Page 290 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk assessment model 131267
Weather A. Atmospheric corrosion
The threat associated with meteorological events can be Atmospheric corrosion potential is a function of facility design,
assessed here. Events such as a wind storm, tornado, hurricane, environment, coating systems, and preventive maintenance
lightning, freezing, hail, wave action, snow, and ice loadings practices. There are many opportunities for “hot spots” as
should be considered. (Note that earth movements such as described in Chapter 4. Many station facilities are located in
earthquakes and landslides are considered in the design index.) heavy industrial areas or near waterways to allow for vessel
A relative, qualitative scale can be used to judge the frequency transfers. Industrial and marine environments are considered to
of occurrence for each possible event and the potential dam- be the most severe for atmospheric corrosion, whereas inland
ages resulting from any and all events. In areas where multiple dry climates are often the least sevcre. Score the potential for
damaging events are possible, the score should reflect the atmospheric corrosion as shown in Chapter 4.
higher potential threats. Mitigation measures can reduce threat
levels. B. Internal corrosion

During normal operations, station facilities are generally
Successive reactions exposed to the same internal corrosion potential as described in
Chapter 4. However, certain facilities can be exposed to corro-
The threat associated with one portion of the facility (or a sive materials in higher concentrations and for longer dura-
neighboring facility) causing damage to another portion of the tions. Sections of station piping, equipment, and vessels can be
facility is assessed here. Examples include vessels containing isolated as “dead legs” for weeks or even years. The lack of
flammable materials that, on accidental release and ignition, product flow through these isolated sections can allow internal
can cause flame impingement or explosion overpressure dam- corrosion cells to remain active. Also, certain product additive
ages (including projectile damages) to adjacent components of and waste collection systems can also concentrate corrosion
the facility. Therefore, portions of a facility that are more sus- promoting compounds in station systems designed to transport
ceptible to such secondary accident effects will show a higher products within line pipe specifications. Score the items for
risk. The threat value associated with this external force is logi- internal corrosion, product corrosivity, and internal protection
cally less since another event must first occur before this event as described elsewhere in this text.
becomes a threat. This reduces the probability of the successive
reaction event. C. Subsurface corrosion
A qualitative scale can be used to judge this risk level includ-
ing the damage potential of the causal event. The type and In some older buried metal station facility designs, little or no
quantity of the material stored determines the damage poten- corrosion prevention provisions were included. If the station
tial. A calculation of the overpressure (blast wave) effects from facilities were constructed during a time when corrosion pre-
an explosion scenario is a valid measure of this potential (see vention was not undertaken, or added after several years, then
Chapters 7 and 14). Where such calculations are not performed, one would expect a history of corrosion-caused leaks. Lack of
an approximation can be made based on the type, quantity, and initial cathodic protection was fairly common for buried station
distance of the nearby flammables. piping constructed prior to 1975. If it can be demonstrated that
Points are assigned based on the vulnerability of nearby corrosion will not occur in a certain area due to unsupportive
facilities. Where protective shields, barriers, or distance reduce soil conditions, CP might not be required. The evaluator should
the likelihood of damage from the causal event, the threat is ensure that adequate tests of each possible corrosion-enhancing
reduced and point assignments should reflect the lower poten- condition at various soil moisture levels during a year have
tial. Protective barriers and shields should be assessed for their been made, before subsurface corrosion is dismissed as a fail-
realistic ability to protect adjacent components from thermal ure mechanism.
and blast effects. Modem stations employ the standard two-part defense of
Note that, for simplicity, the likelihood of failure of the coatings and cathodic protection detailed in Chapter 4.
causal event is usually not considered since such considera- Subsurface corrosion potential can be evaluated as described in
tion involves another complete risk assessment. This addi- that chapter, with consideration for some issues.
tional assessment might not be possible if the causal event can Older, poorly coated, buried steel facilities will have quite
occur from a neighboring facility that is not under company different CP current requirements than will newer, well-coated
control. steel lines. These sections must often be well isolated (electri-
cally) from each other to allow cathodic protection to be effec-
Corrosion index tive. Given the isolation of buried piping and vessels, a system
of strategically placed anodes is often more efficient than a rec-
Depending on the materials being used, the same corrosion tifier impressed current system at pipeline stations. It is com-
mechanisms are at work on pipeline station facilities as are mon to experience electrical interferences among buried
found in buried or aboveground pipe on the ROW. However, it is station facilities where shorting (unwanted electrical connec-
not unusual to find station piping that has little or no coating, or tivity) of protective current occurs with other metals and may
other means of corrosion prevention, and is more susceptible to lead to accelerated corrosion.
corrosion. As in the basic line pipe model, corrosion potential is Even within a given pipeline station, soil conditions can
assessed in the three categories of atmospheric, internal, and change. For instance, tank farm operators once disposed of tank
subsurface. bottom sludges and other chemical wastes on site, which can