A new cost-effective concept for heavy maintenance of Floating Offshore Wind Turbines by use of large OSCV.

Offshore wind is a key facet in the goal to decarbonize the energy sector and meet the decarbonization goals defined by the Paris Agreement. The predictions are well documented by [GWEC, 2022] and [Regjeringen, 2022] to exceed more than 140 GW within 2031. As a result of the EU's ambitious targets for increased offshore wind production, as many as 20,000 wind turbines could be produced and installed in Europe alone in the coming decades. With the trends we are seeing now, it will revolve around giant installations high as the Eiffel Tower with its 324m. For this market growth to occur, floating offshore wind must be further developed to find cost competitive industrialized installation and maintenance solutions for both the world-wide initiatives and Norwegian initiatives for floating offshore wind. There are currently no sustainable solutions for installation and maintenance of floating wind turbines in an offshore environment. During the lifetime of the field, regular maintenance and inspections will be required. The lifting height, vessel motions and FOWT motions are some of the main challenges that must be resolved replacing a FOWT blade and a nacelle at the offshore location. To address a solution to this, it will require scientific mathematical approaches, industry practicality, marine operational experience/intervention methods and expertise on ship design and hydrodynamics. In this project, we will develop a new, cost-effective concept for installation and maintenance of floating offshore wind turbines using large construction vessels. Current status of the project: In Work Package 1, efforts were focused on determining whether a large construction vessel could potentially perform replacements of wind turbine blades or typical components such as gearboxes using its own vessel crane. The idea was that the construction vessel, with the help of a motion-compensated lifting tool, could directly replace components on the wind turbine. Several hydrodynamic analyses were conducted to define the requirements for the motion-compensated lifting tool. The large construction vessel and Windmoor 12MW were used in these hydrodynamic analyses. The conclusion from Work Package 1 was that, with a vessel of similar size to the large construction vessels currently available on the market, it is not possible to perform component replacements directly using the vessel crane. The main reasons for this were the lack of crane capacity (load/height) and the recommendation that it was not possible to compensate for the movements identified in the hydrodynamic analysis results. After the conclusion of Work Package 1, a workshop was held to define the way forward in the project. The proposal from the workshop for further work was to explore a new concept. This concept involves lifting a crane onto the wind turbine from a construction vessel. This would allow the crane on the wind turbine to handle components solely with the movements of the turbine itself. The current work in the project involves further development of the crane concept and method development for lifting the crane from the construction vessel to the wind turbine. Additionally, efforts are being made to explore possibilities for finding motion-damping solutions for the construction vessel.

The lifting height, vessel motions and FOWT motions are some of the main challenges replacing a FOWT blade and a nacelle at the offshore location. The new lifting system and equipment to be developed for removing and installation of blades, will attach/grip near to the midpoint of a vertical blade, and by that reduce the required lifting height. Reducing the lifting height will reduce the relative motions between the OSCV and the FOWT. These motions need to be multi motion compensated. Requirements for installation and replacement of other equipment, such as nacelle will also be investigated. For nacelle installation, alternative crane/winch systems with passive or active motion compensator will be analysed. The challenge and main innovation is to find solutions for dismantling/installing at a moving target from a floating installation vessel. This will require a scientific mathematical approach together with industry practicality and marine operational experience to further develop improved installation and intervention methods. The ship design company will contribute with expertise on ship design and naval architecture. The partners in this project is DOF, Huisman, Vard and Sintef Ocean.