Margin of overpull is additional tension to be applied when pulling the stuck drill string without breaking the tensile limit of the drill string. This is the difference between maximum allowable tensile load of drill string and hook load.
There are several factors when consider about the margin of over pull as listed below;
• Overall drilling conditions
• Hole drag
• Likelihood of getting stuck
• Dynamic loading
The formula for margin of overpull is described below;
Margin of Overpull = Ta – Th
Ta is the maximum allowable tensile strength, lb.
Th is the hook load (excluding top drive weight), lb.
Th can be calculated by the following formula;
Th = Weight of String x Buoyancy Factor (BF)
Weight of string = (Ldp×Wdp) + (Ldc×Wdc) +(Lhwdp×Whwdp)+(Lbha × Wbha)
Ldp = length of drill pipe
Wdp = weight of drill pipe
Ldc = length of drill collar
Wdc= weight of drill collar
Lhwdp = length of heavy weight drill pipe
Whwdp= weight of heavy weight drill pipe
Lbha = length of bottom hole assembly
Wbha= weight of bottom hole assembly
Buoyancy Factor (BF) = (65.5 – MW)÷65.5
The ratio between Ta and Th is safety factor (SF).
SF =Ta ÷ Th
Example: The drill string consists of the following equipment:
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
Lenght 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 town.
Determine the margin of overpull from the information above.
Maximum tension will happen at the surface so 5”DP will get the most tension when pulling and since only 90% of tensile strength is allowed. The allowable tensile (Ta) is as follows;
Ta = 0.9 x 436 = 392 Klb
Buoyancy Factor (BF) = (65.5 – 9.2)÷65.5
Buoyancy Factor (BF) = 0.86
Weight of string = (8,000 × 23.5) + (900 × 58) + (20,000)
Weight of string = 260 Klb
Note: Weight of mud motor and MWD is given at 20Klb.
Tension on surface (Th)
Th = 260 × 0.86 = 223.8 klb
Margin of over pull = 392 – 223.8 = 168.2 Klb
Safety Factor = 392 ÷ 223.8 = 1.75