole <NUM>, e.g., by pushing or pulling on the drill string <NUM>, while collecting and/or recording data from the location probe(s) <NUM>. In embodiments wherein the location probe(s) <NUM> include magnetometers, the borehole mapping tool <NUM> may be stopped periodically to take magnetic locating shots. Such magnetic locating shots may be used as a second verification of the actual location of the borehole <NUM> within the formation. The collected sensor data along with the secondary magnetic locating shots may then be used to produce a map of the borehole <NUM>.

If desired, one or more reamers (not shown) may be mounted to either or both of the first and second ends <NUM> and <NUM> of borehole mapping tool <NUM>. The use of such reamers may reduce the risk of borehole collapse or otherwise reduce the likelihood that the borehole mapping tool <NUM> will become stuck or jammed within borehole <NUM>. In some applications, it may be advantageous to connect together multiple borehole mapping tools <NUM>, <NUM>', and <NUM>" to create borehole mapping tool string <NUM>, as best seen in <FIG> and <FIG>. The borehole mapping tool string <NUM> may then be pushed or pulled through the borehole <NUM> in the manner described herein in order to map the location of the borehole <NUM>.

A significant advantage of the present invention is that it may be used to map the location of a completed borehole <NUM> to determine whether it accurately follows the planned or desired pathway. Significant deviations from the desired pathway may be detected and evaluated in advance of pipeline installation. If necessary or desirable, remedial measures may be taken to correct any significant deviations before the pipeline is installed. Besides ensuring that the installed pipeline will be located within an acceptable tolerance of the defined pathway, any deviations that would result in excessive deformations of the pipeline (e.g., resulting from a radius of curvature that is too small for the planned pipeline) also can be corrected, thereby significantly reducing the likelihood of subsequent in-service failures.

Still other advantages associated with the present invention include the ability to accurately map the centerline of the borehole <NUM>. Such accurate mapping is the result of sizing the outer casing <NUM> so that it is closely received by the borehole <NUM>. Because the location probe(s) <NUM> are located substantially along the centerline <NUM> of the borehole mapping tool <NUM>, the resulting position data will correspond with the centerline of the borehole <NUM>. No additional coordinate transformations or adjustments will be required.

Still other advantages are associated with the nozzles <NUM> that may be provided on the borehole mapping tool <NUM>. The provision of drilling fluid <NUM> to the nozzles <NUM> during the mapping operation will help to reduce the forces required to move the borehole mapping tool <NUM> through the borehole <NUM>. The drilling fluid <NUM> may also help to remove any remaining loose or partially-excavated material that may remain in the borehole <NUM>. If one or more reamers (not shown) are mounted to the borehole mapping tool <NUM>, the provision of drilling fluid <NUM> will also enhance the operation of the reamers, e.g., by providing lubrication, cooling, and removal of reamed material. If multiple borehole mapping tools <NUM>, <NUM>', and <NUM>" are connected together to form a string <NUM>, the resulting borehole map will generally be of increased accuracy. In addition, the use of a string <NUM> of multiple borehole mapping tools <NUM>, <NUM>', and <NUM>" will speed the mapping process in that fewer stops will be required to perform the magnetic survey shots. Of course, the use of multiple borehole mapping tools <NUM> also will provide system redundancy in the event one or more of the locating probes <NUM> fails or otherwise becomes inoperative during the mapping operation.

Having briefly described certain exemplary embodiments of systems and methods of the present invention, as well as some of its more significant features and advantages, various embodiments and variations of the present invention will now be described in detail. However, before proceeding the description, it should be noted that while various embodiments are shown and described herein as they could be used in a horizontal directional drilling operation to map the location of a reamed borehole in advance of pipeline installation, the present invention is not limited to use in such applications. For example, the methods and systems of the present invention could be used in any of a wide range of applications wherein it would be desirable to obtain a highly accurate map of an underground borehole. Consequently, the present invention should not be regarded as limited to use in any particular type of directional drilling operation, environment, or application.

Referring back now to <FIG> and <FIG>, one embodiment of the borehole mapping tool <NUM> may comprise an elongate, generally cylindrically-shaped structure defined primarily by probe casing <NUM>, outer casing <NUM>, and first and second end caps <NUM> and <NUM>. As will be described in further detail below, it is generally preferred, but not required, to configure the borehole mapping tool <NUM> so that it may be readily used with existing directional drilling equipment, such as drilling rigs, drill strings, and drilling fluid delivery systems.

In the particular embodiments shown and described herein, probe casing <NUM> may comprise a generally elongate, cylindrically-shaped member having a first end <NUM> and a second end <NUM>. Probe casing <NUM> is hollow and defines an interior conduit <NUM> of sufficient size to receive one or more location probes <NUM>. The location probes <NUM> may be mounted within the interior conduit <NUM> of probe casing <NUM> by means of one or more probe stabilizer members <NUM> so that the location probes <NUM> are located substantially along a central axis <NUM> of probe casing <NUM>. In most embodiments, the interior conduit <NUM> of probe casing <NUM> will be fluidically connected to a supply of drilling fluid <NUM> via drill string <NUM>.

In embodiments wherein the borehole mapping tool is configured to interface with a conventional drill string <NUM>, probe casing <NUM> may be configured so that the first and second ends <NUM> and <NUM> thereof can be readily connected to drill string <NUM>, e.g., by means of threaded connections. So configuring the probe casing <NUM> will also allow the borehole mapping tool <NUM> to be operatively connected to one or more reamers (not shown), which may be desirable in certain applications. In some embodiments, first end <NUM> of probe casing <NUM> may be provided with an orientation stub <NUM> to allow the borehole mapping tool to be connected to drill string <NUM>.

The overall dimensions (e.g., diameter and overall length) of the probe casing <NUM> may comprise any of a wide range of values depending on the particular application and type of drilling equipment to be used. Consequently, the present invention should not be regarded as limited to probe casings <NUM> having any particular size. However, by way of example, in one embodiment, probe casing <NUM> may have an outside diameter <NUM> of about <NUM> (about <NUM> inches) and inside diameter <NUM> of about <NUM> (about <NUM> inches). Probe casing <NUM> may have an overall length <NUM> of about <NUM> (about <NUM> feet).

Probe casing <NUM> may be fabricated from any of a wide range of materials, such as various metals and metal alloys, that are now known in the art or that may be developed in the future that are, or would be, suitable for the particular application. Consequently, the present invention should not be regarded as limited to any particular material. In embodiments wherein one or more of the location probes <NUM> utilize magnetometers, probe casing <NUM> should be fabricated from a non-magnetic material, such as non-magnetic stainless steel or Monel®. Monel is a registered trademark of the Huntington Alloys Corporation, Huntington, WV (US) for metal alloys containing nickel and copper.

As mentioned, location probes <NUM> may be mounted within the interior cavity <NUM> defined by probe casing <NUM> so that the location probes <NUM> are located substantially along the central axis <NUM> of probe casing <NUM>. By way of example, in one embodiment the location probes <NUM> may be mounted to probe casing <NUM> via a plurality of stabilizer members or 'spiders' <NUM>, as best seen in <FIG>. Location probe(s) <NUM> may also be mounted to a probe extender <NUM> to allow the location probe(s) <NUM> to be readily positioned at about the midpoint of probe casing <NUM>.

Location probes <NUM> may comprise any of a wide range of downhole location probes or measurement-while-drilling (MWD) probes that are now known in the art or that maybe developed in the future that are, or would be suitable, for mapping the location of the probe(s) <NUM>, and by extension borehole mapping tool <NUM>, as it moves within borehole <NUM>. Location probe(s) <NUM> of the type suitable for use with the present invention typically involve a combination of accelerometers and magnetometers to provide the location functionality. Alternatively, other devices are known and may be used as well. However, because such location probes are well-known in the art and could be readily provided by persons having ordinary skill in the art after having become familiar with the teachings of the present invention, the particular location probe(s) <NUM>, as well as any ancillary systems and devices that my be required for their operation, will not be described in further detail herein.

With reference now primarily to <FIG>, borehole mapping tool <NUM> may also comprise an outer casing <NUM>. In one embodiment, outer casing <NUM> may comprise an elongate, generally cylindrically-shaped member having a first end <NUM> and a second end <NUM>. The outside diameter <NUM> of outer casing <NUM> is selected so that outer casing <NUM> will be closely received by the final, reamed borehole <NUM>. Outer casing <NUM> may have an overall length <NUM> that is less than the overall length <NUM> of probe casing <NUM>. This will allow the first and second ends <NUM> and <NUM> of probe casing <NUM> to extend beyond the outer casing <NUM>, as best seen in <FIG>. By way of example, in one embodiment, the outer casing <NUM> may have an outside diameter <NUM> of about <NUM> (about <NUM> inches) and an overall length <NUM>, of about <NUM> (about <NUM> feet).

Before proceeding with the description, it should be noted that, as used herein, the term 'closely received' should be understood to encompass a range of clearances between the outside diameter <NUM> of outer casing <NUM> and the diameter of the reamed borehole <NUM>. The clearance should be sufficiently large so as to allow the borehole mapping tool <NUM> to move within the borehole <NUM> without a substantial likelihood that it will become stuck or jammed within the borehole <NUM>. On the other hand, the clearance should not be so large as to permit the borehole mapping tool <NUM> to move within the borehole <NUM> by an amount that would exceed the allowable positional tolerance for a particular application. Moreover, and because the present invention could be used to map boreholes <NUM> having diameters ranging from a few centimeters to a few meters, and because the boreholes <NUM> could extend though a wide range of formations having a wide range of characteristics, from hard, rocky formations to soft, sandy formations, the present invention should not be regarded as limited to any particular clearance between the borehole <NUM> and the borehole mapping tool <NUM>, expressed either as an absolute measurement or as a percentage or ratio between the diameters of the outer casing <NUM> and borehole <NUM>.

Outer casing <NUM> may be fabricated from any of a wide range of materials, such as metals and metal alloys, that are now known in the art or that may be developed in the future that are, or would be, suitable for the particular application. In embodiments wherein one or more of the location probes <NUM> utilize magnetometers, then outer casing <NUM> should be fabricated from a non-magnetic material, such as non-magnetic stainless steel or Monel®.

Outer casing <NUM> may be mounted to or secured to probe casing <NUM> by a plurality of stabilizers or 'spiders' <NUM> extending between probe casing <NUM> and outer casing <NUM>. See <FIG> and <FIG>. In the particular embodiments shown and described herein, each stabilizer <NUM> comprises a flat, generally plate-shaped member sized to extend between the two casing members <NUM> and <NUM>. The stabilizers <NUM> may be attached to the two casing members <NUM> and <NUM> by any convenient means, such as by welding. In the particular embodiment illustrated in <FIG> and <FIG>, four (<NUM>) stabilizers or spiders <NUM> are mounted around probe casing <NUM> at <NUM>° angles to one another. However, other embodiments may utilize a greater or lesser number of stabilizers <NUM>. For example, another embodiment may use three (<NUM>) stabilizers <NUM> mounted around probe casing <NUM> spaced about <NUM>° apart.

The various stabilizers <NUM> may be fabricated from any of a wide range of materials, such as metals and metal alloys, that are now known in the art or that may be developed in the future that are, or would be, suitable for the particular application. Here again, in embodiments wherein one or more of the location probes <NUM> utilize magnetometers, the various stabilizers <NUM> should be fabricated from non-magnetic materials, such as non-magnetic stainless steel or Monel®.

Borehole mapping tool <NUM> may also be provided with first and second end caps <NUM> and <NUM>. End caps <NUM> and <NUM> close off the interior space <NUM> defined between the probe casing <NUM> and outer casing <NUM>. End caps <NUM> and <NUM> also allow the borehole mapping tool <NUM> to more easily move through the borehole <NUM> during the mapping operation. With reference now primarily to <FIG> and <FIG>, first end cap <NUM> may be mounted to the first end <NUM> of outer casing <NUM>. First end cap <NUM> may be provided with an opening <NUM> therein that is sized to receive probe casing <NUM>. This will allow the first end <NUM> of probe casing <NUM> to extend beyond the first end cap <NUM>. Second end cap <NUM> may be mounted to the second end <NUM> of outer casing <NUM>. Second end cap <NUM> also may be provided with an opening <NUM> therein that is sized to receive the probe casing <NUM> so that the second end <NUM> of probe casing <NUM> extends beyond the second end cap <NUM>.

First and second end caps <NUM> and <NUM> may comprise any of a wide range of shapes, such as conical, ellipsoidal, or hemispherical, to allow the borehole mapping tool to more easily move through borehole <NUM>. By way of example, in one embodiment, the first and second end caps <NUM> and <NUM> are substantially hemispherical in shape.

First and second end caps <NUM> and <NUM> may be fabricated from any of a wide range of materials, such as metals and metal alloys, that are now known in the art or that may be developed in the future that are, or would be, suitable for the particular application. In embodiments wherein one or more of the location probes <NUM> utilize magnetometers, then first and second end caps <NUM> and <NUM> should be fabricated from non-magnetic materials, such as non-magnetic stainless steel or Monel®.

In many embodiments, the borehole mapping tool <NUM> may also be provided with one or more nozzles <NUM> that are fluidically connected to the supply of drilling fluid <NUM>. In the particular embodiments shown and described herein, four (<NUM>) individual nozzles <NUM> are mounted to each of the first and second end caps <NUM> and <NUM>, as best seen in <FIG> and <FIG>. Alternatively, the nozzles could be provided elsewhere on borehole mapping tool <NUM>. As mentioned earlier, the various nozzles <NUM> are fluidically connected to the supply of drilling fluid <NUM> (<FIG>). In embodiments wherein the drilling fluid <NUM> is supplied to the interior conduit <NUM> of probe casing <NUM>, the various nozzles <NUM> may be fluidically connected to the interior conduit <NUM> of probe casing <NUM> via the first and second end caps <NUM> and <NUM>. In such an embodiment, respective first and second isolation bulkheads <NUM> and <NUM> may be used to define respective first and second drilling fluid chambers <NUM> and <NUM> that are sealed or isolated from the interior space <NUM>. Suitable openings <NUM> provided in the probe casing <NUM> to allow drilling fluid <NUM> in the interior conduit <NUM> to pass into the first and second drilling fluid chambers <NUM> and <NUM>. Thereafter, the drilling fluid, which is under pressure, will be ejected from nozzles <NUM>.

Nozzles <NUM> may comprise any of a wide range of drilling fluid nozzles that are readily commercially available and could be easily provided by persons having ordinary skill in the art after having become familiar with the teachings provided herein. Consequently, the nozzles <NUM> that may be used in one embodiment will not be described in further detail herein.

Referring now primarily to <FIG>, the borehole mapping tool <NUM> may be used as follows to map the location of an underground borehole <NUM>. Once the borehole <NUM> is ready to receive the pipeline, i.e., once the pilot and reaming phases have been completed, the borehole mapping tool <NUM> may be positioned within first end <NUM> of borehole <NUM> and attached to a drill string <NUM>. The location probe(s) <NUM> provided within the borehole mapping tool <NUM> may then be energized or otherwise activated so that they can determine the position of the borehole mapping tool <NUM> with respect to the desired coordinate system. The borehole mapping tool <NUM> may then be moved through the borehole <NUM>, e.g., by pushing the drill string <NUM> in the direction of arrow <NUM>, while collecting and/or recording data from the location probe(s) <NUM>. In this regard it should be noted that the borehole mapping tool <NUM> may be either pushed or pulled through borehole <NUM>. In embodiments provided with drilling fluid nozzles <NUM>, drilling fluid <NUM> maybe pumped through drill string <NUM> and thence nozzles <NUM> to assist in the movement of tool <NUM> through borehole <NUM>. In embodiments wherein the location probes include magnetometers, the borehole mapping tool <NUM> maybe stopped periodically to take magnetic locating shots. Such magnetic locating shots maybe used as a second verification of the actual location of the borehole <NUM>. The collected sensor data along with the secondary magnetic locating shots may then be used to produce a map of the borehole <NUM> within the formation.

If desired, one or more reamers (not shown) may be mounted to either or both of the first and second ends <NUM> and <NUM> of borehole mapping tool <NUM>. Drilling fluid <NUM> maybe pumped through drill string <NUM> and nozzles <NUM> to assist the reamers. The use of such reamers may reduce the risk of borehole collapse or otherwise reduce the likelihood that the borehole mapping tool <NUM> will become stuck or jammed within borehole <NUM>.

In some applications, it may be advantageous to connect multiple borehole mapping tools <NUM>, <NUM>', and <NUM>" together to create borehole mapping tool string <NUM>, as best seen in <FIG> and <FIG>. The tool string <NUM> may then be pushed or pulled through the borehole <NUM>, e.g., in the direction indicated by arrow <NUM>, in the manner described herein in order to map the location of the borehole <NUM>. Drilling fluid <NUM> may be pumped through drill string <NUM> to assist in the movement of the tool string <NUM> through borehole <NUM>. If desired, one or more reamers (not shown) may also be attached to tool string <NUM> to further assist the movement of the tool string <NUM> through borehole <NUM> during the mapping operation.

Claim 1:
A borehole mapping tool (<NUM>) for mapping a location of a borehole (<NUM>), comprising:
a probe casing (<NUM>) having first and second ends (<NUM>, <NUM>), said probe casing (<NUM>) defining an interior conduit (<NUM>) therein;
a location probe (<NUM>) mounted within the interior conduit (<NUM>) defined by said probe casing (<NUM>);
an outer casing (<NUM>) having first and second ends (<NUM>, <NUM>), said outer casing (<NUM>) surrounding said probe casing (<NUM>) so that an interior space (<NUM>) is defined between said outer casing (<NUM>) and said probe casing (<NUM>), said outer casing (<NUM>) being sized to be closely received by the borehole (<NUM>);
a first end cap (<NUM>) mounted to the first end (<NUM>) of said outer casing (<NUM>), said first end cap (<NUM>) defining an opening (<NUM>) therein that is sized to receive the first end (<NUM>) of the probe casing (<NUM>) so that the first end (<NUM>) of said probe casing (<NUM>) extends beyond said first end cap (<NUM>); and
a second end cap (<NUM>) mounted to the second end (<NUM>) of said outer casing (<NUM>), said second end cap (<NUM>) defining an opening (<NUM>) therein that is sized to receive the second end (<NUM>) of the probe casing (<NUM>) so that the second end (<NUM>) of said probe casing (<NUM>) extends beyond said second end cap (<NUM>).