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Simultaneous lay and trench with a typical power
cable plough has been the typical cable lay
approach for most buried telecommunications
cable and now subsea power cables, mainly export
cables. This , however, could be a less practical
option when compared to pre-trenching. So says
Lee Screaton of S&A Associates.
“Simultaneous lay and trench is the process of
cable lay whereby the cable is laid into a subsea
cable plough that immediately buries the cable
into the seabed as it is towed behind the cable lay
vessel,” he said
“The problem with this method is that power
cables have a far larger diameter than telecoms
cables with a minimum bend radius (MBR) of
between 4m and 5m. Furthermore, power cables
are arguably more fragile than telecoms cables
due to the combination of large diameter, high
density and construction elements such as the
‘paper’ insulation layer.”
There are numerous advantages of simultaneous
lay and trench.
●
The cable is immediately trenched so
unlikely to be damaged as it waits for trenching
●
The whole lay and trench operation occurs
from one vessel
These advantages were, and still are, great for
tele-communications cables but they don’t
translate as well to the power cable installation
operation.
When viewed from the power cable installation
perspective, however the disadvantages are:
●
Power cable ploughs are larger than
telecommunications ploughs due to the far larger
MBR and usually need 150Te of bollard pull (for
2m+ trenching) making for a more powerful cable
lay vessel or dual vessel laying with an offshore tug
in tandem with the cable lay vessel
●
The standard approach of loading cable
into the plough on the vessel and launching with
the cable through the plough is less desirable as the
launching process can damage the cable outer
serving layer
●
It can be difficult to perform some
ploughing operations, such as changing heading, as
the overboard tension of a power cable should be
lower than telecoms cable due to concerns over the
fragility of the cable.
This can lead to issues such as not being able to lay
power cable directly into the bellmouth of the
plough
●
The back deck of a power cable lay vessel
can be congested and multi-role when compared to
a telecoms lay vessel so it isn’t easy to find space for
the power cable plough. This can lead to additional
systems such as movable mezzanine decks, etc.
Various approaches have been developed to offset
the plough related problems such as subsea loading
using grabs or ‘top loading’ techniques which avoid
the launching issues but they are not common and
have their own problems.
Is the answer to shift the emphasis to pre-trenching
instead?
In the context of this article, the term “pre-
trenching” is intended to imply the use of a plough
that creates a V shaped trench in the seabed that the
cable lay vessel lays the power cable into as a
separate process.
Practically, what this means is a pre-trenching plough
is deployed from a plough support vessel, not the
cable lay vessel, to cut the trench in the seabed.
After the trench is cut, the cable lay vessel lays
LAY AND TRENCH V PRE TRE
TRENCHERS
Fig1: Typical Simultaneous
Lay and Burial Operation