Page 239 - Pipeline Rules of Thumb Handbook

226 Pipeline Rules of Thumb Handbook

Survey results : a signal loss of 14dB over 40m. More detailed signal
Review of all the close interval potential results high- attenuation survey work located two possible coating
lighted some twenty-eight locations where both the defects with signal losses of 10dB over 20m and 48dB
“ON” and “OFF” potentials were less negative than the over 20m. A subsequent Pearson survey and two exca-
protection criteria. Of these 28 locations, 80% would vations revealed 5 small defects in a shrink sleeve weld
not have been apparent from potential measurements joint coating for the 10dB loss and one defect less than
2
recorded at adjacent test posts, thereby allowing the 100mm in the factory applied coating for the 4.8dB.
integrity of the pipeline to be put at risk where these
unsatisfactory levels of cathodic protection coincided
with coating defects. Conclusion
4. Pipeline : Aviation Fuel 6≤, UK
Length : 5 kilometers It is considered that the discussion and case histories illus-
Built : 1960 trated have shown that each of the above ground survey pro-
Coating : Asphalt cedures offer certain advantages and disadvantages, and that
Survey : Signal attenuation survey many of the technical and economic penalties of choosing any
Survey period : 2 days one specific survey may be overcome if used in conjunction
Survey results : with another.
The survey showed consistently high signal losses indi-
cating a generally deteriorated coating. Signal loss values
were of the order of 40dB per km. References
Over 50% of the total signal loss was concentrated over
four areas amounting to some 1,720 meters of pipe. Cor- J. Pearson, Petroleum Engineer 12(10) 1941.
relating close interval potential survey results over the CCEJV “Cathodic Protection Monitoring of Buried
same areas allowed 315 meters to be selected for Pipelines”—State of the Art Report by Task Group E4-2.
rewrapping. N. Frost, “Monitoring Pipeline Corrosion.” Pipes and
5. Pipeline : Crude Oil, Middle East Pipelines International, May 1986.
Length : 132 kilometers
Built : 1986
Coating : Extruded polyethylene Source
Survey : Signal attenuation coating survey
Survey period : 4 days Jones, Glyn, and Thron, Joanna, Advances in Pipeline
Survey results : Protection, Gulf Publishing Co., 1990.
Generally signal losses were found to average less than C. J. Argent et al. “Coating defect location by above ground
1dB per kilometer. One location was found to indicate surveys”—proceedings of UK corrosion, 1986.

Estimate the number of squares of tape for pipe coating (machine applied)

W ¥ A Tape width = 6≤
Squares of Coating =
( WO) ¥ 100 sqft square Overlap = 1≤
-
Pipe surface area = 1.73442sq. ft/ft (Table 7 Pipe Data
Where: W = width of coating, inches
A = pipe surface area, sq. ft. Section 3)
O = overlap, inches
Total surface area = 1.73442 ¥ 5280 = 9,157 sq. ft.
Example:
6 ¥ 9157
Squares of Coating =
1
Pipe OD = 65/8≤ (6 - ) ¥ 100
Length = 1 mile = 110 squares