Page 270 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Corrosion index 12/247
the clearinghouse must have current, complete locations of 111. Corrosion index
all facilities.
See also Chapter 3 for more information. Offshore pipelines are typically placed in service conditions
that promote both external and internal corrosion. In consider-
E. Right-of-way condition (weighting: 5%) ing external corrosion, steel is placed in a very strong elec-
trolyte (seawater), which is a very aggressively corrosive
Along with a damage prevention program, marking of the environment. Because it must be recognized that no pipe coat-
pipeline route provides a measure of protection against unin- ing is perfect, it must also be assumed that parts of the pipe steel
tentional damage by third parties. Buoys, floating markers, and are in direct contact with the electrolyte.
shoreline signs are typical means of indicating a pipeline pres- Scoring for corrosion in offshore pipelines is similar to scor-
ence. On fixed-surface facilities such as platforms, signs are ing for onshore lines. Additional factors for the offshore envi-
often used. When a jetty is used to protect a shore approach, ronment must often be considered however. As with other
markers can be placed. The use of lights, colors, and lettering failure modes, evaluating the potential for corrosion follows
enhances marker effectiveness. logical steps, replicating the thought process that a corrosion
This item is normally only appropriate on shore approaches control specialist would employ. This involves (1 ) identifying
or shallow water where marking is more practical and third- the types of corrosion possible: atmospheric, internal, subsur-
party damage potential is higher. Note that in deeper water face; (2) identifying the vulnerability of the pipe material; and
where this item will probably score low, the activity level item (3) evaluating the corrosion prevention measures used at all
will often indicate a lower hazard potential. These will offset locations. Corrosion mechanisms are among the most complex
each other to some extent. of the potential failure mechanisms. As such, many more pieces
A qualitative scoring scale can be devised similar to the of information are efficiently utilized in assessing this threat.
following:
A. Atmospheric Corrosion
Excellent A 1. Atmospheric Exposures 0-5 pts
At every practical opportunity, high visibility signs and mark- A2. Atmospheric Type 0-2 pts
ers clearly indicate the presence of the pipeline and contact A3. Atmospheric Coating 0-3 pts
telephone numbers for the pipeline operator. All known Total 0-10 pts
hazards are clearly marked.
Fair
Some locations have signs and markers, not all of which are in B. Internal Corrosion
good condition. B 1. Product Corrosivity 0-10 pts
Poor B2. Internal Protection 0-10 pts
No attempt has been to mark the pipeline location, even in Total 0-20 pts
areas where it would be practical to do so. Where marking is
impractical everywhere, use this point level. C. Submerged Pipe Corrosion
c1. Submerged Pipe Environment 0-20 pts
F. Patrol (weighting: 20%) Soil Corrosivity 0-1 5 pts
Mechanical Corrosion &5 pts
As with the onshore case, pipeline patrolling is used to spot c2. Cathodic Protection 0-25 pts
evidence of a pipeline leak, but it is often more usehl as a Effectiveness 0-15 pts
proactive method to prevent third-party intrusions. A poten- Interference Potential 0-10 pts
tial threat does not have to be in the immediate vicinity of c3. Coating 0-25 pts
the pipeline. An experienced observer may spot a dredge Fitness 0-10 pts
working miles away or the movements of an iceberg or the Condition 0-1 5 pts
activity of fishermen that may cause damage in the following
weeks or that may have already caused unreported damage. The general balance of 10% atmospheric corrosion, 20% inter-
The patrol might also note changes in the waterway or shore- nal corrosion, and 70% submerged pipe corrosion will allow
line that may indicate a pipeline exposure due to shifting comparisons among pipelines that are at least partially exposed
bottom conditions. to these hazards. Where no system to be evaluated has any
A small amount of spilled hydrocarbon is not always easy to atmospheric exposure, for example, the evaluator may choose
visually spot, especially from moving aircraft. A variety of to eliminate this component and increase the other hazards by
sensing devices have been or are being investigated to facilitate 5% each. When this is done, each item can be increased propor-
spill detection. Detection methods proposed or in use include tionately to preserve the weighting balances. If onshore and
infrared, passive microwave, active microwave, laser-thermal offshore pipelines are to be compared scoring should be
propagation, and laser acoustic sensors [78]. consistent.
As with the case onshore, offshore patrol effectiveness is a As noted in other chapters, the primary focus of this assess-
product of several factors including speed and altitude of air- ment is the potential for active corrosion rather than time-to-
craft, training and abilities of the observer, and effectiveness of failure. In most cases, we are more interested in identifying
any sensing devices used in the patrol. locations where the mechanism is more aggressive than in
Scores should be awarded based on frequency and effective- predicting the length of time the mechanism must be active
ness ofpatrol on a point scale similar to that shown in Chapter 3. before failure occurs.