Page 87 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/65
Scoring the corrosion potential Atmospheric Corrosion (1 0% ofcorrosion threat = IO pis)
Exposures (50% atmospheric = 5 pts)
All variables consldered here continue to reflect common Environment (25% atmospheric = 2 pts)
industry practice in corrosion mitigatiodprevention. The vari- Coatings (30% atmospheric = 3 pts)
able weightings indicate the relative importance of each item in Fitness (50% of coatings = 1.5 pts)
terms of its contribution to the total corrosion risk. The evalua- Condition (50% of coatings = 1.5 pts)
tor must determine if these weightings are most appropriate for Visual inspection (50% of Condition)
the specific systems being assessed. Nondestructive testing (NDT) (30% of Condition)
In the scoring system presented here, points are usually Destructive testing (DT) (20% of Condition)
assigned to conditions and then added to determine the corro-
sion threat. This system adds points for safer conditions. For
example, under subsurface corrosion of steel pipelines, three Al. Atmospheric exposure (weighting: 50% of
main aspects are examined: environment, coating, and cathodic atmospheric corrosion)
protection. The best combination of environment (very The evaluator must determine the greatest risk from atmos-
benign), coating (very effective), and cathodic protection (also pheric corrosion by first locating the portions of the pipeline
very effective) commands the highest points. that are exposed to the most severe atmospheric conditions.
An alternative approach that may be more intuitive in some Protection from this form of corrosion is considered in the next
ways is to begin with an assessment of the threat level and then variable. In this way, the situation is assessed in the most con-
consider mitigation measures as adjustment factors. In this servative manner, The most severe atmospheric conditions may
approach, the evaluator might wish to begin with a rating of be addressed by the best protective measures. However, the
environment-ither atmosphere type, product corrosivity, or assessment will be the result of the worst conditions and the
subsurface conditions, depending on which ofthe three types of worst protective measures found in the section. This conser-
corrosion is being examined. Then, multipliers are applied to vatism not only helps in accounting for some unknowns, it also
account for mitigation effectiveness. For example, in a scheme helps in pointing to situations where actions can be taken to
where an increasing number of points represents increasing improve the risk picture.
risk, perhaps a subsurface environment of Louisiana swamp- A schedule of descriptions of all atmospheric exposure sce-
land warrants a risk score of 90 (very corrosive). A dry Arizona narios should be set up. The evaluator must decide which sce-
desert environment has an environmental rating of 20 (very low narios offer the most risk. This decision should be based on data
corrosion). Then, the best coating system decreases or offsets (historical failures or discoveries of problems), when available,
the environment by 50% and the best cathodic protection sys- and employee knowledge and experience. The following is an
tem offsets it by another 50%. So, the Louisiana situation with example of such a schedule for steel pipe:
very robust corrosion prevention would be 90 x 50% x 50% =
22.5. This is very close to the Arizona desert situation with no Air/water interface 0 pts
coating or cathodic protection system. This is intuitive since a Casings 1 pts
very benign environment, from a corrosion rate perspective. Insulation 2 pts
can be seen as roughly equivalent to a corrosive environment Supports/hangers 2 pts
with mitigation. Groundair interface 3 pts
Further discussion of scoring options such as this can be Other exposures 4 pts
found in Chapter 2.
None 5 pts
Multiple occurrences detractor -1 pt
In this schedule, the worst case, the lowest point value, gov-
A. Atmospheric corrosion (weighting: 10% of ems the entire section being evaluated.
corrosion threat)
Aidwater interface The air/w'ater interfuce is also known as
Atmospheric corrosion is basically a chemical change in the a splash zone, where the pipe is alternately exposed to water
pipe material resulting from the material's interaction with the and air. This could be the result of wave or tide action, for
atmosphere. Most commonly this interaction causes the oxida- instance. Sometimes called waterline corrosion. the mecha-
tion of metal. In the United States alone, the estimated annual nism at work here is usually oxygen concentration cells.
loss due to atmospheric corrosion was more than $2 billion, Differences in oxygen concentration set up anodic and cathodic
according to one 1986 source [31]. Even though cross-country regions on the metal. Under this scenario, the corrosion mecha-
pipelines are mostly buried they are not completely immune to nism is enhanced as fresh oxygen is continuously brought to the
this type of corrosion. corroding area and rust is carried away. If the water happens to
The potential for and relative aggressiveness of atmospheric be seawater or brackish (higher salt content), the electrolytic
corrosion is captured in this portion ofthe model. The evaluator properties enhance corrosion because the higher ion content
may also include other types of potential degradations of further promotes the electrochemical corrosion process.
exposed pipe such as the effect ofultraviolet light on some plas- Shoreline structures often have significant corrosion damages
tic materials. due to the airiwater interface effect.
A possible evaluation scheme for atmospheric corrosion
is outlined below and described in the following Cusings Industry experience points to buried casings as a
paragraphs. prime location for corrosion to occur. Even though the casing