Page 152 - Offshore Electrical Engineering Manual

Cathodic Protection 139

2. Analysis of samples of all materials used for the minimum performance
required. Insulation thicknesses should be more generous than those for
cables used on land. If screen drain-wires are required, ensure that these
do not have a ‘cheese-wire’ effect on insulation when the umbilical is in
tension.
3. Close control of conductor and insulation diameter and thickness of each cable
during manufacture.
4. Samples of each cable should be taken at the start and finish of each stage of
the cable manufacturing run for analysis, and results of analysis known before
continuing with the next stage.
5. The final testing of each cable should include insulation resistance, high voltage
testing and measurement of total cable resistance, reactance, capacitance and
attenuation over a given frequency range. If dc switching or digital signal are
being transmitted, low attenuation at MHz frequencies may be important in
order to avoid degradation of the signal over the length of the cable. Such read-
ings may also detect faults, if attenuation readings for known healthy cables are
available for comparison.
6. A sample of the completed cable should undergo high voltage testing in water
at the pressure equivalent to at least the maximum depth of water likely in the
installed location. The test is carried out using a pressure vessel with a cable
gland at each end. The cable sample is passed between the two glands which are
then sealed. The vessel is pressurised and the high voltage can then be applied
at the cable ends protruding from the glands. Another high voltage test often
conducted on a sample is to submerge the sample in a tray of salt water with one
exposed conductor in air at either end. A high voltage is then applied between a
submerged conductor (or the tray) and the exposed conductor in air. Both of the
above tests are carried out to the maximum voltage available or until a voltage
breakdown occurs in the sample.
7. Once all the cables have successfully completed their testing, they may be
incorporated into the umbilical. However, if there is a delay between cable and
umbilical manufacture or the cable has to be transported to a different, further
insulation testing will be required immediately prior to umbilical manufacture
to ensure that transit or coiling damage has not occurred. It should also be
remembered that faults due to poor curing of the (plastic) insulation may appear
several weeks after manufacture.

CATHODIC PROTECTION
INTRODUCTION
Cathodic protection provides an effective method of mitigating corrosion damage to
subsea metal structures, whether their surfaces are coated or not.
Impressed current cathodic protection is preferred for long-term protection of
both platform structures and subsea pipelines whereas galvanic or sacrificial anode