Page 291 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

131268 Stations and Surface Facilities
cause highly localized and variable corrosive conditions. In handle unanticipated stresses. Components with complex
addition, some older tank bottoms have a history of leaking shapes are often difficult to calculate. Manufacturer informa-
products over a long period of time into the surrounding soils tion is often used in those cases. Either normal operating pres-
and into shallow groundwater tables. Some materials may pro- sures or maximum operating pressures can be used in
mote corrosion by acting as a strong electrolyte, attacking the calculating stress levels, just as long as one or the other is con-
pipe coating or harboring bacteria that add corrosion mecha- sistently applied. Adjustments for joint efficiencies in tanks
nisms. Station soil conditions should ideally be tested to iden- and piping might also be appropriate.
tify placement of non-native material and soils known to be Materials with a lack of ductility also have reduced tough-
corrosion promoting. ness. This makes the material more prone to fatigue-type fail-
Station piping of different ages and/or coating conditions ures and temperature-related failures and also increases the
may be joined. Dissimilar metals can create galvanic cells chances for brittle failures. Brittle failures are often much more
and promote corrosion in such piping connections. Pipeline consequential than ductile failures since the potential exists for
stations sometimes use facilities as an electrical ground for a larger product releases and increased projectile loadings. The
control building’s electrical system, which can possibly impact potential for catastrophic tank failure should be considered,
the cathodic protection system, corrosion rates, and spark perhaps measured by shell and seam construction and mem-
generation. brane stress levels for susceptibility to brittle fracture.
AC induction is a potential problem in station facilities any-
time high voltages are present. Large compressor and pump B. Fatigue
stations, as well as tank farms, normally carry high-voltage and
high-current electrical loads. Therefore, nearby buried metal As one of the most common failure mechanisms in steel,
can act as a conduit, becoming charged with AC current. fatigue potential is assessed as discussed on pages 000-000.
Although AC induction is primarily a worker safety hazar4 it Instances of high stress levels at very rapid loading and unload-
has also been shown to be disruptive to the station’s protective ing (high frequency of stress cycles) are the most damaging
DC current and a direct cause of metal loss. scenario.
The threat is reduced as cycle frequency or magnitude is
Design index reduced. It is common practice to put extra strength compo-
nents with very high ductility into applications where high
As detailed in Chapter 5, the design index is a collection of fail- fatigue loadings are anticipated. Common causes of fatigue on
ure mechanisms and mitigations related to original design con- buried components and aboveground connections to equip-
ditions. The main variables described there are also appropriate ment include loading cycles from traffic, wind loadings, watex
for a station risk model. Those factors are: impingements, harmonics in piping, rotating equipment, pres-
sure cycles, temperature cycles, and ground freezindthawing
A. Safety Factor cycling. Mitigation options include the removal or reduction
B. Fatigue of the cycles or, as previously mentioned, the use of special
C. Surge Potential materials.
D. Integrity Verification
E. Land Movements Vibration monitoring As a further measure of potential
fatigue loadings, sources of vibration can be assessed. As a
Some additional issues arise regarding the safeqfuctor and prime contributor to vibration effects, rotating equipment
fatigue assessments, as are discussed here. vibrations can be directly measured or inferred from evidence
such as action type (piston versus centrifugal, for example),
A. Safety factor speed, operating efficiency point, and cavitation potential.
Common practices to minimize vibration effects include care-
Although pipeline station facilities are typically constructed of ful attention to equipment supports, PPM practices, pulsation
carbon steel, other construction materials are also used. dampers, and the use of high ductility materials operating far
Because station equipment can be made of a composite of dif- from their maximum stress levels.
ferent materials, it can be useful to distinguish between materi-
als that influence the risk picture differently. In scoring the Incorrect operations index
safe@factor, the evaluator should take into account material
differences and other pipe design factors peculiar to station Human error is a significant factor to consider when scoring
facilities. risk at a pipeline station. Human error is oAen the true root
The stress level of a component, measured as a percentage of cause of facility failures when one considers that proper design,
maximum allowable stress or pressure., shows how much mar- construction, testing, operations, inspection, and maintenance
gin exists between normal operating levels and component should prevent almost all equipment and product containment
maximum stress levels. At stress levels close to absolute toler- integrity failures.
ances, unknown material defects or unanticipated additional A station environment provides many more opportunities for
stresses can easily result in component failure. Systems that are human error but also provides more chances to interrupt an
being operated at levels far below their design levels have a accident sequence through mitigation measures to avoid human
safety margin or safety factor. Many pressure vessels and pipe error. This part of the assessment builds on Chapter 6. Several
components have safety factors of 1.5 to 2.0. When the safety previously described risk variables are discussed here that are
factor is close to 1.0, there is little or no margin for error or to specific to the station environment.