The drill string comprises the entire column of drill pipe, drill collars, and bottom hole assembly components extending from surface to the drill bit. It transmits rotational energy, applies weight to the bit, provides a conduit for drilling fluid circulation, and must withstand tensile, compressive, and torsional loads while maintaining hydraulic efficiency.
Component Specifications
API Specification 5DP defines drill pipe grades by minimum yield strength:
- Grade E-75: 75,000 psi yield strength—standard grade for moderate-depth wells
- Grade X-95: 95,000 psi—intermediate strength applications
- Grade G-105: 105,000 psi—high-stress environments
- Grade S-135: 135,000 psi—deep wells and extended reach applications
Extended Reach Performance
Optimized drill string configurations have enabled record-breaking operations:
- Sakhalin-1 completion runs (SPE-150959): Drill string design enabled 11,943m completion runs in world-record extended reach wells
- Proper grade selection and connection design are critical for managing the combined tensile, torsional, and bending loads in directional wells
Failure Modes
Fatigue is the primary failure mode in directional wells, with failures concentrated at tool joints and upset areas where stress concentrations exist. Regular inspection and rotation of drill string components extends service life and prevents costly downhole failures.
Advanced drilling automation optimizes parameters to reduce the cyclic stress that drives fatigue, extending drill string service life while maintaining peak performance.