Too much noise under water?

Finding better ways to measure and analyse this noise was the focus of PhD candidate Apostolos Tsouvalas’ dissertation titled ‘Underwater noise generated by offshore pile driving’.

During offshore pile driving, a hydraulic hammer hits the top of a pile, generating a stress wave which propagates downwards. Part of the induced energy is spent on the pile progression into the soil, whereas another part is irradiated into the water in the form of pressure waves from the vibrating surface of the shell. A third part of the energy enters the soil and generates elastic waves which propagate through the soil medium.

Nowadays, strict regulations prohibit the installation of foundation piles in the sea without proper consideration of the effects on the marine ecosystem. This leads to considerable time delays and increased costs for the offshore industry. In order to gather data on noise generated from pile driving, Tsouvalas said they undertook several measurement campaigns in the North Sea, in a realistic offshore environment. A typical experiment consisted of the measurement of the sound levels using hydrophone arrays positioned at several distances from the installed pile.

The research produced several interesting results. First, it helped to understand and analyse the principal noise generation mechanisms together with their source characteristics during the installation of a pile offshore. “We now know that the noise can reach the receiver in two ways: by the so-called “primary noise path” which is related to the acoustic energy radiated directly by the vibrations of the pile surface in contact with the surrounding water medium, and by the “secondary noise path” which is related to the energy that leaks back into the water region from the waves travelling in, or along, the seabed,” said Tsouvalas.

Tsouvalas noted that the contribution of the secondary noise path was hardly known prior to this research project, however, it turns out to be the key factor in explaining the inefficiency of several noise mitigation techniques currently being used. Next, several computationally fast models were developed which can be used by the industry for the underwater noise predictions during pile driving. The team also presented an integrated approach to relate the findings of this study to the research done by marine biologists on the impact on marine species. Finally, a model was developed to predict noise reduction by the use of an “air-bubble curtain” around the pile.

Apostolos Tsouvalas
Thesis: Underwater noise generated by offshore pile driving
Phd Supervisor: Dr. A.V. Metrikine (Civil Engineering and Geosciences)
Defence date: November 27, 2015