By Colin Rice
The quillrod is an irreplaceable part of a drilling operation and so it is very important that the causes of quillrod failure are clearly understood and that appropriate management procedures are instituted to ensure that quillrod failures do not occur.
This is the second 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.
A few years ago, I was asked to assist in investigating the occurrence of 5 separate quillrod failures in a single month on a drilling project. In two cases the failures resulted in serious injuries - the remaining 3 miraculously did not. We instituted a simple quillrod management procedure and the problem disappeared. However, it re-emerged about a year later when the management procedures were not followed.
The quillrod represents a significant hazard particularly in a diamond core drilling operation where fatigue can build up at a rapid rate due to the high rotational speeds used in core drilling operations. Rotary percussion and dual-tube reverse circulation drillstrings are rotated at very much slower rotational speeds than a diamond coring drillstring and the wall thickness of the threaded ends is substantially greater than is the case in wireline drill rods and so catastrophic fatigue failure is less likely in these situations. The following discussion is therefore pretty much focussed on quillrods in a diamond core drilling operations.
In normal use the quillrod is not subject to tensile, compressive or bending loads. The quillrod is subject to axial loading only when tripping the drillstring – when tripping out of the hole, the quillrod is subjected to maximum tensile load at the start of the operation and when tripping into the hole at the end of the operation. It is highly unlikely therefore that a quillrod failure under tension will occur and for the purposes of this discussion can be ignored.
A diamond drill quillrod rotates “in the open” - there is no borehole wall to dampen eccentric rotation or vibration and so the amplitude of the cyclic stress reversals experienced by the quillrod can be significantly greater than in the rest of the drillstring. The weakest parts of the drillstring are the threaded connections and so fatigue failure of the exposed pin connection can occur particularly if connections in the quillrod are not correctly pre-torqued. We therefore have to manage the risk of fatigue failures of exposed connections in the quill rod. Mid-body failure should not occur in a drill rod but it can occur through notch fatigue if the appropriate management procedures are not followed.
In most modern hydraulic drills, connections are made-up and broken-out using the synchronised functionality of the chuck and rod clamp or using the rotation head and rod clamp. The quillrod is therefore subjected to the action of the jaws of the hydraulic chuck and rod clamp and if the clamp and / or chuck are poorly maintained, the jaws can slip or not fully open and cause deformation of the outer surface of the drill rod.
The deformations shown in Figure 1 are particularly severe longitudinal grooves scored into the drill rod due to the jaws of the rod clamp not fully opening. These longitudinal grooves are not necessarily an issue, however, if there is simultaneous rotation then a circumferential groove can begin to form and if left unchecked these grooves can become more and more pronounced as shown in Figures 2 and 3. Figure 4 shows an unbelievably severe series of circumferential grooves scored into a quillrod.
These circumferential grooves (or notches) become areas of increased stress and can result in very rapid fatigue failure of the mid-body. We previously discussed how to break a piece of wire when we have no tools to cut it – we merely repeatedly bend it – we create cyclic stress reversals and the wire breaks through fatigue. If we put a small notch into the wire however, it will break after fewer stress reversals - this is exactly the same phenomenon in the quillrod and so requires very careful management.
In the next article in this series, we look at options for quillrod management.