Page 46 - Pipelines and Risers

lntroduciion 19

- Pipelaying Reelship
The pipe reeling method has been applied mainly in the North Sea, 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
installation the pipe is unreeled and straightened using a special straightened ramp. The pipe
is then placed on the seabed in a similar configuration to that used by the laybarge although in
most cases a steeper ramp is used and overbend curvature is eliminated.

The analysis of reeled pipelay can be carried out 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 since the welding process can cause unacceptable work hardening in
higher grade steels.

A major consideration in pipeline reeling is that the plastic deformation of the pipe must be
kept within limits specified by the relevant codes. Existing reelships reflect such code
requirements.

- Pipeline installation by Tow or Pull
In certain circumstances a pipeline may be installed by a towing technique where long
sections of line are made up onshore and towed either on the seabed or off bottom by means
of an appropriate vessel (tug or pull barge). The technique has its advantages for short lines
and for bundled lines where several pipelines are collected together in a carrier. In this case
difficult fabrication procedures can be carried out onshore. The design procedures for towed
or pulled lines are very dependent on the type of installation required. For example, it is
important to control the bottom weight of a bottom towed line to minimize towing forces and
at the same time give sufficient weight for stability. Thus, a high degree of weight
optimization may be needed, which can involve tighter control on pipeline wall thickness
tolerances than for pipelay, for example.

1.5 Pipeline Simulator
The Pipeline Simulator System comprises a new generation of pipeline modeling tools to
replace in-house computer programs developed in the mid-1980s. New technology allows
more accurate FEM analysis of pipeline behavior in order to optimize design and achieve cost
reductions. The Simulator consists of in-place modules (global models), strength modules
(local models) and LCC (life cycle cost) design modules.

The in-place modules (global models) simulate through-life behavior of pipelines, including
the following design aspects:

- installation
- on-bottom stability
- expansion, upheaval and lateral buckling