According to Offshore Wind Outlook 2019*, published in October, investment in offshore wind could reach US$1 trillion by 2040, driven by falling costs, supportive government policy and technological progress, such as larger turbines and floating foundations.
"Today, offshore wind capacity in the European Union stands at almost 20 gigawatts," the International Energy Agency (IEA) says. "Under current policy settings, that is set to rise to nearly 130GW by 2040. However, if the European Union reaches its carbon-neutrality aims, offshore wind capacity would jump to around 180GW by 2040 and become the region's largest single source of electricity."
A deep understanding
However, it is worth remembering that, in December 2013, two of the UK's Round Three offshore windfarm projects, the Argyll Array and the Atlantic Array, were cancelled, partly because of adverse ground conditions.
This highlighted the critical importance of necessary, but ever-more cost-effective, site investigation solutions for these projects, which becomes increasingly difficult as windfarms move into deeper water further offshore.
There is a very real risk that a lack of engineering information relating to the seabed and sub-seabed could lead to more projects being cancelled. Geotechnical data is critical, not only when designing foundations and for cable burial, but also because it is an essential component of the "go, no-go" strategy to design and build the project in the first place.
Ground models
It would be tempting to think there can never be enough site investigation data but, just like any other construction project, there is a point when additional information provides no real added benefit. Ultimately, sound geotechnical judgement is vital to ensure that the data collected is appropriate and costs are controlled.
Obviously, by better defining what is required to quantify the nature of the seabed and sub-bottom geology, and by providing a good baseline, useful information will be obtained, not just for design and installation, but also the working life of the windfarm.
An interactive 3D ground model can give the entire team an overview of morphology, the geology, any likely geohazards and, most importantly, the level of uncertainty throughout the lifetime of a project.
There is already a huge amount of private and public sector data available on the geological and geotechnical properties of potential offshore windfarm sites. By using this data to compile preliminary ground models early in the process, an initial appraisal of the foundation challenges which may lie ahead can be considered before multi-million-pound site investigations are undertaken.
Investigation guidance
The Offshore Site Investigation and Geotechnics (OSIG) committee of the Society of Underwater Technology published its Guidance Notes for the Planning and Execution of Geophysical and Geotechnical Ground Investigations for Offshore Renewable Energy Developments** in 2014, providing a good overview for geotechnical teams and their clients.
Cone penetration testing (CPT) is particularly suited to offshore investigations and has built an excellent track record in oil and gas operations due to its speed, versatility and its ability to generate, and report upon, a wide range of geotechnical data in real-time.
CPT is ideal for gathering data for the design of all types of foundations used in offshore windfarms
CPT is ideal for gathering geotechnical data for the design of all types of foundations used in offshore windfarms, including monopiles in soil and rock, jackets, gravity bases, suction foundations and anchored or tethered foundations.
Over the past decade, technological advances have resulted in improved quality and a more extensive range of in-situ measurements. There are a number of different cones available, from the seismic cone and cone pressuremeter, that acquires soil stiffness information at small to medium strains, to environmental cones that enable engineers to trace and quantify hydrocarbon plumes.
Sampling can also be carried out and, if holes are cased, difficult strata can often be penetrated. However, the accepted rule of thumb is that high-quality sampling is required (through conventional rotary boreholes) next to at least 10 per cent of the CPT locations, to calibrate results.
CPT and borehole drilling is typically carried out from jack-up platforms in water depths of up to 25m. In deeper water, seabed CPT units and drill vessels are preferred, with work carried out from the latter using wireline downhole CPT systems with heave compensation that allows testing to continue in a range of sea conditions.
Wireline systems have production rates of up to 40m a day - double that of conventional top-push units and are also able to cope better with harder, more variable geology and conventional top-push methods. They are now routinely used, not only on drill vessels but also from jack-up platforms. Additionally, combined rotary and sonic drill rigs enable optimum sample recovery, with the ability to switch between the two systems when needed.
Specialist opportunities
The growth of offshore wind shows no signs of slowing and, in fact, looks set to accelerate over the next two decades, presenting an ideal opportunity for geotechnical specialists, particularly those with experience gained in offshore oil and gas.
As the IEA says: "Huge business opportunities exist for oil and gas sector companies to draw on their offshore expertise. An estimated 40 per cent of the lifetime costs of an offshore wind project, including construction and maintenance, have significant synergies with the offshore oil and gas sector."
To take full advantage of these opportunities, it is imperative that the geotechnical industry pushes the message that a better understanding of ground conditions will result in improved decision-making and more accurate design and construction and will continue to deliver benefits throughout the operational lifetime of offshore windfarms.
Eric Zon is MD of Lankelma, which provides on-land, nearshore and offshore CPT services around the world. Lankelma has delivered numerous projects for offshore wind and oil and gas projects over the past 20 years, mobilising CPT equipment at short notice to work on third party platforms and vessels around the world.
*Offshore Wind Outlook 2019 is available at www.iea.org/offshorewind2019.
**The OSIG Guidance Notes for the Planning and Execution of Geophysical and Geotechnical Ground Investigations for Offshore Renewable Energy Developments are available at www.sut.org/wp-content/uploads/2014/07/OSIG-Guidance-Notes-2014_web.pdf.
