Page 106 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/84 Corrosion Index

causing coating or metal

Magnetic or electric
I')r field sets up current
flow in pipeline

Figure 4.8 AC power currents on pipeline

Also fitting into the second scenario might be cases such as especially where such pipe is coated, is a common example of
the potential to create shielding effects. Certain soil or rock
Very low-power AC only types. concrete coatings, and other buried structures (retaining
High-power AC present but at least 3000 ft away walls, culverts, foundations, etc.) are also examples. Any struc-
AC nearby, but regular surveying confirms no induction ture that is very close to the pipe (perhaps <2 ft) should alert the
occurring. evaluator to shielding potential.
Where the evaluator sees potential shielding situations,
Note, however, that significant inductive interference effects points assigned should show a lowered interference score
can be seen as far away as 1.2 miles in high resistivity soils [SO]. (higher interference potential). When the operator is sensitive
In some cases, a more thorough investigation of AC effects to this potential and takes special precautions, points can be
might be warranted. Before a site-specific analysis is done, a awarded. When there is no potential for shielding, as has been
more robust risk-screening tool could be used to highlight the verified by appropriate surveys, maximum points should be
most critical of suspect locations. Breaking the AC interference given.
potential into several items for more detailed scoring might
involve the variables shown inTable 4.6. DC-related interference (weighting: 70% of interference
Preventive measures can be designed for induction or for potential) The presence of other buried metal in the vicinity
ground fault cases or for both. As previously mentioned, of a buried metal pipeline is a potential concern for corrosion
grounding cells can be designed to safely handle the discharge prevention. Other buried metal can short circuit or otherwise
of current from the pipeline. Close monitoring of the situation interfere with the cathodic protection system of the pipeline. In
would be considered part ofthe preventive measures taken. The the absence of a cathodic protection system, the foreign metal
evaluator should be satisfied that the potential AC current prob- can establish a galvanic corrosion cell with the pipeline. This
lem is well understood and is being seriously addressed, before may cause or aggravate corrosion on the pipeline. This can be
credit is given for preventive measures. quite severe: 1 amp ofDC current discharging from buried steel
can dissolve more than 20 pounds of steel per year.
Shielding (weighting: 10% of interference potential) The most critical interference situations occur when electri-
Shielding is the blocking of protective currents. Casing pipe, cal contact occurs between the pipeline and the other metal.