Project profile: Hornsea Offshore Wind Farm One, North Sea

Hornsea Project One is an offshore windfarm proposed by DONG Energy that will be located off the Yorkshire coast within the Hornsea Zone in the southern North Sea. With a total capacity of 1.2GW, it will be the world’s largest offshore windfarm


Offshore windfarm




DONG Energy





Located in the Humber region, the project is part of a £6bn ($8.7bn) investment to transform the region into a hub for the UK’s renewable energy sector.

At 1.2GW, the project will be the world’s biggest offshore windfarm and the first offshore windfarm to have more than 1GW of capacity.

In February 2015, DONG Energy acquired 66.66% ownership share of SMart Wind, a consortium of Mainstream Renewable Power and Siemens Financial Services, to become the sole owner of the project. It is also the owner of future offshore developments, Hornsea Project Two and Hornsea Project Three, which have a potential capacity of a further 3GW.

The final investment decision on the Hornsea Project One was taken in February 2016 and the project is scheduled to be commissioned in 2020. It will have a lifespan of approximately 25 years.

The windfarm is expected to power more than one million UK homes upon completion, and create 2,000 employment opportunities during the construction phase, in addition to 300 more jobs during its operational phase.

Hornsea Project One make-up

Hornsea Project One is being constructed on an area of 407km² and will be equipped with 174 Siemens wind turbines rated at 7MW each.

The turbines will be 190m tall and have a rotor diameter of 178m with a maximum height of 200m to blade tip. They will have a clearance level of 22m above mean high water springs.

The project will also involve construction of a high-voltage AC electrical system comprising more than 900km of cables that will transfer the generated electricity to the shore and the national grid.

The site chosen for the development has favourable water depths between 20m and 40m and provides good wind and ground conditions.

The windfarm will also include three offshore wind generating stations, two offshore accommodation platforms, up to five HVAC collector substations that will be connected through inter-array cables of 30kV to 70kV, up to two offshore HVDC converter stations, and an offshore HVAC reactive compensation substation.

Onshore infrastructure for the Hornsea Project One

The onshore infrastructure will comprise a substation, underground cable, core buildings and compounds, low-voltage electricity equipment and communication cables, as well as roads, paths, drainage and landscaping.

The onshore substation, to be based at North Killingholme in North Lincolnshire, will transfer electricity from the offshore windfarm to the adjacent national grid substation. It is being constructed on an area of 32,200m², and will include transformers, reactors, high-voltage gas-insulted switchgear systems, static VAR compensator or static synchronous compensator and harmonic filters.

Offshore cables will make a landfall at the Horseshoe Point, south of Grimsby, and will be connected to underground onshore cables by means of transition joint bays. The cable route will span 40km underground from the landfall site to the onshore substation of the national grid located in North Killingholme.

Hornsea Project One construction

Construction of the onshore facilities began in early 2016 and will continue until 2019. Offshore construction began in January 2018 and is expected to be completed by 2020. The world’s first offshore Reactive Compensation Station was installed in June 2018.

Offshore installation of foundations is planned to be completed within 24 months. Onshore cables will be installed underground using open-cut trenching techniques, while cables at the landfall will be installed using horizontal directional drilling under the saltmarsh and sea defence. Jointing bays that will be used to connect the cables will have dimensions of 25m x 6m x 2m, and will be constructed at intervals of 750m to 2.5km.

The world’s biggest ever seabed investigation campaign was completed in April 2015 in preparation for the project. It involved 2,800m of seabed cone penetration testing and approximately 5,000m of boreholes on all future wind turbine generators and operational support system positions.

Cover image credit: Orsted/DONG Energy