24
Within the space of a couple of
hours after the cable was laid,
ROV images showed sand being
ejected out from under that
cable. The result was that the
cable rapidly became self-
embedded into the seabed and
thus became profoundly stable.
At adjacent locations on those
cables, where they crossed
existing assets (pipelines, cables)
buried in the seabed, they were
required to install hollow sleeves
on the outside of the cable to
guarantee separation with the
existing assets.
“Those hollow sleeves were
actually free to roll around the
cable which effectively meant
that the CSS, (crossing
separation system) was free to
roll backwards and forwards. This
made it profoundly and
fundamentally unstable.
Using our knowledge of cable
scour meant the crossings could
be installed with confidence,
knowing they would self embed.
“By analysing videos taken on
site, it is possible to observe the
rate at which sand is being
ejected out. This meant the
scouring was occurring
underneath that cable as
predicted.
“Again, within the space of 6–
12hrs, that system self-
embedded into the seabed and
became fundamentally stable
and did not move laterally.”
“When the installation
contractor returned to undertake
permanent stabilisation works of
the crossings, they found that the
cables and CSS had Indeed self-
embedded Into the seabed and
remained stable.
Elsewhere in Scotland, the group
have been looking at the power
cables that connect the UK's first
tidal stream energy project to the
grid - the MayGen tidal stream
project owned by SAE
Renewables.
“It is clearly possible to see the
extent to which these bare cables
- with no added external
protection applied to them, have
naturally found nooks and
crannies in the seabed into which
they have become wedged and
thus become self-stable.
“The seabed is characterised by
hard rocks and thus, the cables
could not buried. Instead, they
have been placed on the bare
rock surface. The rock was not flat
and on a smaller scale, had cracks
and boulders distributed across
the surface.
“Conventional wisdom would
suggest that the cables would be
free to slide across the surface
but upon investigation, the cables
embedded themselves between
the boulders and undulations and
ultimately self-stablised on the
seabed.
“It is critical to understand and to
learn by looking carefully at what
is happening in the ocean,
understanding the relevant
physics and then tailoring the
engineering solutions to suit.
“When applying the relevant
physics to the design, we
wind up being able to show
some rather extraordinary
outcomes that these cables
are actually self-stable.
“This is not what you would
expect if you just inherited
the oil and gas design
methods, and based on the
different sets of ROV footage
we've actually got proof when
we've taken the design
methods that were inspired
by watching the Maygen
cables and we've been able
to apply them to now two
offshore wind farms off the
Atlantic coast of Europe that
have been developed on
entirely rocky seabeds.
“It was a career highlight to
stand on the cliffs at Le
Croisic near St Nazaire and
look out towards France's first
offshore windfarm a few
weeks after it was
commissioned.
This project demonstrates
what we are advocating - that
the offshore renewables
industry actually needs new
design guidance that is
actually fit for purpose for
subsea cables.
They need to be based on a
new mindset a new paradigm
for how we go about setting
the target risk and the target
reliability for subsea cables so
that we one lower the cost
but two improve the
performance of cables.
NEWS