Fraunhofer IWES completes measuring campaign for Baltic Sea

4C Offshore | Tom Russell
By: 23/11/2022 Fraunhofer IWES
The Fraunhofer Institute for Wind Energy Systems IWES (Fraunhofer IWES) has completed a boulder detection campaign off the coast of the island of Rügen in the German Baltic Sea on behalf of 50Hertz. New offshore platform seismic measuring techniques were employed to detect boulders below the seafloor including large rocks up to 100m below the seafloor.

Fraunhofer IWES employed the new method to check whether there are boulders in the subsurface of a wind farm area that had already been presurveyed for 50Hertz’s Ostwind 3 project. The seismic detection can be utilised to clarify whether it might be necessary to change the position of the transformer platform at an early stage to avoid damage during installation.

Using the patented Manta Ray G1, the IWES scientists can localize rocks and boulders at a depth of up to 100m below the seafloor. The measuring system comprises a towed array equipped with seismic sensors (hydrophones) as well as positioning systems. During the survey of an area, the hydrophones pick up the reflected sound waves previously emitted by a signal source. This allows the subseafloor to be recorded step-by-step and mapped in three dimensions. Not only does the measuring system map the sediment layers, but its design also enables the detection of boulders buried in the seafloor. According to Fraunhofer IWES, this is possible thanks to the method of diffraction imaging, through which the acoustic energy diffracted by the boulders can be traced back to its point of origin.

“For us, precise knowledge of subsoil conditions is important for avoiding possible installation risks. It allows us to avoid construction delays and thus save costs. The technique has proved that it is effective and delivers the essential measurements that we need for precise planning,”
said Dr. Henrich Quick, Head of Offshore Projects at 50Hertz.

Dr. Benedict Preu, Head of Department Subsurface Investigation at the Fraunhofer IWES, explained: “We employ our seismic measuring techniques to help 50Hertz to plan the platform as precisely as possible and minimize risks. We are continuously developing the technology further and know from our experience in preceding projects that the method works well and the data acquired are meaningful. This allows us to support the entire offshore industry in acquiring accurate knowledge of the subsoil and thus identify suitable solutions for all construction projects. In doing so, we are driving the expansion of the wind energy sector further.”

The detection on behalf of 50Hertz was performed within just one week aboard the survey vessel MV Situla. During this period, the vessel was mobilised, i.e., all the equipment and technology was made ready for use and tested, the measurements were performed, and the vessel was subsequently demobilised again.