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contain. Leakage has the potential
to cause catastrophic events and
environmental devastation. It was
necessary, especially in the early
days of the industry, to over-
engineer. The underlying
reliability targets inherited and
adopted in the offshore wind
sector have largely derived from
trying to prevent catastrophic
events in pipelines.
“What we need to be looking to
do is move towards bespoke
design guidelines actually geared
towards the fundamental risks and
opportunities of subsea cables."
CABLES
Cables are very important. While
they cost roughly 9% of the
project CAPEX, they represent 80–
90% of insurance claims. Despite
being a relatively modest
proportion of the project cost,
therefore, cables are a
fundamentally unacceptably large
proportion of the reason for the
failure of wind farms to deliver.
“We are proposing that the
industry moves away from a
one-size-fits-all target of
reliability and actually
progresses towards setting
bespoke cable reliability targets
that are actually fit for purpose
in carrying the amount of power
that needs to be transmitted
through each cable.
“We suspect that will drive
fundamentally different cable
layouts as well,” said Griffiths.
“We have been able to identify
that across the offshore wind
sector, cables at the bottom of
the ocean are subject to very
different metocean criteria
because they have very different
response characteristics.
“What we've been able to do on
a number of projects is actually
re-evaluate the metocean
hindcast and redevelop fit for
purpose metocean design
criteria to suit the response
characteristics of the cables.“
CONSERVATIVE DESIGN
Since the early years of the
offshore industry, engineers have
adopted conservative
assumptions in the input
parameters.
“When it comes to subsea cables,
however, we repeatedly find there
is no universally conservative bias
that can be adopted. We can look
at the Goldilocks principle.”
In the eponymous fairy tale,
before Goldilocks went into the
house of the bears, she didn't
know which bowl of porridge or
which chair or which bed was the
right one for her. She had to try all
three.
“Similarly, the way in which we go
about designing subsea cables
needs to also stop making the
erroneous assumption that we can
both second guess and correct for
bias in each of the input
parameters. We need to model
these on a case-by-case basis until
we have reliable information.
“We recently carried out some
investigations on subsea cables
being installed for the Hollandse
Kust offshore wind farm in the
central North Sea to actually look
at what happens to real cables in
the real ocean conditions.
“That cable laying on the surface
was originally predicted to be
unstable, however using the
STABLEpipe design methodology
it was predicted that in reality the
cables would rapidly embed into
the seabed due to scour and
erosion.