Page 129 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

5/106 Design Index
Visual and NDT inspections (see Appendix G) performed However, the inability to detect such flaws is a limitation of
on exposed pipe can be very thorough, but are very localized pressure testing.
assessments. A zone-of-influence approach (see Chapter 8) or By conducting a pressure test at high pressures, the pipeline
ideas taken from statistical sampling techniques can be used is being subjected to stress levels higher than it should ever
to extrapolate integrity information from such localized encounter in everyday operation. Ideally, then, when the
assessments. pipeline is depressured from the hydrostatic test, the only
For an ILI, the assessment should ideally quantify the ability cracks left in the material are of a size that will not grow under
of the 1LI to detect all possible defects and characterize them to the stresses of normal operations. All cracks that could have
a given accuracy. Given the myriad of possible defects, ILI grown to a critical size under normal pressure levels would
tools, interpretation software, and postinspection excavation have already grown and failed under the higher stress levels of
programs, this can be a complex undertaking. In the final the hydrostatic test.
analysis, it is again the largest theoretical undetected defect that Research suggests that the length of time that a test pressure
best characterizes the robustness. is maintained is not a critical factor. This is based on the
One approach is to characterize the ILI program-tool accu- assumption that there is always crack growth and whenever the
racy, data interpretation accuracy, excavation verification pro- test is stopped, a crack might be on the verge of its critical size
tocol-against all possible defect types. and, hence, close to failure.
When both a pressure test and an ILI have been done, the The pressure level, however, is an important parameter. The
scores can be additive up to the maximum allowed by the termpressure reversal refers to a scenario in which, after a suc-
variable weighting. cessful pressure test, the pipeline fails at a pressure lower than
the test pressure. This occurs when a defect survives the test
Pressure test pressure but is damaged by the test so that it later fails at a lower
pressure when the pipeline is repressurized. The higher the test
A pipeline pressure test is usually a hydrostatic pressure pressure relative to the normal operating pressure, the greater
test in which the pipeline is filled with water, then pressur- the safety margin. The chances of a pressure reversal become
ized to a predetermined pressure, and held at this test pres- increasingly remote as the margin between test and operating
sure for a predetermined length of time. It is a destructive pressures increases. This is explained by the theory of critical
testing technique because defects are discovered by pipe crack size discussed earlier.
failures during the test. Other test media such as air are Immediately after the pressure test, uncertainty about
also sometimes used. Tests with compressible gases carry pipeline integrity begins to grow again. Because a new defect
greater damage potential since they can precipitate failures could be introduced at any time or defect growth could accel-
and causemore extensive damage than by testing with an erate in a very localized region, the test’s usefulness is tied to
incompressible fluid. other operational aspects of the pipeline. Introduction of new
The test pressure exceeds the anticipated operational maxi- defects could come from a variety of sources, such as corro-
mum internal pressure to prove that the system has a margin of sion, third-party damages, soil movements, pressure cycles,
safety above that pressure. It is a powerful technique in that it etc., all of which are contributing to the constantly changing
proves the strength of the entire system. It provides virtually risk picture. For this reason, pressure test data have a finite
indisputable evidence as to the system integrity (within the test lifetime as a measure of pipeline integrity. A pipeline can
parameters). However, pressure testing does not provide infor- be retested at appropriate intervals to prove its structural
mation on defects or damages present below its detection integrity.
threshold. Such surviving defects might later worsen and cause Interpretation of pressure test results is often a nontrivial
a failure. exercise. Although time duration of the test may not be critical,
As noted previously, all materials have flaws and defects, if the pressure is normally maintained for at least 4 hours for prac-
only at the microscopic level. Given enough stress, any crack tical reasons, if not for compliance with applicable regulations.
will enlarge, growing in depth and width. Under the constant During the test time (which is often between 4 and 24 hours),
stress of a pressure test, it is reasonable to assume that a temperature and strain will be affecting the pressure reading.
group of flaws beyond some minimum size will grow. Below This requires a knowledgeable test engineer to properly inter-
this minimum size, cracks will not grow unless the stress pret pressure fluctuations and to distinguish between atransient
level is increased. If the stress level is rather low, only the effect and a small leak on the system or the inelastic expansion
largest of cracks will be growing. At higher stresses, smaller of a component.
and smaller cracks will begin to grow, propagating through The evaluation point schedule for pressure testing can con-
the material. When a crack reaches a critical size at a firm proper test methods and assess the impact on risk on the
given stress level, rapid, brittle failure ofthe structure is likely. basis of time since the last test and the test level (in relation to
(See previous explanations of fracture toughness and crack the normal maximum operating pressures). An example sched-
propagation in this chapter.) Certain configurations of rela- ule follows:
tively large defects can survive a hydrostatic test. A very nar-
row and deep groove can theoretically survive a hydrostatic (I) Calculate H, where H = (testpressureh4OP)
test and, due to very little remaining wall thickness, is more H< 1.10(1.10=testpressure lO%aboveMOP) 0 pts
susceptible to failure from any subsequent wall loss (perhaps 1.11 <H< 1.25 5 pts
occurring through corrosion). Such defect configurations are 1.26<H< 1.40 10pts
rare and their failure potential at a pressure lower than the test H> 1.41 15 pts
pressure would require ongoing corrosion or crack growth. or a simple equation can be used: