Pipe-in-Pipe

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.

The first known pipe-in-pipe system was installed in 1973 by Pertamina Offshore Indonesia. This pipeline was 8 miles long extending from shore to a single point mooring facility. The outer and inner diameters of this pipeline were 40" and 36" respectively. Up till now nearly 36 pipeline bundles have been installed by controlled depth tow method (CDTM). The first one was installed at the Murchison field in 1980. The longest pipeline bundle is the one being designed, constructed and installed in Norwegian Sector by Rockwater. This bundle is 14 km long with 46" carrier pipe and three production lines.

Pipe-in-pie

Source: http://www.offshore-mag.com/content/dam/offshore/print-articles/Volume%2072/feb/islay2-1202off.jpg

Why Pipe-in-pipe Systems

There are several conditions under which pipe-in-pipe systems may be considered for a particular flowline application over a conventional or flexible pipeline.

a) Insulation- high pressure high temperature (HP/HT) reservoir conditions
HP/HT 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 HP/HT field, typically
150°C 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
The bundle concept (pipes-in-pipes) is a well established one and a number of advantages can
be achieved by grouping individual flowlines together to form a bundle. For specific projects the complete bundle may be transported to site and installed with a considerable cost saving
relative to other methods. The extra steel required for the carrier pipe and spacers can be
justified by a combination of the following cost advantages.

Schematic of electrically heated pipe-in-pipe system

Source: http://www.offshore-mag.com/content/dam/etc/medialib/new-lib/offshore/print-articles/2011/sept/75700.res/_jcr_content/renditions/pennwell.web.600.300.jpg

• A carrier pipe can contain more than one flowline. Common applications have also contained control lines, hydraulic hoses, power cables, glycol lines etc.
• Insulation of the bundle by the use of gel, foam or inert gas is usually cheaper than individual flowline insulation.
• In most cases there is no trenching or burial requirement due to the carrier pipe's large diameter. Since there are multiple lines within the carrier, seabed congestion within the filed is also minimized.

Bundle installation is commonly carried out through use of the Control Depth Tow Method
(CDTM). The main limitation to the CDTM is the permissible length of bundle that can be
installed, currently around 7.8 km. This is due to a combination of construction site and inshore
launch area size.

c) Trenching and Rock-dumping
Traditionally, flowlines less than 16-inch in diameter are trenched and/or buried. When contained within a sleeve pipe, which could be anything from 18-inch to 24-inch in diameter for single pipe-in-pipe systems and much larger for bundles, a reasoned argument for nontrenching can be made demonstrating that the line will not pose a risk to human life or the environment, nor will it become a hazard to other users of the sea. The cost associated with needing to trench, backfill and rock dump is often greater than that of the installation cost of the pipeline. By not trenching, buckling of the pipeline will only occur in the lateral direction across the seabed and there are methods to control such an event, e.g. mid-line spools or laying in a 'snaking' configuration. Upheaval buckling through the seabed, which is the more severe situation, can only be controlled through sufficient over burden being placed on the line in the form of rock dumping. These issues are addressed later.

In terms of impact from trawl boards or fishing gear, the external pipe acts as the first line of defense and although it may be breached, the integrity of the flowline will not compromised. For certain applications, pipe-in-pipe systems offer significant cost saving over conventional pipelines, particularly when the need to trench, backfill and rockdump can be eliminated with additional mechanical and structural benefits as well.

Configuration

Various configurations of pipe-in-pipe can be used. The followings should be considered when determining the configuration.

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

Failure Modes

1. Bursting
The burst capacity of the pipe-in-pipe system is determined based on the inner pipe subjected
to the full internal pressure, and the outer pipe subjected to the full external pressure.

2. Fatigue and fracture
Pipe-in-pipe systems are subjected to both low cycle and high cycle fatigue due to daily
operational fluctuations and start-up/shut-down conditions. One area particularly prone to
fatigue is the weld joint. Typically, the weld joint for pipe-in-pipe systems comprise butt weld
on the internal pipe, and either split shells or some form of sleeve arrangement for the external
pipe connection. Special attention should be given to the fatigue assessment for the inner face
of internal pipe since it is subjected to corrosive environment, and the outer face of the
external pipe subjecting to seawater environment.

3. Global buckling
Due to effective axial force and the present of out-of-straightness (vertically and horizontally)
in the seabed profile, pipe-in-pipe systems are subjected to global buckling, namely upheaval
buckling and lateral buckling. The upheaval buckling should be investigated if the pipe-inpipe
system is intermittent rockdumped. Lateral buckling should be investigated for all the
case.





Source:
Subsea Pipelines and Risers - Yong Bai (2005)