Offshore wind farms are crucial in reducing reliance on fossil fuels, cutting greenhouse gas emissions, and combating climate change. However, the construction phase raises environmental challenges, not least noise pollution.
The construction phase is the noisiest part of a wind farm's life cycle. Installation work for the foundations (especially pile driving) and noise generated by ship engines are the main sources of sound emissions.
Increased underwater noise levels affect all marine species, with marine mammals and sea turtles being the most studied due to their high auditory capacity. Consequently, evaluating and mitigating noise is at the core of installation campaign planning.
The type of foundation chosen determines the noise level during the construction phase. Monopile foundations are the noisiest due to activities such as pile driving and drilling. Installing jacket foundations, which use smaller-diameter piles, generates lower noise levels. Finally, gravity-based structures are the least noisy. Regulations often require noise mitigation techniques.
Foundation Installation Techniques for Offshore Wind Farms
Offshore wind farm foundation installation relies on several techniques, each suited to specific seabed conditions and environmental considerations. The most common methods, hammering, drilling, and vibro-hammering, vary in their approach, noise generation, and effectiveness.
Hammering
The conventional hammering method involves driving the monopile into the seabed with a large hydraulic or diesel hammer. The hammer repeatedly strikes the top of the pile, gradually forcing it into the ground.
This method is effective in softer seabeds like sand or clay. However, it generates high levels of underwater noise, which is related to the pile diameter. A larger-diameter pile requires more mechanical energy and, therefore, a more powerful hammer to be installed. Thus, the pile diameter is an indicator of the hammer's power.
Jetting
Jan de Nul and Ørsted recently tested a new installation technique at Gode Wind 3. The new low-noise process developed by Ørsted consists of a jetting technology attached to the monopile. This lowers the resistance of the surrounding sandy soil, allowing the foundation to sink into the seabed.
The method reduced noise by 34 decibels (dB) from conventional hammering. The jetting technology is adapted to soft seabeds.
Drilling
Drilling is reserved for rocky or heterogeneous seabeds and can be used as an alternative to pile driving for piles with smaller diameters. The noise generated is continuous and lower than that of pile driving.
During the process, the drilling machine creates a hole, sometimes with a casing, and the monopile is lowered into position. Cement or grout may be used to stabilize the monopile in the drilled hole.
Drilling has been observed to be more time-consuming than hammering or vibro-piling.
Vibro-hammering
A vibratory hammer shakes the monopile at a high frequency, reducing friction between it and the soil and allowing it to penetrate the seabed.
Vibro-driving is generally less noisy than impact hammering, with noise levels 15 to 20 dB lower on average. However, the noise generated by vibro-hammering, which consists of continuous (vibrations) and impulsive (oscillations) emissions, is not directly comparable to the impulsive noise of pile driving when considering the impact on marine life.
The technique can be used with impact driving, particularly for softer soils or when the total depth can’t be reached with impact driving alone.
Leading Solutions for Secondary Noise Mitigation
The noise pollution generated by foundation installation campaigns disrupts the marine ecosystem to various degrees. Therefore, solutions have been sought and implemented to address this issue.
These noise mitigation solutions are currently divided into two main categories: primary noise mitigation, which counteracts noise generation directly at the source (e.g., vibro-hammering), and secondary noise mitigation, which reduces noise radiation by placing noise barriers some distance from the pile.
The second category includes the most common solutions on the market today. These include, but are not limited to, big bubble curtains, isolation casings, dewatered cofferdams, and hydro-sound-damper systems.
Big Bubble Curtains (BBC)
A bubble curtain creates a barrier of bubbles in the water to block or reduce underwater sound waves. Vessels generate these bubbles by pumping compressed air through perforated hoses placed around the foundation on the seabed. The bubbles rise and form a screen that reflects and absorbs sound.
It is the best-tested and proven noise mitigation technique and has been used in many projects. However, it is limited by currents, which can scatter the bubbles, reducing their effectiveness. Greater depths are also challenging, requiring higher pressure and more energy to produce bubbles of the correct size.
Hydro-Sound-Damper System (HSD)
Hydro-sound dampers (HSD) consist of small gas-filled elastic balloons and durable polyethylene foam elements attached to nets or frames around piles. This system operates on the same principle as a bubble curtain but allows for adjustable noise reduction frequencies by varying balloon sizes. The method relies on the excitation of resonant frequencies to create scattering and absorption.
Ocean currents do not heavily affect this method, and it has, for example, been used at the Wikinger wind farm.
Bubble Curtain Use in Foundation Installation Campaigns
Out of 26 installation campaigns in the past three years, 16 campaigns used a vessel to perform noise mitigation using a big bubble curtain.
All jackets-based projects used a bubble curtain; none of the gravity-based structure (GBS) or suction bucket jackets (SBJ) did. This is because the installation of regular jackets requires a noise mitigation system. A BBC is appropriate for this installation, while HSD is unsuitable for these specific operations given the number of piles installed, often with a piling template. Meanwhile, gravity-based structures and suction bucket jackets do not require noise mitigation systems because they do not penetrate the seabed. The nature of these installation processes means they emit less noise.
The results were mixed in monopile-based projects, with 12 projects using a bubble curtain and eight not using one. Projects not using a bubble curtain included relatively shallow wind farms close to the shore, such as Vesterhav Syd and Nord in Denmark, where a hydro sound damper system was used instead.
Deploying bubble curtains at some worksites has been more challenging due to water depth or currents. These projects include Ocean Winds' Moray West, which lies in water depths between 36 and 52 meters. At Moray West, installation contractor DEME used a vibrohammer to reduce noise.
Fleet analysis: many actors, little pressure
Spinergie has developed an algorithm to identify the specific pattern of vessels performing BBC installation during foundation campaigns, providing insights into the bubble curtain job market.
The bubble curtain market represents a small volume of vessel days, as not all wind farms opt to use one. For those that do, a single vessel is required during the piling installation campaign.
Over the last seven years, approximately 30 vessels have been used in Big Bubble Curtain campaigns for offshore wind farms. Most vessels mobilized were Platform Supply Vessels (PSVs), with fewer Anchor Handling Tug Supply vessels (AHTS) completing the list. BBC jobs do not require specialized contractors. Tidewater, a major PSV owner, leads in vessel days, particularly in Taiwanese bubble curtain projects at Formosa 2, Zhong Neng, and Yunlin.
Spinergie has identified around 1,600 active vessels among a global fleet of 2,120 PSVs. The material needed for a BBC campaign can be mobilized on existing vessels with little preparation ahead of the campaign. With so many vessels available, the fleet is not expected to face significant pressure from the projected growth in offshore wind demand.
In conclusion, the impact of noise pollution during offshore wind farm construction is a key industry challenge. However, significant advances have been made in installation techniques such as jetting and vibro-hammering. Furthermore, secondary noise mitigation solutions such as bubble curtains and hydro-sound dampers have emerged. These methods demonstrate the sector’s commitment to minimizing its ecological footprint.
However, deploying these solutions has limitations, such as varying seabed conditions, water depths, and the suitability of existing vessel fleets. Technology providers must continue to develop and grow alongside the rest of the wind sector to stay ahead of environmental expectations.
Reporting by: Helia Briaud, Maelig Gaborieau and Yvan Gelbart
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