Page 475 - Pipelines and Risers

442 Chapter 24

With the use of pipe-in-pipe systems come additional design features that are not present in
conventional pipeline design. Challenging engineering problems rang from structural design
of spacers and internal bulkheads to the understanding of the structural behavior both globally
and locally under a variety of loading regimes. Due to the increased number of components in
a pipe-in-pipe system compared with conventional pipelines, the design process is therefore
more iterative in nature as the interactions of the components may necessitate design
alteration.

A pipe-in-pipe system is essentially made up of an insulated inner pipe and a protective outer
pipe. The function of the inner pipe is to convey fluids and therefore is designed for internal
pressure containment. The inner pipe is insulated with thermal insulation materials to achieve
the required arrival temperature. The outer pipe protects the insulation material from external
hydrostatic pressure and other mechanical damage. Concrete weight coating is not normally
required due to high submerged weight and usually low ocean current speeds in deepwater
areas.

For the exploitation of HP/HT reservoirs, pipe-in-pipe system can provide the necessary
thermal insulation and integrity for transporting hydrocarbon at high temperature (above
12OoC) and high pressure (in excess of 1OOOOpsi). Pipein-pipe system comprises a rigid steel
flowline inside a rigid sleeve pipe. The two pipes are kept apart by some form of spacer at the
ends of each joint, and by bulkheads at the ends of the pipeline. The various proprietary
systems in the market differ in the details of the spacers and bulkhead arrangements. The air
gap between the inner and outer pipes provides the means of achieving the high thermal
insulation. This air gap accommodates the insulation, which typically consists of either
granular material poured into the inter-pipe annulus, or of a blanket form, which is wrapped
around the inner pipe. In either case, the insulation material needs to be kept dry in order to
maintain its insulation properties.

24.2.2 Why Pipe-in-Pipe Systems
There are several conditions under which pipe-in-pipe systems (including bundles in this
definition) may be considered for a particular flowline application over a conventional or
flexible pipeline.

a) Insulation- HP/HT reservoir conditions

HFVHT flowlines require thermal insulation to prevent cool down of the wellstream fluid to
avoid wax and hydrate deposition. There are many thermal coatings available that can be
applied to conventional steel pipe but they tend not to be particularly robust mechanically and
have not been proven at the temperatures now being encountered in HPlHT field, typically
15OOC and above. A similar problem exists for flexibles in this respect. An alternative is to
place the flowlines(s) inside another larger pipe, often called a carrier or outer sleeve pipe.
The annulus between them can then be used to contain the insulating material whether it be
granular, foam, gel or inert gas.

b) Multiplicity of flowlines