Page 478 - Pipelines and Risers

Pipe-in-Pipe and Bundle Systems 445

noncompliant system force transfer occurs at discrete locations. The structural design of
pipe-in-pipe system is more complex than that of a single pipe system. The number of
components in the system, comprising inner the outer pipes, spacers and bulkheads, and the
increased amount of welding required.

The structural behavior of a pipe-in-pipe system is dependent on both the overall behavior of
the system, and the mechanism of load transfer between inner and outer pipes. The overall
effective axial force developed in the system is dependent on the operating conditions of
temperature and pressure, and if the pipeline is in the end expansion zone, on the friction
forces developed between the outer pipe and the soil. The stresses that develop within the
pipe-in-pipe assembly are governed by the type of system used, i.e. compliant or non-
compliant, and the presence of end bulkheads.
A pipeline lying on the seabed will develop effective axial compressive forces within the
system when subjected to operating temperature and pressure. As the pipeline expands under
operating conditions, soil friction forces between the outer pipe and the seabed oppose the
free thermal expansion of the assembly and results in an overall effective axial compressive
force developing within the system. The magnitude of the maximum overall effective axial
force depends on whether or not the pipeline develops full axial constraint. If the pipeline is
operating in the end expansion zone, then the overall effective axial force is a function of the
soil friction and submerged weight and distance from the spool, given as:
Peff =JWS.P~ +R (24.1)

where, Pe~ the overall effective axial force (compression positive), Ws is the submerged
is
weight, is the pipe-soil axial friction coefficient and R is the resistance provided by the
spool. The overall effective axial force increases from the spool location up until it develops
full axial constraints, given by:
Peff = -erue 4 - P, (24.2)
f
where:
PmC- true wall force,
Pi = pi.Ai - force due to internal pressure,
P, = p,.& - force due to external pressure,
pi - internal pressures,
pe - external pressures,
Ai - inside areas of the inner pipe,
A, - outside mas of the outer pipe.

The true wall forces for a pipe-in-pipe system comprises both contribution from the inner and
outer pipes, i.e.
<me = Ptl+ 42 (24.3)

where:
Pt, true wall forces in the inner pipe