51
NCHING
power cable into the pre-cut trench.
Once the power cable has been laid into the trench,
the same plough is reconfigured for backfill and is
deployed again from the plough support vessel to
backfill the trench thereby covering the power cable.
This same plough could also be used for route
clearing ahead of the pre-trenching however that
depends on whether there are boulders or other
obstructions on the cable lay route.
S&A have produced a generic Standard Operating
Procedure (SOP) storyboard drawing that outlines
how pre-trenching can be approached.
The advantages of this pre-trenching approach are
●
The trenching, which is notoriously slow, can
be completed by an operationally cheaper vessel
than the lay vessel. Typically, a plough support vessel
is an offshore tug with only the ships crew and
plough team
●
The trenching can be performed off the
critical path of the project, for example when the
cable lay vessel is loading cable or deployed
elsewhere
●
The lay vessel can be a significantly less
expensive vessel in that it could be a basic lay barge
with anchors for propulsion or basic dynamic
positioning (DP)
●
The lay vessel aft deck can be optimised for
cable lay instead of requiring space for the plough,
tow cable, umbilical/ umbilical winch, LARS, etc.
●
If the trench is required to be deep or
problems are encountered during the pre-trenching
operation, the plough support vessel can perform
multiple passes until the trench is as required. The
same applies to complex cable routes that feature
direction changes that can be problematic when
installing large power cable running through a
standard power cable plough
●
The issue of MBR and damage to the cable
is effectively eliminated as the cable does not pass
through the plough
There is an obvious disadvantage to pre-trenching
in that the trench may infill while it waits for the lay
vessel or the cable, once laid into the trench, is
effectively unprotected until the backfill operation
completes. But with appropriate organisation of the
vessel availability it is believed that these periods
can be mitigated against.
It is also worth noting the increase in reports of
damaged power cable that is believed to have
occurred during installation.
“With the activity in the Far East and USA there is a
push for reducing costs and implementing "lessons
learned" from the European installation
experience,” said Screaton. “There is a lot of
newbuild activity but is bigger, more powerful, more
expensive, etc. the way to go?
“Perhaps pre-trenching could prove to be the more
cost effective, cable friendly and practical approach
for future export power cable installation?”
Figure 4: Backfilling
the trench
Figure 3: Laying
into the trench
Figure 2: Creating
the trench