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Cable Installation for
Deep Wells

Cable Tension During
Installation

Operational Situations

Working Loads

Receiving & Handling

Installation & Spooling

Operating and Maintenance

Common Abuses

Damage Caused by
Excessive Tension

Wire Line Spooling

Installation Tension for
Well Logging Cables Chart

Cable Damage Due
to Drum Crush

Figures 1 - 5

Example for a 7H42 Cable

Example for a 7H47 Cable

Cable Installation
Tensions

Installation Tension
Graph

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DAMAGE TO EM CABLES CAUSED BY EXCESSIVE TENSION

TO: CARL DODGE
FROM: BILL BOWERS
SUBJECT: DAMAGE TO EM CABLES CAUSED BY EXCESSIVE TENSION

The breaking strength of an EM cable as listed in all manufactures catalogs is the actual tension at which a new EM cable will break in a straight, non-rotating pull. The maximum working strength of a cable varies with a number of factors including; sheve wheels, rotation, spooling methods, etc., but there are some general rules that can be applied for most oil field cables and operating methods. These rules are:

1. An EM cable can be pulled an unlimited number of times to  50% of its breaking strength with out any compromising or deteriorating of the quality of the cable, (i.e. at 50% of breaking strength all elements of the EM cable are operating within their elastic limits.)
   
2. Any pull on the cable above 50% of breaking strength starts to cause some deterioration of the cable as elements begin to yield, and plastic insulation begins to cold flow. In most cases, a few pulls over 50% can be tolerated with only a slight loss in expected cable life. The more frequent the pulls over 50% and the amount above 50% will 11 result in a significant loss of cable life.
   
3. Any pull of a cable to 75% of the breaking strength will cause permanent and irreversible damage to the cable. The damage will be in the form of electrical leaks, loose or high armor wires. In many cases, the damage will not be apparent at the time of the excessive tension, but will show up after a few more runs of the cable.

The general rules for cable operating tensions  and damage were developed by extensive testing at Schlumberger Cable Engineering Department for 12 years and conducted literally hundreds of bench test pulls of cables to various high tensions and then dissected the samples to determine the damage. Further, hundreds more samples of field failures were examined and correlated with laboratory test results. It was based on this data that the above operating rules were developed. In later years, while in charge of engineering and manufacturing at Vector Cable, ITT Cable and Camesa,  I found that good cable performance and cable failures followed closely the general rules of tension limits as stated.

The mode of failure at high tension starts when the plastic insulation in the core begins to cold flow between the inner armor wires. The armor wires are wound in a helical manner around the core assembly and as they are pulled under tension, they generate a radial force or pressure squeezing the core. As the tension in the cable Increases, the squeezing pressure increases in direct proportion. The cable cores will withstand squeezing pressures that occur when the cable is pulled up to 50% of breaking strength. When the cable tension exceeds 50%, then the plastic insulation starts to be squeezed through the core tape binder an into the interstitial spaces between adjacent inner armor wires. As tension is further increased, the plastic will be forced between the armor wires. If sufficient plastic is displaced, electrical failure results. In any case, the insulation has become thin in some places and if electrical failure does not occur immediately, it will develop in subsequent runs.

Once the plastic is forced out, the effective core diameter is reduced and the armor wires become loose when tension is returned to normal. This looseness will, in subsequent runs, be "milked" down the cable and appear as high wires. These high wires abrade on the casing until they break. 

Certain cable designs are more prone to damage by excessive tension than others. Where very high breaking strengths are required with a minimum cable diameter, it is necessary to use a smaller electrical core and larger and fewer inner wires. The smaller core means that there is less  plastic insulation and fewer large inner armor wires have larger cusps or spaces between the adjacent armor wires. This combination is more likely to deteriorate with excessive tensions than some other designs.

To increase the cables tolerance to excessive tension, it is necessary to use a larger core with thicker plastic and cover the core well with a larger number of smaller inner armor wires. Cables of this design, though more tension tolerant, will have a much lower breaking strength, and working strength for the same diameter of finished cable. 

In choosing a cable design for a particular application, a number of factors and compromises must be considered. For operation on a North Sea platform, where deviated holes require a-stiff and very strong cable, the Camesa 7H47 would be an excellent choice. With its large armor wires, it-develops the stiffness and strength required within a tolerable diameter. On the other hand, the large armor wires and small core will make this cable more likely to be permanently damaged if subjected to tensions above the working tension of 11,500 Ibs.

Carl. I wiIl be happy to examine the cable sample you have, but unless it has been cut from the section of the cable where it was subjected to excessive tension, there will be very little to be learned.

If you send the sample, I will check it and give you a report.

William E. Bowers, P.E.

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