Page 122 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 5/99
tection under certain circumstances, this is not thought to be a Many anomalies will be of a size that do not require repair
common failure mechanism. because they have not reduced the pipe strength from required
There is no proven method of predicting the failure pressure levels. However, a risk assessment that examines available
level of a preexisting blister and no proven method to calculate pipe strength should probably treat anomalies as evidence of
its crack-driving potential from the standpoint of fatigue [86]. reduced strength and possible active failure mechanisms.
The potential for laminations surviving pressure tests, adding A complete assessment ofremaining pipe strength in consid-
weaknesses to the pipe wall, and contributing to a future failure eration of an anomaly requires accurate characterization of the
can be considered by the evaluator when deemed appropriate. anomaly-its dimensions and shape. In the absence ofdetailed
remaining strength calculations, the evaluator can reduce pipe
Construction issites strength by a percentage based on the severity of the anomaly.
Higher priority anomalies are discussed in Appendix C.
Similar to the discussion on pipe manufacturing techniques, the
methods for welding pipe joints have improved over the years. Stress calculations
Girth welds today must pass a more stringent inspection than
welds from the original construction of the pipeline. Welding Calculation of the required wall thickness for a pipeline to
standards such as API 1104 (incorporated by reference into withstand anticipated loads involves several steps. First,
US. regulations) specify additional and different potential Barlow’s formula for circumferential stress is used to deter-
weld defects to be repaired than the standards from previous mine the minimum wall thickness required for internal pres-
periods. sure alone. This calculation is demonstrated in Appendix C.
It is not certain that girth weld defects, as defined by today’s Barlow’s calculation assumes a uniform material thickness and
welding inspection standards, increase the probability of strength and requires the input of a maximum allowable stress
weld failure in an inspected and tested pipeline. However, this for the pipe material. It yields a stress value for the extreme
issue illustrates an improving safety and risk-awareness evolu- fibers ofthe pipe wall (for the stress due solely to internal pres-
tion over time, presumably rooted in actual experience and sure). By starting with a maximum allowable material stress,
supported by engineering calculations. the wall thickness needed to contain a given pressure is calcu-
Arc bums, created during welding, are of concern due to the lated. Alternately, inputting a wall thickness into the equation
possibility of tiny cracks forming around the “hard spot” that yields the maximum internal pressure that the pipe can with-
can be created from the arc burn. A common procedure among stand. These calculations assume that there are no weaknesses
pipeline operators is to remove arc burns. in the pipe.
Some previous construction techniques might have permit- Allowable material stress levels are normally specified in
ted miter joints. wrinkles in field bends, certain branch rein- pipe purchase agreements and verified by material test reports
forcement designs, certain repair methods, and other aspects accompanying the purchase. These reports are usually called
not currently acceptable for most pipeline construction. These miN certifications of pipe material composition and properties
should be considered in evaluating the strength ofthe system. and are issued by the pipe manufacturing steel mill.
Offsetting these concerns to some extent might be the evi- In the absence of mill certificates, reliable pipe specification
dence of a pipeline system in continuous and reliable operation documents (or recent pressure test data+specially if the mate-
for many years. In other words, incorporating “withstood the rial ratings are questioned) regarding the maximum pressure to
test of time” evidence may be appropriate. which the pipe has been subjected (usually the preservice
hydrostatic test) can be used to calculate a material allowable
Damages during operations stress. That is, we input the maximum internal pressure into
Barlow’s formula to calculate a material allowable stress value.
Failure modes and potential damage can occur when the From this allowable stress value. we can then calculate a mini-
pipeline is in operation. These include damage from corrosion. mum required wall thickness.
dents, gouging, ovality, cracking. stress corrosion cracking
(SCC), and selective seam corrosion. These are generally rare Scoring thepipe safetv factor
phenomena and involve simultaneous and coincident failure
mechanisms. Potential corrosion damage and SCC are The procedure recommended here is to calculate the required
addressed in Chapter 4. pipe wall thickness and compare it to the actual wall thickness
Selective seam corrosion is a possible, but rare, phenomenon (see Figure 5.4), adjusted by any integrity assessment informa-
on low-frequency ERW pipe. However, the possibility cannot tion available. The required wall thickness calculation is more
be dismissed entirely. It is an aggressive form of localized straightforward if it does not include standard safety factors.
corrosion that has no known predictive models associated This is not only in the interest of simplicity. but also because
with it. Not all low-frequency ERW pipe is vulnerable since, some of the reasons for the safety factors are addressed in other
apparently, special metallurgy is required for increased suscep- sections of this risk analysis. For instance, regulations often
tibility [86]. base design safety factors on nearby population density.
Damages can be detected by visual inspection or through Population density is part of the consequences section (see
integrity verification techniques. Until an evaluation has shown Chapter I, Leak Impaci Factor) in this evaluation system and
that an indication detected on a pipe wall is potentially serious, would cloud the issue of pipe strength if considered here also.
it is normally called an anomalv. It is only a defect if it reduces Consequences are examined in detail separately from probabil-
pipe strength significantly-impairing its ability to be used as ity-of-failure considerations, for purposes of risk assessment
intended. clarity and risk management efficiency.