Open Hole

Cable armor wire is classified as "galvanized extra improved plow steel". It has a tensile range of from 270 to 300 thousand PSI.

1. Guaranteed Minimum Breaking Strength  .  The manufacturer s rated minimum breaking strength applies to a new cable, pulled straight with no rotation.  For the 7J46 type cable this rated breaking strength is 18,000 lbs.  Actual factory break tests performed on each new cable typically result in values greater than 19,000 lbs.

    

2. The actual breaking strength for a cable may be less than the rated value for the following reasons:
   1. Wear
   2. Corrosion
   3. Bending
   4. Fatigue
   5. Torque
   6. Rotation
   7. Physical damage
   8. Defective tension device

       

3. Weak Spots. Cables do not have "weak spots". If they break at tensions below the guaranteed minimum breaking strength, it is for one of the above reasons. Individual wire breaks can be the result of a weak point or defect in the wire, but these occur less than one every million feet of wire, so the probability of all 48 wires in a 7J46 cable having a defect at the same point is virtually impossible. Individual wire breaks can occur for all of the above reasons and in addition:

    

   1. Faults or inclusions in the steel structure
   2. Butt welds
   3. Damage during manufacturing respooling

       

4. Field Failures.  Experience over the years clearly indicates that by far most cable breaks in field use are the result of:

    

   1. Physical damage
   2. Rotation
   3. Inaccurate tension device
   4. Cable or tool becomes stuck near the surface

    

5. Operating Strength.  The guaranteed breaking strength of the cable is the guaranteed minimum pull the cable will stand before parting. A 7J46 cable has a rated breaking strength of 18,000 lbs. For normal operations the following guidelines should be remembered.
   1. The cable when properly installed can withstand unlimited pulls to 50% of its rated strength. This is 9,000 lbs. for the 7J46 cable.
   2. The cable when properly installed can withstand 75% of rated strength, (13,500 lbs.) with only minor damage to the cable, but repeated pulls to this tension over sheave wheels or measuring devices will cause permanent and irreversible damage to the cable.
   3. Any pull on the cable above 75% of rated strength will cause serious and un repairable damage to the cable, which may not show up until several runs after the cable has been pulled to this tension.

    

6.  Stuck.  When "stuck in the hole" the following suggestions are offered.

   1. Check your tension device and depth to stuck point by comparing it to cable stretch.  The cable can be flagged or marked and the change in the length measured when the tension is increased.

      5 L = K L 5T

      5T = 5L / (K L)

      L = 5L / (K T)

      5L = change of length

      5T = change in tension

      L = length to stuck point

      K = stretch coefficient 0.77 ft/1000ft/1000 lbs.

   2. Check all devices the cable physically contacts. Be sure the correct sheave wheel (proper diameter and groove shape) is being used and that trucks and sheave are properly aligned.  Any pinching, bending or scraping of the cable can significantly reduce the cable strength.

   3. Move your "set up" distance so that the armor wires are not fatigued by repeated bending in the same area by the sheave wheel or measuring device.

   4. Use  common sense in "spudding".  "Spudding" means using the inertia of the logging instrument to break through solidified drilling mud the is blocking the well bore.  The cable was designed to work in tension   not compression.

7. Splices.  Open hole cables are frequently spliced, which will be discussed later but with regard to cable strength the following points should be remembered.

   a.  A properly made splice, either shim or welded, will develop at least 90% of the strength of an unspliced cable in a straight pull.

   b.  Splices will not tolerate spudding.  Drastic reductions of strength of a splice can occur if is put in compression.

   c.  Splices fatigue rapidly in bending around sheave wheels and measuring devices. The smaller the sheave diameter, the more rapid the deterioration of the splice.

8. Torsion and Rotation.  All cables currently in use in oilfield operations generate a specific torque when subjected to a load.  When permitted, the cable will rotate many revolutions.  For 7J46 cable the approximate values are:

   a.  Torsion: 2 ft. lbs/1000 lbs tension change

   b.  Rotation: 7 revolutions/1000 ft/1000 lb tension change

   When the cable is first put into use it will "spin-out" in response to the tension profile it experiences. Once the cable has "spun-out", there will be a torque generated and tendencies for the cable to rotate only when there is a change in the tension profile. 

   The cable experienced a change in tension profile every time the cable goes into and comes out of the hole as a result of the frictional drag on the tool and cable. If the cable were lowered and pulled out at a uniformly slow speed, there would be virtually zero frictional drag. Under those conditions, a seasoned cable would have no tendency to rotate. Under practical operating conditions the tension going into the hole is several hundred or even a thousand pounds less than the tension coming out.  This results in significant torque and rotation in the cable during every round trip into and out of a borehole.

   There in no limit to the speed at which the cable can be spooled except as how it affects the tension in the cable and the resulting torque and rotation.

   To avoid any cable problems resulting from cable torque, the tension at any given depth should never be less than 1/2 of the tension at any given depth should never be less than 1/2  of the tension going into the hole at that same depth coming out of the hole.  When the tension drops to 1/3, there is loss of contact between the inner and outer armor and mud lumps can be generated.

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