Page 93 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/71
B. Internal corrosion (weighting: 20% of corrosion wall. The greatest threat exists in systems where the product is
threat) inherently incompatible with the pipe material. Another threat
arises when corrosive impurities can routinely get into the
Internal Corrosion (20%,20pts) product. These two scenarios can be scored separately and then
Product corrosivity (50% of internalcorrosion = 10 pts) combined for an assessment of product corrosivity:
From potential upsets (70% ofproduct corrosivity = 7 pts)
Equipment (30% of 7 pts -2 pts) Product corrosivity = [flow stream charactenstics] +[upset conditions]
O&M (3OYoof 7 pts, 2 pts)
Flow velocity (40% of 7 pts, 3 pts) These components are added since the worst case scenario
From flow stream characteristics (30% of product corro would be a case where both are active in the same pipeline-
sivity, 3 pts) both a corrosive product and potential for additional corrosion
Solids related (40% of 3 pts, 1 pt) through upsets. The weighting of the two is situation specific,
Water related (60% of 3 pts, 2 pts) but because hydrocarbons are inherently non-corrosive and
Preventions (5oy0 of internal corrosion, 10 pts) most transportation of hydrocarbons strives for very low prod-
Measured corrosion rate (adjustments) uct contaminant levels, a weighting emphasizing upset poten-
tial might be appropriate for many hydrocarbon transport
In this section, an assessment is made of the potential for scenarios. The following example point scores uses a 30 / 70%
internal corrosion. Internal corrosion is pipe wall loss or dam- weighting scheme, emphasizing product corrosivity episodes
age caused by a reaction between the inside pipe wall and the originating from unintentional contaminations-upsets.
product being transported. Such corrosive activity may not be For convenience, the term contaminant is used here to mean
the result of the product intended to be transported but rather a some product component that is corrosive to the pipe wall, even
result of an impurity in the product stream. Seawater intrusion though some amounts of the component might he allowable
into an offshore natural gas stream, for example, is not uncom- according to the product specification.
mon. The natural gas (methane) will not harm steel, but saltwa-
ter and other impurities can certainly promote corrosion. Other Normalflow stream characteristics The normal flow stream
corrosion-promoting substances sometimes found in natural characteristics should represent a measure of the corrosivity of
gas include CO,, chlorides, H,S, organic acids, oxygen, free the products transported in the pipeline. This measure assesses
water, solids or precipitates, or sulfur-bearing compounds. corrosion potential from normal contact between flowing prod-
Microorganisms that can indirectly promote corrosion uct and the pipe wall, based on product specifications andor
should also be considered here. Sulfate-reducing bacteria and product analyses. A “no-flow’’ condition might aggravate oth-
anaerobic acid-producing bacteria are sometimes found in oil erwise harmless contact between product and pipe wall. An
and gas pipelines. They produce H,S and acetic acid, respec- example is the higher concentrations of dropout contaminants
tively, both of which can promote corrosion [79]. that occur during no-flow or low-flow conditions, such as water
Pitting corrosion and crevice corrosion are specialized forms accumulation in low spots. These scenarios can be considered
of galvanic or concentration cell corrosion commonly seen in here (as normal flow conditions) or they might be more effi-
cases of internal corrosion. Corrosion set up by an oxygen con- ciently handled under the evaluation of corrosivity due to upset
centration cell can be accelerated if certain ions are present to conditions (where they are considered to be abnormal flow con-
play a role in the reactions. The attack against certain stainless ditions).
steels by saltwater is a classic example. Erosion as a form of In many cases, the flow stream characteristics can be divided
internal corrosion is also considered here. into two main categories-water related and solids related-
Product reactions that do not harm the pipe material should for purposes of evaluating corrosivity [94]. These categories do
not be included here. A good example of this is the buildup of not precisely reflect the role or transport state of the various
wax or paraffin in some oil lines. While such buildups cause contaminants, but might be useful for organizing variables.
operational problems, they do not normally contribute to the
corrosion threat unless they support or aggravate a mechanism Flow stream characteristics = [water related] + [solids related]
that would otherwise not be present or as severe.
Some of the same measures used to prevent internal corro- Water-related contamination potential might include an
sion, such as internal coating, are used not only to protect the assessment of the concentrations of components such as
pipe, but also to protect the product from impurities that may be
produced by corrosion. Jet fuels and high-purity chemicals are Water content
examples of pipeline products that are often carefully protected Oxygen
from such contaminants. PH
The assessment ofthe threat from internal corrosion is evaluated H,S
by an examination ofthe product characteristics and the preventive Temperature
measures being taken to offset certain product characteristics. Chlorides.
BI. Product corrosivity (weighting: 50% of internal Solids-related contamination potential might include meas-
corrosion potential) uring the concentrations of components such as
This is an assessment of the relative aggressiveness of the MIC
pipeline contents that are in immediate contact with the pipe Suspended solids (see also discussion of erosion potential)