Page 210 - Pipelines and Risers

Instullution Design 183

Apart from this the handling, welding and lay down of pipe is perfomed in the same manner
as discussed for semisubmersibles (see Figure 12.1).

Installation CapabilitiedConstraints

Pipelay ships have very similar installation capabilities as pipelay semisubmersibles. This
includes the wide range of pipeline diameters in water depths from 15m to over 1000m.

The monohull pipelay ships have poorer seakeeping capabilities than the semisubmersibles.
This results in greater periods of downtime and reduces the total time per season during which
pipe can be installed.

The main advantage of the pipelay ship is the cost: the relatively smaller, dynamically
positioned ships can operate without anchor handling vessel assistance. Presently none of the
existing dynamic positioned semisubmersibles are equipped for pipe laying.

If the work could be confined to the summer season then a small dynamically positioned ship
would provide a more economical means for installation of short pipelines than would any of
the large existing semisubmersibles. If smaller dynamically positioned semisubmersibles were
equipped for pipelaying purposes, however, the relative cost effectiveness between the
application of a ship and a semisubmersible could be different, also in the summer season.

12.2.3 Pipelay Reel Ships
There is presently one reel ship in the UK sector of the North Sea (Stena Apache). This vessel
has provided an economical tool for installing short, small diameter pipelines (see Figure
12.1).

General Principle
The pipe reeling method is applied for line sizes up to 16-inch. The pipeline is made up
onshore and is reeled onto a large drum on a purpose built vessel. During the reeling process
the pipe undergoes plastic deformation on the drum.

During the installation the pipe is unreeled and straightened using a special straight ramp. The
pipe is then placed on the seabed in a similar configuration to that used by the laybarge (S-
lay) although in most cases a steeper ramp can be used and overbend curvature is eliminated,
(Le. J-lay). Using the J-lay method very deep water depths can be achieved.

The analysis of reeled pipelay can be achieved using the same techniques as for the laybarge.
Special attention must be given to the compatibility of the reeling process with the pipeline
steel grade and the welding process used. Recent tests have indicated that the reeling process
can cause unacceptable work hardening in higher grade steels.