By Colin Rice
Pro-active detection of potential drill rod failure should be a key element of day-to-day drill site safety management, however, quillrod management is complicated by the different procedures used by different contractors.
This is the third article of Part 3 of our Technical Series on Drill Rod Safety. Click here for an outline of the entire Technical Series on Drill Rod Safety.
Several different types of quillrod are commonly used in diamond core drilling operations, in the past, contractors using mechanical spindle drills used BW or NW conventional drill rods as quills. The mid-body and the threaded connections of these drill rods are relatively heavy walled and so are capable of managing high levels of cyclic stress loading. In recent years however, with the rapid change to hydraulic longstroke and top drive drills, contractors now use 3-meter lengths of standard wireline (“Q”, “T”, “MO” or “RQ”) drill rod as quills.
We must recognise that the wall thickness of any of the standard wireline drill rod irrespective of manufacturer, is exactly the same and so the issues with fatigue at the threaded ends remain even in heat treated drill rods. Similarly, issues around notch fatigue remain irrespective of the manufacturer of the drill rods.
Pro-active detection of potential drill rod failure should therefore be a key element of day to day drill site safety management. Quillrod management however is complicated by the different procedures used by different contractors.
When using a hydraulic longstroke drill, there are two procedures commonly used. In the first procedure; at the end of a corebarrel run, the drillstring is pulled off bottom, the quillrod is backed-off and moved to the side. A drill rod is then added to the drillstring, the drillstring lowered to bottom and the quillrod made up onto the drillstring. In this way, the same drill rods remain as the quillrod.
In the second procedure; at the end of the corebarrel run the waterswivel is removed from the quillrod and added to the “new” quillrod which is then added to the drillstring. In this way, what was the quillrod becomes a part of the drillstring and effectively a “new” quillrod is used for every corebarrel run.
In the first scenario, it is relatively easy to manage the quillrod because the same drill rods remain. In the second scenario however, the quillrod could be made up of very old drill rod / s with high levels of fatigue. Obviously, this represents a much more significant hazard that scenario 1.
When using a top-drive drill, it is common that a new drill rod is added to the drillstring after every corebarrel run and so effectively this procedure is the same as scenario 2 above.
Irrespective of the procedure used by the contractor, the following controls are recommended to mitigate the risk associated with quillrods:
1. If using standard wireline drill rods to make up the quillrod:
a. Only brand new drill rods must be used to make up a quillrod and a protocol to replace the quillrod at regular intervals must be introduced.
We know that fatigue is cumulative and that it is impossible to determine the accumulated level of fatigue in a drill rod, we cannot therefore use previously used drill rods as quill rods. In drill site safety standards that I have written for a number of mining companies, we have stipulated that the quillrod is changed on a monthly basis. This can be done as the last operation at the end of the month before off-time and it can be signed off by the Supervisor in the last daily report for the month. This provides the necessary documentation to certify that the procedure has been followed. The cost of this to the contractor is minimal because the “old” quill can be built into the normal drillstring.
b. Threaded connections must be correctly pre-torqued as per the manufacturers’ specifications.
c. A saver-sub should be used on the bottom-most connection to protect the pin thread on the bottom-most drill rod.
d. Any there are any indications of deformation of the surface of the quill rod, the source of the deformation must be found and eliminated and if there is any evidence of circumferential notching, the quill rod must be changed out with brand new drill rods as per point 1 above.
Chuck or rod clamp slippage will normally be noticed by longitudinal scoring on the quill rod. Longitudinal marks are not by themselves an issue however, if the drill can make longitudinal grooves, it just as easily make circumferential grooves and so they are an indication of a problem with the chuck or rod clamp and the source of the scoring must be found and rectified. In other words, the chuck and / or rod clamp must be repaired.
I am frequently asked “how deep must the circumferential groves be before the contractor is instructed to change the quill rod?”. It is not possible to answer this question and it would be foolish to try to answer it; a 0,5mm deep groove in a quill rod that has done 22 million rotations may be more or less severe than a 1mm groove in a drill rod that has done 1 million rotations – it all depends upon the degree of accumulated fatigue in the drill rod and, as we have already discussed, this is impossible to measure.
My response therefore is; a correctly set and properly maintained chuck or clamp should not deform the quill rod and so if it does then, there is something wrong and the equipment must be repaired. Therefore, if you can see longitudinal marks or grooves on the quill rod the operation must be stopped and the cause of the deformation identified and rectified. If you can see circumferential grooves the quillrod must be changed. If the contractor has properly repaired the offending chuck or clamp the problem will disappear and everyone will be happy! If the chuck or clamp has not been correctly repaired then the problem will reoccur and further attempts to rectify the problem must be made until the problem is solved.
2. If a 6 metre drill rod is used with a top drive drill then there will always be an exposed connection in the mast and if drilling in compression, this joint may be subjected to large amplitude stress reversals. However, because the quillrod is made up into the rotation head, it is very unlikley that it will fall in an uncontrolled way should the joint break.
If, drilling with a 3 metre corebarrel, this issue disappears because there will not be an exposed joint in the quillrod.
We must not be complacent about the risk with top drive drills – refer to the Hazard Alert section where we have summarized the findings of a fatal accident involving a top drive drill.
3. If procedure 1 is used, it is possible to use a heavy duty (CHD or NDBR) drill rod in preference to standard wireline drill rod as a quillrod. The heavier wall thickness of these heavy duty drill rods will be more tolerant of cycylic stresses than a standard wireline drill rod and so a less frequent change out strategy can be applied, perhaps change the quillrod every 3 months instead of every month. The mid-bodies of these heavy duty drill rods is exactly the same as for standard “N” series wireline drill rods and so the surface deformation and notching issues are equally applicable.
4. On hydraulic longstroke and mechanical spindle drills, the hoist rope must be connected to the waterswivel at all times to prevent a failed quillrod from falling in an uncontrolled way.
In the next article in this series, we explore the care and maintenance of wireline drill rods.