Page 550 - Corrosion Engineering Principles and Practice

512 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 513

• Soil/environmental conditions
• Corrosion protection data
• Operating parameters/history
The data collected for preassessment are similar to the information
required for risk assessment, which may be performed concurrently.
The feasibility of using ECDA must be evaluated based on available
data. Other factors that, besides the lack of information, could
preclude ECDA from being the assessment method of choice include
• Electrical shielding from disbonded coating
• Rocky terrain and backfill
• Paved surfaces
• Inaccessible locations
• Paralleling buried metallic structures
If an operator determines that ECDA is an appropriate assessment
methodology, the pipeline then must be divided into ECDA regions and
two indirect inspection techniques need to be selected for each region.
For example, on a cathodically protected pipeline with good coating, a
close-interval potential survey (CIPS) will provide information
concerning the level of protection, interference, and current shielding. A
direct current voltage gradient (DCVG) survey performed in conjunction
with the CIPS will detect specific locations of damaged coating (see
Chap. 13 for additional details). However, it is crucial that, independent
of the two techniques selected, the ECDA be performed along the entire
length of the specified section of pipeline with both techniques.
Indirect Inspection
The purpose of indirect inspection is to identify the locations of
coating faults, insufficient cathodic protection, electrical shorts,
interference, geologic current shielding, and other anomalies along
the pipeline. Indirect inspection also permits to identify the areas
where corrosion may be occurring or has occurred. Typical pipeline
survey techniques that may be used are [19]
• Close-interval on/off potential surveys (see Chap. 13)
• DCVG surveys
• Electromagnetic current attenuation surveys
• Alternating current voltage gradient surveys
All four survey techniques are well established. Each has its advan-
tages and disadvantages, depending upon the type of anomaly that needs
to be detected and the condition of the pipeline right-of-way. However, it
is important that the inspections with the two chosen techniques be car-
ried out at approximately the same time and that the data be tied to all