Bubbles JIP Completed

WHY

During installation of monopiles and jackets at sea, the noise generated by high-energy piling may harm the marine environment. To mitigate the risk of noise for marine life, governments have adopted noise limits for pile-driving operations in their permits for offshore construction projects. Air bubble curtains reduce the sound-levels. A looped hose on the seabed - pressurized by air compressors – generates the bubble curtain. Currently, engineers base the bubble curtain designs on previous experience and not on scientific research. Companies experience large variations in the current performance of bubble curtains. A better understanding of the noise generation and mitigation mechanisms would enable better engineering of the bubble curtains, leading to screens that are more effective and to lower costs. Improved screens will control better the noise levels and will reduce the risk not to comply with the specific noise requirements per piling project.

WHAT

Bubbles JIP contributed to more efficient and effective designs of bubble curtains to reduce noise during offshore installation. This project researched the current practice of bubble curtain generation. It also researched the sound propagation of piling noise through water and soil and the physical mechanism of noise attenuation by air bubbles. We developed technology to measure the bubble size distribution and concentration over the water depth, also in relation to waves and current.

EXPECTED RESULTS

Based on this improved knowledge and understanding we developed a computer model to calculate the effect of improved configurations of air bubble curtain on the noise levels during offshore installation. For an efficient application of the knowledge gained within this project, we developed guidelines for the design of bubble curtains, which we reported in a “Best Practice” reference document, providing:

• Obtained understanding of bubble curtain design and noise mitigation mechanisms.
• Recommendations for bubble curtain configuration and the associated parameters, like distance from the pile and hose dimensions, airflow, and nozzle spacing and size. The parameters will depend on the local conditions as like current, waves and water depths.
• Recommendations for engineering the bubbles screens for application in offshore wind turbine installation.

The project partners benefited from the improved understanding of the working principles and performance dependencies of noise mitigation by a bubble curtain. It also allows making a more accurate budget and schedule estimates for wind farm installation.