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.

Feb 1st, 1996
01off21

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.

More in Subsea