Page 238 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 225

interval potential surveys would seem the most appropriate of the signal attenuation coating survey. Bearing in mind the
and perhaps the only survey system to consider. probably high quality coating of a new pipeline, and because
Experience has shown however that the repair of signifi- of the comparison basis employed by the signal attenuation
cant coating defects will benefit the overall level of cathodic coating survey, even small coating defects will become readily
protection achieved, possibly permitting reduction in output noticeable.
levels and/or increasing the distance protected from any one At the other extreme, with poorly coated old pipeline, a
protection station. In addition, the maintenance of a “two- Pearson survey may have an almost continuous response
level” protection system also ensures the highest integrity is making it impossible to discern individual defects or the most
maintained for the pipeline. With this purpose in mind, the significant coating breakdown. The signal attenuation coating
advantages of the coating surveys become apparent. survey would, however, be able to give a rapid assessment
If we consider the requirements of a newly laid pipeline, of coating condition and indicate where limited rewrapping
it has been considered important to ensure as near perfect would be of greatest benefit.
a coating as is possible at the start of the pipeline”s life. It In general, therefore, experience has shown that it is cost
has therefore been common practice to “Pearson” survey the effective for the more detailed surveys of coating condition
pipeline after installation. As described above, this survey and cathodic protection to be undertaken in conjunction with
requires a two-man team and commonly progresses at some a signal attenuation coating survey. A number of case histo-
4–6km per day. This procedure may be accelerated by the use ries are described below.

Case histories

1. Pipeline : Multiproducts UK Survey Period : 10 days—Signal attenuation coating
Length : 115 kilometers survey
Built : 1982 1 day—Pearson survey
Coating : Coal tar reinforced enamel with self- 4 days—Close interval potential survey
adhesive tape at field joints. Survey Results :
Survey : Signal attenuation coating survey Initial signal attenuation coating survey identified 30
Survey period : 9 days (4 days for disconnection of all locations of significantly high signal losses, of these 12
“temp” magnesium anodes, 5 days for were attributed to interpipeline bonds and sacrificial
survey). anodes, the remaining 18 were subject to more detailed
Survey Results : investigation.
Survey indicated that over some 80km of the pipeline Close interval potential surveys showed that a satisfac-
the coating was satisfactory and further survey work was tory level of cathodic protection was being achieved at
not recommended for those areas. Signal losses over each of the locations where defects had been pinpointed
these sections ranged between 1.5dB/km and 3dB/km. using Pearson equipment.
Some 9 sections, amounting to a total of some 10km Three particular locations were excavated, in the first a
were given first priority for further investigation signal coating defect of some 800mm associated with a welding
losses were as high as 17.6dB over distances of 1.5km. arc burn was found within an area previously showing
A further 2 sections, amounting to some 21km were a signal loss of 6dB over less than 100m section. In
classified as second priority items. the second area, showing a similar signal attenuation, a
It was decided that this information gathered be corre- number of small coating defects were found together
lated with the results obtained from a proposed intelli- with a box of welding rods touching the pipe. At the third
gent pig survey before further action. location a badly poured bitumen field joint had been
2. Pipeline : White Oil products, UK indicated by a signal attenuation loss of 10dB.
Length : 115 kilometers 3. Pipeline : Oil, UK
Built : Approx 1963 Length : 210 kilometers
Coating : Coal tar enamel, tape or bitumen flood Built : Approx. 1970
field joints Coating : Coal tar enamel with tape wrapped field
Survey : Signal attenuation coating joints.
survey Survey : Close interval potential survey
Pearson survey Recording both “ON” and “OFF”
Close interval potential potentials at 5 meter intervals.
survey Survey period : 40 days for 3 man team