The general stress design criteria for pipeline systems in this Standard are limited to elastic stress analysis for the design conditions in Clause 4.2 and provide design specifications for
a) design wall thickness;
b) maximum allowable temperature differential in restrained sections;
c) maximum allowable freely supported spans for axially restrained sections;
d) minimum required flexibility in partially or fully unrestrained sections;
e) maximum allowable support spacings for stress design of unrestrained sections; and
f) maximum allowable cold-sprung reactions on equipment attached to flexible piping.
4.6.2 Stress design of restrained and unrestrained portions of pipeline systems Due to the fundamental differences in loading conditions and structural behaviour for restrained portions of pipeline systems and unrestrained portions not subject to substantial axial restraint,different limits for allowable longitudinal expansion stresses and analysis methods are necessary, andsuch limits and methods shall be as specified in Clauses 4.7 and 4.8.
Note: In this Standard, “unrestrained” means that the pipe is able to strain along its length and move laterally. Pipe that does not meet both of the above requirements is referred to as “restrained”. Typically, long straight lengths of buried pipe and aboveground pipe on closely spaced rigid supports are classified as restrained, whereas buried pipelines adjacent to bends or unanchored end caps can be regarded as restrained or unrestrained, depending on specific circumstances.
4.6.3 Discontinuity stresses
The stress design criteria in this Standard are not applicable to gross structural discontinuities such as those that are present at small attachments and partial-penetration welds. The designer shall provide protection against membrane, ratcheting, and fatigue failure for discontinuity conditions, using sound engineering practices.
Note: For thick-wall components, the equivalent mean diameter pressure-design formula and stress limits for design based on stress analysis from the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, may be used.
4.6.4 Supplemental stress design
Supplemental structural limit criteria as described in Clause 4.3.1 shall be given due consideration for installation, pressure testing, operation, monitoring, and maintenance.
4.6.5 Hoop stress
The hoop stress used in stress analysis for any location on the pipeline system shall be calculated using the following formula:
Sh = PD/2tn
Sh = hoop stress, MPa
P = design pressure, MPa
D = outside diameter of pipe, mm
tn =I pipe nominal wall thickness, less allowances specified in Clause 4.3.10, mm
4.6.6 Steel properties
Flexibility calculations shall be based upon the modulus of elasticity at the lowest expected pipe temperature. For carbon and high-strength low-alloy steels for temperatures up to 230 °C, the following values may be used:
a) 207000 MPa for the modulus of elasticity (Ec);
b) 12 × 10–6 °C^-1 for the linear coefficient of thermal expansion (α); and
c) 0.3 for Poisson’s ratio (ν).
CSA Z662-15 (Oil and gas pipeline systems)
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