September/October 2022 - 110

and installation and construction risk
management. For example, the same
geophysical dataset may be used to
facilitate cable engineering, foundation
design, identification of archaeological
artefacts, understanding the benthic
habitat and providing project definition
for siting of the wind turbines. Given
the complex number of end uses, it is
essential that all geo data is collected to
the required specifications to address
all the project requirements.
Development timelines vary globally
for offshore renewable energy developments
depending on funding models,
development frameworks (centralized
or developer led) and local regulatory
regimes. As a generic guide and considering
site selection has already been
undertaken, we can consider a subsequent
5-year development cycle to
reach final investment decision. In some
jurisdictions, this
may be longer due to
regulatory and or permitting hurdles.
The Importance of a Survey
Strategy
Geophysical, geotechnical and related
surveys are the biggest development
expenditure that an offshore wind
development will face prebid. Furthermore,
unlike onshore projects, it is not
feasible for offshore projects to undertake
survey work " on the fly " if unexpected
conditions emerge. Therefore it is
essential that the ground conditions are
accurately captured ahead of time and
derisked to avoid " unforeseen ground
conditions " emerging during the works.
Establishing a schedule and projected
costs early on is essential to achieve the
project objectives and mitigate risk, and
also to minimise early expenditure, i.e.,
presite exclusivity, presubsidy power
purchase agreement and prefinancial
investment decision. Managing budgets,
timelines, procurement requirements
and interfaces with all geophysical and
geotechnical survey work between
initial planning and construction is
necessary to ensure investment is in
place at the appropriate times.
Site Investigation - Past,
Present and Future
Site investigation for offshore wind farm
projects involves geophysical surveys
using seismic techniques, side scan
sonar, multi-beam echo sounder (MBES)
and magnetometers, in addition to
shallow sampling, deep borehole
sampling and cone penetration testing.
High quality soil and rock samples are
subsequently used for a range of lab
tests including classification, mechanical
strength and or stiffness testing, and
more advanced cyclic testing. The
quality of the lab results is directly
related to the quality of the samples and
therefore high-quality sampling is
essential to ensure that the data
facilitates optimum design processes.
Poor-quality samples can make advanced
lab testing redundant and can
drastically increase the size and cost of
the foundations.
When producing and evaluating
specifications for offshore sampling of
soils and coring in rock, the entire
survey spread needs to be considered,
including the vessel capabilities as well
as the equipment specifications. It is
often observed that the objectives are
focused on the sample category and the
relative sample diameter required. For
example, the drilling technique for rock
Two sets of rock cores recovered using marine coring systems
is selected such that a category A
technique (ISO 22475-1) and 100 mm (4
in) diameter core size is defined to
assist with overcoming size effects
during laboratory testing. The result is
that a triple tube coring system, such as
Geobor S, is specified. Contractors'
proposals are often evaluated in the
same vein with only high-level consideration
to the means of deployment
and often without critical appraisal
of the complete marine and geotechnical
spread.
For example, two cores were
recovered from within the same
investigation area and geology at
similar depths only meters apart,
however it can be seen that the quality
of cores obtained by one coring system
is superior to the other. In retrospect it
can be argued that one coring system
demonstrates Applicability Class A and
the other Class B. The lower class
relates disturbance by small heave
induced vertical motions of the drill bit
during coring and the higher class had
all vertical vessel motions isolated.
This case of heave induced drilling
disturbance influences the site characterisation
in terms of quantities of
suitable samples for advanced testing
and, in cases, the associated sample
disturbance causes changes in the
natural state and structure. As a result,
key des i gn parame t ers can be
influenced by sample disturbance,
which is crucial when assessing the
axial pile response to load and the
interface strength between grouted
piles and rock.
It is essential when specifying and
evaluating rock coring for offshore
investigations that consideration be
given - by a suitably experienced
offshore investigation specialist - to
deployment modes and technical
aspects beyond those focusing solely on
the core barrel type and diameter.
Thorough specification and evaluation
of the vessel drilling systems, which
a r e inf l uenced by movement s
associated with surface environmental
and metocean conditions, are required
110 * DEEP FOUNDATIONS * SEPT/OCT 2022
September/October 2022

Table of Contents for the Digital Edition of September/October 2022
