IN Brief:
- Stillstrom by Maersk has built and factory tested a modular 11kV Vessel Container Unit, with DNV witnessing the work.
- The unit houses a marine connector, control functions and safety interlocks for offshore wind-to-vessel electricity transfer.
- A complete offshore charging arrangement remains under development, with extended sea trials expected in summer 2027.
Stillstrom by Maersk has completed construction and factory acceptance testing of a modular vessel connection unit intended to allow offshore service operation vessels to charge from wind farms at sea. The company’s Vessel Container Unit, or VCU, incorporates an 11kV connection package and associated controls. DNV witnessed critical stages of the factory testing, although the complete offshore charging arrangement has yet to undergo its planned sea trials.
The vessel-mounted container brings the electrical connector, guidance hardware and its control interface together in one package. Stillstrom says it can be fitted near the bow, stern or beneath a deck, depending on the vessel’s layout. Offshore-generated electricity must still pass through compatible supply, protection and conversion equipment before it can charge the vessel’s batteries; the 11kV connector alone is not a complete shipboard charging system.
The modular design is intended for new vessels and retrofits, with the possibility of moving a unit between ships where the required onboard cabling is already available. That qualification is significant in engineering terms: an existing vessel may need electrical distribution, protective devices and energy-management changes before it can accept a high-voltage supply. Stillstrom has not identified a standard retrofit package or disclosed a vessel-specific installation time.
Factory acceptance testing establishes that a manufactured unit has been exercised against an agreed test programme in controlled conditions. DNV witnessed critical steps, including work on the connector and its safety functions, but this does not establish that a charging connection has been proven at sea. Stillstrom expects the complete system, including equipment installed at the wind farm, to be ready for extended offshore trials in summer 2027.
At 11kV, the system uses a voltage suitable for moving substantial electrical power without requiring the very high currents that would accompany the same transfer at low voltage. The actual charge rate will depend on the connected infrastructure, conversion equipment and receiving vessel. Stillstrom has not provided a validated offshore transfer rate or reported operational charging hours from the new hardware, so neither can be inferred from its voltage rating alone.
A marine electrical connector must function despite vessel motion, repeated handling and exposure to salt spray. Stillstrom’s integrated funnel is intended to guide the connection while allowing the ship to maintain a suitable heading; the vessel’s dynamic positioning operator remains responsible for manoeuvring and connection or disconnection operations. The equipment must also be able to cope with credible loss-of-power and emergency-disconnection scenarios when integrated into a vessel.
Safety interlocks are designed to control when the circuit may be energised and disconnected. The offshore and onboard systems must agree on a safe operating state because an electrical fault, loss of communication or unexpected vessel movement may require charging to stop. The company has not published a final approval for unrestricted commercial service; DNV witnessing at the factory should not be confused with certification of an operational offshore installation.
On the offshore side, Stillstrom has a separate development arrangement with Semco Maritime for a local equipment room containing a transformer, medium-voltage switchgear, communication equipment and safety systems. Siemens is expected to supply the switchgear and controls under that arrangement. The equipment room is intended to govern the transfer from the wind farm to the vessel, and the company previously targeted manufacture in Esbjerg with completion in spring 2027. These are linked but distinct engineering workstreams.
Earlier Stillstrom trials focused on automatic cable control while a vessel uses dynamic positioning, along with communications between the connection system and the ship. The completed VCU test extends that work to manufactured vessel-side hardware. A future integrated trial must establish that the offshore source, cable-control system and vessel interface operate together across the conditions specified for the installation; none of those stages can be inferred solely from successful factory testing.
Charging a service operation vessel directly from a wind farm could reduce the need to generate onboard electricity from liquid fuels while vessels remain on station. Whether it delivers a particular fuel or emissions saving will depend on the vessel’s power demands, battery capacity, time spent connected, wind-farm availability and the electricity supplied during an actual operating cycle. The latest test does not establish those operational savings.
Stillstrom is also developing a second version of the VCU to move the existing semi-automatic alignment process towards full automation. More automated connection would still need to maintain safe manual intervention and emergency disconnection provisions, particularly when changes in sea state or vessel position affect cable loading. No deployment date has been confirmed for that subsequent design.
Stillstrom, which has teams in Copenhagen and Aberdeen, is developing a second VCU design intended to progress from semi-automatic to fully automated alignment. It has not announced a commercial deployment date for that version. Its immediate next milestone remains integration and testing of the full offshore charging arrangement in the planned 2027 trials, rather than the start of routine charging by service operation vessels.
