Page 496 - Pipelines and Risers

Pipe-in-Pipe and Bundle Systems 463

with respect to sliding and overturning with combinations of dead-weight, maximum
environmental and accidental loads applied. The design of towhead structure shall be in
accordance with relevant structural design code like MI RP2A.

Bundle Appurtenances Design
Relevant design codes should be applied for the design of bundle appurtenances based on
their functional requirements.

Fubrication and Construction Design
Design check and analysis shall be performed to confirm the adequacy of the selected pipe
wall thickness to withstand loads during fabrication and construction phases and to ensure that
the pipe stress values remain within the specified limits.

24.3.9 Installation by CDTM
The most feasible and reliable way of bundle installation is by use of Controlled Depth Tow
Method (CDTM), which is a subsea pipeline installation system.

The principle of the CDTM involves the transportation of the bundle towed between two lead
tugs and one trail tug. By controlling the tow speed in combination with the tension
maintained by the trailing tug the trailing towhead, the bundle configuration and its
deflections are kept under control during the tow. The essential parameters are continuously
monitored during the two, and adjusted if necessary to maintain the desired bundle
configuration, well clear of the seabed, nominal position during tow is some 30 m below sea
surface.

The complete installation of bundle system includes the following main activities

Launch
Upon completion of fabrication and testing, the bundle will be outfitted for tow and
installation with ballast chain, telemetry system and other installation aids. Breakout and pull
forces during various stages of the launch shall be calculated and assessed for the bundle
system.

Pre-tow preparation
The pre-tow preparation will commence with the activities including towhead inspection,
trimming, bundle submerged weight check and tow preparation.

Tow to field
The bundle system will be towed to the field by use of CDTM along a pre-surveyed route.
During tow, the drag on the ballast chain creates a ‘lift force’, and so reduces the bundle
submerged weight. This lift force will result in a complete lift off from the seabed into CDTM
mode. When the two anives near the field, the bundle will be lowered to the seabed in the
designated parking area situated in front of the bundle installation area.