Page 205 - Pipelines and Risers

178 Chapter 12

12.2 pipeline Installation Vessels
Pipeline installation methods have significantly changed over the last twenty years. This is
pertinently enforced by the recent replacement of the BP Forties 170km trunkline. When it
was first installed in 1974 it took two lay barges more than two summers, and each lay barge
suffered 60% downtime due to weather. In 1990 it took one (relatively old) pipelay vessel to
install the replacement pipeline (and the pipewall was significantly thicker- 28.5mm
compared to the original 19mm). The significant increase in layrate is due to a combination of
factors including:

0 Improved welding techniques;
0 Improved survey capabilities;
0 Improved anchor handling techniques;
0 Improved procedure.

The methods available to install pipelines are discussed under the following headings (see
Figures 12.1 to 12.4 from Langford and Kelly (1990)). Different vessel types are used
depending on the pipelay method and site characteristics (water depth, weather etc.).

S-lay/J-lay semisubmersibles;
S-lay/J-lay ships;
0 Reel ships;
0 Tow or pull vessels.

12.2.1 Pipelay Semi-Submersibles
Pipelay semisubmersibles were developed as a direct response to the large weather downtime
being experienced by the monohull pipelay barges (especially in the North Sea). These
vessels have excellent weather capabilities and can provide a stable platform for pipelaying in
seas experiencing Beaufort force 8 conditions. It is usually the limitations of the anchor
handling vessels which prevent the semi-submersible from operation in rough weather.

There are presently several such vessels operating in the North Sea (see Figure 12.2 for
typical vessel- Semac).