Page 348 - Pipelines and Risers

Route Optimization, Tie-in and Protection 319

Installation Capabilitidconstraints
The main advantage of this system is that it can trench a large range of flowline sizes (up to
24-inch diameter) operated from a DSV. The trench rates can be very high depending on the
soil conditions.

The system is probably the only system that can bury flowlines in one operation, (should it be
required). It should be noted, however, that some operators prefer rock or imported material
to be used as backfill.

The main disadvantage of this system is that it has a limitation on the depth which can be
excavated. To date, the maximum trench depth is 1.5m. An additional disadvantage is that the
plough system can cause damage to flowlines, especially on those lines not protected by
concrete coating. However, this system is better than most.

This system usually requires divers for plough placement and retrieval, but in some cases it
can be performed without divers.

17.4.3 Mechanical Cutters
Mechanical cutters have been developed as a diverless option to trenching for small diameter
flow lines (see Figure 17.8).

General Principles
There are many varied and different types of mechanical diggers available for subsea flowline
trenching. However, the methods are all based on the same basic principle. The controls and
power source is onboard a surface vessel, which via an umbilical powers a subsea machine.
This machine moves along the seabed on tracks.

Installation CapabilitidConstraints
These machines can usually handle only small diameter flowlines (and preferably flexible).
Since they provide their own traction the machines require reasonably firm soil. They cannot
trench in very soft soil or very hard clayhock.

17.5 Flowline Rockdumping

Rockdumping, like the other installation activities for offshore, has become more specialized
in the last 17 years. The rockdumping vessels were designed to deposit large quantities of
rock in localized areas. Along with the requirement of small quantities of rock being placed
over pipelines, new vessels have been developed (see Figure 17.9).