Germany’s Fraunhofer Institute for Wind Energy Systems IWES has completed a boulder detection campaign off the coast of the island of Rügen in the German Baltic Sea for the offshore transmission system operator 50Hertz’s new offshore platform project.
Fraunhofer IWES used seismic measuring techniques to check whether there are boulders in the subsurface of a wind farm area. The method, which allows the identification of large rocks up to 100 metres below the seafloor, can be utilised to check whether it might be necessary to change the position of the transformer platform at an early stage to avoid damage during installation.
“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.
The process itself comprises a towed array equipped with seismic sensors (hydrophones) and positioning systems and starts with the hydrophones picking up reflected sound waves previously emitted by a signal source. The subseafloor is then recorded step-by-step and mapped in three dimensions. Finally, a method of diffraction imaging, through which the acoustic energy diffracted by the boulders can be traced back to its point of origin, allows accurate localization of rocks within the mapped seafloor sediments.
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 minimise 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 area had already been presurveyed for the operator’s Ostwind 3 project, a grid connection project of the Windanker offshore wind farm, where 50Hertz plans to construct and operate a submarine cable that will connect its transformer platform to the national grid.
The detection campaign was performed within one week aboard the survey vessel MV Situla, which was demobilised after the campaign finished.