🎓 Lesson 18
D5
Drilling Cost Breakdown Anatomy
Drilling cost breakdown anatomy is a detailed look at all the expenses involved in drilling blast holes—like labor, equipment, fuel, and bits—and how each part affects the total cost per meter drilled.
🎯 Learning Objectives
- ✓ Calculate drilling cost per meter by summing and allocating direct and indirect cost components
- ✓ Analyze the sensitivity of total drilling cost to bit wear rate and penetration rate using parametric models
- ✓ Design a cost-optimized drill pattern by balancing hole depth, spacing, and bit selection against unit cost targets
- ✓ Explain how equipment utilization rate and fleet availability impact effective cost per meter
- ✓ Apply industry-standard cost allocation methods (e.g., ISO 14040-based LCA principles) to compare drilling alternatives
📖 Why This Matters
Drilling typically consumes 25–40% of total blast-related costs—but it’s often managed as a black box. A single mis-specified drill bit or underutilized rig can inflate unit costs by 15–30%. Understanding the anatomy of drilling cost isn’t just about accounting—it’s about making engineering decisions that directly affect fragmentation quality, downstream loading/hauling efficiency, and overall mine profitability. In modern mines, this breakdown powers digital twin simulations and autonomous drill fleet optimization.
📘 Core Principles
Drilling cost anatomy rests on three pillars: (1) Cost classification—direct (traceable to a specific hole, e.g., bit, fuel, operator wage) vs. indirect (allocated, e.g., supervision, workshop overhead, insurance); (2) Time-based cost drivers—drilling time, setup time, move time, and downtime; and (3) Equipment lifecycle economics—depreciation (straight-line or usage-based), maintenance escalation curves, and consumable failure modes. Critically, cost is not static: penetration rate drops exponentially with bit wear, while energy consumption rises non-linearly with rock compressive strength (UCS > 150 MPa increases kWh/m by ~40%). Real-world cost modeling must therefore integrate geomechanical data, equipment telematics, and maintenance logs.
📐 Total Drilling Cost per Meter (CDR)
CDR is the weighted average cost to drill one linear meter of hole, aggregating all cost categories over a defined period or project phase. It serves as the primary KPI for drill fleet productivity and forms the basis for comparative analysis across rigs, shifts, or rock types.
Total Drilling Cost per Meter (CDR)
CDR = (C_{bits} + C_{fuel} + C_{labor} + C_{maintenance} + C_{depr} + C_{overhead}) / L_{total}Calculates the average cost incurred to drill one meter of blast hole, enabling benchmarking and optimization.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| C_{bits} | Bit and consumables cost | USD | Total cost of drill bits, rods, adapters, and flushing agents consumed during period |
| C_{fuel} | Fuel and power cost | USD | Diesel for mobile rigs or grid electricity (kWh) converted to USD at site rate |
| C_{labor} | Labor cost | USD | Wages, benefits, and payroll taxes for drill operators, helpers, and shift supervisors directly assigned |
| C_{maintenance} | Maintenance cost | USD | Preventive, corrective, and predictive maintenance—including parts, labor, and workshop overhead |
| C_{depr} | Depreciation and financing cost | USD | Straight-line or usage-based depreciation plus interest on financed equipment |
| C_{overhead} | Allocated overhead | USD | Indirect costs (planning, QA, safety, administration) assigned using activity-based costing |
| L_{total} | Total drilled length | m | Net linear meters drilled (excluding redrills, test holes, and non-blast holes unless specified) |
Typical Ranges:
Soft sedimentary rock (sandstone): $22 – $34/m
Medium-hard rock (dolomite, limestone): $28 – $42/m
Hard rock (granite, quartzite): $38 – $52/m
Abrasive/alteration zones (clay-rich shear zones): $48 – $75/m
💡 Worked Example
Problem: A hydraulic rotary drill operates 5.8 hrs/shift (net drilling time = 4.2 hrs). Over 30 days, it drills 2,850 m. Total monthly costs: $127,600 (includes $38,200 bit consumption, $29,500 fuel, $24,800 labor, $18,300 maintenance, $16,800 depreciation & overhead).
1.
Step 1: Confirm total drilled meters = 2,850 m (given).
2.
Step 2: Sum all monthly costs = $127,600.
3.
Step 3: Divide total cost by total meters: $127,600 ÷ 2,850 m = $44.77/m.
4.
Step 4: Validate against typical range for hard rock (granite, UCS ≈ 180 MPa): $38–$52/m — result falls within acceptable band.
Answer:
The result is $44.77/m, which falls within the safe range of $38–$52/m for hard rock surface drilling with modern DTH rigs.
🏗️ Real-World Application
At Newmont’s Boddington Mine (WA, Australia), a 2022 cost anatomy study revealed that bit-related costs accounted for only 12% of total CDR—but contributed to 63% of unplanned downtime due to premature failure in altered granodiorite. By switching from standard tungsten-carbide inserts to polycrystalline diamond compact (PDC) bits and adjusting rotation speed based on real-time torque feedback, they reduced bit cost/m by 18% *and* increased average penetration rate by 22%, yielding a net CDR reduction of $6.40/m. This change was validated using ISO 50001-aligned energy and cost tracking across 14,200 m of production drilling.
📋 Case Connection
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