Offshore wind turbine design - Wind Power Generation and Wind Turbine Design - page 371

demand to collect borehole soil samples at each turbine locations. However, this
increased the plant development costs signifi cantly. Moreover, it is not clear if a
geotechnical program comprising boreholes at every turbine site, which is referred
to as “full coverage”, would signifi cantly reduce the overall geotechnical risk.
Studies performed by the oil and gas industry show the full coverage might be
unnecessary as long as adequate information exists regarding the overall geologi-
cal structure in the area, which can be synthesized utilizing non-intrusive geophys-
ical measurement techniques. There exists a need in the wind industry to quantify
the increased risk associated with less than full coverage of all turbine sites based
on the synthesized geological and geophysical knowledge on the general plant
area. Subsequently the marginal risk reduction from each additional borehole can
be traded-off with the increasing investigation costs. Some boreholes will always
be needed to get an idea about the real soil conditions across the site. However,
there may not be a need for a borehole under each turbine.
4.4.5 Mud-line evaluation
At the end of foundation design life or immediately after severe storms, no
techniques exist to evaluate foundation conditions below the mud-line. Effec-
tive above-the-mud-line evaluation techniques do exist to identify cracks using
eddy circuits or visual observations. However, these techniques cannot be eas-
ily transferred to foundation sections below the mud-line. Moreover, there are
no techniques to assess soil condition, which is an important element of the
overall system condition. Technologies such as digitally extract soil density
images with high-frequency ultrasound techniques before and after severe
storms need to be developed to answer the critical questions over wind power
plant residual life.
The above challenges are barriers to synthesize accurate design envelope that
can be used to design the wind power plant system including the turbine and the
foundation. These challenges stem from the uncertainty associated with the off-
shore wind environment. In developing that design envelope along with the uncer-
tainty associated with the above challenges, it is the key to work within a
probabilistic framework that provides feedback on the changing risks and costs.
This will aid wind power plant developers, owners and design engineers to trade-
off effectively based on cost and risk for decisions.
4.4.6 Sediment transportation
At the offshore wind power plant seabed level, the wave and current con-
stantly transport the sediments around to morph the landscape. This effect
could expose the embedded portion of the turbine foundation and produce
problem later on during the plant life along with scoring. Anti-scoring typi-
cally involves expensive operations to lay stone layer around the foundation.
New innovations utilizing plastic/rubber mat material has been tested to miti-
gate this problem. Its long-term effectiveness need to be checked. More global
sediment transportation effects need to be evaluated additionally during the
site survey period.