Optimizing the Rotary Drill String
In rotary drilling, careful selection of drill string components is vital to achieve accurate holes, optimal rock fragmentation and operational efficiency—parameters that affect total operational costs
By Rick Meyer
When developing a rotary drilling system, most of the attention is usually given to the drill rig—the capital equipment that requires significant investment and hence a planned payback. The second priority in the system tends to be choice of rotary tricone drill bit. However, to utilize the full power and capacity of the rig and the bit, and at the same time increase service life and productivity, consideration should be given to the entire drill string. The optimal drill string includes a shock absorber at the top, a rotary deck bushing to centralize the drill string as it passes through the deck of the rig, strong and straight drill pipes and finally a hole-stabilizing roller stabilizer or bit sub-adaptor to optimize performance. Giving the necessary attention to every part of the drill string will lead to the lowest total operating costs in rock excavation and fragmentation.
The primary purpose of the drill string is to transmit the rotational torque and weight from the power source—the rotary head of the rig—to the rockbreaking drill bit. As with every rock drilling method, the power must be transmitted as efficiently as possible, and return as few vibrations as possible, as these cause unnecessary wear on the rig and reduce penetration rates. When selecting components for the drill string, attention must be given to the different roles of the support tools in the string. The aim can be to:
- Absorb damaging vibrations travelling back up the drill string
- Improve transmission of energy from the rotary head to drill bit
- Centralize the drill bit within the hole
- Achieve longer bit life
- Reduce friction as the drill string passes through the drill rig deck
- Stabilize the hole wall to prevent hole caving
- Increase penetration rates and lower drilling costs
- Achieve blast hole accuracy for improved blasting efficiency
- Improve the end result—the fragmentation of the blasted rock.
At the very top of the drill string, between the rotary head and drill pipe, a shock absorber is commonly used. As the name indicates, the intention of this tool is to reduce the negative effects of harmful vibrations that travel back up the string as a result of the drilling process.
The benefits of using a shock absorber include:
- Improved torque control
- Increased drilling penetration rates
- Better drill rig availability and extended drill rig drive head and mast life
- Longer service life of drill bits.
To guide the drill string and reduce the risk of wobbling, a rotary deck bushing is used at the drill rig deck opening. The deck bushing guides the pipes to prevent reduction of rotary head torque and assists with the final straightness of the hole.
The deck bushing contains an outer housing with a top flange that allows it to fit perfectly into the deck opening. A series of roller bearings allow the inner sleeve to rotate with the drill string. Wear of the deck bushing occurs primarily on the inner sleeve as cuttings are blown upwards, between the drill pipe and the inner sleeve.
The role of the drill pipe is to transfer sufficient amounts of rotational torque and weight to the drill bit. The goal is to establish an optimal rate of penetration while still achieving an acceptable life of the tricone bit. The use of strong and straight alloy drill pipe is one of the best ways of preventing wobbling of the drill string and hole deviation. Drill pipe is subjected to a severe and abrasive environment, due to the rapid evacuation of drilling cuttings through the annulus of the hole, causing a sandblasting effect on the drill pipes. It is logical, therefore, to use only the best alloy steel for both the threaded connections and the body of the drill pipe. Special wear protection material is applied to the most critical areas of erosion at the bottom of the drill pipe. The drill pipes can, in most cases, be refurbished to prolong service life.
To connect the bit to the drill pipe, a wear-protected bit sub adaptor is generally used when the rock formation is relatively competent, and not in need of stabilization within the hole. In some softer, fractured rock formations, it is worthwhile to consider the use of a stabilizer as an alternative. A roller stabilizer contains three roller assemblies which provide support against the hole walls, serving to both guide the drill bit in a straight direction and pack the wall of the hole to prevent caving. The use of either straight or spiral-bladed stabilizers is strongly discouraged as this causes excessive friction when these blades are at full gauge diameter, plus they also lose gauge diameter rapidly—rendering them virtually useless as a stabilizer after only a few shifts. In addition, a spiral-bladed stabilizer slows down evacuation of the cuttings. So, to achieve improved hole straightness, hole wall integrity, and at the same time increase the effective life of the stabilizer, only stabilizers with rollers fitted with cemented carbide inserts are recommended.
All in all, when you consider the significant amount of capital invested in a rotary blasthole drill rig and the annual investments in tricone drill bits, the selection of the best quality rotary drill string tools that are suited to the application is critical to the success of the drilling program. The rotary drill string tools should not just be considered as mere support tools, but rather as an essential, integrated part of the total rotary drilling system.
The following basic criteria should be considered when deciding which rotary drill string tools will best optimize overall drilling performance and cost effectiveness:
- Are quality materials and innovative design used to address specific drilling problems?
- Can the tools be refurbished for an economical second run?
- Does the tool supplier offer application and follow-up service?
In conclusion, straight blast holes drilled exactly to the pre-planned hole bottom positions pave the way for lower total operating costs, taking into account the entire process: drilling, blasting, secondary breaking, loading, haulage and crushing/screening.
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Rick Meyer
