Page 477 - Pipelines and Risers

444 Chapter 24

0 Gap thickness between the internal and external pipes: This should be optimized to
maintain the heating.
0 Thermal stability
Overall feasibility

24.2.4 Structural Design and Analysis
There are four main structural parts to a pip-in-pipe system, these being the flowline, the
sleeve pipe, the sleeve pipe connection (field joint) and internal components such as
insulation material, spacers and bulkheads. Each component is designed to individual
specifications initially and then the combined system must be analyzed to ensure that the local
and global response to various loading regimes is satisfactory. In this way the interaction of
all the components is checked, which is important as one component’s behavior may affect
the behavior of others. The design of pipe-in-pipe systems is therefore a more iterative
process than the systematic approach used for conventional pipelines.

The camer or sleeve pipe is line pipe but is sized in accordance with the requirements of the
overall system. Diameter is usually dependent upon volume of insulating material required
and wall thickness is generally determined on a hydrostatic collapse criteria, i.e. operating
water depth. Sleeve pipe dimensions have a direct economic impact in that a larger pipe,
whether it be diameter or wall thickness, means more steel and probably longer offshore
welding time at each station on the installation barge.

As the sleeve pipe is not a pressure containing structure it is not subject to the same design
codes as the flowline. In fact there is no applicable design code, as it is only a structural
member, and therefore the design requirement is fitness for purpose. A general basis of 2%
strain can be used for limiting design as this is of the order of strain seen by reeled pipe.
Obviously, it is not desirable for the sleeve pipe to be at this level of strain for the duration of
its lifetime but short excursions to this level can be tolerated, such as during installation in the
limiting sea states.

If the flowline is designed to 0.1% plastic strain and this governs the limiting installation sea
state (i.e. maximum permissible bend radius), then the sleeve pipe will be at a higher level of
strain due to its larger diameter.

Of all the components it is the design of the sleeve pipe that is most flexible for achieving
specific system characteristics. Optimization of sleeve pipe size and other advantages that are
to be gained from a particular size sleeve pipe are dependent on the global behavior of the
system which is addressed later in this section.

In terms of structural behavior, pipe-in-pipe system is categorized as being either compliant or
non-compliant, depending on the method of load transfer between the inner and outer pipes.
In compliant systems, the load transfer between the inner and outer pipes is continuous along
the length of the pipeline, and no relative displacement occurs between the pipes, whereas in