Page 395 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

370 Sample Pipeline Risk Assessment Algorithms
Within the paper, each of the above parameters is defined, and where iPF is the probability factor due to a particular failure
their respective equations provided. The resulting equation for mode, i. Breaking down the individual failure modes allows
the consequences of failure is as follows. one to identify the influence of each mode on the entire
pipeline. The authors also include an equation that takes the
CF = [20(DS1) + ST + 5(OS’PD) + 5(TT)] PR +40( 1 - PR)(D/8)1/2 probability of a particular failure mode and further breaks it
down into specific risk factors:
where
iPF = iSSF x iSVF
20(DS 1) = class location effect
ST = security of throughput where
5(OS*PD) = ductile fracture propagation
5CW = transition temperature of the material iPF = probability factor due to failure mode i
PR = multiplier reflecting product type. iSSF = susceptibility factor due to failure mode i
iSVF = severity factor due to failure mode i.
Again, a detailed description of each of the above is pro-
vided. The end result for calculating the relative risk combines The next section of the paper discusses the consequences of
the above factors as failure and the consequence factors. The consequence of failure
is the damage or cost incurred when a pipeline fails, as defined
RR=PF*CF as the sum ofall the feasible consequence factors.
This equation can be used to prioritize pipelines for main- CF = Z jCF
tenance, in-line inspection, hydrostatic retesting, or rehabili-
tation. Parameters can also be omitted to suit special The authors give consequence factors as risk to life, damage to
situations. The paper also provides a sensitivity analysis to property, loss of service, cost of failure, and environmental
justify the selection of the coefficients in the equations and effects. These factors are not weighted against each other; rather,
10 example problems where the algorithm is applied to vari- weighting is decided for each factor by the pipeline operator.
ous pipeline situations. Finally, the limitations of the algo- These above equations combine to form the full relative risk
rithm are discussed, mainly that it is only a ranking equation as follows:
mechanism, no number calculated should be thought of as an 1 1 .
absolute risk. RR = - ZiPF X - ZICF
7 5
Model 2 or
I 1 .
Z
Kirkwood, M. G., and Kamm, M.. ‘2 Scheme for Setting RR = - (ISSF X iSVF) X - ZICF
Pipeline Repair; Maintenance and Inspection Priorities,” 7 5
presented at Pipeline Risk Assessment, Rehabilitation and The 7 (the number ofprobability factors) and the 5 (the number
Repair Conference, September 12-15,1994. of consequence factors) in the denominators are used to aver-
age the total probabilities of each. The authors also suggest that
The focus of this paper is to provide the reader with a strategy this scheme can be used to examine risk of failure due to a spe-
to maintain and repair a pipeline using relative risk assessment. cific factor. For example, the risk of failure from internal corro-
Risk is defined as the combination of the probability of occur- sion can be determined from:
rence of a hazard and the magnitude of the consequences of the
1
.
1
failure. Both quantitative and qualitative risk are defined and CIR = - (ICPF + ECOF) X - ZICF
S
the paper provides a method that utilizes qualitative data, thus 2 5
producing risk within pipeline segments relative to one another. where
Using quantitative data produces absolute risk, rather than rela-
tive, but oftentimes not enough statistical data exist to properly CIR = a corrosion inspection rating
determine the risk. The relative risk method uses engineering ICPF = probability factor for internal corrosion
knowledge, experience, and awareness. ECPF = probability factor for external corrosion.
The next portion of the paper provides a detailed description
of the pipeline hazards used in the risk assessment. These haz- The parameters used in the priority rating are presented in a list
ards include internal corrosion, external corrosion, fatigue, as questions for the operator to answer. A value is then calcu-
stress-corrosion cracking, mechanical damage, third-party lated for each of the risk factors taking into account all of the
intervention, and loss of ground support. This is not an exhaus- parameters that have an effect on the pipeline. The parameter
tive list, but the examples are characteristic of the considera- list was compiled using references, a review of pipeline failure
tions required. data, and expert opinion. These input parameters are normally
The total probability of failure (PF) is given as the sum of assigned a value in the range of 0.0 to 1 .O. An example is pro-
each of the individual probability factors: vided showing this system, calculating the risk factor for
fatigue (FSSF).
PF=ZiPF Once the relative ranks are calculated, the given scheme can
be used in two ways: