Page 125 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

5/102 Design Index
Note that no credit is given for weaker components that are of 2700 psig and are rated for 1800 psig in normal operation.
protected from overpressure by other means. These scenarios Again, the evaluator has no knowledge of the exact strength of
are examined in detail in the incorrect operations index the valves and meters, so he uses the normal operation rating
(Chapter 6). The reasoning here is that the entire risk picture is of 1800 psig.
being examined in small pieces. The fact that there exists a The weakest component, the pipe, governs; therefore,
weak component contributes to this piece of the risk picture,
regardless of protective actions taken. Even though a pressure 1700/900 = 1.89, which yields a point value of 3 1.2 points
vessel is protected by a relief valve, or a thin-walled pipe sec-
tion is protected by an automatic valve, the presence of such Note that in the preceding examples, the pipeline segments
weak components in the section being evaluated causes the being evaluated have a mixture of components. An alternative
lower design-to-MOP ratio and hence the lower point values. and often-preferable segmentation strategy would create a sep-
Of course, the evaluator may insert a section break if she feels arate pipeline segment, to be independently scored, for each
that a higher pressure section is being penalized by a lower component present. This avoids the blending of dissimilar risks
rated item when there is adequate isolation between the two. within a segment scores. It has the further benefits of allowing
Regardless of his choice, the adequacy of the isolation will be similar components to be grouped and compared-”apples to
evaluated in the incorrect operations index (Chapter 6). apples.” See discussions segmentation strategy (Chapter 2) and
also risk evaluations of station facilities (Chapter 13).
Example 5.3: Calculating the safety factor for
non-pipe components
B. Fatigue (weighting: 1%0)
The evaluator is examining a section of a jet fuel pipeline. Fatigue failure has been identified to be the largest single cause
The MOP of the pipeline is 1200 psig. This particular section of metallic material failure [47]. Historical pipeline failure data
has an aboveground storage tank that is rated for 1000 psig does not indicate that this is a dominant failure mechanism in
maximum. The tank is the weakest component in this section. It
is located on the low-pressure end of the pipeline and is pro- pipelines, but it is nonetheless an aspect of risk. Because a
tected by relief systems and redundant control valves such that fatigue failure is a brittle failure, it can occur with no warning
it never experiences more pressure than 950 psig. This effec- and with disastrous consequences.
tively isolates the tank from the pipeline system and does not Fatigue is the weakening of a material due to repeated cycles
require that the pipeline be down-rated to a lower operating of stress. The amount of weakening depends on the number
pressure. These safety measures, however, are not considered and the magnitude of the cycles. Higher stresses, occurring
for this item and the design-to-MOP ratio is as follows: more often, can cause more damage to the material. Factors
such as surface conditions, geometry, material processes, frac-
ture toughness, temperature, type of stress applied, and welding
Weakest component + system MOP = 1000/1200 = 0.80
processes influence susceptibility to fatigue failure (see
This is based on the fact that the weakest component can Cracking: a deeper look, in this chapter).
withstandonly 1000 psig.Thisratesapoint scoreof-l0points. Predicting the failure of a material when fatigue loadings
are involved is an inexact science. Theory holds that all mate-
rials have flaws--cracks, laminations, other imperfections-
Example 5.4: Calculating the safety factor for if only at a microscopic level. Such flaws are generally too
non-pipe components small to cause a structural failure, even under the higher
stresses of a pressure test. These flaws can grow though,
In this section, the only components are pipe and valves. The enlarging in length and depth as loads (and, hence, stress) are
pipe is designed to operate at 2300 psig by appropriate design applied and then released. After repeated episodes of stress
calculations. The overall system is rated for a MOP of 800 psig. increase and reduction (sometimes hundreds of thousands of
The valve bodies are nominally rated for maximum pressures these episodes are required), the flaw can grow to a size large
of 1400 psig, with permissible hydrostatic test pressures of enough to fail at normal operating pressures. Unfortunately.
2200 psig. The evaluator rates the weakest component, the predicting flaw growth accurately is not presently possible
valve bodies, to be 1400 psig. Because he has no exact informa- from a practical standpoint. Some cracks may grow at a con-
tion as to the strength of the valve bodies, he uses the pressure trolled, rather slow rate, while others may grow literally at the
rating that is guaranteed by the manufacturer for long-term speed of sound through the material. The relationship
service. The design-to-MOP ratio is, therefore, between crack growth and pressure cycles is based on fracture
mechanics principles, but the mechanisms involved are not
1400/800 = 1.75, which yields a point value of 26.3 points completely understood.
For the purposes of risk analysis, the evaluator need not be
able to predict fatigue failures. He must only be able to identify,
Example 5.5: Calculating the safety factor for in a relative way, pipeline structures that are more susceptible to
non-pipe components such failures.
Because it is conservative to assume that any amount of
Here, a section has valves, meters, and pipe. The MOP is cycling is potentially damaging, a schedule can be set up to
900 psig. The pipe strength is calculated to be 1700 psig. compare numbers and magnitudes of cycles. Stress magnitudes
The valve bodies and meters can all withstand pressure tests should be based on a percentage of the normal operating pres-