Page 67 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

3/46 Third-party Damage Index
Serious damage to a pipeline is not limited to actual punc- ment). Three feet of cover is a common amount of cover
tures of the line. A mere scratch on a coated steel pipeline dam- required by many regulatory agencies for new construction.
ages the corrosion-resistant coating. Such damage can lead to Credit should also be given for comparable means of pro-
accelerated corrosion and ultimately a corrosion failure per- tecting the line from mechanical damage. A schedule can
haps years in the future. If the scratch is deep enough to have be developed for these other means, perhaps by equating
removed enough metal, a stress concentration area (see Chapter the mechanical protection to an amount of additional earth
5) could be formed, which again, perhaps years later, may lead cover that is thought to provide equivalent protection. For
to a failure from fatigue, either alone or in combination with example,
some form of corrosion-accelerated cracking.
This is one reason why public education plays such an impor- 2 in. ofconcrete coating = 8 in. of additional earth cover
tant role in damage prevention. To the casual observer, a minor 4 In. of concrete coating = 12 in. of additional earth cover
Pipe casing = 24 in. of additional cover
dent or scratch in a steel pipeline may appear insignificant- Concrete slab (reinforced) = 24 in. of additional cover.
certainly not worthy of mention. A pipeline operator knows the
potential impact of any disturbance to the line. Communicating Using the example formula above, a pipe section that has 14
this to the general public increases pipeline safety. in. of cover and is encased in a casing pipe would have an equiv-
Several variables are thought to play a critical role in the alent earth cover of 14 + 24 = 38 in., yielding a point value of
threat of third-party damages. Measuring these variables can 38 + 3 = 12.7.
therefore provide an assessment of the overall threat. Note that Burial of a warning tape-a highly visible strip of material
in the approach described here, this index measures the poten- with warnings clearly printed on it-may help to avert damage
tial for third-party damage-not the potential for pipeline fail- to a pipeline (Figure 3.3). Such flagging or tape is commer-
ure from third-party damages. This is a subtle but important cially available and is usually installed just beneath the ground
distinction. Ifthe evaluator wishes to measure the latter in a sin- surface directly over the pipeline. Hopefully, an excavator will
gle assessment, additional variables such as pipe strength, oper- discover the warning tape, cease the excavation, and avoid
ating stress level, and characteristics of the potential third-party damage to the line. Although this early warning system
intrusions (such as equipment type and strength) would need to provides no physical protection, its benefit from a failure-
be added to the assessment. prevention standpoint can be included in this model. A deriva-
What are believed to be the key variables to consider in tive of this system is a warning mesh where instead of a single
assessing the potential for third-party damage, are discussed in strip of low-strength tape, a tough, high-visibility plastic mesh,
the following sections. Weightings reflect the relative percent- perhaps 30 to 36 in. wide is used. This provides some physical
age contribution of the variable to the overall threat of third- protection because most excavation equipment will have at
party damage. least some minor difficulty penetrating it. It also provides addi-
tional protection via the increased width, reducing the likeli-
hood of the excavation equipment striking the pipe before the
Assessing third-party damage potential warning mesh. Either system can be valued in terms of an
equivalent amount of earth cover. For example:
A. Minimum depth of cover (weighting: 20%)
Warning tape = 6 in. of additional cover
The minimum depth of cover is the amount of earth, or equiva- Warning mesh = 18 in of additional covet
lent cover, over the pipeline that serves to protect the pipe from
third-party activities. As with all items in this risk assessment system, the evalua-
A schedule or simple formula can be developed to assign tor should use his company’s best experience or other available
point values based on depth of cover. In this formula, increasing information to create his point values and weightings. Common
points indicate a safer condition; this convention is used situations that may need to be addressed include rocks in one
throughout this book. A sample formula for depth of cover is as region, sand in another (is the protection value equivalent?) and
follows: pipelines under different roadway types (concrete versus
asphalt versus compacted stone, etc.). The evaluator need only
Amount of cover in inches - 3 =point value up to a maximum of
20 points remember the goal of consistency and the intent of assessing
the amount of real protection from mechanical damage.
For instance, Ifthe wall thickness is greater than what is required for antic-
ipated pressures and external loadings, the extra thickness is
42 in. of cover = 42 + 3 points = 14 points available to provide additional protection against failure from
24 in. of cover = 24 + 3 points = 8 points external damage or corrosion. Mechanical protection that may
be available from extra pipe wall material is accounted for in
Points should be assessed based on the shallowest location the design index (Chapter 5).
within the section being evaluated. The evaluator should feel In the case of pipelines submerged at water crossings, the
confident that the depth of cover data are current and accurate; intent is the same: Evaluate the ease with which a third party
otherwise, the point assessments should reflect the uncertainty. can physically access and damage the pipe. Credit should be
Experience and logic indicates that less than one foot of given for water depth, concrete coatings, depth below seafloor,
cover may actually do more harm than good. It is enough cover extra damage protection coatings, etc.
to conceal the line but not enough to protect the line from even A point schedule for submerged lines in navigable water-
shallow earth moving equipment (such as agricultural equip- ways might look something like the following: