Page 476 - Pipelines and Risers

Pipe-in-Pipe and Bundle Systems 443

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.

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.
0 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 in-
shore launch area size.

c) Trenching and Rock-dumping

Traditionally, flowlines less than 16-inch in diameter are trenched andor 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 non-
trenching 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.

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