West Seno Facilities and Pipelines, Using Conventional Vessels for Deepwater Installation
Authors: Hebert Chuck 
Published: October 2008

This paper describes the installation of the West Seno Field Production Facilities and Pipelines. Because of the unique design and the location of the Field, it was possible to perform the installation utilizing conventional vessels and using conventional techniques. There were several valuable lessons learned through the life of this project that are shared in this paper.

The paper is divided into the following sections:Shallow Water Pipelines - conventional approach, lay barge layout, dual pipe lay, moorings, beach pull, pipe crossingsDeepwater Pipelines - route selection, installation, welding, AUT, SCRs, weldersConductor and Tendon Piles - Conductor Guide Frame (CGF), tolerance, CGF design features, pile installation, Deepwater Lowering System (DWLS)Tendon and TLP - Tendon make up tower, Tendon length, damage control, Tendon Support Buoys (TSB), ballast system, rapid deballastSteel Catenary Risers (SCR) The project was scheduled to allow the installation contractor latitude to sequence installation activities to best suit preferred methodology and equipment availability. It was of course driven by material deliveries as well. The sequence was as above.

Shallow Water Pipelines

The shallow water portion was designated as from the shore line to approximately kilometer 12. This was established as the point where the installation changed from a spread moored lay vessel to a dynamically positioned (DP) vessel. This will be discussed later. The water depth at this location is approximately 35 meters, which was the limit of the shallow water lay barge's capacity. The pipe was 12" × 0.500" X52 ERW, double random joints.

A conventional installation approach was used. The lay barge, Clough Challenge, was a flat bottom barge with temporary lay equipment and accommodations. It is about 50 meters by 21.3 meters and equipped with an eight point mooring system.

There were three welding stations and an X-Ray/coating station on each firing line. The innovative aspect of the layout was that the lay ramp and work stalls were elevated. This gave more deck space and created a safer work environment.

With the work being performed above the deck, personnel moving about didn't interfere with the work and didn't have to enter and exit the work areas to go from one side of the barge to the other.

The special, although not new, approach was to lay the gas and oil pipelines simultaneously. Because of concerns that the pipelines might cross, it was decided to "bundle" the two lines with stainless straps. Spacer blocks were used to maintain separation and be sure the anodes were not damaged.

Figure 1 Pipe Spacers for Dual Pipe Lay (Available in full paper)

The shallow water scope also included pulling the lines ashore and crossing existing pipelines. Also, the route took a course parallel to several other existing pipelines. To prevent damage to the existing pipelines from the anchor cables, wire ropes weren't used for moorings. Polypropylene rope was used and actually floated with the use of buoys. This kept the mooring lines off the existing pipelines and assured no damage to existing lines or other subsea obstructions.