Page 119 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

5/96 Design Index
portions might never see pressures even close to design limits. Each aboveground span can be visually inspected to verify
This approach might be more appropriate for operational risk the existence ofan adequate number ofpipe supports, such that
assessments where differences along the pipeline are of most pipe spans do not exceed precalculated lengths based on
interest. applied dead load (i.e., load due to gravity) and the internal
Pressure cycling should be a part of the assessment since the pressure. Pipe coating and pipe supports can also be inspected
magnitude and frequency of cycling can contribute to fatigue for integrity. Historical floodwater elevations can be identified
failure mechanisms. This is discussed elsewhere. based on field inspections andor against floodplain maps.
Calculating pipe stresses from internal pressure is discussed The maximum allowable pipe spans can be calculated based
in Appendix C. on accepted industry standards such as ASME B31.4, Liquid
Transportation Svsteins for Hydrocarbons, Liquid Petroleum
External loadings Gas, Anhvdrous Ammonia and Alcohol, that specify require-
ments for gravity loads and internal pressure. Allowable span
External loadings include the weight ofthe soil over the buried lengths can conservatively be based on the assumption of a
line, the loadings caused by traffic moving over the line, possi- beam fully restrained against rotation at its supports when
ble soil movements (settling, faults, etc.), external pressures calculating the applied stresses in the span.
and buoyancy forces for submerged lines, temperature effects
(these could also be internally generated), lateral forces due to Third-party daniage
water flow, and pipe weight. Stress equations for some of these
are shown in Appendix C. Loadings from third-party strikes are not normally included
The diameter and wall thickness of the pipe combine to in pipe design calculations, but the pipe’s design certainly influ-
determine the structural strength of the pipeline against most ences the pipe’s ability to withstand such forces. According
external loadings. Pipe flexibility is also a factor. Rigid pipe to one study, pipe wall thickness in excess of 11.9 mm can
generally requires more wall thickness to support external only be punctured by 5% of excavator types in service as of
loads than does flexible pipe. This chapter focuses on steel pipe 1995 and none could cause a hole greater than 8Omm.
design. See Chapter 11 for a discussion of other commonly Furthermore, no holes greater than 80 mm have occurred in
used pipe materials. pipelines operating at design factors of:0.3 with a wall thick-
ness greater than 9.1 mm [%]. These types of statistics can be
Overburden useful in assessing risks, either in a relative sense or in absolute
terms (see Chapter 14).
The weight of the soil or other cover and anything moving over
the pipeline comprises the overburden load. In an offshore envi- Buckling
ronment, this would also include the pressure due to water
depth. Uncased pipe under roadways may require additional Pipe buckling or crushing is most often a consideration for
wall thickness to handle the increased loads. offshore pipelines in deep water. Calculations can estimate
Often, casing pipe is installed to carry anticipated external the pressure level required for buckling initiation and buckling
loads. A casing pipe is merely apipe larger in diameter than the propagation. It is usually appropriate to evaluate buckle poten-
carrier pipe whose purpose is to protect the carrier pipe from tial when the pipeline is in the depressured state and, thereby,
external loads (see Figure 4.2). Casing pipe often causes diffi- most susceptible to a uniformly applied external force (see
culties in establishing cathodic protection to prevent corrosion. Appendix C). Recognition of buckling initiation pressure is
The effect of casings on the risk picture from a corrosion stand- sometimes made since less pressure is needed to propagate a
point is covered in the corrosion index (see Chapter 4). The buckle, compared with initiating.
impact on the design index is found here, when the casing car-
ries the external load and produces a higher pipe safety factor Other
for the section being evaluated (see The case fodagainst
casings). The pipe’s ability to resist land movement forces such as those
generated in seismic events (fault movement, liquefaction,
Spans shaking, ground accelerations, etc.) can also be included here.
Soil movements associated with changing moisture conditions
An unsupported pipe is subject to additional stresses compared and temperatures can also cause longitudinal stresses to the
with a uniformly supported pipe. An unsupported condition pipeline and, in extreme cases, can cause a lack of support
can arise intentionally-an aerial crossing of a ditch or stream, around the pipe.
for instance, or unintentionally-the result of erosion or sub- The potential for damaging land movements is considered in
sidence, for example. From a risk perspective. the evaluator a later variable, but whether or not such forces are “damaging”
should be interested in a verification that all aboveground depends on the pipeline’s strength. The diameter and wall thick-
pipeline spans are identified, adequately supported vertically, ness are often good measures of the pipeline’s ability to resist
and restrained laterally from credible loading scenarios, includ- land movements.
ing those due to gravity, internal pressure, and externally applied Loss of support is covered in the discussion of spans as well
loads. Especially in an offshore or submerged environment, this as in the evaluation ofpotential land movements.
must include lateral loads such as current flow and debris Hydrodynamic forces can occur offshore or in any situa-
impingement. Resistance to stresses from unsupported spans is tion where the pipeline is exposed to forces from moving water,
generally modeled using beam formulas (see Appendix C). including water-borne debris.