Page 33 - Reliability and Maintainability of In service Pipelines

22 Reliability and Maintainability of In-Service Pipelines

The internal pressure decreases ring formation within the pipe after installation
and pressurization. By this, the deflection of the pipe is not greater than the verti-
cal compression of the surrounding soil. However, once the critical deflection,
which is the vertical deflection at which soil slip occurs, exceeds this vertical
compression, the pipe will collapse (if flexible pipe).
The deflection of the pipe however, may be more limited than that for critical
soil compression as coatings and linings in pipes may play a role in the process.
Deflection can be controlled more by controlling the material in the embedment
and the installation processes, than the properties of the pipe itself, such as pipe
stiffness.
steel is ductile and is known to perform in a ductile range, the above factors
should be considered carefully in pipe design, as steel pipes are subject to factors
including bending, buckling, cracking, and collapsing that significantly affect
their structural integrity. The next section outlines how these factors contribute
and result in damage and failure mechanisms.

1.5 Loads and Stresses on Pipelines

1.5.1 LOADS ON PIPELINES

All pipes shall be designed to withstand the various external and internal loadings
to which they are expected to be subjected, during construction and operation.
The external loadings include loads due to the backfill, most severe surface sur-
charge or traffic loading (live load) likely to occur, and self-weight of the pipe
and water weight. The internal pressure in the pipeline, if different from atmo-
spheric, shall also be treated as a loading.

1.5.1.1 Earth Load
Beginning in 1910, Marston and Anderson developed a method for calculating
earth loads above a buried pipe based on the understanding of soil mechanics at
that time. Marston’s formula is considered for calculation of earth load on buried
pipes in codes of practice and manuals (such as BS EN 1295-1 and ASCE
No.60). The general form of Marston’s equation is:
W 5 CγB 2 ð1:12Þ

where W is the vertical load per unit length acting on the pipe because of
gravity soil loads, γ is the unit weight of soil, B is the trench width or pipe
width, depending on installation condition, and C is a dimensionless load