Page 234 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 221

Example: N = 140e 11¥0.113 = 485 leaks
N 1 = number of leaks in first year (i.e., 1979) = 140
N = number of leaks in year considered (1986) = 310
n = 86 - 79 = 7
310 = 140e 7A Source

Hence, coefficient A = 0.113. The estimated number of leaks Ahmad, Hayat, Gas Pipeline Renewal Insertion Technology, 3
in 1990 will be: and 4 (1990).

ADVANCES IN PIPELINE PROTECTION

Specialized Corrosion Surveys for Buried Pipelines: Methods Experience
M. D. Allen, CEng, MIM, MICorrST; N. R. Barnes, BTech, CEng, MIM, MiCorrST Spencer and Partners, U.K.

Synopsis provide an acceptable indication of the overall level of corro-
sion protection being achieved on the pipeline.
The external corrosion protection of a pipeline is commonly These measurement locations however, commonly at
achieved by a system comprising an insulating coating and least 1km apart, are generally selected for ease of access
cathodic protection. The performance of this system is nor- and thus only provide a valid assessment where pipeline
mally assessed by regular monitoring of pipe to soil poten- coating is consistent, ensuring a constant attenuation of
tials at selected intervals along the pipeline. Three more cathodic protection levels between each measurement
detailed pipeline survey procedures are available to provide point. Where the coating is variable in quality, it cannot be
additional monitoring information and the case histories assumed that satisfactory levels of protection are maintained
described indicate the experience and value gained from their between measurement locations and a deficiency in the pro-
selective use. tection system may remain undetected allowing corrosion to
occur.
Coating defect surveys have also been used to identify areas
Introduction where pipeline coating quality is questionable. Historically,
these surveys have almost universally taken the form of
The external corrosion protection of a buried steel pipeline Pearson type surveys. These surveys have entailed a two-man
is generally achieved by a system comprising an insulating survey team walking the length of a pipeline identifying
coating and cathodic protection. On modern pipelines, the coating defects and subsequently marking the points with
coating is regarded as the primary means of protection with pegs for further investigation.
the cathodic protection systems providing control of corrosion Surveys of this type have two inherent disadvantages,
where the coating has failed or has been damaged. namely that the survey procedures are time consuming and
The performance of the cathodic protection system is no indication of cathodic protection levels at the defect loca-
usually assessed by regular monitoring of pipe to soil poten- tion is obtained.
tials at selected intervals along the pipeline. With these shortcomings in mind two other survey proce-
From these results conclusions may be drawn concerning dures have been developed, the signal attenuation coating
the level of cathodic protection being achieved and by infer- survey and the close interval potential survey. The former
ence the performance of the insulating coating. Under most provides a rapid assessment and record of pipeline coating
circumstances, such measurements at selected locations will condition and more readily identifies areas where a defect or
defects are present. Identified areas may then be surveyed in
detail by close interval potential techniques which cannot only
Paper first presented at the 7th International Conference on the Internal and
External Protection of Pipes. Held in London, England. Organized and spon- confirm coating defect locations, but also establish the status
sored by BHRA, The Fluid Engineering Centre © BHRA, The Fluid Engi- of the cathodic protection system at close intervals through-
neering Centre, Cranfield, Bedford, MK43 0AJ, England 1987. out the chosen areas.