VISSKA – influence of the installation of offshore foundations on harbour porpoises

Until now, the foundations for offshore wind turbines are usually driven into the seabed using a hydraulic hammer (impulse pile driving). The new method of vibratory pile driving uses vertically directed vibrations to install the foundations in the ground.

VISSKA is a German acronym referring to vibratory pile driving at the Kaskasi II offshore wind farm, in terms of installation, noise emissions and impacts on harbour porpoises.

The research project accompanied vibration pile driving for the Kaskasi II offshore wind farm before, during and after the construction phase with several investigations. These included passive acoustic monitoring, aerial transect surveys and noise monitoring. 

BioConsult SH focussed on analysing the impact of noise emissions on the marine environment using the harbour porpoise as an example. The harbour porpoise is an indicator species for the assessment of anthropogenic noise emissions. 

Aim was to investigate whether vibratory pile driving is a low-noise and environmentally friendly alternative to the currently used impulse pile driving method in terms of installation time and noise emissions. 

Due to technical challenges, vibratory pile driving could only be used on six of 32 foundations in offshore wind farm Kaskasi II. Therefore, the focus of the project was extended to include the effects of continuous noise from different sources (vibration pile driving and ship traffic) on the presence of harbour porpoises. 

Furthermore, the effects of vibratory pile driving could be directly compared with the effects of impulse pile driving. 

The following research questions were defined against the background of the changed framework conditions during the construction phase:

• What influence does the method used to install the foundations (impulse or vibratory pile driving) have on the presence of harbour porpoises during the installation process?
• What influence do impulse and vibratory pile driving have on the presence of harbour porpoises when including ship traffic?
• How does the noise level near the C-PODs (Porpoise Detectors) (low and high frequency noise) affect the presence of harbour porpoises?
• How does the harbour porpoise presence change in relation to the distance to the next ship (all ships or ships with a noise-intensive dynamic positioning (DP) system (ships with thrusters))?
• How does the number of construction vessels (all vessels or specifically those with DP systems) within a radius of 5 km affect the presence of harbour porpoises?

To answer the research questions, data was collected using various methods:

• Five aerial transect surveys were carried out using the HiDef system. These surveys covered a large area around the Kaskasi II offshore wind farm. They took place before the start of the construction phase and during vibration and impulse pile driving and allowed to determine the spatial distribution of the animals and to calculate absolute harbour porpoise densities. 

  The results of the aerial surveys were used in various statistical analyses (e.g. GAMs, INLA-SPDE).

   

• Passive acoustic monitoring was carried out using 21 C-PODs at 17 stations at different distances from the respective foundations of the Kaskasi II offshore wind farm.

  The collected data documents the presence and relative abundance of harbour porpoises before and during construction work in relation to the respective position of the measuring device and distance from the various sources of noise (vibratory pile driving/impulse pile driving/ships). They were used in GAM analyses of the impacts of pile driving and ships in the construction phase.

   

• An exact to the minute data set of AIS data for the entire study area including the transect area of the aerial surveys was used to analyse the amount of ship traffic.

Influence of impulse pile driving and vibratory pile driving

The displacement effect of impulse pile driving without noise control is 15–20 km and that of silenced impulse pile driving is 7–14 km.

Despite the limited data available, the results indicate a significantly decreased displacement of harbour porpoises by vibratory pile driving with a verifiable displacement range of 2–6 km. 

It can be assumed that the smaller displacement ranges for vibratory pile driving is due to the in comparison to impulse pile driving lower-frequency sound. The hearing abilities of harbour porpoises is significantly better in the high-frequency range.

Influence of ship traffic

Across all ship types, harbour porpoise displacement effects occurred primarily when the nearest ship was within 2 km of the C-POD. 

For DP-capable ships (ships with an automatic positioning system using permanently active thrusters), effects were observed up to a distance of 3 km. Apparently, the high continuous noise of DP-capable ships over a wide frequency range had a stronger displacement effect on harbour porpoises near Kaskasi II offshore wind farm than the usual, lower-frequency ship noise.

The number of all ships within a radius of 5 km also had a negative effect on the harbour porpoise detection rates. Displacement increased with an increase in the number of ships within a 5 km radius. Ship speed and length, on the other hand, did not play a role in any model.

The VISSKA project shows that a smaller displacement range of harbour porpoises can be achieved using vibratory pile driving. 

Ship traffic, in particular the number of ships as well as ships with active DP systems, lead to a displacement effect of harbour porpoises up to a distance of 5 km.

Overall, the project was able to provide valuable data on the effects of various noise sources in connection with the foundation work for offshore wind farms on harbour porpoises. The study thus is an important contribution to an environmentally friendly further development of offshore wind energy.