Page 479 - Pipelines and Risers

446 Chapter 24

Pa true wall forces in the outer pipe

At full constraint, the true wall forces are given by:
(24.4)

and

(24.5)
where:
E Young's modulus,
cross sectional area of the pipe wall,
a coefficient of linear expansion,
AT temperature of the pipe wall,
v Possion's ratio,
pi internal pressure,
pe external pressure,
D pipe diameter,
t wall thickness of the pipe,
Ai cross sectional area of the inside of the pipe.

subscripts 1 and 2 refer to inner and outer pipes respectively.

24.2.5 Wall-thickness Design and Material Selection
Compared to conventional pipeline, there are several issues practical considerations
associated with the pipe-in-pipe system including insulation methods and insulation
capabilities currently available, material and construction costs, ease of repair, and structure
integrity issues.

The inner pipe will be designed to resist bursting under internal operating pressure and
hydrotest pressure. The inner pipe may also be designed to resist collapse under external
hydrostatic pressure and local buckling in case of leakage in the outer pipe.

For the outer pipe, the governing criteria is usually collapse and local buckling under
combined loading of hydrostatic pressure and bending. Resistance to busting may also be
required so hat fluid containment can be maintained in case of leakage in the inner pipe. This
would be a contingency measure and would not be considered as a normal operating
condition. For deepwater pipeline, the use of buckle arrestors is more economical to limit the
extent of a buckle than having a thick wall to resist buckle propagation. This is paaicularly
true for pipe-in-pipe systems self-weight needs to be kept low to ensure that the pipeline is
installable.