The Kriegers Flak BC (Boulder Clearance) project concerns the boulder removal operations at predefined footprints for the subsequent jack-up barge operations for the WTG installation at the Danish Kriegers Flak OWF.

This windfarm of 72 locations, developed, designed, built and operated by Vattenfall, is located in the Baltic Sea area. In specific, the DKF site is situated in the most eastern part of Danish territorial waters and Danish Exclusive Economic Zone, directly next to the maritime border to Germany and Sweden. The closest distance to the Danish coastline (island of Møn) ranges between ca. 13 km and 39 km.

The presence of surface boulders at Danish Kriegers Flak site is widespread throughout. This would pose several potential hazards to jack up vessel operations and subsequently a boulder mitigation strategy was required.

Vattenfall has awarded ASSO a contract for the execution of boulder clearance works. The scope of works was performed by TSV Aethra and MSV Astrea, as per below split. 

Summary of Aethra’s works:

  • 120 Plough Passes executed to clear a total area of 16.606 m2 from boulders and boulder clusters.
  • 1230 Tine/Clamshell Grabs performed to remove individual Boulders and Spoil Heaps utilizing a boulder-grabbing ROV (AssoGrapple).
  • 3 large boulders (up to 17Te) removed from the clearance area with the use of a crane-operated Orange Peel Grab.
  • 30km of multibeam echosounder (MBES) and sidescan sonar (SSS) survey to verify the final state of the seabed.

Summary of Astrea’s works:

  • Total targets removed 3,490 consisting of boulders and a few debris by chartered Tine Grab

ASSO’s scope of work included:

  • Preparation Activities // Mobilization activities
  • Boulder removal
  • As-built (MBES/SSS) survey 

 Vessel/Equipment used:

  • Aethra
  • Astrea 
  • MF Plough 
  • AssoGrapple (equipped with a grabber and Clamshell)
  • Orange-peel Grab
  • Atom ROV
  • Chartered Tine Grab


Project Specifications

Kriegers Flak BC

Asso.subsea Ltd.
Service Provided:
Boulder Removal
Vattenfall Vindkraft Kriegers Flak P/S
Baltic Sea
Max Water Depth: