Survey report for vibrocore investigations, 2023: Geological screening for offshore wind farms, the Danish Energy Agency

GEUS has carried out fieldwork in 2023 in Danish waters as part of a geological screening for offshore windfarms for the Danish Energy Agency (ENS). The purpose of the survey was to acquire sediment cores with vibrocore equipment widespread in the survey areas for correlation with the seismic data acquired in 2022 and 2023 and verification of the geological interpretations. A secondary purpose was to perform CPT measurements in a dedicated case study area for more detailed correlation between downhole information and seismic data and for a first correlation between large scale geological interpretation and geotechnical parameters. Thus, the acquired data will serve to establish a better basis for developing conceptual geological models and mapping geological units of importance for offshore windfarm development.

The survey, hereafter referred to as the ENS 2023 vibrocore and CPT survey, comprised most of the Danish waters. The activities were carried out using the survey vessel Fortuna Crane and was divided into five survey legs. Mobilization of the vibrocore equipment took place in Esbjerg on August 14 and the investigations started August 15. The survey finished on September 16 at Bornholm.

Leg 1 started August 14 and included 23 vibrocores in the North Sea. Some technical problems with the anchor winches of the vessel were encountered as well as a short period of weather standby and repair in Hvide Sande. Before planned crew change and repair in Esbjerg on August 21, 7 vibrocores were taken as part of another project at Blåvandshuk.

Leg 2 started out August 21 with acquisition of 5 vibrocores on the Blåvandshuk project mentioned above and subsequently 45 vibrocores were taken in the North Sea. Two periods of weather standby were encountered, one offshore and one in Hirtshals also with repair due to continued technical problems with the anchor winches. A planned crew change took place in Hirtshals on August 30.

Leg 3 started August 30 and comprised 73 vibrocores in Kattegat, south of Ærø, southeast of Lolland and Falster and east of Sjælland. Additionally, 4 vibrocores were taken on another project in Langelandsbælt, Fakse Bugt and Kattegat. A short period of rough weather was used for transit from Kattegat to south of Ærø. A planned crew change took place in Grenaa on September 7.

Leg 4 started with mobilization of CPT equipment and repair of anchor winches in Grenaa on September 7 and 8 and included 2 CPT measurements on September 9 and 10, respectively, before demobilization of the equipment and crew change in Grenaa on September 10. Technical problems with the CPT equipment resulted in one damaged CPT cone on September 9 and a broken CPT rod on September 10 resulted in loss of the second and last CPT cone.

Leg 5 started with transit to Bornholm on September 11 and subsequently 24 vibrocores were taken around Bornholm. A short period of rough weather was used for taking 11 vibrocores on another project in a more sheltered area southwest of Rønne before resuming the ENS survey and finishing on September 16. After the survey, 3 additional vibrocores were taken in Kattegat off Grenaa during another project.

A total of 168 vibrocores and 2 CPT measurements were carried out during the ENS 2023 vibrocore and CPT survey. Details on the vibrocore and CPT positions are provided in the survey log included as Appendix A.