Page 336 - Pipelines and Risers

Route Optimization, Tie-in and Protection 309

173.2 Lateral Pull
General Principle
Lateral deflection involves positioning the flowline end to one side of the target structure and
then pulling it laterally into position. This has two disadvantages compared with a direct pull-
in.

- Alignment is more difficult to achieve;
- A clear (swept) area is required to one side of the tie-in site.

The flowline may be pulled towards the target by a single wire, or a series of wires may be
developed through dead-man anchors to give greater control of alignment. A bell mouth or
stab-in guides usually assist final alignment.

For large diameters such as export lines or bundles, it is necessary to make a length of
pipeline neutrally buoyant, This gives greater flexibility and reduces the pull forces, but can
expose the pipe to large current forces.

One development of this technique is the use of vertical deflection rather than lateral
deflection. The required initial shape could be attained by local adjustments to buoyancy,
pull-in being again by a system of wires. The principal advantage of this method is that it does
not require the same amount of seabed space.

In addition, it should be possible to devise initial configurations which it would be difficult to
create laterally by laying or towed installation. Direct pull for second end tie-ins may then
become available by creating a vertical slack loop behind the pullhead.

Installation CapabilitiedConstraints
This method is usually utilized when direct pull-ins (i.e. J-tube, connect and lay away or stalk-
on) are not feasible options. However, this method is frequently used as direct pull-ins are
usually not possible. For instance J-tube pull-in and connect and lay away can only be
performed by a vessel laying away, and not when a vessel lays down. Stalk-ons can only be
performed in shallow water.

The main disadvantages of this method are:

- Requires extensive diver intervention;
- Difficult operation to perform; several experienced diving operators have buckled
flowlines using this method;
- If connected directly to the tie-in point then all the pipeline expansions will be fed into
that point. The tie-in point must either take high axial loads or large deflections.