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Monopiles are tough on the environment

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onsdag 31. oktober 2012

Underwater noise from monopile installations is an inherent problem linked to the choice of substructure. Much research and many proposals have been made with limited effect.

By Kent Krøyer
Several reports show that underwater noise from hydraulic hammering of monopiles into the sea floor can be harmful to the wildlife of the ocean if not restricted.

Many whales such as the harbor porpoise depend on their hearing when they navigate and hunt for food, therefore deafened whales cannot hunt. Harbor porpoises are common in coastal areas where offshore wind farms are being erected.

This problem is well understood and several research projects have tried to mitigate the noise with different kinds of experimental sound barriers.

However, the banging noises from the hydraulic hammers are so intense that it has been very difficult to dampen it, because sound propagates very effectively in water even over long distances, due to the physical properties of water, i.e. high density and low compressibility.

A large crane is necessary to lower the heavy IHC Merwede noise reduction system in place. Besides protecting the whales the system acts as a precision guide for the monopile as it is driven down into the mud. The yellow tubes are the final transition pieces on which the wind turbine tower will be mounted. Photo: Riffgat Offshore Windpark

Extreme sound waves
According to a report from Delft University of Technology 2011, commissioned by the North Sea Foundation, the sound exposure in the water close to a six meter monopile being installed is around 247 dB re 1 µPa.

The threshold of pain for a human ear in atmospheric air is 130 dB re 20 µPa, the same level as a jet engine at 100 feet distance. Permanent hearing damage occurs at 120 dB.

To compare the sound pressures above and below water one needs to add 26 dB to the “re 20 µPa” level to get a comparable “re 1 µPa” level. To compare sound intensity an additional 36 dB has to be added due to the difference in the characteristic impedance of air and water.

However, the dB scale is logarithmic, which means that 247 dB under water is still an extremely loud sound wave.

The sound wave attenuates under water as a function of distance. However, the sound is reflected both by the seabed and by the sea surface. At at distance of 500 meter the sound exposure level from a large hydraulic hammer is still 174 dB re 1 µPa, according to the report. A single event of this magnitude could cause temporary hearing loss for marine mammals such as harbor porpoises and seals. Repeated exposure can cause permanent damage according to the report.

Installation method needs re-examination
This noise problem is closely connected to the use of monopiles and jackets as substructures for wind turbines. Monopiles are the popular choice for most of the offshore wind farms globally. As wind turbines grow in size so do their substructures and consequently the need for even bigger hydraulic hammers.

The North Sea Foundation report states that: “The wind energy represents a green and environmental friendly future while the use of monopiles with current installation techniques is contradictory to the whole wind energy philosophy and needs to be re-examined.”

The noise is generated as the hydraulic hammer hits the top of the monopile, often with a blow energy of 2300 kJ. This creates a longitudinal wave in the monopile, a wave that travels along the monopile to the seafloor and back, while creating a sound wave in the surrounding water. Some of the noise propagates from the air above the hammer and some from the vibrating seabed.

Current solutions to underwater noise are few
Proposals for mitigating the noise has been many: specially designed injection-style tips for the monopiles, contact damping between hammer and monopile, prolonged contact time via adjustment of the hammer, bubble curtains, confined bubble curtains, air pillow systems and finally the IHC noise mitigation system with a double-walled isolating sleeve combined with a confined bubble curtain.

The North Sea Foundation report also mentions a substitution of the hydraulic impact hammer with a less noisy vibratory hammer, as tested by Menck at the Bard 1 wind farm installation. Another idea is to drill down a concrete monopile, an unproven concept.

Leaving the monopile substructure in exchange for guyed structures, suction buckets, gravity based structures or floating structures could be future solutions. However, this could increase the cost of wind energy which must come down to be competitive.

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