In order to monitor cathodic protection, we
must be able to contact the metal of the subject pipeline or structure.
There was a time when contact was achieved
by driving a steel rod into the ground from above the pipeline, and making
temporary contact by piercing the coating. The main reason for discontinuing
this practice was the physical damage that was possible to the pipe metal
itself, and that damage that was caused to the coating.
Most pipelines now have provision for
contact through electrical conductors connected to the subject metal in a
variety of ways.
The most common is a process known as
cadwelding, which can result in a low resistance, permanent, electrical bond
between the copper conductor and the steel of a pipeline.
The disadvantage of this type of connection
is that it needs considerable skill to achieve a good connection, in some field
conditions. The joint must be carefully
inspected and the integrity of the coating must be tested before backfilling.
We are all aware that copper and brass are
more noble than steel which will tend to disolve if coupled together in an
electrolyte. Cadwelding introduces such a ' bi-metalic coupling' to the surface
of the pipe and care must be taken that all metal is separated from the
electrolyte by a chemically impervious, electrically resistant coating. This coating must be compatible tho the
pipeline coating and to the insulation on the copper conductor cable.
The use of copper conductor cables also
introduces the possibility of a bi metalic coupling if its insulation is not
perfect. It should be remembered that
the voltage that we are measuring is between the potential of copper in a
saturated solution of its own salts, and steel in the local environmental
electrolyte.
If the conductor to the pipe is severed,
then the voltage that we measure will be that between copper in a saturated
solutionof its own salts and copper in the salts that are present in the local
environment. This voltage will be very low, as the difference between the
potentials, will be small.
Readings can be very confusing however as
they are sometimes affected by the cathodic protection current. Charges will be passing onto the broken
copper tail which is still attached to the steel of the pipe, due to the
galvanic activity, and this will cause a variation in the potential of the
ground in the immediate vicinity. The extent of this area of influence depends
on the area of contact between the copper and the electrolyte, and the
resistance of that electrolyte.
If the conductor is not completely severed,
it will definitely draw currentfrom the ground, and this will have a
significant effect on the measured voltage if the insulation damage is close to
the electrode position.
The contact between the conductor and the
subject metal must have a low electrical resistance, as it may be used for
measuring current.
The best test facility is direct contact
with the pipeline at a riser but there are many sections of pipeline which are
buried with no riser.
There was a period when some test posts
were connected to two conductors which contacted the pipeat two locations
exactly 100m apart. The purpose was to
enable current direction readings to be taken, but although I saw several
attempts to do this, I never saw it done successfully, or was never able to
obtain meaningful readings myself. I
read and understand the theory behind this type of measurement, but the
application seems impractical in field work.
The electrical resistance of the pipeline
itself is extremely low, for example a 4" dia. steel pipeline is 0.141
ohms per mile and a 24" dia. pipeline is an incredibly low 0.0161 ohms
per mile.(Peabodies) If we are dealing
with other structures such as storage tanks we can never consider the
resistance of the metal itself, as a significant feature in cathodic protection
calculations.
It therefore follows that the POTENTIAL of
the pipe metal does not vary significantly, over a 2km section of continuous
welded steel pipeline, with the diameters quoted. This matter was debated during the application
of over-the-pipeline potential surveys, conducted in the UK, on high pressure,
welded steel, gas mains.
I was part of a team that carried out the
field test which resolved this matter, on a 2km section of 24" dia. welded
steel, coal tar enamel coated, buried pipeline.
This pipeline was protected by impressed current cathodic protection
which was switched on continuously during these tests.
The negative pole of a high resistance
voltmeter was connected to the test post conductor terminal at the top of a
test post at location A.
A standard copper/copper-sulphate electrode
was placed in a fixed position at location A.
A reel of armature wire was used to connect
the electrode to the positive pole of the high resistance voltmeter and the
reading was noted.
The wire was reeled off the spool and used
to make contact with a standard copper/copper-sulphate electrode at location B,
which was 2km distance from location A.
A changed voltage was noted on the meter,
which was still connected to the test post at location A.
The change of standard electrode positions
had significantly altered the recorded voltage.
The armature wire was then used to connect
the negative pole of the voltmeter to the distant test post at location B.
The positive pole of the voltmeter was then
reconnected to the electrode at location A.
The voltage on the meter was identical to
the first reading.
Altering the position of contact to the
pipeline, by a distance of 2 km, had no detectable influence on the voltage
measured.
The meter was taken to location B and
connected between that test post terminal and electrode B.
The voltage recorded was identical to the
second voltage of the test, confirming that the location of the electrode is
the only significant feature.
The positive pole of the meter, at location
B, was then connected to the armature wire which was connected to the electrode
in the fixed position at location A.
The voltage recorded was identical to the
first voltage recorded confirming, once more that the point of contact to the
pipeline has no detectable effect on the recorded voltages.
The discussions culminating in this test,
resulted in a re-appraisal of test post locating within the operating
company. It was decided that fewer test
posts were needed, and that the priority importance was access to the test post
locations.
The best form of test post, for a steel
pipeline consists of a steel bar welded directly onto the pipeline metal and
protruding through the surface of the ground directly above. This is protected
by encasement in a concrete block, which includes a vertical 4" dia.pipe
filled with the local ground material.
The standard electrode would always be placed in the top of 4" pipe
for the purposes of periodic voltage measurements.
