Page 124 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 5/101
checks have revealed thicknesses as low as 0.55 in. This is regulatory safety factors be removed from MOP calculations
confirmed by documents in the files. Additionally, there (see page 00). It may be difficult, however. to separate the
may be weaknesses related to the low-frequency ERW pipe safety factor from the actual pressure-containing capabilities of Provide
manufacturing process. No integrity verifications have been the component.
performed recently. so there is justification for a conserva- A flange, for instance, may be rated by the manufacturer to
tive assumption of pipe weaknesses. The evaluator chooses operate at a pressure of 1400 psig. It can be safely tested for
to apply a somewhat arbitrary 12% de-rating of pipe short periods at pressures up to 2160 psig, as certified by the
strength due to possible weaknesses and uses 0.48 in. as the manufacturer. It is not obvious exactly how much pressure the
actual ‘effective’ wall thickness. flange can withstand from these numbers and it is a nontrivial
6. The actual-to-required-wall thickness ratios are therefore matter to calculate it. For purposes of this risk assessment, the
0.48 0.46 = 1.04 and 0.48 + 0.44 = 1.09 for sections with value of 1400 psig should probably be used as the maximum
and without uncased road crossings, respectively. These flange pressure even though this value certainly has a safety
ratios yield point values of 1.4 and 3.2, respectively. factor built in. The separation of the safety factor would most
Conservatism requires that the evaluator assign a value of likely not be worth the effort. It also makes the comparison to
1.4 points for this section of pipeline. pipe strength (MOP) more valid when safety factors are
removed from each.
On the other hand, the design calculations for a pressure ves-
.-lltemntivepipe strength scoring sel are usually available. This would allow easy separation of
the safety factor. Again, if these calculations are not available,
An alternative scoring approach for the sufep./uctor, could add the best course is probably to use the rated operating pressure.
pipe diameter as a variable to further consider structural This will yield the most conservative answer. Again. consis-
strength. Pipe strength. from an external loading standpoint, is tency is important.
related to the pipe’s wall thickness and diameter. In general, As in the pipe analysis. a ratio can be used to show the differ-
larger diameter and thicker walled pipes have stronger load- ence between what a system component can do and what it is
bearing capacities and should be more resistive to external presently being asked to do. This can be the pressure rating of
loadings. A thinner wall thickness and smaller diameter will the weakest component divided by the system maximum oper-
logically increase a pipe’s susceptibility to damage [48]. ating pressure. When this ratio is equal to I. there is no safety
Some risk evaluators have used D/t as a variable for both factor present (discounting some component safety factors that
resistance against external loadings and as a susceptibility-to- were not separated). This means that the system is being oper-
cracking indicator. As D/t gets larger, stress levels increase- ated at its limit. Ifthe ratio is less than 1, the system can theoret-
increasing failure potential and risk. ically fail at any time because there is a component of the
Another risk measure of pipe strength has been proposed system that is not rated to operate at the system MOP A ratio
[38] as a pipe geometry score, derived from a relationship greater than 1 means that there is a safety factor present; the
where failure probability is estimated to be proportional to system is being operated below its limit.
A simple schedule can now be developed to assign points. It
(l/r2+12,d) may look something like this:
where
t =pipe wall thickness (in.) Design-to-MOP Ratio
(I= pipe diameter (in. ), -7.0 3s pis
1.75-1.99 28pts
As this number gets higher, the relative risk of failure from 1.50-1.74 -71 pts
external forces increases. 1.25-1.49 llpts
Either of these relationships is readily converted into a risk 1.10-1.24 7pts
scoring scheme similar to the one described using simple wall 1.00-1.09 Opts
thickness ratios. <1.00 -1 0 pts
An equation can also be used instead of the point schedule:
Non-pipe coniponents
[(Design-to-MOP ratio) - I] x 35 =points
The evaluation of safety jactur should also include non-pipe
components whenever they are part of a segment being The steps for the evaluator are therefore:
assessed. If a non-pipe component is the weakest part of the
pipeline segment being evaluated its point score should govern. I. Determine the pressure rating of the weakest system
Components include flanges, valve bodies. fittings, filters, component.
pumps, flow measurement devices, pressure vessels, and others. 2. Divide this pressure rating (from step I) into the system-
Each pipeline component has a specified maximum operat- wide MOP.
ing pressure. This value is given by the manufacturer or 3. Assign points based on the schedule.
determined by calculations. The lowest pressure rating in the
system determines the weakest component and is used to set This is equivalent to the previous pipe strength evaluation
the design pressure. Ideally, the design pressure as it is but uses pressure instead of wall thickness. Because pressure
used here should not include safety factors for the individual and wall thickness are proportional in a stress calculation,
components for the same reasons it is recommended that pressure could also be used in the pipe strength analysis.