Unspiralisation stage and installation stage.

The unwinding and the installation of the pipeline will occur at the similar time. To
have no constraints at installation, the part of the pipeline between the spiral and the
installation barge should have a minimum length as a work buffer. This length
depends on the pipe diameter, the number of windings in the spiral, the water currents
and the speed at which the installation barge can install the pipe.
Compared to the traditional methods of laying larger diameter pipe the speed of laying
can be considerable higher. The basis for this is that no welding and ultrasonic
inspection has to be performed on the pipe while the pipe is being transferred over the
lay barge since these procedures have been done in the onshore pipeline
construction yard.

 Lay barge with central stinger laying pipe from spiral. The central stinger gives the lay barge better
stability and therefore the pipe can be laid to a greater depth.

The motive that the lay barge is being employed is that at the point where the pipe
submerges, there is a need for guarded buoyancy so the pipe can be launched in the
water in a controlled way. Depending on the positioning system, the size of the
tensioners, type of stinger and floating capacity of the barge the pipes can be lowered
to a certain depth. The higher the capacity of the barge the more depth can be
reached.
The barge also functions as safety precaution in case a wet buckle would occur
during the lay process.
The barge is relatively simply in its lay out. Due to the pipe being fed from the spiral
there is no need for an enormous amount of workspace around the pipe. Instead of a
stinger at the bow it is possible to lower the pipe at the centre of the barge. The
weight of the pipe under the barge is better distributed and the angle of the heavy
stinger can be accurately positioned and supported by thebarge. The barge consists
of a lifting crane, a minimum of two tensioners (one at the stern and one at the
stinger), an abandonment and recovery system , a positioning system (preferable a
dynamic positioning system that can interact with the dynamic positioning system
of the centre barge in the spiral), a barge stabilise and ballast system, a small
workshop and living accommodations.
It might be an option to have a single welding station and coating station on the barge
in case the pipeline to be laid is for a project with a longer distance. As a
consequence the pipeline can then be delivered in several lengths (more spirals).

Model of lay barge with central stinger for (ultra) deep water and stinger at stern for
picking up pipe from spiral

When arrived at the installation location the spiral will be unwound using the centre
barge and auxiliary boats connected to the spiralled pipe.
The speed of laying the pipe is independent of the diameter of the pipe or on the
depth of the seabed. However a very good cooperation between the lay barge crew
and the team unwinding the spiral is of high importance. Specially the unlashing of the
links which keep the layers on the spiral together should keep pace with the speed of
laying. Speeds of laying can be expected to reach 20 to 25 kilometres per day.
Flotation devices which are used to keep the spiral afloat during storage and transport
have to be released before the pipe enters the lay-barge. They will be stored and
shipped back for storage to be re-used on a following operation.