Page 117 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

5/94 Design Index
such under- and over-responses have changed the risk picture, Chapter 6, but the evaluator should carefully define MOP for
and then relate both of these to the consequences at specific purposes of risk assessment.
areas along the pipeline route.
V. Risk variables and scoring
Maximum pressure
The Design Index is more technically complex than most of the
The terms maximum operating pressure (MOP), maximum other components of the evaluation. If the evaluator does not pos-
allowable operating pressure (MAOP), maximum permissible sess expertise in matters of pipeline design, outside help may be
pressure, and design pressure are often used interchangeably beneficial. This is not a requirement, though. By making some
and indeed they are used interchangeably in this text. They all conservative assumptions and being consistent, a nonexpert can
imply an internal pressure level that comports with design do a credible job here. He must, however, be able to obtain
intent and safety considerations-whether the latter stem from some calculated values. Where original design calculations are
regulatory requirements, industry standards, or a company’s available, few additional calculations are needed.
internal policies. The following paragraphs describe a risk assessment model
MOP is normally calculated. For purposes of risk assess- that captures and evaluates design-related risk variables. All
ment, MOP can incorporate any and all design safety factors, or variables are listed together at the beginning of this chapter for
it may exclude the operating safety factors that are mandated by quick reference.
govemment regulations. It should not exclude engineering
safety factors that reflect the uncertainty and variability of A. Safety factor (weighting: 35%)
material strengths and the simplifying assumptions of design
formulas since these are technically based limitations on In this part of the assessment, the overall strength of the
operating pressure. These include adjustment factors for pipeline segment and its stress levels are considered. This
temperature, joint types, and other considerations. Regulatory includes an assessment of loads, stresses, and component
operating safety factors, however, usually go beyond this to strengths. Known and foreseeable weaknesses in pipe due to
allow for errors and omissions, deterioration of facilities, previous damage or suspect manufacturing processes are also
and extra safety margins in general. Such allowances are considered here. In effect, we are calculating a safety factor or a
certainly needed in pipeline operation, but can be confusing margin of safety, comparing what the pipeline can do (design)
if they are included in the risk assessment. The actual margin versus what it is currently being asked to do (operations).
of safety exists between the maximum stress level caused by The evaluation process involves an evaluation of loadings:
the highest pressure and the stress tolerance of the pipeline.
Measuring this directly without determining the margin Internal pressure
between a regulated stress level and stress tolerance, makes External loadings
the assessment more intuitive and useful when differing reg- Special loadings
ulatory requirements make comparisons more complicated.
Regulatory safety factors may therefore be omitted from the System strength (resistance to loadings) is also evaluated
MOP calculations for risk assessment purposes. As with all
elements of this risk assessment tool, such distinctions are Pipe wall thickness
ultimately left to the evaluator. Because a picture of risk relative Pipe material strength
to other pipelines is sought, any consistent definition of MOP Pipe structural strength
will work. Possible weaknesses in pipe
Surge (water hammer) pressures may be included in maxi- Other components.
mum pressure determination or alternatively, can be part of a
separate risk variable, as shown in this proposed model. Surge Internal pressure
potential is discussed in Appendix D. Pipe wall damages or sus-
pected weaknesses-anomalies-may impact pipe strength When calculating stresses due to internal pressure, evaluators
and hence allowable pressures or safety margins. Anomalies may use either the maximum (design) pressures or the normal
are discussed in Appendix C. Reductions of MOP resulting operating pressures, depending on the type of risk assessment
from pipeline anomalies are normally based on remaining being performed (see previous discussion of MOP definitions).
effective wall thickness calculations and conform The former is the most conservative and is appropriate for char-
to approaches described in industry standards such as ASME/ acterizing the maximum stress levels to which all portions of
ANSI B3 1 G, Manual for Determining the Remaining Swngth the pipeline might be subjected, even if the normal operating
of Corroded Pipelines, or AGA Pipeline Research Committee pressures for most of the pipe are far below this level. This use
Project PR-3-805, A Modified Criterion for Evaluating the of design pressure or MOP might be more appropriate when
Remaining Strength of Corroded Pipe. characterizing an entire pipeline as one unit. It also avoids the
It may also be important to make a distinction between a potential criticism that the assessment is not appropriately
safety-system-protected MOP from one that is impossible to conservative.
exceed due to the absence of adequate pressure productio- The second alternative, using normal operating pressures,
where it is physically impossible to exceed the MOP because provides a more realistic view of stress levels along the
there is no pressure source (including static head to temperature pipeline. Portions immediately downstream of pumps or com-
effects) that can cause an exceedance. This is covered more in pressors would routinely see higher pressures, and downstream