GALLERY: 'First' offshore repowering completed

Other developers have opted simply to decommission aged offshore wind farms as they reach the end of their life-cycle. Danish company Momentum, however, has finished a project in Swedish waters that it claims is the first partial repowering of an offshore wind farm.

by Craig Richard 3 December 2018

  • Bockstigen comprised five 20-year-old 550kW Wind World turbines

    Bockstigen comprised five 20-year-old 550kW Wind World turbines

  • Momentum replaced the nacelles with used 660kV Vestas models

    Momentum replaced the nacelles with used 660kV Vestas models

  • Repowering was completed less than a year after the option was first proposed

    Repowering was completed less than a year after the option was first proposed

  • The process cost less than €5 million

    The process cost less than €5 million

  • Blades from a refurbished Vestas V47 turbine were installed

    Blades from a refurbished Vestas V47 turbine were installed

  • The rotor diameter increases by 10 metres with the new turbines

    The rotor diameter increases by 10 metres with the new turbines

  • Momentum expects repowering will cause annual output to double from 5.5GWh to 11GWh

    Momentum expects repowering will cause annual output to double from 5.5GWh to 11GWh

  • Repowering will increase nameplate capacity from 2.75MW to 3.3MW

    Repowering will increase nameplate capacity from 2.75MW to 3.3MW

  • Baltic Sea wind speeds of more than 20m/s created challenges for crews responsible for construction

    Baltic Sea wind speeds of more than 20m/s created challenges for crews responsible for construction

  • Crews used a mobile crane installed on a jack-up platform to carry out installation

    Crews used a mobile crane installed on a jack-up platform to carry out installation

  • Momentum claims that Bockstigen is the first offshore wind farm to be partially repowered

    Momentum claims that Bockstigen is the first offshore wind farm to be partially repowered

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The developer has replaced the nacelles and blades of the five 20-year-old, 550kW Wind World turbines at its Bockstigen project with components from used V47-660kW Vestas units increasing the site's nameplate capacity from 2.75MW to 3.3MW.

The turbines’ towers, foundations and cabling remained untouched, however.

As such, Momentum claimed Bockstigen, four kilometres off the coast of Sweden’s largest island, Gotland, is the world’s first partial repowering of an offshore wind farm.

The developer initially wanted to decommission Bockstigen, which has been online since 1998, and replace it with a new wind farm consisting of twelve 4MW turbines.

But transmission system operator (TSO) Svenska kraftnät’s plans for a 400kV DC connection between Gotland and mainland Sweden that could have helped integrate 500MW of wind power — including that generated by Momentum’s new project — were cancelled due to cost concerns.

Unable to distribute power from its planned 48MW site, Momentum decided to repower instead, company CEO Kim Madsen explained.

Despite Baltic Sea waves, wind speeds of more than 20m/s, and the challenges created by replacing the hardware using a mobile crane installed on a jack-up platform secured by cables, the upgrade was completed in November.

Repowering was completed less than a year after the option was first proposed and at a cost of less than €5 million, Madsen claimed. Repowering also did not require a new permit.

Crucially, Madsen added the upgrade will extend the project’s life by 15 years and double its output from 5.5GWh/year to 11GWh/year, but will not place much extra burden on connections between Gotland and Sweden.