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Cased Hole

STRENGTH

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

1. Guaranteed Minimum breaking strength:   The manufacturers rated minimum breaking strength applies to a new cable, pulled straight with no rotation allowed. For the 1N32PP type cable this rated strength; is 11,000 Ibs. Actual factory break tests performed on each new cable typically result in values closer to 11,500 Ibs.
   
   

2. The actual breaking: Strength for a cable may be less than the guaranteed minimum breaking strength 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 weak point or defect in the wire, but these occur less than one in every 2 million feet of wire. The probability of all the wires in a cable having a defect at the same point in the lenght 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 minimum guaranteed  breaking strength of the cable is the minimum pull the cable will withstand before parting. 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 5,500 Ibs. for the 1N32PP cable.

2. The cable when properly installed and maintained can withstand 75% of rated strength, (8,250 Ibs.) with only minor damage to the cable. Repeated pulls to this tension will cause permanent and irreversible damage.

3. Any pull on the cable above 75% of rated strength will cause serious and un-repairable damage to the "-cable.

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 5T)

   5L = change of length

   5T = change in tension

   L = length to stuck point

   K = stretch coefficient 1.2 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 a bridge that is blocking the well bore. The cable was designed to work in tension - not compression.

7. Splices: Cased hole cables are sometimes spliced. With regard to cable strength the following points should be remembered.

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

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

3. 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 logging and perforating generate a specific torque when subjected to a load. When permitted, the cable will rotate many revolutions. For the 1N32PP cable, the approximate values are:

1. Torsion - 1.3 ft.-lbs/1000 Ibs tension change

2. Rotation - 5 revolutions/1000 ft/1000 Ib tension change

When the cable is first put into use it will "spin-out" in response to the tension profile it experiences. That is, the cable will rotate in the direction of the outer armor layer winding. 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 experiences a change in tension profile every time it goes into and comes out of the well as a result of the frictional drag on the tool and cable. If the cable were lowered and pulled out at an 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 several 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 well.

There in no limit to the speed at which the cable can be spooled except as how if 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 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 out armor layers, in this condition, the cable can accumulate wellbore material between the armor layers, develop "high wire", or "bird caging" of the arm can occur.

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