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Learning experience: Ground conditions for offshore wind farms

The publication of new guidelines on site investigations for offshore renewables in July has highlighted the need for a better understanding of seabed ground conditions for offshore wind farm projects.

The UK government’s green light for construction of the East Anglia One offshore wind farm in June underlined the importance of offshore wind to the UK’s future energy mix. The facility will have up to 240 turbines and be one of the world’s largest, generating enough electricity to power about 820,000 homes.

Wind farm

Two Gardline vessels worked simultaneously to survey the East Anglia Offshore Wind site over 380 days and provided more than 19,000 line kilometres of survey

However, it is worth remembering that, in December last year, two of the UK’s Round Three offshore wind farm 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.

The situation has worsened because, although UK government subsidies for the development of offshore wind are set to increase (with a consequent reduction for onshore projects), it is clear future schemes must be delivered more cheaply than at present, to help lower overall costs to levels comparable with other forms of energy generation. This is increasingly difficult as wind farms move into deeper water further offshore.

Those with experience of supporting both the oil and gas industry and offshore renewables projects fear a lack of engineering information relating to the seabed and sub-seabed is leading to projects being cancelled, with a huge potential to “throw the baby out with the bath water”.

Fundamentally, if ground conditions were more fully understood, the decision whether or not to proceed could be better informed.

Within the UK Continental Shelf there is already a huge amount of private and public sector data available on the geological and geotechnical properties of potential offshore wind farm sites. Using this data to compile preliminary ground models early on means an initial appraisal of the potential foundation challenges can be made before multimillion pound site investigations are undertaken.

Wind farm

Cone penetrating testing delivers important geophysical data

Much of the early survey work focused on environmental issues, specifically the impact that an offshore wind farm would make on benthic, pelagic, seabed, cetacean and ornithological communities.

This meant that some early project scopes of work lacked precision in understanding the seabed and sub-seabed conditions. This was odd, given this data is critical when designing foundations and for cable burial and because it is also an essential component of the “go, no-go” strategy to design and build the project in the first place.

Fortunately, there is now recognition of the need to acquire, process and interpret seabed data, which can be used more effectively in the design of wind farms.

In July, 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. It is hoped this document will help improve understanding of the processes needed to better gather and interpret data.

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 the design and installation, but also the working life of the wind farm.

Survey contractors have been working on gathering terabytes of data for offshore wind farm projects over the last few years, using hydrographic, marine geophysical and geotechnical techniques. Vessels like Gardline’s MV Ocean Discovery have been engaged to gather soils data, primarily to provide control to remote sensing data.

Investigations historically involved four or five days of fieldwork over a 3km by 3km exploration survey area. In contrast, when Gardline undertook a survey for the East Anglia Offshore Wind site, two vessels working simultaneously took a total of 380 vessel days and more than 19,000 line kilometres of survey data was acquired. This created more than 10 terabytes of data, which needed to be analysed and interpreted.

This creates another issue: how to present this data in an informative way, that will give a clear picture of site conditions.

Wind farm

3D visualisations for Front End Engineering Design

An offshore wind farm, like any offshore engineering project, requires considerable planning, research and decision making to be able to move from concept to construction. 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.

The 3D visualisation of geotechnical properties aids the processes involved with Front End

Engineering Design (FEED) and engineering decision making. This is very useful in ascertaining the engineering problems related to any offshore infrastructure development, such as wind turbines, oil and gas structures, pipelines and cables.

Based upon techniques used in the oil and gas sector, the ground model can be developed further, to model geotechnical proprieties of each of the soil types and the uncertainty associated with these estimates. Geophysical data, insitu test results (from cone penetration testing) and laboratory testing can also be incorporated.

The GIS environment has proven to be a reliable and portable mechanism to store, update and present data throughout the project lifecycle. By having the data in one place, it can be managed easily by qualified administrative staff, for use by all project personnel, including the engineering team.

Three-dimensional modelling and GIS have already demonstrated their ability to provide useful, accurate and viable information to aid in the design of offshore wind farm. They are also useful during the operational phase.

For example, many of the wind farms built in Rounds One and Two reported considerable outages, caused by failure of the infield and inter afield cable arrays. Better understanding of the nature and complexity of the seabed means these types of failures can be rectified more easily.

It would be tempting to think that 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.

And while the offshore wind industry has matured in its own right, it is clear that projects could benefit further from experience gained in the oil and gas sector. A better understanding of the ground conditions will result in improved decision making, more accurate design and construction and will continue to deliver benefits throughout the operational lifetime of the wind farm.

Eric Zon is managing director of Gardline Geosciences.

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