IN Brief:
- ABL will oversee marine warranty aspects of cable, converter platform and offshore infrastructure installation.
- Ostwind 4 will provide 2GW of transmission capacity through a 525kV HVDC connection in the German Baltic Sea.
- Around 110km of subsea cable and 4km of land cable will connect the offshore system to Stilow.
ABL has been appointed by German transmission system operator 50Hertz to provide marine warranty survey services for the 2GW Ostwind 4 offshore grid connection in the German Baltic Sea.
The scope covers transportation and installation activities for subsea cables and offshore infrastructure, including the converter platform and associated assets. ABL’s Hamburg team will review technical documents and procedures, assess the proposed marine spread, provide marine risk and assurance services and attend warranted operations on site.
Ostwind 4 is the first German Baltic Sea offshore grid connection planned around a 525kV high voltage direct current system. The project combines an offshore converter platform, an onshore converter station and a cable route linking offshore generation northeast of Rügen with 50Hertz’s transmission network at Stilow.
50Hertz states that the connection will provide 2GW of final transmission capacity. Around 110km of subsea cable will run from the offshore converter platform to the landing point near the port of Vierow, followed by approximately 4km of underground cable to the onshore converter and substation at Stilow.
The connection is intended to serve TotalEnergies’ planned OstSee Energies offshore wind farm in area O-2.2. The wind project itself has a target capacity of 1GW, while the 2GW design of Ostwind 4 also provides for wider development in the Baltic offshore grid. 50Hertz expects the connection to be completed by 31 May 2031.
Marine warranty surveying provides independent technical review of high value marine construction and transportation operations before and during execution. Surveyors assess vessels, procedures, lifting arrangements and installation methods against the planned work, with approval required for specified warranted activities.
Ostwind 4 spreads that assurance requirement across several interfaces. The programme combines long subsea cable installation with transportation and installation of a large converter platform, bringing vessel capability, weather limits, lifting procedures and seabed conditions into the same delivery schedule.
The converter platform also changes the transmission architecture used for this part of the Baltic. Electricity collected from the offshore wind farm as alternating current will be converted offshore to direct current for transmission to land. At Stilow, an onshore converter will return the power to alternating current before it enters the 380kV network.
A 525kV HVDC system can move large power flows over a long cable route while avoiding the charging-current constraints associated with an equivalent high-capacity AC subsea connection. That architecture brings converter technology, HVDC cable systems, offshore integration, protection and control into the critical path alongside conventional marine installation work.
50Hertz has already placed major contracts across the package. GE Vernova and Drydocks World were selected in 2024 for the converter system, covering offshore and onshore converter infrastructure, while NKT is supplying the offshore and onshore cable systems. ABL now adds an independent assurance function around transportation and marine installation.
The cable corridor also intersects infrastructure developed for neighbouring Baltic connections. Protective conduits for Ostwind 4’s land cable were installed as part of work associated with the Ostwind 3 connection, reducing the need to reopen sections of the route during later construction.
Offshore, 50Hertz intends the new cable to run as parallel as practicable to Ostwind 1, 2 and 3 infrastructure to limit additional environmental disturbance. Reusing corridors can reduce duplicated civil work and seabed impact, although it places greater emphasis on coordination between existing assets, future cables and installation windows.
Landfall routes, substations, converter sites and established transmission corridors can constrain offshore grid projects long before turbine installation begins. The 2GW connection therefore depends on synchronising civil, electrical and marine programmes across several contractors and jurisdictions.
Three principal permitting procedures remain, covering the German Exclusive Economic Zone, coastal waters and the onshore section including the converter. Regulatory work will continue alongside engineering and procurement as the project moves towards construction.
ABL’s appointment covers the marine execution risk within that wider programme. The company has also secured assurance work on the 976.5MW Gennaker offshore wind farm in the German Baltic, giving its Hamburg operation another major offshore programme in the region.
For Ostwind 4, the work is narrower and more immediate: technical review of the transportation and installation methods that will eventually place the connection’s cables and offshore infrastructure into service.
