Page 36 - Pipelines and Risers

10 Chapter I

Longitudinal Stress

The longitudinal stress (a,) is the axial stress experienced by the pipe wall, and consists of
stresses due to:

- Bendingstress (011,)
- Hoopstress (ob)
- Thermal stress (03
- End cap force induced stress (03

The components of each are illustrated in Figure 1.4.

The longitudinal stress can be determined using the equation:
+
+
=
6, 0.30, + 6,, b,, 6,
It should be ensured that sign conventions are utilized when employing this equation (Le.
Tensile stress is positive).

0 Equivalent stress

The combined stress is determined differently depending on the coddstandards utilized.
However, the equivalent stress ( 0,) can usually be expressed as:
+
2
2
0, = do/# 6, - 0*01 + 32,2 (1.2)
where:
oh =hoopstress
= longitudinal stress
zlh = tangential shear stress

The components of each are illustrated in Figure 1.4.

1.4.3 Span Analysis
Over a rough seabed or on a seabed subject to scour, pipeline spanning can occur when
contact between the pipeline and seabed is lost over an appreciable distance (see Figure 1.4).
In such circumstances it is normal code requirements that the line is investigated for:

Excessive yielding;
Fatigue;