CARE AND HANDLING
Packing Features:
- Drill rods and casing are packaged and sold in “bundles” which offer product protection during shipping and handling.
- Protective cardboard caps cover box and pin threads sealing in thread lubrication.
- Heavy duty galvanized hexagon bundle end caps protect the drill rod ends.
- Drill rods are coated with rust inhibitor to reduce surface oxidation during shipping. When storing rods for long periods of time, it is recommended you reapply a rust inhibitor to protect the rods from oxidation.
LUBRICATION AND CLEANING
Boart Longyear® drill rod threads are coated with thread compound (lubricant) for shipment from thefactory. For initial use, it is neither necessary nor desirable to remove this thread compound unless contamination has occurred. Thereafter, each time the rods are used, clean and re-lubricate the threads with a Boart Longyear recommended compound (BOL or Esso Z50). Use enough compound to cover both thread and shoulder surfaces. A 40 to 50 mm (1.5 to 2 inches) brush is excellent for applying compound.
Note: Keep the compound and brush clean
Note: While occasional mixing of the compound is recommended to avoid settling, dilution of any kind (e.g. Diesel, gasoline or oil) will render the compound ineffective.
The thread compound is critical to the wear life of the joint. In order to prevent wear, metallic particulate in the compound forms an inner-layer that is able to withstand the contact pressure and prevent the mating surfaces from interacting. A poor choice of compound or diluted compound will allow the mating surfaces to interact, resulting in adhesion or abrasion wear.
The thread compound is also critical to the strength of the joint. The interaction of the metallic particulate and the surface determines the frictional resistance to torque loads. This in turn determines the joint load efficiency: how much torque is transferred through the joint versus how much is absorbed by the joint. A poor choice of compound or diluted compound will provide insufficient friction, decreasing efficiency loading to overload failure.
Compounds containing 50% zinc particulate generally provide a higher friction factor (higher torque capacity) and get better resistance than those containing similar amounts of copper, lead or graphite particulate. Environmentally friendly compounds must contain non-toxic, bio-stable, solid particles of similar properties and performance characteristics to that of typical zinc particles in order to perform.
Note: Use of compounds without solid particulate will void the warranty.
Note: Metal compounds will react in acidic water leading to hydrogen embrittlement and reduce fatigue life (see glossary) however, copper is less reactive than zinc.
In addition, lubricating the rod body with grease is recommended to reduce hole friction, drilling torque, and midbody wear.
THREAD WEAR
The wear of sliding steel-on-steel surfaces, such as in a rod or casing joint, is well defined in engineering literature. Galling is the common industry term given to thread wear which mainly consists of adhesion and abrasion wear as a result of making and breaking (see glossary). While some wear can be tolerated without compromising performance, worn surfaces are prone to further wear. Unattended, the degree of wear can worsen to the point where it can cause premature failure or, in the case of mating surfaces of similar hardness, seize the joint. Alternatively, a worn thread can damage a good thread.
The rate of wear to be expected in a sliding metal-to-metal system can only be determined by considering all of the following variables:
- Lubrication or wear factor: published values are greater for poor lubrication; less for mating surfaces of dissimilar hardness
(see lubrication and cleaning) - The hardness of the softer surface
- The distance of contact slide
- The contact load or pressure
Less wear resistance can be achieved by:
- Cleaning and lubricating joints regularly; preferably after every break. Dry lubrication coatings are available but these wear off and must also be cleaned and lubricated (see lubrication and cleaning)
- Choosing joints with mating surfaces of dissimilar hardness. Published data shows that given equal contact pressures and equal hardness on the softer surfaces, a system with a harder mating surface (dissimilar hardness) can provide several times the wear life
- Choosing joints with greater hardness on the softer thread
- Reduce the sliding contact distance by choosing joints with greater taper
- Reduce or eliminate the contact pressure by adjusting the feed rate and rotation speed during make and break to match the thread pitch and compensate for rod and drill head weight Another source of rod joint wear is worn accessories. All threaded accessory equipment, such as Kelly (drive) rods, drive head adapter subs, hoist plugs, water swivels and cross-over adapter subs should be inspected prior to use to ensure they are in good condition. Use only genuine Boart Longyear accessories to ensure proper fits and maximum wear life. Boart Longyear tooling and gauging adhere to an uncompromising global standard.
BOX AND MIDBODY WEAR
Similar to the steel-on-steel wear systems of the joint, the box and midbody are subject to relative sliding contact with the wall of the casing or hole. In the case of wear against the wall of the hole, the surface of the hole may be of significantly greater hardness and roughness (not to mention cuttings suspended in the drilling fluid) potentially resulting in rapid wear rates. However, in many applications the cause of retirement of a drill rod is due to localized wear resulting from the deformation of the box out of a ‘flush’ position or of the typical midbody out of straight.
In typical joints, it is inherent for the box and box end shoulder to elastically deform radially or ‘bulge’. This is due to radial and hoop stresses (see glossary) imposed by conventional threads which add to drilling load stresses. This is evident by a thin section in the box shoulder
and/or a small polished area on the side of the joint where thread engagement begins. As the wear progresses, the box becomes weaker and the deformation more pronounced, increasing the wear rate. RQ® style joints however, mimic the load response of a solid tube in that radial and hoop stresses imposed by the thread subtract from drill load stresses, virtually eliminating “bulging”.
It is inherent for a rod string to respond to significant drilling loads and rotation in a three dimensional corkscrew shape, a phenomenon first identified and defined by Boart Longyear as ‘helical whirling’. As loads or rotation increase, the contact pressure between the string and the hole increases contributing to an increased midbody wear rate.
Given sufficient contact pressure and speed, the heat generated between the rod string and casing or hole can cause heat-check cracking (see glossary) which ultimately appears as an axial crack, typically on the box end.
The bending stresses associated with this helical whirling become significant under high load or rotation, especially in oversize holes or ‘caves’, and may cause permanent bending of the string. Boart Longyear drill rods incorporate enhanced tubing processing which doubles the bend strength of the midbody virtually eliminating permanent bending.
The use of ‘rod grease’ to reduce friction between the rod string and the casing or hole is common (see lubrication and cleaning) however the only effective solution to reduce midbody abrasion wear is to significantly increase the hardness through case hardening.
LOADS AND DEVIATED HOLES
Fatigue failures are brittle failures or cracks that occur under stress or load levels that are significantly below static load ratings; however, the loads are applied or cycled a large number of times. An example of this type of load is where a rod string is rotating in a deviated hole, the surface of the rod undergoes both tension and compression in each revolution. Where the rod is deviated at significant depth, this bending load is superimposed on a constant pullback load resulting in a fluctuating tension load on the rotating surface. Another example is in oversized holes or caved hole sections wherein the string can bend or buckle, significantly increasing bending stresses.
Due to the reduced cross-sections of material in the threaded ends, the joints between mated rods in the string are significantly weaker than the rod midbodies – regardless of heat treatment or thread design (despite the interlocking thread, RQ® joints for example, are weaker and are not
stiffer than their midbody). Also, joints are pre-loaded (make-up) and have interference fits which further reduce the deviation capacity of the joint.
A further limitation on the ability of a drill rod joint to perform through a bend is due to a peculiarity of the steel material itself. If there is a constant tension load applied in addition to a cyclical load, the fatigue strength is even further reduced. In the case of drill rod joints, if the joint is properly made up the pin end will always be under a greater tension load than the box end (see make-up torque). As a result, the pin end is the weakest part of a drill rod and is the typical location of failure under an excessive cyclic load. Boart Longyear utilizes a full scale cyclic bend load test to evaluate joint designs and to ensure manufacturing quality.
A fatigue failure crack always occurs perpendicular to the cyclic load or stress. Therefore the most common failure is a circumferentially oriented crack which indicates that the cyclic load or stress was axially oriented which can only be caused by bending. If the crack is axially oriented it is either the result of heat-check cracking (see glossary) or indicates that the cyclic load was circumferentially oriented and this can only be caused by improper fit of a joint in terms of make-up, deformation, foreign debris, or wear.
Fatigue failures can be avoided by limiting the level of cyclic loads with consideration for the pullback load. Limit the build angle or rate of hole deviations checking that the deviation rating per rod length is not exceeded rather than the deviation per 30 m (100 ft), for example, which can be significantly less. Deviation should be further limited as pullback increases with increasing hole depth.
Note: Standard weight wireline drill rods have limited deviation capacity. Lightweight or internally upset rods are recommended for greater deviation.
