The frictional heating is the result of ‘rubbing’ contact or ‘drag’ against a mating surface. Friction heat can build to exceed the transformation temperature of the steel (or ‘austenite transformation start temperature’, ~750degC/1350degF), followed by rapid cooling from surrounding steel or cooling fluid, hardening, and embrittlement. When this cycle repeats frequently, the heating and cooling create rapid expansion and contraction which leads to fatigue failure, seen as perpendicular cracks that propagate from the surface.  

Figure 1 – Oil & Gas Tool Joint with heat check hardening and cracking

While this phenomenon is well documented in engineering texts, the problem has been prominent in the oil and gas exploration industry since the 1940’s. The API (American Petroleum Institute) describes heat check cracking as, “Formation of surface cracks formed by the rapid heating and cooling of the component” (API ‘RP 7G-2, Recommended Practice for Inspection and Classification of Used Drill Stem Elements’, and ‘RP 96, Deepwater Well Design and Construction’.)

A 1992 IADC/SPE paper on heat check cracking described full-scale simulations to prove that the heating and hardening are easily achieved, but that fatigue cracking only results from the rapid heating and cooling associated with each rotation of the drill string.

As shown in these pictures of wireline coring drill rods, heat check cracking can actually be easily identified, visually in the field. Heat check cracks are unique in that they follow the axis of the rod (longitudinal or ‘axial’ orientation), and are located near the female or box end shoulder, and are associated with a shiny, polished wear area. This section of the box always protrudes slightly more than any other area on a wireline drill rod and grows or ‘bulges’ under high drilling loads.

‘Box bulging’ is the result of a) the interference fit between the pin and the box (which is responsible for keeping the joint closed under deceleration ) and, b) the compression of the box shoulder under torsion and any radial loading from the thread-form. The positive load flank angles of traditional thread-forms, such as Q threads, generate radial load components that can increase box bulging to the point of separation, whereas premium thread forms with reverse-angle load-flanks, such as RQ and XQ, actually limit box bulging.

Since heat check cracks are the result of fatigue loading, they always start from the surface and form perpendicular to the direction of the expansion and contraction which is longitudinal or axial on a drill rod and easily pass across turns of thread. Fatigue failures that result from excessive drilling loads or excessive deviation always produce cracks that start internally and form perpendicular to the axis, or circumferentially, and typically follow the thread. In other words, since there are no drilling loads that act circumferentially, the only way to form longitudinal or axial cracks is through rapid heating and cooling.

Also, consider that the fatigue strength of any steel is less than only 50% of its normal ‘yield strength’ and that hardened steel is much more brittle than tempered steel. When subjected to excessive fatigue loading, the maximum expected life of any steel is less than three million cycles of alternating load. In terms of a spinning wireline drill rod, this represents less than a few days operation—at most. This same material limitation is behind fatigue failures in drill rod joints when subjected to excessive deviation (refer to our earlier article, “Drill Rod Bending Capabilities and deviated hole applications).

These limitations hold true for all wireline drilling industry carbon and alloy steel grades, all tube forming processes, all parallel and variable wall tubing, and all heat treatment configurations. This is because the transformation temperature is determined by carbon content, and does not worsen significantly without abnormally high levels of carbon (for example, the grade AISI/SAE 1541 has excessive carbon and a reduced transformation temperature, which was a common grade before heat treatment became popular). Also, whether a drill rod was initially heat treated or not, steel will transform when frictionally heated above its critical temperature, regardless of previous material properties.

Figure 2 – Various wireline coring drill rod samples with heat check hardening and cracking

Further, these material limitations hold true regardless of whether a drill rod is new or used, i.e. transformation properties do not change with use.  Operators who suffer from heat check cracking often claim to have older rods that did not crack, which is just a coincidence. That is to say, whether or not a heat check cracking incident occurs depends on a difference in the loading – possibly in the same hole – rather than any difference in the drill rods.

In summary, ‘heat check cracking’ is the result of an application issue where there is heat generated by a) lateral contact pressure and, b) insufficient lubrication, leading to fatigue failure.

Lateral contact pressure, or ‘drag’, can obviously be created with a misaligned drill rig or when the drill rod string passes through a deviation, but more typically is associated with drill string buckling, due to excessive drilling torque, rotational speed, and thrust or ‘weight on bit’. A reduction in any of these operating parameters will reduce the lateral contact pressure. However, in some cases drill rod midbodies may have been permanently twisted or ‘bent’, due to dynamic overload or due to roller rod handlers, which adds to lateral contact pressure. Always minimize fatigue and lateral loading by limiting hole deviation “dog-legs” (total deviation in dip and azimuth) to less than 1 degree per rod length, monitoring with tight hole survey intervals, and corrective reaming operations (limit to less than 0.6deg per rod in larger diameter holes). Avoid dynamic loading and rod twisting by maximizing spacing between multiple deviations and by eliminating neighboring deviations with opposing or divergent directions. Alternatively, select a free-cutting drill bit which will allow for reduction in torque and thrust. Use a Boart Longyear shoulder thickness wear gauge to inspect for un-even wear around the box shoulder circumference, which can provide an early warning to adjust operating parameters or improving lubrication.

It is helpful to apply and maintain an appropriate lubricant or grease coating on the outer surface of the drill string which can directly reduce the friction factor and the heat generated. During rod pulls, visually inspect how the coating has worn away as a good indicator of the degree of lateral contact. ‘Shiny one-sided’ wear patterns, either at the box ends or with a ‘slow’ one-quarter spiral turn over the rod length, indicate excessive loading.

While drilling fluids contribute to heat check hardening by quenching heated surfaces, maintain a high fluid pressure and consider polymer additives to improve lubrication, and reduce frictional heating and quench severity.

Finally, consider this recommendation from the IADC/SPE, “When users are confronted with the cause of failure, the initial reaction is often disbelief…It is difficult to believe that tool joints can be heated above their critical temperature while drilling…in the presence of mud circulation. From time to time, knowledge of the in-hole friction heating and cyclic quenching phenomena and characteristic longitudinal cracks need to be reintroduced to the field. Users and inspectors need to recognize the evidence and cull out the affected joints.”

Chris Lambert has a bachelor’s degree in mechanical engineering and has been with Boart Longyear since 2008. He has been the company’s global product manager for coring tools, including rods, in-hole tools and diamond bits, since 2012. Prior to that, he was the engineering manager for diamond products. His 20-year engineering and product development career has included materials, hydraulics, pumps and diesel engines.

Our new host, Jon Peterson (Geo Jon for fun), works for the Products division as a Customer Service Representative. Jon graduated from the University of Utah in 2015 with his Bachelors of Science in Geology. He’s worked for Boart Longyear for the last two and a half years and interned as a geologist with an exploration company for about six months prior to coming to work for Boart Longyear. Jon has been married for a year and a half and in his free time enjoys trail running, hiking, working on his yard, spending time with family, and playing with his black lab Dexter. 

Learn more about the Longyear bits.

We would love to hear your questions and comments below. Thanks for listening and if you liked this episode, share it on LinkedIn, Facebook, or Twitter. 

This article originally appeared in GeoDrilling International, October 2018 edition, pages 16-17.

The MDR500 was the long-term brain child of Geoff Moroney (Drilling Services Asia Pacific Fleet & Asset Manager) and John Kirkwood (Drilling Services Asia Pacific Regional Director). The project was given the go ahead in the third quarter of 2016 after market research indicated that there was a desire from sections of the Australian market for a medium depth underground diamond drill rig which was in the most part self-sufficient and could move quickly from hole to hole. The concept design kicked off at the start of December 2016 and the first completed rig rolled out of the workshop in early December 2017.

The basis of the idea was to convert the existing MDR150s in the Australian fleet into MDR500s. The original MDR150 was designed and built by Geoff Moroney and Nathan Kaesler (now Global R&D Engineering Manager) nearly 10 years earlier, back when Geoff was working on tooling and Nathan was a fresh, keen, new engineer at Boart Longyear. The MDR150 consists of a 75kw power pack and a nimble 400 series feed frame mounted to the telescoping boom of a very solid Normet carrier, finished off with 144m of on board rod storage. It is perfect for quick, short holes on sites where the manual handling of rods into the drill string is allowed. The MDR500 meets the increasing demand within the market for a mobile rig for deeper holes combined with the ever increasing push to reduce manual handling. That combined with the MDR150’s being almost 10 years old, most in need of a major rebuild and with several key control system components outdated, it made sense to convert these MDR150’s into new MDR500s.

With the project coinciding with the first prototype tests of the upcoming LMi underground power pack platform it was an easy decision for the design group to choose the new 110kW LMi design to power and control the drill that consists of a Boart Longyear HQ high speed drilling head mounted on a 700 series feed frame and utilizing a standard underground rod handler. After the basic concept was decided upon, the project became somewhat of a complex packaging task for the Asia Pacific Drilling Services Engineering team. Finding ways to shoehorn all of the LMi components along with the 500m of rods onto the carrier and then design an arm that could hold the much heavier drill out the front of the rig while still allowing a full 360 degree horizontal and plus 90 to negative 90 degree drilling angles to be achieved was no easy task.

An innovative concept from Stewart Jones allowing for the front rod bin to sit over the front axle during transit and then tilt into a vertical configuration on either side of the rig during use allowing for ergonomic rod handling was an additional bonus along the way. The project was certainly not without its challenges with some strengthening of the Normet chassis and a larger front axle being required to take the substantially heavier load now carried by the front of the machine.

The tight project timeline meant that the design and build had to happen concurrently and the team in the workshop, under the direction of the lead fitter on the project, Darren Watson, did a superb job of building and assembling the first rig despite only having half-finished designs and partial drawings in the beginning. Designing the hosing runs for the MDR500 was a mammoth task given the amount of movement required from the front arm and the workshop team were pivotal in the design and build of this, much of it being designed in the workshop at the rig.

As the heavy lifting on behalf of the mechanical team was nearing completion the design baton had well and truly already been passed over to the then products engineering electrical and control systems duo of Bernie Chia and Glen Verrall to make this rig come to life with all control of both the setup functions and the drill handled through either the touch screen interface in the cab of the carrier or from the LMi electronic control panel. Glen worked his magic to weave the additional MDR functions seamlessly into a modified version of the LMi software.

Throughout the project the engineering team was in close contact with the end users in the field, chiefly Matt Barnes who leads the operational team at the MMG Dugald River Mine in Queensland where the first two MDR500s were deployed. This collaboration occurred through every phase of the project starting with a site trip by the engineering project team to scope out the requirements of the rig with Matt and his team to ensure that the end result would be just what was required. The team’s involvement continued throughout with Matt being an invaluable subject matter expert and also a key participant in the design reviews and risk assessments. The project was closed out with an engineering site visit in early January 2018 and then again in March coinciding with the operational deployment of each of the MDR500s to Dugald River. The primary purpose of these visits being to train the operators on the features of the new rig and to be at hand to fix any issues immediately should they arise.  Luckily there were no significant issues and Glen was able to use the trips as a valuable source of driller feedback for further LMi software development.

The first MDR500 arrived on site and after going through its routine site clearance inspections basically went straight to work drilling its first hole without issue. This was in no small part due to the significant contributions from the operations team throughout the project and the quick adoption of the new rig by the drillers. The drillers at Dugald River were quick to pick up the MDR500’s new LMi control system and their level of acceptance of the interface has been very pleasing. The Dugald River team were quickly outperforming their meter rate achieved with the previous MDR150s and are now averaging an impressive 2 additional meters per hour.

A third MDR500 is nearing completion in the Adelaide Workshop and when complete, will be used as a carrot for attracting new work with several potential sites currently being discussed. If the market demand remains strong the intention is to steadily convert the aging fleet of 8 remaining MDR150s into MDR500s over the next few years with the possibility of sourcing additional carriers to make even more if required.

The MDR500 has and is continuing to be a success in the field and a key point of interest for the Asia Pacific Drilling Services team when negotiating underground drilling contracts. The success of the project was obviously the result of a lot of hard work, but also the excellent teamwork and collaborative effort exhibited by the various Boart Longyear functional groups involved, with many more people than could be mentioned here playing an important part in the project.

Currently employing nearly 40 engineers to design solutions for the resources drilling industry, Boart Longyear’s engineering team might be one of the company’s best kept secrets, although not on purpose.

It’s not a common business model, where a contract drilling services company also sells drilling products. But Boart Longyear does exactly that—the company provides a variety of professional drilling services to exploration and mining companies; it also designs, manufactures, and sells reliable drilling rigs, quality tooling, and accurate instrumentation.

Thanks to its team of engineers located throughout the world, the company is able to develop products and put them through rigorous testing in the hands of its own Drilling Services personnel. Tools and equipment are not only tested in the lab but also in the field. The collaboration between members of the global engineering team and Boart Longyear’s drillers elevates the performance of both Boart Longyear Drilling Products and Boart Longyear Drilling Services.

Some engineers are dedicated to Drilling Services projects – focused on developing solutions and safety innovations specifically for our contract Drilling Services teams. Other engineers are dedicated to developing solutions and safety innovations for commercialization for the Drilling Products division that then sells these products to the market. However, subject matter experts and synergies are leveraged across divisions for maximum potential as well.

Drilling Services brings technical issues or suggestions for improvements to engineers, who then research, design, and develop solutions. Solutions are also suggested by engineering –using the latest technologies, sometimes on problems that Drilling Services didn’t know they had. Engineered solutions are tested, tweaked to perfection, produced by our manufacturing team and implemented by Drilling Services.

The Drilling Products division focuses on designing solutions that help drilling contractors achieve more with less, safely. Boart Longyear drilling products are known for being high-quality products with proven results. The Products business works closely with Drilling Services to test new products before going to market. Products designs new bits, coring rods, in-hole tools, and production drilling tools and then uses Drilling Services to test, measure results for validation, and receive feedback. The engineering team generates valuable proprietary technologies which are instrumental to the global products business and protects those innovations with patents or maintains them as trade secrets.

This partnership between Boart Longyear engineers and Drilling Services benefits everyone in the industry as many of the solutions developed are also considered for commercialization. Boart Longyear Drilling Services uses the highest-quality, safest and most innovative equipment and tooling available, developed by Boart Longyear Drilling Products. As a result of the unique Boart Longyear business model and the collaboration with Drilling Services, Drilling Products customers know they’re using thoroughly tested equipment and tooling.

Here are just a few examples...

Over a six month period (May – October 2013) Boart Longyear used an LM75 drill rig with a Drill Control interface (DCi) to drill a total of 5,726.3 meters using a BQTK size diamond core bit. This was an increase of 907.7 meters drilled over the previous six-month average, which resulted in a unit cost improvement of $2.90 per meter average and monthly productivity increase of 13.5 percent. Read more…

Using patented technology, the new Longyear bits have diamonds that are chemically bonded to the matrix creating a more versatile, faster penetrating, and a longer-lived bit. The new Longyear bits retain the smooth drilling characteristics drillers prefer, resulting in more core in the box on every shift for every crew. Available in 16mm more open, express geometry for even higher cutting speeds as well as 16mm Stage and 25mm Stage 3 configurations. Read more…

Recently, a mining client in Canada requested that Boart Longyear develop a mobile drill rig to solve the issues faced in moving their existing operating StopeMaster equipment safely and quickly through the mine. Read more…

Boart Longyear combined proven technology from its most popular surface coring drill rigs to create the powerful LF160 which was fully tested and vetted with Drilling Services prior to commercialization.

When paired with the FL262 FREEDOM Loader, the LF160 combination is ideal for targeting sophisticated surface drilling exploration contracts that stipulate some of the highest safety standards, without compromising on productivity. Learn more…

Drill rod joints need to maintain their strength by resisting wear against the hole, and the wear of make-ups and break-outs due to bit changes.

With the pending release of Boart Longyear’s third generation XQ drill rod, this article reviews how each generation delivered breakthroughs in load capacity and wear life, substantially boosting reliability and productivity. The original Q drill rod joint is renowned for introducing coarse, tapered threads with a self-locking “interference fit,” but it had room for improvement.

As the thread’s load flank (loaded face of each thread) wore, the joint load capacity dropped, leading to failure: threads jumped, box ends expanded and climbed, or pin ends snapped from fatigue. To address these types of issues, Boart Longyear added hardening to the top half of the pin thread to maximum hardness (60 HRC), creating a significant hardness difference between pin and box. This hardness difference minimized wear compared to the rapid adhesive wear on threads of similar hardness.

Wear debris clings to a failed non-case-hardened joint after 12 cycles. 

Case hardening is unmatched in protecting the thread from wear, while keeping the toughness required to carry drilling loads. Competing drill rods are typically through-wall hardened to 32 HRC. Without case hardening (no “difference in hardness”), wear life is limited and load capacity degrades with each make and break.

In simple repeated “make and break” comparison testing, competing non-case-hardened rods were recently shown to fail at between 12 and 73 cycles, whereas Q and RQ joints withstood more than 150 and 300 cycles, respectively. The Q thread profile also had room for improvement. The positive-angled load flank (+15 degrees) tries to open or expand the box end under tension, which limits depth capacity and accelerates wear against the hole.

XQ threads. 

 A short-lived design variation, the MQ, variations of which are still offered by some competitors today, reduced the load flank angle (+2 degrees) to address this issue. However, the MQ was soon superseded by the second-generation RQ drill rod in 1999. The patented RQ joint improved load capacity and reduced joint expansion with a reverse-angled load flank (-10 degrees), which tries to shrink or close the box end, mimicking the natural load response of a continuous tube.

Wear life was doubled by reducing the number of turns during make and break. Many in the industry have applauded the RQ as being the most reliable rod for demanding, deep or deviated holes, both surface and underground. The next breakthrough meant overcoming two limits inherent to all threaded joints.

First, all threaded joints inherently suffer uneven load distribution, favoring the first point of mating contact because the male portion is under tension and the female portion is under compression. The stress and strain at this first point of contact produces uneven joint expansion and wear, commonly exhibited as leakage or a thin portion of the outer shoulder.

Patented XQ joints feature opposite double start threads that provide a balanced load response and double the contact area, which means half the contact pressure. This advancement doubles the make-and-break test cycles over the RQ. Additionally, XQ significantly increases load capacity by utilizing a load flank angle of -20 degrees, double that of the RQ, allowing for depth ratings beyond 4,000 meters, or 30 percent more than the RQ.

Second, the start of each thread typically requires gradual partial thread transitions that lead to rapid wear and wedging, followed by jamming and cross-threading, which would be problematic for simple double-start threads.

The new XQ holds up during “make and break” comparison testing: 168 cycles, right, compared to new, left.

XQ features an innovative self-aligning thread start geometry, ensuring mating threads engage smoothly, without wedging or jamming. Smooth starting, in combination with case hardening, significantly improves both productivity and wear life.

Finally, consider Boart Longyear’s V-Wall tubing option that made drill strings up to 30 percent lighter, thanks to a thinner wall mid-body. Operators welcomed this feature, as heavier rods can reduce productivity, especially in deep or deviated hole applications.

XQ joint. 

A lighter drill string reduces driller fatigue from manual handling and increases drill rig depth capacity, and the enlarged mid-body significantly reduces inner tube tripping time, all of which improve safety and productivity.

However, the thinner midsection is prone to uneven wear distribution concentrated at the mid-point. The new NXQ and HXQ drill rods exclusively feature patent-pending double- upset W-Wall tubing, with overall weight reduction and tube tripping capability equivalent to V-Wall.

A midplaced section of standard thickness improves mid-body wear life and stiffness, approximating standard wall tubing. Similar to the RQ, W-Wall tubing is cold-drawn from high quality, North American alloy steel, uniquely processed to Boart Longyear specifications.

As global field trials progress, so does our confidence and excitement about the advantages the new XQ rod will bring. Its strength, wear life and easier make and break will reduce downtime, lower operating costs, increase productivity and expand drilling capabilities, as well as maintain the operator’s reputation with clients.