Firm foundations for floating offshore wind success

At present, most offshore wind farm developments are situated close to shore in shallow waters, typically less than 40 metres. However, more than 80% of the world’s offshore wind resource potential lies in water depths greater than 60 metres. It’s still early days for the floating offshore wind industry but the potential is huge, and interest is growing around the world.

In Australia, floating offshore wind will be an option where the continental shelf drops steeply or where communities have concerns about the visibility of offshore wind turbines from shore.

Heading into deeper waters

As developments move into water depths around 60 metres or more, fixed-bottom foundations are less economical, and floating offshore wind turbines become a viable alternative. Designing and installing efficient and cost-effective mooring and anchoring systems for these turbines will be critical to an offshore wind project’s success.

Thanks to learnings from the offshore oil and gas and aquaculture sectors, a range of anchoring concepts exist for floating offshore structures. These same concepts can be applied for floating wind turbines. Potential options include:

• Anchor piles (typically either driven or drilled and grouted)

• Suction piles / caissons

• Gravity anchors 

• Drag embedment anchors 

• Dynamically embedded anchors 

• Plate anchors (including drag embedded, suction embedded, or dynamically embedded) 

• Helical piles. 

The last factor is relatively unique to floating offshore wind, where developers try to minimise the footprint by attaching multiple mooring lines to individual anchor points. Certain anchor types are better at accommodating this than others, because complex load dynamics can be introduced at the anchor attachment point.

Optimising the design depends on geotechnical understanding 

As with any foundations, optimising the anchoring design depends first and foremost on obtaining good quality geotechnical and geophysical data at the site. This can be challenging in deep water and in certain seabed conditions. However, methods have been refined over decades of use in the oil and gas industry.

From a geotechnical perspective, the key design principles applied in calculating the geometry of an anchor for an oil and gas development are the same as those for an offshore wind turbine. And, based on learnings from these industries, an ever-increasing database of field observations and measurements can be applied to calibrate the design of offshore anchoring systems.

Overcoming installation challenges 

The design of an anchoring system for any floating structure must always balance the achievement of a suitable holding capacity with the ability for the anchor to be installed in an efficient and cost-effective manner. There's little point designing an anchoring system that can only be installed by one particular vessel in the world! With certain specialist offshore wind farm installation vessels fetching day rates in the order of $1 million, foundation installation costs can have a significant bearing on the overall CAPEX costs.

Certain anchor types require more external input during installation than others. Hence, anchor concept selection and design can play a vital role in limiting offshore time and therefore installation costs.

The anchor design needs to account for issues that can arise during installation, such as: 

• Pile free-fall during driving 

• Pile tip buckling, leading to premature refusal 

• Refusal of suction anchors on strong/stuff soil layers 

• Set-up effects which can increase the installation resistance during unexpected delays 

• Drag anchors skipping off harder or cemented materials during installation resulting in a failure to achieve target preload levels 

• Environmental concerns relating to pile driving, but also sediment plumes if drilling is required. 

Applying lessons from offshore oil and gas to offshore wind 

Floating offshore wind introduces some new challenges. However, the fundamental principles associated with anchor design and installation are well understood from decades of technical evolution in the oil and gas and aquaculture industries in Australia, and around the world, that we can apply the lessons to offshore wind projects.

In one project, we demonstrated the feasibility of installing anchor piles using impact driving, which avoided the need for a much more expensive drilled and grouted pile system and significantly reduced offshore installation time. In another project, we developed a novel drive-drill-drive installation method, achieving safe, efficient, and cost-effective results in highly variable seabed soil conditions.

In a different project, where water depths ranged from approximately 135 to 1,200 metres, our geological desktop study and anchor concept screening assessment addressed the challenging conditions of steep seafloor gradients as the site extended off the continental shelf. We also considered the potential presence of sub-surface palaeochannels, submarine canyons, and weak seabed soils.

WSP’s specialist offshore teams leverage local and global expertise to support offshore wind development across the entire project lifecycle. We assist with every aspect of project development, from feasibility assessment and engineering to environmental impact assessments and geotechnical and geophysical surveys. Our geoscience team, drawing on experience in designing and analysing anchoring systems from other industries, helps offshore wind developers overcome challenges and find optimal solutions for their unique projects. 

We're looking forward to seeing this emerging industry reaching its potential for a more sustainable and resilient energy future. 

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