Page 123 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

511 00 Design Index
The comparison between the actual and the required wall additional stresses anticipated. Surge pressures, extreme
thickness is most easily done by using a ratio of the two num- temperatures, or other loadings are extremely unlikely.
bers. Using a ratio provides a numerical scale from which The total required wall thickness is therefore 0.60 + 0.08 =
points can be assigned. If this ratio is less than one, the pipe 0.68 in.
does not meet the design criteria-there is less actual wall The actual pipe wall thickness installed is a nominal 0.88 in.
thickness than is required by design calculations. The pipeline Manufacturing tolerances allow this nominal to actually be
system has not failed either because it has not yet been exposed as thin as 0.79 in. No documented thickness readings indi-
to the maximum designconditions or because some error in the cate that the line is any thinner than this 0.79-in. value and
calculations or associated assumptions has been made. A ratio recent integrity verifications indicate no defects, so the
greater than one means that extra wall thickness (above design evaluator uses 0.79 as the actual wall thickness.
requirements) exists. For instance, a ratio of 1.1 means that The ratio of actual to required wall thickness is therefore
there is 10% more pipe wall material than is required by design 0.79 + 0.68 = 1.16. Therefore, 16% of additional protection
and 1.25 means 25% more material. against external damage or corrosion exists.
The actual wall thickness should account for all possible The point value for 16% extra wall thickness is 5.6, using the
weaknesses, as discussed earlier and again in the integrity veri- equation given earlier.
fication variable. This can be done using detailed stress calcula-
tions (see Appendix C) or through derating factors devised by
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the evaluator. Example 5.2: Calculating the safety factor
When all issues have been considered, a simple point sched-
ule such as that shown in Table 5.1 can be employed to award Another section of cross-country steel pipeline is being eval-
points based on how much extra wall thickness exists. This uated. Hydrocarbon liquids are being transported here. In this
schedule uses the ratio of actual pipe wall to pipe wall required case, original design calculations are not available. The line is
and calls this ratio t. 35 years old and is exposed to varying external loadings. The
A simple equation can also be used instead ofTable 5.1. The evaluator proceeds as follows:
equation
1. Because of the age of the line and the absence of original
(t- 1) x 35 =point value documents, the most recent hydrostatic test pressure is used
to determine the maximum allowable stress for the pipe
yields approximately the same values and has the benefit of material. Using the test pressure of 2200 psig, the stress
more discrimination between differences in t. level is calculated to be 27,000 psi (see Appendix C). The
evaluator is thus reasonably sure that the pipeline can with-
Table 5.1 Point schedule based on extra wall thickness stand a stress level of 27,000 psi. The maximum operating
pressure of the line is 1400 psig. Using this value and a
stress level of 27,000 psi, the required wall thickness (for
f Points
internal pressure only) is calculated to be 0.38 in.
11.0 -10 WARNING 2. Using some general calculations and the opinions of the
1.0-1.1 3.5 design department, the evaluator feels that an additional
1.1 1-1.20 7 10% must be added to the wall thickness to allow for exter-
1.21-1.40 14 nal loadings for most conditions. This is an additional
1.41-1.60 21 0.04 in. He adds an additional 5% (total of 15% above
1.6 1-1.80 28
>I21 35 requirements for internal pressure alone) for situations
where the line crosses beneath roadways. This 5% is thought
to account for fatigue loadings at all types of uncased road
crossings, regardless of pipeline depth, soil type, roadway
Some examples to illustrate the pipe component ofthe safety design, and traffic speed and type. In other words, 15%
factor follow. wall thickness above that required for internal pressure only
is the requirement for the worst case situation. This is an
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Example 5.1: Calculating the Safety Factor additional 0.06 in. for sections that have uncased road
crossings.
A cross-country steel pipeline is being evaluated. The 3. Water hammer effects can produce surge pressures up to 100
pipeline transports natural gas. Original design calculations are psig. Such surges could lead to an internal pressure as
available. Pipe is the only type of pipeline component in the high as 1500 psig (100 psig above MOP). This additional
segment being assessed. The evaluator feels that no extraordi- pressure requires an additional 0.02 in. of wall thickness.
nary conditions exist on the line and proceeds as follows: 4. The requiredminimum wall thicknesses are therefore 0.38 +
0.06 + 0.02 = 0.46 in. for sections with uncased crossings,
1. He uses information from the design file to determine the and 0.38 + 0.04 + 0.02 = 0.44 in. for all other sections.
required wall thickness. A MOP of 2000 psig using a grade 5. The evaluator next determines the actual wall thickness.
of steel rated for 35,000-psi maximum allowable stress Records indicate that the original purchased pipe had a
yields a required wall thickness of 0.60 in, for this diameter nominal wall thickness of 0.65 in. When the manufacturing
of pipe (see Appendix C). External load calculations show tolerance is subtracted from this, the wall thickness is 0.58
the need for an additional 0.08 in. in thickness to handle the in. Field personnel, however, mention that wall thickness