Page 288 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk assessment model 13/265
the most part aboveground, such as terminals, tank farms, and Incorrect Operations Index
pump stations, and are usually on property completely con- A. Design
trolled by the owner, the approach described in those chapters B. Construction
should be somewhat modified. Some suggested modifications C. Operations
are designed to better capture the risks unique to surface facili- D. Maintenance
ties, while maintaining a direct comparability between these
facilities and the pipe-only portions of the pipeline system. The Leak Impact Factor
basic components of the risk score for any station facility are Product Hazard
showninTable 13.3. Spill Size
Dispersion
Receptors
Risk model components
[Index Sum] = [External Forces] + [Corrosion] +[Design] + [Incorrect
In the revised model, variables in the corrosion, design, and Operations]
incorrect operations indexes are scored as described in [Relative Risk] = [Index Sum] / [LIF]
Chapters 4 through 6, respectively, with only minor modifica-
tions. The leak impact factor (LIF) is similarly scored with only Given the many types of stations that might be evaluated with
a slight possible modification, as described later. The main dif- this model, an additional adjustment factor, to take into account
ference in the revised model entails the treatment of certain the relative size and complexity of a station, is recommended.
external forces. In Chapter 3, an index called the third-party This is called the equivalent surface area, discussednext, and it
damage index is used to assess the likelihood of unintentional is used to adjust the index sum.
outside forces damaging a buried pipeline or a small above-
ground component such as a valve station. A different set Equivalent surface area
of outside forces can impact a surface facility so this index
title has been changed to External Forces for use in station In this risk assessment approach, the failure probability of a
assessments. station is thought to be directly proportional to the station’s
Comparisons and references to the basic model are made in complexity and density of more “problematic” components.
the descriptions of scorable items that follow. After customiza- The facility dimensions, adjusted for components that his-
tion, the risk model for pipeline station facilities could have the torically are more problematic, provide a relative “area of
following items: opportunity” for failures. Specifically, larger surface areas
result in more chances for corrosion, traffic impacts, fire
External Forces Index impingement, projectile loadings, wind loadings, and often
Corrosion Index complexity-which can lead to human error. It is reasonable to
A. Atmospheric Corrosion believe that more tankage, more piping, more pumps, more
B. Internal Corrosion vessels, etc., lead to more risk of failure. Under this premise,
C. Subsurface Corrosion stations will show higher failure probabilities overall as they
Design Index become larger and more complex, compared to cross-country
pipe or smaller stations. This is consistent with commonly held
A. Safety Factor beliefs and seems to be supported by many company’s incident
B. Fatigue databases.
C. Surge Potential A measuring scale can be developed to capture the relative
D. Integrity Verification complexity and nature of facilities. This scale is called the
E. Land Movements equivalent surface area. It selects a base case, such as 1 square
foot of aboveground piping. All other station components will
Table 13.3 Basic components of a risk score for a station facility be related to this base case in terms of their relative propensity
to initiate or exacerbate leaks and other failures.
Risk model component @pe of informution needed The equivalent surface area measure first evaluates the
physical area of assessed facilities. Actual surface area is calcu-
Probability lated based on facility dimensions: combined surface areas of
Probability vanables Conditions and activities that are integrity all piping, tankage, compressors, etc. Adjustments are then
threats; qualities ofvariables and made for higher leak-incident components by converting a
weightings
Area of opportunity Physical equipment and material sizes; counts count of such components into an equivalent surface area. Table
of more problematic components 13.4 is a sample table of equivalencies for some commonly
Consequence encountered station components.
Product hazard Acute and chronic product hazards; stored The relationships shown in Table 13.4 are established based
energy quantities on any available, published failure frequency data (in any
Spill size Volumes stored; leak detection capabilities; industry) or on company experience and expert judgment oth-
secondary containment erwise. Table 13.4 implies that, from a leak incident standpoint,
Receptors Population, environmental receptors, high- 1000 ft2 of above-ground piping = 200 fiz of tank bottom = 1/2
value area considerations; rangeability; loss
control systems of a Dresser coupling = 5 other mechanical couplings = 20 tan-
dem pump seals. This reflects a belief that couplings and tank
Risk score = probability x consequence = [Index Sum] / [LIF] bottoms cause more problems than aboveground piping.