What lies beneath
David Higgs, geotechnical lead at Ossian and Roger Birchall, geophysical lead at Ossian
Being able to build a floating offshore wind farm, rather than a fixed wind farm, means that we have access to more of the seabed, the deeper parts of the seabed. Floating offshore wind turbines have a different type of contact with the seabed compared to a fixed turbine. A floating turbine will be on a floating base made of steel, or concrete, and it will be anchored to the seabed by mooring lines, rather than one large structure.
The anchor for the mooring lines needs to be sturdy enough to hold the large structure in place in the roughest of seas and the strongest of winds. We also need to ensure that we are installing the mooring lines in a suitable position with no obstruction, and importantly that the anchor lines have minimum impact on the seabed during installation and while in situ.
To ensure responsible installation we need to carry out survey work to inform the design decisions.
“To boldly go where no one has gone before…”
In the survey work that Ossian has, and will, be carrying out we will be surveying some parts of the seabed that have never been “seen” before. To collate the relevant information on subsurface conditions we use two different types of ‘geo’ surveys – geophysical surveys and geotechnical surveys. Together these types of surveys will collect thousands upon thousands of data points, which will complement each other and will help us build a ground model to map out ‘what lies beneath’.
Geophysical surveys
Geophysical surveys help us understand the seabed habitat before we install the anchors. This understanding will allow us to ensure that we can design our wind farm around these habitats, so we can consider any sensitive areas during construction. With the use of non-invasive sensors, geophysical surveys can also help us identify any hazards which may lie on the seabed such as boulders, steep slopes, peat or even shipwrecks and unexploded ordinance. The results from bathymetric and geophysical surveys will help inform the next steps for the geotechnical surveys.
Geotechnical surveys
Geotechnical surveys are used to understand the physical properties of the seabed. The geotechnical surveys to date have shown us general characteristics of the site soils, next we will need to carry out more work to give us a specific indication of where we might install the anchors for the floating wind turbines. The process includes collecting samples from bore holes drilled in the seabed and taking these samples to a lab for analysis. In these samples we are looking out for certain properties of the ground that we are surveying to see how firm it is in order to understand how it will react with our anchors.
The data from the geophysical survey allows optimisation of subsequent geotechnical survey with regards to cost, duration and environmental impacts.
Working together
Geophysical and geotechnical surveys complement each other, we need them both to successfully design the floating offshore wind farm. Generally, interpretation of geophysical surveys provide an indication of areas to avoid and where to site subsea infrastructure such as anchors and subsea cables. Geotechnical surveys will help us select an anchor appropriate to the specific conditions at each location. Both types of surveys inform design, and one could not succeed without the other.
Proper siting of offshore infrastructure is required to mitigate potential hazards to foundations, moorings, and anchor systems for floating offshore wind turbines, buried cables, and other offshore infrastructure. Site investigations should account for the area’s water depth, geology, and proposed facilities.
The future
When all survey work is concluded, we will be one step closer to delivering the Ossian wind farm. When complete, Ossian will be one of the largest floating offshore wind farms in the world with up to 3.6GW of potential capacity – enough to power up to 6m homes annually. Ossian is one of the largest lease areas of the ScotWind projects, occupying 858km2 of seabed 84km off the east coast of Scotland.
The scale of Ossian, alongside the floating technology we use, will make it a game changer in the UK renewable energy sector and a critical driving force behind the journey to net zero offsetting up to 7.5m tonnes of carbon emissions.