When a drill string is submerged in drilling fluid, the fluid exerts an upward buoyant force that reduces the apparent weight of the string. The buoyancy factor (BF) is a multiplier applied to the in-air weight of the drill string to calculate the buoyed weight, which is the weight the derrick actually supports. Understanding buoyancy is essential for calculating hook loads, determining available weight on bit, and ensuring the rig has adequate hoisting capacity.
This guide covers the buoyancy factor formula, hook load calculations, weight on bit determination, and practical applications in drilling operations.
The Buoyancy Factor Formula
The buoyancy factor for steel drill string in drilling fluid is:
BF = 1 − MW / 65.5
- BF = buoyancy factor (dimensionless, always less than 1.0)
- MW = mud weight (ppg)
- 65.5 = density of steel (ppg)
The value 65.5 ppg corresponds to a steel specific gravity of 7.85 (65.5 / 8.34 = 7.85). For non-steel components (aluminum drill pipe, titanium tools), use the appropriate material density in ppg. The buoyancy factor decreases (more buoyancy) as mud weight increases. In 10 ppg mud, BF = 0.847. In 16 ppg mud, BF = 0.756. In 20 ppg mud, BF = 0.695.
The buoyed weight of any component is simply its in-air weight multiplied by the buoyancy factor. This applies to the entire drill string, individual components, or the total string weight.
BF = 1 − MW / 65.5
8.6 ppg (freshwater): BF = 1 − 8.6/65.5 = 0.869
10.0 ppg: BF = 1 − 10.0/65.5 = 0.847
12.0 ppg: BF = 1 − 12.0/65.5 = 0.817
14.0 ppg: BF = 1 − 14.0/65.5 = 0.786
16.0 ppg: BF = 1 − 16.0/65.5 = 0.756
18.0 ppg: BF = 1 − 18.0/65.5 = 0.725
Drill String Buoyancy Factor Calculator
Calculate buoyancy factor, buoyed weight, and hook load for drill strings in drilling fluid. Essential for weight-on-bit planning and rig capacity checks.
Hook Load Calculations
The hook load is the total weight suspended from the hook, measured by the weight indicator on the rig floor. It is the primary tool for monitoring downhole conditions during drilling and tripping. The calculated hook load for the string hanging free (no WOB, no friction) is:
Hook Load = (String Weight in Air) × BF
In practice, the actual hook load deviates from the calculated value due to friction (drag) between the drill string and the wellbore. In a deviated well, the drill string rests against the low side of the hole, creating friction that adds to the hook load when pulling (overpull) and subtracts when running in (slack-off). Monitoring the difference between calculated and actual hook load reveals the friction or drag in the system.
Overpull = Actual hook load (tripping out) − Calculated free-hanging weight. If overpull exceeds the planned maximum (typically 30,000–50,000 lb), the string may be sticking and corrective action is needed. Slack-off = Calculated free-hanging weight − Actual hook load (tripping in). Excessive slack-off can indicate a ledge, keyseated hole, or differential sticking.
Drill string: 10,000 ft of 5" DP (19.50 lb/ft)
+ 500 ft of 8" DC (147 lb/ft)
Mud weight: 12.0 ppg
Air weight: (10,000 × 19.50) + (500 × 147)
= 195,000 + 73,500 = 268,500 lb
BF = 1 − 12.0/65.5 = 0.817
Hook load = 268,500 × 0.817 = 219,385 lb
Drill String Buoyancy Factor Calculator
Calculate buoyancy factor, buoyed weight, and hook load for drill strings in drilling fluid. Essential for weight-on-bit planning and rig capacity checks.
Weight on Bit (WOB) and Available Weight
Weight on bit is the downward force applied to the drill bit to make it cut rock. It is generated by allowing a portion of the buoyed drill collar weight to rest on the bit. The available WOB is limited by the buoyed weight of the collars (and HWDP) below the neutral point:
Available WOB = (Collar Weight in Air) × BF
The neutral point is the location in the drill string where the axial load transitions from compression (below) to tension (above). Drill collars below the neutral point are in compression, providing WOB. Drill pipe above must remain in tension at all times. Putting drill pipe in compression causes fatigue failures, connection washouts, and twist-offs.
A safety factor of 10–15% is applied to ensure drill pipe stays in tension. If you need 40,000 lb WOB: Required collar buoyed weight = 40,000 / 0.85 = 47,060 lb. In 12.0 ppg mud (BF = 0.817), air weight needed = 47,060 / 0.817 = 57,600 lb of collars.
600 ft of 8" drill collars at 147 lb/ft
Air weight = 600 × 147 = 88,200 lb
MW = 14.0 ppg, BF = 0.786
Available WOB = 88,200 × 0.786 = 69,325 lb
With 85% safety factor: 69,325 × 0.85 = 58,926 lb
Maximum recommended WOB = 58,926 lb
This keeps drill pipe above collars in tension.
Drill String Buoyancy Factor Calculator
Calculate buoyancy factor, buoyed weight, and hook load for drill strings in drilling fluid. Essential for weight-on-bit planning and rig capacity checks.