Page 218 - Pipelines and Risers

hiallation Design 191

12.4.4 Hydrostatic Pressure
The pipeline is exposed to hydrostatic external pressure when it is submerged. There is no
internal pressure during installation. The external pressure has an effect on the pipeline
response. The radial pressure will induce an axial strain via the Poisson’s ratio effect.
V
E, =--(oh +u, ) (12.10)
E
where:
exx = Axial strain
v = Poisson ratio
o,= Hoopstress
or = Radial stress
E = Young’s modulus

The hoop and radial stresses are given by the Lame’s equation. If the pipe ends are free, the
strain will not introduce any stress. However, if the ends are constrained, axial force will
develop. This effect is similar to thermal loads.

When the pipe ends are capped, a force will be induced:
Tp =Po A, -Pi 4 (12.1 1)

where:
PO = external pressure
pi = internal pressure
A0 = outside cross-sectional area
Ai = inside cross-sectional area

The distributed pressure on a deflected pipeline will alter the tension-stiffening effect and
indirectly affect the pipeline curvature.
The effective axial tension T, in the pipeline is defined as, see Figure 12.9.
T, =T, +Tp (12.12)

Figure 12.9 Effective axial tension.