In all this time, this system (seen in photo on the right) has had the same basic issues:

Spearhead Handling: in up-hole applications underground, a driller is required to manually push the head assembly into the hole by the spearhead. Since it has a pointed end and pivots by design, it can be difficult to handle this operation comfortably.

Inner Tube Handling: When hoisting the inner tube assembly, elastic action of the wireline cable or accidental impact during handling can un-load cable tension and overcome spring loads which allows the hooked lifting dogs to accidentally release the spearhead. The surface ‘Ezy-Lock’ overshot includes a twist-sleeve that locks onto the spearhead even without cable tension, whereas competing overshots require cable tension to maintain a lock.

Lifting Dog and Spearhead Wear: To balance strength and wear resistance, lifting dogs and spear points are heat treated to a medium hardness. However, it’s difficult to visually evaluate or functionally test the degree of wear, especially in underground applications. 

Boart Longyear currently provides a secondary safety pin that clips through the overshot, passing just under the spearhead tip. This adds an extra layer of protection in case the lifting dogs are excessively worn or deformed. However, spearheads are loaded cyclically and often loaded ‘off-pivot’, which deforms the components over time, to the point of disassembly. While the more recent MKII version of the spearhead assembly is much more robust, in the case of spearhead failure, the head assembly will release from the overshot regardless of lifting dog or safety pin use or condition.

This patent-pending overshot leverages our previous experience with Roller Latch head assemblies to create a more reliable and longer lasting system that eliminates spearheads and lifting dogs entirely. The spearhead assembly is replaced by a one-piece socket receptacle (spearhead adapter) that accepts the overshot itself, which has rollers that latch into an internal groove in the spearhead adapter.

Swapping the pointed, jointed spearhead for a simple cylindrical socket makes for much easier handling of head assemblies in up-holes. Surface Quick Descent Roller Latch head assemblies don’t even require the spearhead adapter since the internal groove geometry was pre-built into their design.

The increased toughness and hardness of the bearing quality latch rollers have a proven history of outlasting traditional pivoting latches for wear life. The new overshot will also feature the same Nitreg-ONC surface treatment as Roller Latch head assemblies that drastically improves corrosion resistance (Nitreg is a trademark of Nitrex Inc.).

Safety pin integration in the new underground Quick Pump-In overshot now pulls double duty of both locking the overshot from accidentally releasing while hoisting, as well as holding the head assembly and overshot together in case of component failure due to excessive wear. Also, the socket and rollers are not affected by side loading and ‘off-pivot’ loading during tube handling outside the hole, eliminating gradual deformation or disassembly. The new surface overshot will also include a one-hand twist-lock sleeve to maintain a locked position while hoisting outside the hole, even with a loss of wireline cable tension.

It’s also easy to use.  Instead of pushing the backs of the lifting dogs together, the driller pushes the two halves of the assembly together, retracting the rollers and releasing the head assembly. This operation takes about the same amount of force as the current overshot, so drillers won’t miss a beat.

Additional benefits have been included[CA1]  apart from the elimination of the spearhead and lifting dogs. While the current design uses a solid pivot pin that is peened into place (making it difficult to re-build), the Roller Latch Overshot has no pins whatsoever. Everything is held in place by simple threaded connections for easy maintenance.

First, in the event of a stuck tube, the driller needs to disengage and retrieve wireline cable in order to pull rods. Today, that is done by overloading and breaking a shear pin placed just underneath the cable swivel. In theory this pin breaks at under half the wireline cable’s max load capacity, but in practice its strength is highly variable because shear pins are inherently weak and ductile. Many operators remove the shear pin, which removes release capability and may result in excessive wireline cable replacement.

The Roller Latch Overshot features a brand new pump-in cable release system, originally conceptualized and prototyped by one of our expert underground drillers in Canada. A slotted sleeve and pumping seal assembly is placed over the wireline and pumped up to the overshot. The sleeve engages a quick-release mechanism and releases the wireline. This system has proven to be much more reliable, and may be the feature drillers are most thrilled to have going forward.  Reports of fewer broken wirelines have been received from several sites testing the pump-in cable release system.

Second, while Q/P Roller Latch head assemblies with built-in brake features have had great success in stopping runaway tubes and creating a safer drilling environment underground, they are perhaps “too” successful. Currently, when retrieving the head assembly from an inclined hole, pressure has to be applied to disengage the brake. Getting this pressure and procedure exactly right can be difficult, especially with hydrostatic pressure at depth.

To combat this and make Q/P Roller Latch easier to use while maintaining its safety features, a ‘brake release spring’ was created. This spring assembles quickly inside of the spearhead adapter on the head assembly. While tripping on its own, the head assembly brake works normally, but when the overshot latches into it this spring is compressed, disengaging the brake. This feature has also been received very positively by drillers in the field.

A surface-style overshot is also in development in B/N/H sizes. In addition to many of the features outlined in this article, the aim is to add more innovations, including:

• An improved lock sleeve to disable accidental head assembly release and stop drillers from accidentally sending the overshot down the hole while locked.

• A built-in 360° pivot and shorter overall length for increased ease of handling.

Excitement is high as testing continues. Drillers are noting the various positive developments: it’s easier to use, saves on wireline, and makes working with Q/P Roller Latch head assemblies much easier in difficult conditions. We’re looking forward to further field success as testing begins on the surface design.

For more information and downloads, visit: 
Roller Latch Quick Pump-In Overshot - Boart Longyear

The drill string’s resistance to deviation—otherwise known as ‘stiffness’—can be determined by its material and mechanical properties. Since all wireline tubular components are made from cold-drawn steel tubing, they all have the same fundamental properties. Specifically, regardless of chemistry, heat treatment, or hardness, all steel grades respond with the same amount of bend to a given load (the ratio of stress (load) to strain (bend) is known as the “modulus of elasticity”). Furthermore, any two steel tubular components with equal dimensions will have equal stiffness, even if produced by different suppliers, regardless of the steel grade, heat treatment, or hardness.

The directional response or sensitivity of the drill string to changes in drilling loads or speeds, or in formation changes, depends heavily on the drill string stiffness. The stiffness of wireline drill rods more than doubles in moving to the next larger system (e.g. BQ to NQ, NQ to HQ, etc.). As a result, larger systems drill straighter but have much more resistance and greater lateral loading when drilling through borehole deviations. Given a typical impregnated coring bit, and constant drilling parameters (assuming no formation changes), the borehole will tend to form a slow helix that is determined primarily by the stiffness of the drill string. With borehole friction, the drill string itself can become unstable, buckling into a helical shape which tightens or loosens with changes in drilling loads and speeds, but then elastically returns straight when unloaded.

The Q wireline core barrel was originally designed to utilize an outer tube with a substantially larger diameter and wall thickness than the unstable string of drill rods behind it. Standard outer tubes provide approximately 40% greater stiffness, and full-hole style outer tubes provide approximately 70% greater stiffness! The outer tube can then act as a stabilizing bearing or collar. The greater the increase in stiffness, the more effective a directional control to resist changes in the formation, drilling parameters, or drill string stability. This control can be enhanced with stabilized reaming shells, stabilized adapter couplings, and stabilized locking couplings.

Consider the directional impact if the outer tube is replaced with another drill rod, completely removing any difference in drill string stiffness, and the related directional control. Originally developed in the 1980’s in an attempt to direct borehole deviations, core barrel configurations, known as “flexi-barrels,” replace the outer tube with an equivalent length assembly of a drill rod and adapters. However, the lack of directional control combined with the lack of directional predictability, typically results in erratic deviations requiring either corrective deviation attempts or reaming to reduce excessive deviations. Therefore, the use of flexi-barrels is not recommended.

When planning holes, consider the potential impact of rod deviation. The stiffness of steel tubes is relatively high, and as mentioned before, increases with system size and section thickness. As a result, the drill string will respond with high side loads against the borehole wall, especially just before and after a deviation. For example, an NQ size drill rod deflected to the recommended maximum deviation of 1.0 degrees over its length produces approximately 9kN (2,000lb) of side load, and an HQ produces 18kN (4,000lb) at only 0.8deg per rod length. Depending on the formation, these high side loads can produce high torque, heavy rod wear or even ‘heat check cracking.’ Additionally, these contact points generate drag and ‘stick-slip’ conditions, which can produce a dynamic response sufficient to permanently deform the drill string into a helical shape. In extreme cases, where the drill string completes enough rotations approaching or exceeding maximum deviation, fatigue failures will occur. Using the minimum deviation possible to hit target and sticking to NQ size rod will reduce the side loads, torque and chances of twisting or cracking rod.

Originally published in Australasian Drilling Magazine, February/March 2020

Chris was also recently recognized by Mining Magazine in Exploration as part of their Mining Magazine Awards.

On this episode, ahead of the official launch, Chris discusses the benefits of XQ rods and how the new features compare to RQ rods as well as giving a shout out to Design Engineer, Anthony Lachance, and the North Bay manufacturing plant.

The top three benefits of XQ rods are:

1.   Easier and faster make and break with double auto-start threads
2.   Longer thread life – double over RQ rods
3.   Up to 60% stronger than RQ rods

Learn more about XQ Rods

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. 

However, Boart Longyear has recently made improvements to the system, significantly increasing reliability and performance.

Primarily, wireline systems require a sample-receiving inner tube completely independent to the rod string, while located at the bottom of the rod string, in the outer tube behind the drill bit.

Mounted at the top of the inner tube assembly is the head assembly, which includes the critical latch mechanism required to hold the inner tube while receiving the core sample and to release the filled inner tube for wireline retrieval.

A device called the overshot is attached to a wireline cable and lowered or pumped into the hole until it captures the head assembly, allowing the inner tube to be pulled or hoisted back to the drill rig by use of a winch or hoist.

Full Hole Locking Coupling

Stabilized Locking Coupling

The Latch Seat

In order to receive the latches deployed from the head assembly, couplings are inserted into the drill string to provide an enlarged interior opening or seat. In the original system, the latch seat was formed by mating two couplings. The first coupling, mated to the drill rods above the inner tube, is known as the locking coupling. The lower coupling, known as the adapter coupling, mates between the locking coupling and the outer tube, below which the inner tube is housed, and in turn to the reaming shell and drill bit. The end face of the male end of the locking coupling serves as the load bearing face for the latch mechanism.

Unfortunately, all pivoting latch mechanisms require a significant amount of “play” or axial clearance in the latch seat to allow for the pivoting latch movement, which results in inner tube play and poor system performance in difficult ground. In fact, an inner tube with a wedged core, or an overfilled inner tube, can be sufficiently loaded up against the latch seat such that the latches are unable to pivot in retraction. This often results in a stuck tube, potentially requiring the costly retraction of the drill string.

 In more recent Roller Latch systems, an improved integrated locking coupling eliminates the secondary joint and incorporates a mid-placed interior groove which serves as the latch seat. Additionally, roller latches drop away, eliminating the need for axial play and improving system performance in difficult ground.

Primarily, the latch seat must resist the reaction of the full thrust load from the drill whenever the driller is pushing through difficult, “blocky” ground. That is to say, when the core sample temporarily sticks or wedges in the inner tube (or when the tube is full), the thrust in the drill string (weight on bit) is fully resisted by the inner tube, the latch mechanism, and the locking coupling. However, the seat has a depth of about a third of the drill string thickness, which limits potential load-carrying capacity.

Latch Mechanisms and Locking Coupling Material

The head assembly of the original Q system incorporated a latch mechanism consisting of a pair of pivoting latches, deployed by a wire “butterfly” spring and retracted by the impact with the bottom of a slot in the latch retracting case when retracted by the wireline overshot.

Now obsolete, these latches had only 8mm (5/16”) thickness, which produced a very small mating contact area on the latch seat. This resulted in contact pressure and material stresses which often exceeded the strength of the latch seat when drilling difficult ground conditions. In some cases, the latch seat material would yield and allow the latches to push into the locking coupling, resulting in a stuck tube. At a minimum, the high wear rate of the latch seat was a maintenance issue.

In 1998, Boart Longyear once again secured its position as the leading innovator in wireline technology by introducing the patented Link Latch mechanism. This innovation virtually eliminated “stuck tubes” by providing mechanical leverage during wireline retraction, directly pivoting the latches into the retracted position, whereas conventional technology attempted to indirectly push the latches while fighting poor mechanical leverage and interference with the latch seat.

Additionally, the latch thickness was doubled to 16mm (5/8”), which cut the latch seat contact pressure and stresses in half, improving wear life and reliability. However, inner tube play is required for the pivoting latches.

In 2012, the launch of Roller Latch technology included a significant upgrade to all Boart Longyear locking couplings. Heat-treated, alloy steel material provided substantial increases in strength (40% increase) and hardness (wear resistance). This provided much greater thrust capacity and reliability in pushing through difficult drilling conditions, but also significant improvements for other locking coupling features as discussed below.

Locking Coupling Wear Pads

Situated at the top of the outer tube, the locking coupling can also act to stabilize the outer tube and reduce hole deviation. This is achieved through the addition of wear pads which act as a bearing surface against the drilled hole.

Boart Longyear offers two styles of locking couplings, “full-hole” and “stabilized.” While both are cut from the same high-quality alloy steel tubing material used for wireline drill rods, full-hole locking couplings have an over-sized outer diameter with four equispaced flats that are cold-drawn along each length.

While the annular area between the flats and the drilled hole provide passage for drilling fluid and cuttings, the over-sized rounded portions are induction case-hardened to provide hard, long-lasting, abrasion-resistant wear pads over the body length.

Full-hole locking couplings perform well in competent ground conditions where the hole is uniform and cuttings are fine. Conversely, poor ground conditions can significantly limit drilling performance and cuttings circulation, or generate excessive drill string torque or feed requirements.

Laser clad wear pad detail

Laser clad wear pad detail

Magnified section showing tungsten carbide in matrix

Locking Coupling Wear Pads

Situated at the top of the outer tube, the locking coupling can also act to stabilize the outer tube and reduce hole deviation. This is achieved through the addition of wear pads which act as a bearing surface against the drilled hole.

Boart Longyear offers two styles of locking couplings, “full-hole” and “stabilized.” While both are cut from the same high-quality alloy steel tubing material used for wireline drill rods, full-hole locking couplings have an over-sized outer diameter with four equispaced flats that are cold-drawn along each length.

While the annular area between the flats and the drilled hole provide passage for drilling fluid and cuttings, the over-sized rounded portions are induction case-hardened to provide hard, long-lasting, abrasion-resistant wear pads over the body length.

Full-hole locking couplings perform well in competent ground conditions where the hole is uniform and cuttings are fine. Conversely, poor ground conditions can significantly limit drilling performance and cuttings circulation, or generate excessive drill string torque or feed requirements.

Quick pump-in roller latch stabilized locking coupling section showing latch groove and brake groove

Laser clad wear pad detail

Stabilized locking couplings (formerly known as “conventional” style) utilize sets of wear pads consisting of tungsten carbide-bearing materials. Tungsten carbide offers wear resistance that is orders of magnitude greater than that of hardened steel, providing a reliable, longer-lasting bearing surface than the full-hole style.

Also, greater spacing between the wear pads, and in the annular area between the coupling body and the hole, ensures efficient passage of drilling fluids and cuttings in all ground conditions.

Originally, stabilized coupling wear pads were applied through a laborious manual process where welding technology was used to melt welding rods, consisting of tungsten carbide in a metal matrix known as “hard facing,” and bond the rod material to the coupling body.

A large amount needed to be added to be able to grind back down to a reasonably sized flat wear pad, and the resulting shape was difficult to control, which degraded the capacity to pass fluid and cuttings. 

In addition, an excessive amount of heat was required, which softened the steel body and weakened the coupling. On some couplings, hard facing was also applied to the latch seat face for wear resistance, but due to the manual welding process, this could soften the underlying latch seat to the point where latches could protrude through the seat under normal loads.

Again, the launch of Roller Latch technology included another significant upgrade to all Boart Longyear stabilized locking couplings. Laser cladding technology enables the precise application of wear pads containing a significantly greater density of tungsten carbide and with very little heat.

Stabilized wear pads are provided in an efficient spiral shape, which promotes passage of drilling fluid and cuttings. Comparative laboratory wear testing has shown an improvement of over 10 times (testing to ASTM G65), which typically translates to more than double the wear life in the field.

Locking Coupling and Head Assembly Interaction

While Boart Longyear has improved the wear resistance of latches and latch seats, abrasive wear is caused by their interaction during normal drilling operations.

Significant relative rotational motion can occur between the Link Latch head assembly and the drill string when there is insufficient loading or mating contact friction. This can result in rapid wear between the latch seat and latches, as well as between the head assembly landing shoulder and the outer tube landing ring.

Locking couplings for the Link Latch head assembly are optionally available with a small drive key or “tang” which is an integral, partial extension of the male end shoulder which protrudes beside deployed latches. As such, rotation of the drill string will drive rotation of the head assembly in unison.

Conversely, the patented Roller Latch mechanism is self-locking in rotation. As the rollers are centrifugally deployed, if there is any relative rotational movement with the drill string, the rollers are wedged between the locking coupling and retracting case to ensure the head assembly is always driven with the drill string.

This self-locking action provides a significant increase in the wear life and reliability of the landing ring, landing shoulder, latch rollers, and the locking coupling latch seat.

Drill Rod String Connection

Locking couplings include both a male outer tube thread and a female wireline drill rod thread connection, in order to mate directly to the drill rod string.

The outer tube is significantly thicker and stiffer than the drill rods, and is stabilized by the drill bit, reaming shell, and locking coupling wear pads. Conversely, the drill rod string is supported only by the formation, and is subject to significant dynamic loading as a result of drilling loads, vibration, and system harmonics.

As such, the locking coupling connection is critical to performance in demanding drilling conditions, wherein the recently improved heat-treated material provides strength and wear resistance.

Boart Longyear’s proprietary Q and RQ wireline drill rod joints utilize tapered threads for easy make and break, and feature engineered thread forms, a precise interference fit, and a unique combination of heat treatments to maximize load strength and wear resistance.

Boart Longyear facilities produce these precision thread connections to exacting global standards, controlled to a proprietary master gauging system.

As is well understood across the wireline drilling industry, in order to avoid joint failures, never inter-mix genuine Boart Longyear drill rods, couplings, or adapters with products produced by unlicensed third parties.

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