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Magnetic Marking of Cables

Brief Summary

From observation of actual stretch in new cables which have been returned for remarking after ten or fifteen runs, we know that a new 15/32" seven conductor cable accrues a 'permanent stretch' of about 2 1/2 inches per 100 feet. This equates to a permanent stretch of about 0.21%. In order to compensate for this stretch we have been marking brand new cables at a tension of 4,000lbs, rather than at 1,000lbs at which most cables are normally marked. (On a 15/32" cable this additional 3,000lbs overpull actually stretches the cable 3 x 0.77ft per 100ft = 2.31"/100ft, and therefore, if the length was to be checked at 1,000lbs., the marks are actually being put on the cable 2.31" short.)

Naturally, the total permanent stretch of 0.21% does not all build up on the first run in the well, but builds up progressively over the first few runs. From tests carried out on new cables after the manufacture and during research and development testing, we know that average cumulative values for permanent stretch for successive pulls on a new cable are approximately as follows:

1st Pull 60%
2ns Pull 75%
3rd PUll 85%
4th - 8th Pull - not measured
9th Pull 98%

It is evident that, if one corrects to the marks, the depth measuring device is assuming that the cable is fully stretched. However, since on the first few runs, the cable has not fully stretched, the measuring device will show a reading that is too deep and a negative correction must be made.

We therefore propose that engineers use the following adjustment after making the correction given by the stretch charts:
1st Run Subtract 40% of 0.21%(=-0.0840%) of Cable Depth
2nd Run Subtract 25% of 0.21%(=-0.0525%) of Cable Depth
3rd Run Subtract 15% of 0.21%(=-0.0315%) of Cable Depth
4th Run Subtract 10% of 0.21%(=-0.0210%) of Cable Depth
5th Run Subtract 5% of 0.21%(=-0.0105%) of Cable Depth
6th and subsequent runs, use no further adjustment