Page 388 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Appendix C/363

Appendix C
Pipe Strength

Determination

Some equations and design concepts are presented in this sec- manent deformation of the pipe. After permanent deformation,
tion to give the evaluator who is not already familiar with the pipe may no longer be suitable for the service intended.
pipeline design methods a feel for some of the commonly Permanent deformation occurs through failure modes such as
used formulas. This section is not intended to replace a design bending, buckling, crushing, rupture, bulging, and tearing. In
manual or design methodology. Used with the corresponding engineering terms, these relate to stresses of shear, compres-
risk evaluation sections, this appendix can assist the nonengi- sion, torsion, and tension. These stresses are further defined by
neer in understanding design aspects of the pipeline being the directions in which they act; axial, radial, circumferential,
examined. tangential, hoop, and longitudinal are common terms used to
refer to stress direction. Some ofthese stress direction terms are
used interchangeably.
Stresses Pipe materials have different properties. Ductility, tensile
strength, impact toughness, and a host of other material proper-
Minimum pipeline wall thicknesses are determined based on ties will determine the weakest aspect of the material. If the
the amount of stress that the pipe must withstand. Design pipe is considered to be flexible (will deflect at least 2% with-
stresses are determined by careful consideration of all loadings out excessive stress), the failure mode will likely be different
to which the pipeline will be subjected. Loadings are not lim- from a rigid pipe. The highest level of stress directed in the pipe
ited to physical weights such as soil and traffic over the line. A material’s weakest direction will normally be the critical failure
typical analysis of anticipated loads for a buriedpipeline would mode. The exception may be buckling, which is more depend-
include allowances for: ent on the geometry of the pipe and the forces applied.
Another way to say this is that the critical failure mode for
Internal pressure each loading will be the one that fails under the lowest stress
Surge pressures level (and, hence, requires the greatest wall thickness to resist
Soil loadings (including soil movements) the failure). Overall then, the wall thickness will be determined
Traffic loadings. based on the critical failure mode of the worst case loading
scenario.
Additional criteria are considered for special installation cir-
cumstances such as drilled crossing and overhead spans. These
criteria include Internal loadings
0 Bending Stresses (Overhead crossings and Drilled Internal pressure is often the governing design consideration
Crossings) for pressurized pipelines. The magnitude of the internal pres-
Tensile Loads (Drilled Crossings) sure along with the pipe characteristics determines the magni-
Bouyancy. tude of stress in the pipe wall (due to internal pressure alone),
which in turn determines the required wall thickness. This
For each of these loadings, failure must be defined and all fail- stress (or the associated wall thickness) is calculated using an
ure modes must be identified. Failure is often defined as per- equation called the Barlow formula: