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Technical Insight: Boulder Clearance Strategy at Danish Kriegers Flak Offshore Wind Farm – by Francis Dick

Thursday 10th December 2020

OceanIQ’s parent company, the Global Marine Group (GMG), was appointed by Vattenfall to deliver the route clearance and cable installation campaign at Danish Kriegers Flak, an offshore wind farm located in the Baltic Sea, southeast of Copenhagen. The new development consists of 72 turbines linked to two offshore substations, splitting the site into two areas: Kriegers Flak A and Kriegers Flak B. Together the sites called for a total of 72 inter array cable routes; a large-scale project requiring detailed analytical work in order to determine the most economically viable routes for the entire site.

Project details

An overview of the Danish Kriegers Flak Offshore Wind Farm. Kriegers Flak A (to the left of the site plan) consists of 24 turbines and inter array cables, arranged in 6 strings. Kriegers Flak B (to the right of the site plan) comprises 48 turbines and inter array cables, arranged in 12 strings.

The full scope of work assigned to GMG for the project included:

  • Route engineering of all 72 inter array cable routes across the two sites.
  • Route clearance and trenching using Global Offshore’s PLP240 Pre-Lay Plough in boulder clearance mode.
  • Removal of the larger, heavier boulders that couldn’t be ploughed, with the Utility ROV (UTROV) Grab.
  • Installation of the inter array cables.
  • Cable pull in at the turbines.

OceanIQ was responsible for performing in-depth analysis on over 6,000 boulders and a vast amount of bathymetry data in order to find the safest possible route for the PLP240, and the inter array cables themselves, between the wind turbines at the site.

The PLP240 Pre-Lay Plough

Formulating a plan

Before route engineering could begin, design parameters needed to be established with consideration for the capabilities of the equipment intended for boulder clearance and the digging of the cable trenches.  The PLP240, engineered alongside Vattenfall and Osbit specifically for this project, would be able to achieve a clearance corridor of 16 m, remove boulders up to a maximum of 2 m in width and make cable turns on a 50 m radius.

The UTROV Grab was confirmed as being able to pick a boulder that was a maximum of 2.5 m in length and weighed a maximum of 10 tonnes, as long as two opposing arms could safely close around it. This meant that boulders greater than 2.5 m in length but under 2.5 m in width, or vice versa, could still be picked if they weighed less than 10 tonnes.

Final project deliverables

The ultimate goal of the boulder analysis work was to identify where on the cable routes the PLP240 would be able to clear boulders, where the UTROV Grab would be able to pick them, and where any remaining boulders too large for either of those activities would be left, requiring an amendment of the cable route itself. The process was iterative, with the cable route designed with all boulder and bathymetry data collated from within the PLP240’s 16 m cable corridor analysed, and the cable route adjusted based upon the results. This process was repeated until all immovable boulders were avoided and all other boulders could be ploughed or picked, which would then provide the team with the final cable routes.

Application of the methodology

To achieve the final boulder listing, the findings from the route survey provided by Vattenfall – which incorporated side scan sonar and multibeam data – had to be refined in order for the route engineering team to only see those boulders which fell within the parameters of the PLP240’s clearance corridor. A 16 m buffer was added around the initial cable route position lists (RPLs), in accordance with the width of the PLP240’s blade, and spatial intersection was conducted to highlight the auto-picked survey targets (boulders) that fell within the buffer. These boulders were then analysed and categorised by their size and mass to determine the clearance method required.

The left image displays the initial route survey with survey targets (boulders) identified in and around the proposed cable routes (signified by the red lines) and the 16 m buffer zone (highlighted in green) surrounding each route. The right image shows the refined targets after spatial intersection was completed.

A significant number of boulders in the corridor were deemed too large to be removed by the UTROV Grab, however the auto-picking software that generated the original listing misidentified some clusters of small boulders as large, singular boulders. High-res bathymetry of the site was therefore inspected in order to clarify the findings and reclassify those which had been miscategorised.

Once the recategorisation was complete, the few immovable boulders that remained were routed around and avoided by the PLP corridor. With the final routes engineered, the final boulder listing was established resulting in a full boulder pick list for the grab and a list of boulders for the PLP240 to clear.

Large boulders deemed immovable required the cable route to be amended in order to avoid these obstacles and find the most viable route. Once these decisions had been made, the final route position lists (RPLs) could be generated.

This detailed analysis led to a boulder risk mitigation strategy based primarily upon avoidance of larger or heavier stones whilst retaining routes which maximised the burial potential and protection for the cables. The inter array cable routes also had to avoid the potential unexploded ordinance (pUXO) targets identified by a thorough UXO survey campaign.

Supplementary project work

A series of additional tasks were also completed by the OceanIQ team in support of the project, which were as follows:

  • A number of boulders around the offshore substations were not identified by the auto-picking software and needed to be manually identified from the high-res bathymetry. The width and length of these particular boulders could be measured easily, but the height was harder to obtain as the bathymetry only provided the water depth at the location of the boulder. The boulder height was therefore approximated by calculating the difference between the depth of the water at the top of the boulder and the mean depth of the surrounding seabed (within a 6 m buffer radius around each point). Using this method, the process could be automated and the utilisation of Geographic Information System (GIS) software made the process much quicker.
Boulder Density Heat Map
  • Boulder density heat maps and route profile charts, which could be used to predict the PLP240’s performance and speed across the site, were used to create charts depicting 1 km segments of each cable route. The charts, which heavily utilised GIS software to automate the process of creating 234 individual charts covering all cable routes across the entire wind farm, provided a useful source of information that the crew aboard the vessels could refer to during plough operations.
Boulder Density & Route Profile Charts presented in 1 km segments along all cable routes.
  • As this marked the PLP240’s first project, some trials of the asset were undertaken on similar seabed conditions ahead of the main work beginning. A small amount of each of the tasks involved in the project was completed on separate trial routes that the PLP240 could complete final testing on.


In total, the route engineering and boulder clearance work on the Danish Kriegers Flak project took almost 2 years to complete and afforded the team a great deal of experience that positively influenced the project and will be invaluable to future route engineering work too.

The investigative work completed by the OceanIQ team has enabled all 72 inter array cable routes to be successfully engineered and the cable trenches to be ploughed. Global Offshore are currently installing the cables on site and completing the cable pull ins at the turbines, with OceanIQ continuing to support, as required, to ensure the ongoing success of the project.

Francis Dick joined the Global Marine Group in 2018 and works as a Route Engineer for OceanIQ. Find out more about Francis on LinkedIn.

OceanIQ’s highly knowledgeable route engineering department provide a wide range of services designed to support subsea cable networks across the lifecycle of each system. Head to the Route Engineering services page here to see how OceanIQ can ensure the longevity and viability of your system, whether it be in the initial planning phases or in an existing system that is currently operational.

This article was published in pages 82-84 of PES Wind 41 Magazine