Page 497 - Pipelines and Risers

464 Chapter 24

Infield installation
The infield installation of the bundle system will be carried out by remote intervention, which
will be carried out directly by ROV.

The bundle will be towed at a slow speed in off-bottom mode into the installation area. After
adding weight to the bundle, the off-bottom tow can commence. During the off-bottom tow
the bundle position must be monitored at all times.

The bundle will be pulled in at a straight line. A temporary target box will be determined for
the leading towhead.

When the towheads and bundle position have been confirmed, flooding down of the bundle
can commence.

CDTM involves the transportation of prefabricated and fully tested flowlines, control lines
and umbilicals in a bundle configuration suspended between two tugs. A further vessel
accompanies the tow as a patroYsurvey vessel. To maintain control during tow, the bundle is
designed and constructed within specified tolerances with respect to its submerged weight.

The bundle is designed to have buoyancy, this being achieved by encasing the bundled
pipelines, control lines, umbilicals etc. inside a carrier pipe. Ballast chains are attached to the
carrier pipe at regular intervals along its length to overcome the buoyancy and provide the
desired submerged weight.
The tow speed has a direct lift and straightening effect on the bundle. By controlling the tow
speed in combination with the tension exerted by the tugs the bundle tow characteristics and
deflections are maintained.

The tow is controlled by adjustment of the tow wire length, tow wire tension, tow speed and
the tug’s relative positions. In this manner the tow depth, catenary shape, stresses and
movement are kept within specified operational limits under given environmental conditions.
During tow the bundle is kept well clear of the seabed to enable a safe and unobstructed
passage. The towheads are kept below the surface to minimize the effect of surface waves.
The towhead depth is normally about 3Om below the surface but this controlled depth can be
increased or reduced by adjustment of the tow wire lengths.

On atrival in the field the bundle is gradually lowered by adjustment of the controlling
parameters (tow wire length, forward speed and tension) and the bundle settles in a position of
equilibrium above the seabed with the lower portion of the chains resting on the seabed. Once
in this position the bundle can easily be maneuvered in the off-bottom mode to its final
position and the towheads located in the required target areas. The carrier annulus is flooded
with inhibited seawater and the bundle settles on the seabed.