Test-rig has loads of potential

DRILLSTRING failures, the bane of underground and surface drillers everywhere, may one day be less of a problem. Sarah Belfield writes

A humble-looking piece of research equipment, designed in recent years to mimic the stresses that a drill hole dog-leg places on rotary drilling rods, may prove useful in helping the drilling industry to whittle down its drillstring failure rate.

But for that to happen, drill-rod manufacturers would probably need to take the machine under their wings, according to University of Queensland mechanical engineering researcher Martin Veidt, because he has no immediate plans to use it.

The machine, which effectively destroys the drilling rods under study, is a spin-off of Veidt’s main research interest — the non-destructive testing of materials using techniques such as ultrasonics.

Veidt used the machine to non-destructively analyse what was happening to the drilling rods while they were being fatigued to the point of partial or complete failure.

He can see benefits in bringing non-destructive testing to the drilling industry.

For instance, to help prevent drillstring failures, drilling rods that have been used in an operation should be checked for any damage before they are re-used.

“At the moment, when these drill pipes come out, the inspection is very limited I would say.”

According to Veidt, England and the US are leaders in using non-destructive inspection techniques to check oil- and gas-carrying pipelines, and pipes in refineries and large ships.

In Europe, prior to his work in Australia, Veidt says he was involved in research that applied non-destructive techniques to inspect newly manufactured reinforced steel bars for defects.

He says the program was “very successful”.

“The geometry of drill pipes is a little bit different, but the concept is exactly the same, and I was hoping that we could make a similar type of development for the drilling industry; we’re still working on that.”

He says that when this work began, he needed flawed drill rods to help calibrate the inspection techniques.

In particular, he needed samples of drilling rods damaged by fatigue cracking, the phenomenon understood to be the most common reason for drillstring failures.

And so the machine in question, the four-point bending test-rig, was born.

The test-rig

The test-rig’s name comes from the four points at which two joined drilling rods are supported.

The two innermost supports are linked to the same bending device. It means that when the device is activated, a constant bending load is applied along the 60 cm drill rod section lying between them.

In turn, the constant loading means no matter where the drilling rods’ join is located, as long as it is within the 60 cm section it will receive the correct bending stress during an experiment. Fatigue cracking in drilling rods can therefore be studied under controlled conditions that replicate the bending stresses induced by dog-legs, or deviations, in a drill hole’s trajectory.

A few years ago, Veidt and Masters student Alex Berezovski used the four-point bending test rig to look at how fatigue cracks initiate and how they can ultimately lead to drilling rod breakages. They looked at NQ sized drill rods.

They found that when rotating drilling rods experience bending loads, stress is concentrated at joins because of their structural complexity there.

Veidt says simulations by other researchers indicated that a join can involve stresses around three times higher than levels seen elsewhere along the rods.

The male half of a join — which features external screw-threading — bears the brunt of the stresses.

In each of 18 run-throughs they did, Veidt and Berezovski found the initial fatigue crack sets itself up in the thread loop nearest the join’s hilt.

The crack then propagates at each end, travelling around the rod’s circumference in both directions by following the helical pattern of the threading. In cases of complete drill-rod failure, the final stage of fatigue cracking involves a final, quick tearing off, which leaves a square tag of metal behind.

Potential uses

Veidt and Berezovski’s experimental results for how NQ drilling rods withstood bending loads give an indication of the possible uses for the test-rig.

The pair found that relatively moderate maximum bending stresses (of around 100 megapascals) prompted NQ rod joins to consistently fail before attaining 300,000 cycles. (One cycle is equivalent to a full rotation of a drilling rod that is under a bending load.)

Data points obtained this way can be compiled to produce S-N curves for any given drilling rod type, Veidt says. Such curves plot stress (S) as a function of the number of cycles (N).

A given stress level can be converted into a corresponding dog-leg severity number.

If enough data were to be collected for a wide variety of rod sizes and manufacturers, then perhaps drillers could draw upon the results to optimise the performance of the rods they use. If drillers know a certain drilling rod doesn’t perform well in more extreme dog-legs, then they can make informed decisions about drilling plans and materials.

What Veidt says may be easier to achieve initially is using a bending test-rig to run comparative product testing — to help in designing drilling rods that have better fatigue resistances.

For instance, drill-rod manufacturers may want to compare a new thread design with thread designs already in use.

Or they may have a new type of heat treatment process and therefore want to know if it really is an improvement over the way things are currently done.

He says even if manufacturers largely use numerical simulations for this kind of design work, there’s likely to be a benefit in carrying out some real-world testing using something like his four-point bending test rig to confirm that computer-modelled improvements are in fact seen in the real world.

According to Veidt, it would be possible to adapt the test-rig’s design to make it simpler or cheaper to build.

For instance, the need for very heavy bearings could be avoided by making the rig slightly longer than the original length of about 1.5 m. This adaptation would mean a given bending load on drill rods can be achieved by a smaller applied bending force.

While Veidt may have no current need for his test-rig, he appears excited about the prospects for the non-destructive inspections of drill rods.

He says if the new drill rod materials currently being developed make their way onto the market — such as fibre composites — then there will be a need to check that these rods are being manufactured defect-free.

Martin Veidt

University of Queensland


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