Page 127 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

W104 Design Index
a large role in determining the microstructure of the steel. A negative pressure wave travels downstream from the point of
The microstructure of two identical compositions that interruption. The pressure wave that travels back upstream
were heat treated in different manners may be completely dif- along the pipeline adds to the static pressure already in the
ferent. One may be brittle (lacks toughness), and the other pipeline. A pipeline with a high upstream pressure might be
might be ductile at normal temperatures. The welding process overstressed as this pressure wave arrives, causing the total
forms what is known as the hear-affected zone (HAZ). This is pressure to exceed the MOP.
the portion of the parent metal adjacent to the weld that has an The magnitude of the pressure surge depends on the fluid
altered microstructure due to the heat of the welding operation. modulus (density and elasticity), the fluid velocity, and the
The HAZ is often a more brittle area in which a crack might speed of flow stoppage. In the case ofa valve closure as the flow
initiate. stoppage event, the critical aspect ofthe speed of closure might
Because the HAZ is an important element in the structural not be the total time it takes to close the valve. Most of the
strength of the pipe, special attention must be paid to the weld- pressure spike occurs from the last 10% of the closing of a gate
ing process that creates this HAZ. The choice of welding tem- valve, for instance.
perature, speed of welding, preheating, post-heating, weld From a risk standpoint, the situation can be improved
metal type, and even the type of weld flux, all affect the cre- through the use of surge protection devices or devices that pre-
ation of the HAZ. Improper welding procedures, either vent quick flow stoppages (such as valves being closed too
because of the design or execution of the welding, can create a quickly). The operator must understand the hazard and all pos-
pipeline that is much more susceptible to failure due to sible initiating actions before corrective measures can be cor-
cracking. This element of the risk picture is considered in the rectly employed. The evaluator should be assured that the
potential for human error in the incorrect operations index operator does indeed understand surge potential (see Appendix
discussion in Chapter 6. D for calculations). He can then assign points to the section
So-called “avalanche” or “catastrophic” fractures, where based on the chances of a hazardous surge occurring.
crack propagation extends literally for miles along the pipeline, To simplify this process, a hazardous surge can be defined as
have been seen in large-diameter, high-pressure gas lines. In one that is greater than 10% of the pipeline MOP. It may be
these “rapid-crack-growth” scenarios, the speed of the crack argued in some cases that a line, in its present service, may
growth exceeds the pipeline depressurization wave. This can operate far below MOP and hence, a 10% surge will still not
lead to a violent pipe failure where the steel is literally flattened endanger the line. A valid argument, perhaps, but perhaps also
out or radically distorted for great distances. From a risk stand- an unnecessary complication-removing a risk variable that
point, such a rupture extends the release point along the might be important as the operations change-in the risk
pipeline, but probably does not materially affect the amount of assessment. The evaluator should decide on a method and then
gaseous product released. An increased threat of damage due to apply it uniformly to all sections being evaluated.
flying debris is present. Preventive actions to this type of failure The point schedule can be set up with three general cate-
include crack arresters-sleeves or other attachments to the gories and room for interpolation between the categories. For
pipe designed to slow the crack propagation until the depressur- instance, evaluate the chances of a pressure surge of magnitude
ization wave can pass-and the use of more crack-resistant greater than 10% of system MOP:
materials including multilayer wall pipe.
If the evaluator is particularly concerned with this type of High probability 0 pts
failure and feels that it can increase the risk picture in her sys- Low probability 5 pts
tems, she can adjust the spill score in the leak impact factor Impossible 10 pts
(Chapter 7) by giving credit for crack arrester installations, and
recognizing the increased susceptibility of large-diameter, High probability exists where closure devices, equipment,
high-pressure gas lines (particularly those lacking material fluid modulus, and fluid velocity all support the possibility of a
toughness). pressure surge. No mechanical preventers are in place.
Operating procedures to prevent surges may or may not be in
C. Surge potential (weighting: 10%) place.
Lowprobability exists when surges can happen (fluid modu-
The potential for pressure surges, or water hammer effects, is lus and velocity can produce the surge), but are safely dealt
assessed here. The common mechanism for surges is the sud- with by mechanical devices such as surge tanks, relief valves,
den conversion of kinetic energy to potential energy. A mass and slow valve closures, in addition to operating protocol. Low
of flowing fluid in a pipeline, for instance, has a certain probability also exists when the chance for a surge to occur is
amount of kinetic energy associated with it. If this mass of only through a rather unlikely chain of events.
fluid is suddenly brought to a halt, the kinetic energy is con- Impossible means that the fluid properties cannot. under
verted to potential energy in the form of pressure. A sudden any reasonable circumstances, produce a pressure surge of
valve closure or pump stoppage is a common initiator of such magnitude greater than 10% MOP.
a pressure surge or, as it is sometimes called, a pressure spike.
A moving product stream contacting a stationary mass of Example 5.7: Scoring surge potential
fluid (while starting and stopping pumps, perhaps) is another
possible initiator. A crude oil pipeline has flow rates and product characteris-
This pressure spike is not isolated to the region of the initia- tics that are supportive of pressure surges in excess of 10% of
tor. In a fluid-filled pipeline, a positive pressure wave is propa- MOP. The only identified initiation scenario is the rapid closure
gated upstream of the point where the fluid flow is interrupted. of a mainline gate valve. All of these valves are equipped with