Page 211 - Pipelines and Risers

184 Chapter 12

A major consideration in pipeline reeling is that the plastic deformation of the pipe must be
kept within limits specified by the relevant codes. The existing reelship reflects such code
requirements.

Installation CapabilitiedConstraints
Due to the requirement to reel the pipeline onto a small diameter drum, the pipeline
experiences some plastic strain. The permissible amount of strain (and ovalisation of the pipe)
limits the maximum diameter of the pipeline that can be installed using this method. Usually,
depending on the wall thickness, the maximum diameter is 16-inch. Also, due to the limited
size of drum, only short lengths of pipe can be laid (usually 3-15 km depending on pipe
diameter). However, it is possible to install larger lines if more drums of pipe are available.

Accepting the constraints of this method, the reel method of installation has been proven to be
a reliable and economical method of installing pipelines. The main advantages of the system
are:

Short offshore installation duration;
Minimum offshore spread (no anchor handlers).

12.2.4 Tow or Pull Vessels
This method is applied to short lines, usually less than 4 km, (7 km has been laid), which
would prove difficult, impossible, or more expensive to install from a pipelay vessel.

General Principle
The pipeline is fabricated onshore, and towed into the sea when it is completed (see Figure
12.4). The buoyancy of the line is selected and designed to verify that a controlled depth tow
can be performed. Usually two rigs tow the pipeline to location; one leading and one trailing.
On location the pipeline is positioned and flooded.

This method of installation is usually used when several flowlines are fabricated together (i.e.
a bundle).

Installation CapabilitidConstraints
The main advantages of fabricating a pipeline onshore are:

Low equipment costs compared to fabricating offshore;
Long fabrication duration's permitting more difficult fabrication techniques to be applied.
Some fabrication techniques, such as using bundling (several pipelines tied together and
sometimes installed into a carrier pipe), cannot be performed by offshore vessels.
Pipeline fabrication is not prone to weather interruptions.

However, the constraints of using the tow/pull methods are: