PIPELINES
Vessel-deployed, free span support system speeds Transmed installation
Atlantis pipeline
mechanical support system readied for operation on the Transmed gas line.
Stabilization of
submarine pipeline free spans along uneven sea bottoms is conventionally
performed using gravel dumping, post trenching or mattresses. However, as SEIC,
based in Fano, Italy points out, these technologies merely support the
pipeline: they cannot lift it.
SEIC has developed a
new technique which has been applied to support free spans along the 26-inch
diameter Transmed gas lines crossing the Sicily Channel in water depths down to
510 metres. The technology is based on the pipeline mechanical support system
Atlantis with its installation module Pegasus: it was developed to cater for
requirements such as short installation time, simple interface with the support
vessel and pipeline lifting capacity.
Reduced installation
time is achieved by automatic operational procedures, including an auto heading
function, which are acoustically controlled from the surface. No umbilical
cable, winch, slip ring or power distribution unit are necessary, meaning that
a vessel equipped with a conventional crane and a support ROV are sufficient to
install the system.
The Pegaso module is
powered by dedicated battery packs and the ROV is used to provide video images
during marine operations and to drive Pegaso hydraulic functions in emergency
conditions. Three types of supports, with different leg length and
minimum/maximum clamping distance, were built and installed along Transmed in
order to satisfy intervention requirements on the expected as-laid
configurations.
Atlantis can be
installed and subsequently adjusted or removed in water depths down to 1,000
metres. Two different installation procedures, one at pre-set lifting displacement
and the other at pre-set lifting force, can be employed to solve free span
problems in terms of pipeline stress level.
No hydraulic
component is installed on Atlantis, the mechanisms of which are operated
automatically by hydraulic actuators placed onboard Pegaso. Redundancy has been
achieved by implementing four different operating modes - automatic, manual,
mechanical and emergency - at different levels of system fault.
According to SEIC,
the Atlantis/Pegaso system has numerous other advantages over competing
techniques. Gravel, it points out, is subject to scouring and may not ensure a
firm support to the pipeline; and post-lay trenching is often impossible due to
bottom morphology and geotechnical characteristics. The adjustment and recovery
procedures of the SEIC system brings operational flexibility compared with
other passive supporting systems, which cannot be removed once installed.
Copyright 1996
Offshore. All Rights Reserved.
02/01/1996
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