Overpull Drilling is the maximum pull exerted on a stuck drill pipe without causing failure in the Drill String. In this article, we shall learn more about the margin of drill pipe overpull calculations, and there is also an online calculator for it.
Maximum Overpull In Drilling
The maximum overpull exerted on stuck drill pipe shall not exceed 80% of the maximum tensile strength of the weakest grade Drill Pipe. In other words, you must know which is the lowest grade drill pipe to get its tensile strength, then multiply it by 80%, and this will be the maximum overpull you can reach during any stuck.
The maximum pull for each crossover point must be calculated to determine the full pull exerted on the Drill String. The calculation procedure is illustrated in the following example.
Assume the Drilling Bit at 14000 ft & the drilling Bottom Hole Assembly BHA would consist of the following:
| Section | 80% Tensile Section Strength | Length, ft | Air weight, lb |
| Bottom Hole Assembly | – | 1200 | 150,000 lb |
| 5″ Grade G Drill pipe | 443,066 | 12,063 | 270,934 lb |
| 5″ Grade G Drill pipe | 569,656 | 737 | 17,245 lb |
Calculations Steps
1- we check the maximum overpull on the weakest drill pipes.
The maximum overpull exerted on the top of grade G drill pipes without exceeding 80% of Grade G tensile strength is 443,066 lb.
2- Now calculate the maximum overpull in drilling string that can be exerted on top of grade S drill pipes,
Max overpull = 80% of Tensile Strength + weight of Grade S Drill Pipes
Max overpull = 443,066 + 17245 = 460,311 lb.
Since 460,311 lb is less than 80% of the grade S tensile strength (569,656 lb), then it is safe to pull 460,311 lb. So, when we pull 460,311 lb at the surface.
- The pull exerted on top of Grade S DP is 460,311 lb.
- The pull exerted on top of Grade G DP is 460,311 minus the weight of Grade S DP, or 460,311-17245 = 443,066 lb.
Margin Of Overpull In Drilling Calculations
The Margin of Overpull definition in drilling is the difference between the maximum overpull in the drilling string and the current weight of your drilling string. Of course, you shall consider the last recorded pick-up weight as it is the weight of your Drill String. Now, Let’s convert all these words above to an equation.
Overpull Margin= Ta – Th
Where
- Th: is the hook load (excluding top drive weight), lb= Drill String Weight x Buoyancy Factor (BF)
- Ta is the maximum allowable tensile strength, lb.
Furthermore, We can use the below equation to calculate the drill string weight:
Drill String Weight = (Lbha × Wbha) + (Ldc×Wdc) +(Lhwdp × Whwdp) + (Ldp × Wdp)
- Wbha= Bottom Hole Assembly BHA weight
- Lbha = bottom hole assembly length
- Wdc= drill collar weight
- Ldc = drill collar length
- Whwdp= heavy weight drill pipe weight
- Lhwdp = heavy weight drill pipe length
- Wdp = drill pipe weight
- Ldp = drill pipe length
- Buoyancy Factor (BF) = (65.5 – MWT) ÷ 65.5
Example On How To Calculate Margin Of Overpull In Drilling
Perform the drill pipe overpull margin calculations from the below drill string data:
- 5” DP S-135, 4-1/2” IF connection, adjusted weight of 23.5 ppf = 8,000 ft
- 5” HWDP S-135, 4-1/2” IF connection, adjusted weight of 58 ppf = 900 ft
- Mud motor and MWD weight = 20 Klb
- Length of mud motor and MWD = 90 ft
- Mud weight is 9.2 ppg
- Tensile strength of 5” DP S-135 (premium class) = 436 Klb
- Tensile strength of 5” HWDP S-135 (premium class) = 1,100 Klb
- 90% of tensile strength is allowed to pull without permission from the town.
Solution
1- Calculate the maximum Tension that will happen at the surface
5″ DP will get the most Tension when pulling since only 90% of tensile strength is allowed. The allowable tensile (Ta) is as follows;
Ta = 0.9 x 436 = 392 Klb
2- Calculate the buoyancy factorBuoyancy Factor
(BF) = (65.5 – 9.2) ÷ 65.5
Buoyancy Factor (BF) = 0.86
3- Calculate The String Weight from the following equation:
Drill String Weight = (Lbha × Wbha) + (Ldc×Wdc) +(Lhwdp × Whwdp) + (Ldp × Wdp)
Note: The weight of the mud motor and MWD is given at 20 Klb.
Drill String Weight=(8,000×23.5)+(900×58)+(20,000)
Drill String Weight = 260 Klb
4- Calculate Tension on surface (Th)
Th= Drill String Weight x Buoyancy Factor (BF)
Th = 260 × 0.86 = 223.8 klb
5- Margin Of Overpull Calculations
Margin Of Overpull = Ta – Th
The Overpull Margin = 392 – 223.8 = 168.2 Klb
Margin Of OverPull Calculator
The purpose of the online calculator for overpull calculations is to help drilling engineers and professionals accurately calculate the maximum overpull exerted on stuck drill pipes without exceeding the maximum tensile strength of the weakest grade drill pipe. This calculation is important in ensuring the safety and efficiency of drilling operations. The calculator allows users to input the relevant data and variables, such as drill string weight, buoyancy factor, and tensile strength, and it produces the maximum allowable overpull in the drilling string.
