Page 120 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Risk variables and scoring 5/97
Buoyancy and buoyancy mitigation measures can introduce its design limits. This is quite common in the industry and
new stresses into the pipeline. happens for a variety of reasons:
Cyclic loadings and fatigue from external forces should be a
consideration in material selection and wall thickness determi- Downstream portions are intended to operate at lower
nation as discussed elsewhere. stresses. even though they are designed for system maximum
Temperature effects can occur through internal or external levels.
changes in temperature. The maximum allowable material Diminishing supplies reduce flows and pressures.
stress depends on the temperature. Hence, temperature extremes Changes in service result in decreased stresses.
may require different wall thicknesses. Such changes introduce
longitudinal stresses as discussed in Appendix C. Regardless of the cause, any extra strength, beyond the cur-
In composite pipelines. such as a PE liner in a steel rent operational requirements. can be considered in the risk
pipe. many more complexities are introduced. Often used evaluation (Figure 5.4).
to handle more corrosive materials, such composites may Research indicates that in design factors of 0.5 and below,
have a layer of corrosion-resistant or chemical degradation- the failure of a typical corrosion or material defect flaw will fail
resistant material and a layer of higher strength (structural) in the through-wall direction, leading to a leak rather than a rup-
material. Because two or more materials are involved, the ture. In design factors of 0.3 and below, all flaws, even those
stresses in each and the interaction effects must be under- with sharp edges, will likewise fail in a similar manner[58]. A
stood. Such calculations are not easily done. Original design through-wall failure mode leading to less product release (and
calculations must be used (or re-created when not available) hence. lower consequences) is discussed in Chapter 7.
to determine minimum required wall thicknesses. The evalua- However, research also indicates that increased wall thick-
tor must then be sure that the additional wall thickness of one ness is not a cure-all. Increased brittleness, greater difficulties
or more of the materials will indeed add to the pipe strength in detecting material defects, and installation challenges are
and corrosion resistance, and not detract from it. It is conceiv- cited as factors that might offset the desired increase in damage
able that an increase in wall thickness in one layer may have resistance [58].
an undesirable effect on the overall pipe structure. Further, As previously noted certain wall thicknesses are also
some materials may allow diffusion of the product. When this thought to reduce the chances of failure from excavating equip-
occurs. composite designs may be exposed to additional ment. Some wall thickness-internal pressure combinations
stresses. provide enough strength (safety margin) that most conventional
excavating equipment cannot puncture them (see page 96 and
.4cc-ountingji1r ex-ternal loads Chapter 14). However, avoidance of immediate failure is only
part of the threat reduction-nonlethal damages can still
If detailed calculations are not deemed to be cost effective, the precipitate future failures through fatigue and/or corrosion
evaluator may choose to use a standard percentage to add to the mechanisms.
wall thickness required for internal pressure to account for all When evaluating a variety of pipe materials. distinctions in
other loadings combined. For instance, 10 or 20% additional material strengths and toughness can be made In terms of
wall thickness, beyond requirements for internal pressure external damage protection, a tenth of an inch of steel offers
alone, would be conservative for most steel pipe under nor- more than does a tenth of an inch of fiberglass. The evaluator
mal loading conditions. This percentage should of course must make this distinction if she wants to compare the risks
be increased for sections that may be subjected to additional associated with pipelines constructed of different materials.
loadings or where ‘diameter to wall thickness’ ratios suggest An important consideration is the difference between nominal
diminished structural strength. or “specified” wall thickness and actual wall thickness. Pipe
strength calculations assume a uniform pipe wall, free from any
Pipe wall thickness defect that might reduce the material strength. This includes pos-
sible reductions in effective wall thickness caused by defects such
The role of increased wall thickness in risk reduction is intuitive as cracks, laminations. hard spots, etc. Pipeline integrity assess-
and verified by experimental work. Some general conclusions ments are designed to identify areas of weaknesses that might
from this work can be incorporated into arisk analysis. Pipe wall have originated from any of the several causes. Differences
thickness is assumed to be proportional to structural strength- between nominal and effective wall thickness include:
greater wall thickness leads to greater structural strength (not
always linearlytwith the accompanying assumption of Allowable manufacturing tolerances
uniform material properties and absence of defects. Manufacturing defects
Most pipeline systems have incorporated some “extra” wall Installatiodconstruction damages
thickness-beyond that required for anticipated loads-in the Damages suffered during operation.
pipe, and hence have extra strength. This is normally because of
the availability of standard manufactured pipe wall thicknesses. Manufacturing issues
Such “off-the-shelf” pipe is often more economical even
though it may contain more material than may be required for Strength It is commonly accepted that older manufacturing
the intended service. This extra thickness will provide some and construction methods do not match today’s standards.
additional protection against corrosion. external damage. and Technological and quality-control advances have improved
most other failure mechanisms. Excess strength (and increased quality and consistency of both manufactured components and
margin of safety) also occurs when a pipeline is operated below construction techniques. These improvements have varying