By Colin Rice
The consequences of a failed drill rod will differ depending upon the design of the drill rod and the drilling method being used. In diamond drilling operations, where the drillstring is rotated at extremely high rotational speeds, the consequences can be catastrophic particularly if the drill rod that fails is a quillrod. A very good understanding of the modes of failure of drill rods is therefore very important so that we can take the necessary steps to reduce or eliminate the risk of a catastrophic drill rod failure.
This is the first 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.
Previous parts of the Drill Rod Safety Series, dealt with the "Manufacture and mechanical properties of drill rod materials" and the "Depth rating of drill rods and thread design". In this article we discuss a very important aspect of the use of drill rods – why drill rods break.
modes of Drill rod and drill pipe failure
Drill rods typically fail through one of three mechanisms:
Failure under tension
The first part of this Technical Series explained that drill rods are manufactured from steels of different grades and so have different mechanical properties. We also saw that it is possible to model the depth at which a particular grade of wireline drill rod will fail under its own weight based on the mechanical properties of the steel.
We know that the weakest parts (the zone of greatest stress) of a wireline drillstring are the threaded connections and so we expect a standard wireline drill rod to fail under tensile load at the threaded connection that is under the greatest stress.
Heavy duty wireline drill rods have friction welded tool joints which are much thicker walled than a standard wireline drill rod and so these connections are substantially stronger than standard drill rods.
Rotary percussion and dual-tube reverse circulation drillpipe is either butt welded or friction welded and, like heavy duty wireline rod, the heavy wall thickness of the tool joints makes the connections particularly strong. Under tensile load therefore, we would expect failure at the weld rather than in the mid-body of the pipe.
Failure under torsion
Failure under pure torsion can also occur and is particularly common in oil and gas operations where very significant torque is applied to the drillstring – these failures are commonly called “twist-offs”. Failure under pure torsion is not common in diamond drill rods unless the quality of machining of the threaded connections is very poor.
It is important to bear in mind that a drill rod thread under torsion will fail under a lower tensile load than a drill rod thread under pure tensile load.
Failure due to fatigue
In the first part of this Technical Series, we discussed the mechanical properties of drill rod materials, one of the key properties that we discussed was the “fatigue resistance” of the material and we defined fatigue resistance as the resistance to failure due to continuous cyclic stress reversals.
Fatigue failure is a very common phenomenon and affects every metallic component or structure to a greater or lesser degree even extremely large structures like ships. The image to the right shows a ship, effectively split in two due to a fatigue crack in its hull.
Fatigue failure of wireline drill rod occurs frequently and so an understanding of the mechanism of fatigue failure is important.
When a drill rod rotates through a bend, it suffers cyclic stress reversals and the more severe the bend, the greater the amplitude of the stress reversal and so the more rapidly the drill rod will begin to fatigue. This concept is extremely important in drilling operations – particularly in core drilling operations where the drillstring rotates at very high rotational speeds.
Fatigue damage will occur at a number of highly stressed points in the drillstring and as the fatigue increases, tiny stress cracks will begin to form and they will propagate as the stress level increases or as the number of stress reversals increase until the drill rod fails. The areas of greatest stress in a wireline drill rod are at the areas of smallest cross-sectional area and these occur at the threaded ends of the rod. Fatigue failure therefore typically occurs at the threaded ends particularly at the pin thread.
It is important to note that every rotation of a drill rod results in two reversals of stress, this is illustrated in figure below. When bent, the inside surface of the drill rod is under compression and the outer surface is under tension. Once the drill rod has rotated through half a revolution, the inside surface will have moved to the outside and so will now be under tension. Similarly, the outside surface will have moved to the inside and so will be under compression. Half a revolution has therefore resulted in a cyclic stress reversal and so a full rotation will result in two stress reversals.
Consider a drill rod rotating at 800 rpm for 9 hours per shift on a 2-shifts per day basis. We can calculate the total number of revolutions and therefore the number of stress reversals as follows:
Total number of rotations = 800 x 60 x 9 x 2
Total number of rotations = 864 000 rotations per day
And, if we assume that we drill for 25 days per month, then on a monthly basis:
Total number of rotations = 864 000 x 25 rotations per month
Total number of rotations = 21,6 Million rotations per month
Since each rotation causes 2 cyclic stress reversals, we can expect the drill rod to suffer nearly 43 million stress reversals per month! Fatigue can therefore build up very rapidly in rotating diamond drill rod particularly if the amplitude of the stress reversals are large.
It is very important to recognise that fatigue is progressive and cumulative – it causes irreversible structural damage. Every drill rod will have some level of accumulated fatigue but it is impossible to determine the degree of fatigue in the drill rod unless a crack has begun to develop and is detected through some form of inspection. It is therefore impossible to estimate the remaining life of a drill rod based on its rotating hours or metres drilled.
In the next article in this series, we look at quill rod failures.