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Route Opfimization, Tie-in and Protection 315

17.35 Stalk-on
General Principle
The stalk-on method is primarily used in shallow water applications (less than 40 m) and
hence would only be applicable in the Southern North Sea. The method involves laying the
flowline down adjacent to the jacket it shall be tied into. The vessel maneuvers over the
flowline, lifts it up and welds on (or flanges on) the jacket riser. The pipeline and riser are
then lowered onto the seabedjacket. The jacket clamps are subsequently closed around the
riser. See Figure 17.1 for illustration.

Installation CapabiIitiedConstraints
The primary advantage of this method is that the same vessel that installs the flowline can
also perform the stalk-on. However, the disadvantages are:

- The riser will be subjected to the expansion of the pipeline, as no expansion, as no
expansion spool is used;
- The operation can only be used in shallow waters.

17.4 Flowline TrenchingBurying
The development of trenchinghurying equipment has, like the flowline installation
equipment, changed significantly over the last twenty years. The trend has been to move away
from dedicated trenching vessels to equipment that can be used from Diving Support Vessels
(DVS’s). The flowline trenchinghurying equipment is discussed under the following
headings:

- Jet sled;
- Ploughing;
- Mechanical cutter.

17.4.1 Jet Sled
This method is the traditional method of trenching a pipeline. Dedicated vessels with turbine
engines were built to provide jet sleds that would trench through most soils (see Figure 17.6).