Assessing the ability of filtering unwanted sound pressure peaks via a Meta Cushion during pile driving of a monopile: An analysis using small-scale testing

Abstract

One of the biggest challenges of the century is the fight against global warming. In all sectors people are doing their best to reduce their carbon footprints and their impact on the environment. In the energy sector this is done by making use of renewable energy sources (RES), such as wind turbines, instead of fossil fuels. This transition to RES and the growing demand of energy results in extensive off-shore wind farms containing large wind turbines. During the installation of these wind turbines, the monopile (MP) foundation is driven into the seabed using an impact hammer, which dissipates a high amount of energy into the water in the form of sound pressure waves. The intensity of these sound pressure waves can be expressed in sound levels and is used to quantify the effects of such waves in underwater environment.

These high underwater noise levels have a large impact on its marine life as marine mammals rely on sound waves, particularly low-frequency wave. The noise from pile driving damages their auditory systems and can lead to multiple injuries making them more vulnerable to predators. For this reason, it is crucial to research methods for predicting underwater noise and reducing the noise during off-shore pile driving operations.

The mitigation techniques currently in use or in development lack the ability to filter the sound pressure peaks at a specific frequency band. For this reason, an elastic meta material-based structure, called Meta Cushion, is designed to reduce low-frequency noise during off-shore impact pile driving. To assess the functionality of the Meta Cushion in reducing low-frequency noise during offshore impact pile driving, a small-scale pile driving test setup was designed and built. The small-scale test aimed to validate the numerical results and to provide a better understanding of the cushion's effectiveness. This test was a joint effort of Delft University of Technology and Huisman Equipment B.V..

The small-scale impact test setup was scaled using scaling laws on the large-scale appliance, and the instrumentation was selected to perform the required measurements to quantify wave propagation. The test setup was built using a test tank with sound-absorbing foam attached to its walls and soil at the bottom to reduce the reflection waves of the tank sides. In the middle of the tank a small-scaled MP was placed on top of a damping plate, to protect the MP and the bottom of the tank. This damping plate was changed during the experiments, because the initial plate caused a large backlash on the MP after impact. On top of the MP, different cushions were placed, and on the MP wall, a set of strain gauges and an accelerometer was attached to measure the behaviour of the MP throughout the tests. In the tank, a hydrophone was placed to measure the sound pressure as a result of the impact force.

The small-scale test setup had three test rounds; the first round assessed the functionality of the meta-material unit cell by testing a modular Meta Cushion made from three different materials: aluminium, acrylic, and nylon, while using a pivot impact hammer; the second and third test rounds were performed on two aluminium non-modular cushions, which were designed and built to withstand the stresses during an impact test with a higher impact force, using a drop weight impact hammer. The FRF-experiments were conducted to extract the actual TL frequency and the attenuation caused by it for each cushion...