A closer look at biofouling on dynamic cables at floating wind farms

Floating wind power cable graphic


01 Sep 2023


Sophie van Zanten - Global lead, power cable routing and design

Stefania De Gregorio - Principal Marine Ecologist

Many offshore floating wind farm operators are concerned about the accumulation of unwanted marine growth, known as biofouling, on the dynamic cables that transfer electricity from the turbines to the substation. If this marine growth is not accounted for in the cable design and monitored, it can add considerable maintenance costs over the lifetime of the wind farm due to repeated cleaning and may even result in cable failure if significant build up occurs.

First things first: what exactly is biofouling? It’s a term used to describe the complex process that involves an accumulation of marine organisms on structures immersed in seawater, which can affect biodiversity, the design tolerance of submerged assets and require additional maintenance. Broadly speaking there are two types: hard fouling like mussels; and soft fouling like algae and sponges.

Why is biofouling on dynamic cables a cause for concern?

At offshore structures around the world, various marine species can attach themselves to various components of the assets, such as dynamic cables. As the extent of the biofouling increases, the cables may be affected by:

  • Structural fatigue – too much biofouling may push the weight, mass and diameter of the cable beyond the parameters of the original engineering design. This may change the structural drag and added mass coefficients, affecting how the cable moves in the water in response to tides, currents and storms. Biofouling may also start or accelerate corrosion. Over time, the structural integrity of the cable may reduce. And if left unchecked, this could lead to cable failure;

  • Inefficient heat transfer – the biofouling may act as an added layer of insulation that prevents the cable from dissipating heat effectively. This creates a risk of overheating. Thermal conductivity of the soil is a key design parameter for buried cables. A dynamic cable’s ability to transfer heat will be affected by the type of biofouling.

Biofouling from five metres in the water at Milford, UK after a six month immersion period

Biofouling from five metres in the water at Milford, UK after a six month immersion period

So many unanswered questions that cause risk

Floating wind farms are in their infancy, so the industry still has lots to learn about how biofouling affects dynamic cables. For example:

  • How much does the biofouling weigh and what effect does it have on buoyancy and stability? And is the weight evenly distributed along the cable?

  • How long does it take for a species to dominate a biofouling community?

  • How do electromagnetic fields affect their growth rate?

  • What are the most effective prevention and removal methods?

  • How often should the cables be cleaned?

  • What happens to the biofouling that’s removed? If left to drop on the seafloor, how will it affect the marine habitat in the long term?

  • Can the removed biofouling be recycled, such as to be used as fertiliser? 

It’s a complex area with many anomalies. For example, why should an engineer who’s designing dynamic cables for a floating offshore wind turbine in California be required to base their calculations on biofouling growth rate standards that stem from oil and gas operations in the North Sea? It’s far from ideal.

There’s a growing awareness of the challenges associated with biofouling on dynamic cables at floating wind farms. Although there are patches of data and knowledge about these challenges within the industry, it’s largely internal, scattered and fragmented. There’s no high-level overview and little evidence of information-sharing.

Biofouling power cables - Bracklesham 2014 - below the surface after 6 months immersion period

Biofouling from below the surface in the water at Bracklesham, UK after a six month immersion period

How to better understand biofouling and reduce risks

Having robust and site-specific data on the engineering effect of biofouling onto submerged assets will allow planning and design closer to the environment. This is because biofouling is influenced by site-specific environmental variables.

We have supported many environmental offshore wind studies in recent years. For example, together with project partners, we are developing innovative methodologies and technologies for remotely collecting environmental DNA samples in the North Sea. This will help us measure the impact of turbine structures on biodiversity by monitoring, characterising and analysing species. Similarly, we are undertaking research studies on biofouling development with our environmental consultants and laboratories. We are currently involved in commercial studies that are exploring biofouling and its implications for engineering design of an offshore wind farm.

If you’re a floating wind farm operator who’s looking to mitigate the risks associated with biofouling on dynamic cables, here are some recommendations:

  • Take a joint approach from the outset – gather the developer, engineers and ecologists together to discuss the risks of biofouling. Agree to tackle it jointly and establish the parameters that need to be investigated (duration, type of settlement plates to be used, water depths and so on);

  • Act quickly – start gathering data on your site-specific biofouling characteristics as soon as you can. A six-month study won’t produce a dataset that’s robust enough for modelling the total lifetime. In general, to produce a five-year prediction, you’ll need a one-year study. For a 10-year prediction, you’ll need a two-year study, to ensure capturing seasonal and temporal variation of marine growth;

  • Cover a lot of ground in one-go – to build a wind farm, you’ll have to do an initial metocean research campaign, so why not do your biofouling research at the same time through experimental settlement panels or through scraping of metocean equipment at a range of depths to the same buoy. By ‘piggybacking’ onto the mandatory baseline data-gathering process you’ll create an efficient and easy way to collect a biofouling dataset covering several years;

  • Feed the data into your cable design – by combining the metocean and biofouling datasets, your engineer will be able to optimise cable design with site-specific parameters, avoiding unnecessary additional costs generated by overengineering.

A challenge we love to take on

Are you looking to understand the site-specific biofouling risks for your planned offshore wind farm? The key is to gather and collect your datasets early in the site appraisal phase to reduce uncertainty in the design phase. This provides the opportunity to integrate the data acquisition with other field work for efficiency and cost reductions.

Did you know?

  • It’s estimated that there are more than 4,000 different marine biofouling organisms

  • The development of biofouling can triple the weight of the dynamic cable

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Power cable routing and design

No power cables, no power transmission. But how do you protect your offshore wind farm’s cables? And make sure they work properly? One way is to select the right route and design. That’s where Fugro comes in.

Find out more