Page 100 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/78 Corrosion Index
Table 4.4 0 Operating stress > 60% specified minimum yield strength
Operating temperature > 100°F
Soil factor Relative weighting (%) Distance from compressor station < 20 miles
0 Age > 10 years
Soil resistivity 30 Coating system other than fusion bonded epoxy (FBE).
25
gril moisture 25
MIC“ 15 An automatic assessment incorporating these criteria can be
STATSGO~ steel corrosivity rating 5 set up in a computer environment.
~
Soil corrosivity scorec 100 Stress Tensile stress at the pipe surface is thought to be a nec-
aMIC =evaluation of the potential for microbially induced corrosion. essary condition for SCC. The stress might be residual, however,
bSTATSGO = State Soil Geographic (STATSGO) soils data compiled and hence virtually undetectable. The higher the stress, the more
by the Natural Resources Conservation Service of the US. potential for crack formation and growth. Fluctuations in stress
Department of Agriculture. level are also thought to play an aggravating role since such fluc-
tuations produce fatigue loadings that can increase crack growth.
It is reasonable to assume that all pipelines will be under at least
cracking (HSCC), sulfide stress corrosion cracking (SSCC), some amount of stress. Because internal pressure is often the
hydrogen-induced cracking (HIC), or hydrogen embrittlement, largest stress contributor, pipelines operating at higher pressures
corrosion fatigue, and erosion. In the United States, stress cor- relative to their wall thickness are thought to have more suscepti-
rosion cracking (SCC) reportedly caused more than 250 bility to SCC. Thermally induced stresses and hendmg stresses
pipeline failures in the 1965-1985 period [52]. Some failure can also contribute to the overall stress level, but, for simplicity’s
investigators think that these numbers represent an under- sake, the evaluator may choose only internal pressure as a factor
reporting of the actual number of SCC related failures since in assessing potential for SCC.
such failures are often very difficult to recognize.
Stress corrosion cracking can occur under certain combina- Environment High pH levels close to the steel can be a con-
tions of physical and corrosive stresses. Evidence shows that tributing factor in classic SCC. This may be caused by a high pH
three conditions must be present: tensile stress, a susceptible in the soil, in the product, or even in the coating. Chlorides, H,S,
pipe material, and a supporting environment at the pipe surface. CO,, and high temperatures are more contributing factors. The
SCC is sometimes referred to as an “environmentally assisted presence of certain bacteria will increase the risk. Persistent
cracking” phenomenon. A breakdown in both coating barrier moisture and coating disbondment are also threatening condi-
and cathodic protection must occur before SCC initiates [63]. tions. In general, any environmental characteristic that promotes
Two different forms have been identified: high-pH SCC (classi- corrosion should be considered to be a risk contributor here.
cal) and near-neutral, low-pH SCC. These are similar in many This must include external and internal contributors.
ways and differ in the role of temperature, electrolyte character-
istics, and cracking morphology [63]. Both types are charac- Steel type A high carbon content (20.28%) increases the
terized by formation of corrosion-accelerated cracking in areas likelihood of stress corrosion cracking. Low ductility materials
of the pipe wall subjected to high tensile stress levels. The pres- with low fracture toughness are more susceptible. Sometimes
ence of corrosive substances aggravates the situation. Certain the rate of loading determines the fracture toughness-a mate-
types of steel are more susceptible than others. In general, a rial may be able to withstand a slow application of stress, but
steel with a higher carbon content is more prone to SCC. not a rapid application (see the design index discussion in
Characteristics ofthe steel that may have been brought about by Chapter 5). This further complicates the use of material type as
welding or other post-manufacturing processes may also make a contributing factor.
the steel more susceptible. Materials that have little fracture
toughness (see the design index discussion in Chapter 5) do not A schedule can be developed that employs these contributing
offer much resistance to brittle failure. Rapid crack propagation factors in an assessment ofthe potential for SCC. Low stress in
brought on by corrosion and stress is more likely in these mate- a benign environment is the best condition, whereas high stress
rials. Note that SCC is also seen in plastic pipe materials. in a corrosive environment is the most dangerous condition.
Stress corrosion cracking is difficult to detect and SCC fail- Stress level can be expressed as a percentage of maximum
ures are not predictable. The effects can be highly localized. allowable operating pressure (MAOP) or specified minimum
Even a fairly non-corrosive environment can support a SCC yield strength (SMYS) of the pipe-the highest normal operat-
process. A previous history of this type of process is, of course, ing pressure divided by MAOP or SMYS.
strong evidence of the potential. In the absence of historical A history of stress corrosion cracking should be seen as the
data, the susceptibility of a pipeline to this sometimes violent strongest evidence ofthis risk and should accordingly score the
failure mechanism should be judged by identifying conditions section at 0 points.
that may promote the SCC process. Predictive models have
been developed and have been effective in prioritizing excava-
tions to find higher occurrences than would be discovered C2. Cathodic protection (weighting 25% of
under a plan of investigations during routine maintenance [63]. corrosion threat)
ASME/ANSI B31.8 notes the following as high risk factors,
where further investigation may be warranted if all of the fol- The branch of the risk assessment leading to the variable
lowing are present in a segment: cathodic protection is as follows: