Technology Pipeline:
Floating Turbine On-site Large Component Exchange
Floating Wind Farm O&M Requirements
Maintenance is to keep system operations safe and smooth. Offshore wind farm maintenance includes scheduled and unscheduled activities and requires regular transfers of personnel and equipment to the wind farm site. The target is to keep the turbine’s uptime, despite weather and access restrictions, in a range of 95% to 98%.
For a typical floating wind farm, the Operations and Maintenance (O&M) cost is around 1/3 of its total Life-Cycle Cost (LCC), with an annual expenditure of about 110 k€/MW. Approx 40% of the O&M cost, or 14% of LCC, is attributed to the maintenance of wind turbines; this is equivalent to buying a new turbine.
Failure of turbine large components, like blades, gearbox, and generator parts, may occur once every 5 to 10 years. For a 15MW wind turbine, the heaviest component is about 230 tons, with an elevation of 150m from the water surface. Therefore, Large Component Exchange (LCE), which is beyond the capacity of turbine internal cranes, will be a major strategic task and expenditure in O&M.
The purpose of O&M for an offshore wind farm is to maximize electricity generation while minimizing the operational expenditure. A proper maintenance strategy shall consider the availability of service vessels, needed equipment, spare parts, skilled personnel, and workable weather windows; and on top of these, very importantly, the possible extended downtime of turbines, i.e., the loss of production – 11.5 MWh/MW-turbine/day on average in the North Sea.
LCE Challenges & Current Solutions
For floating offshore wind turbines (FOWT), conventional on-site LCE methods are no longer applicable:
- The operating water depth is too deep for using a jack-up vessel, which is currently the solution for offshore bottom-fixed wind turbines
- The motion of the FOWT is significant considering the hub elevation of over 150m — even at calm seas with a roll of 0.5°, the oscillation range will be >2.6m — this is too much for floating-to-floating lifts with an offshore crane vessel
For the time being, the only viable solution is to tow the FOWT to a service port or sheltered water, then do the LCE with a land or jack-up based crane. During this process, the FOWT needs to be dis- and re-connected to its power cables and mooring system, and these steps alone will cost about 2-3 M€ each time. The tow-to-port solution will not be favored for future commercial floating wind farms, due to the high cost, potential delay, and dependency on port facilities.
Performing LCE on-site would be the goal for the future. Currently, most developments focus on the self-erecting turbine-mounted cranes, like Liftra and LiftOff. These cranes can be temporarily installed onto the turbine tower or nacelle when needed. However, for lifting capacities larger than 100 tons, reinforcement of turbine structures and additional erection equipment would be required. This would increase the cost and complexity of this method.
Our Solution
To minimize the relative motion between the LCE crane and turbine, our solution is to use a crawler crane onboard the FOWT floater; with this, no modification of the turbines is required. The crawler crane can simply be a land-based but offshore-certified one, which will avoid investment in a new crane.
A temporary crane working platform, approx 30 x 30 m2, needs to be installed in advance onto the floater, of which the bracing structure may require some adjustments to support the working platform. The fixtures of the platform to the floater braces can be designed demountable and exchangeable, so that the platform can be reused for other FOWTs with similar floater designs.
The crawler crane will be transported to the FOWT site by a supply vessel having a dynamic positioning (DP) system. The transfer procedure is illustrated with the figures on the right. Our proprietary technologies are the special transfer bridge and the heave-compensated platform onboard the supply vessel:
- The bridge must be able to support the crane with a self-weight of 600-700 tons (incl. ballast), and meanwhile, tolerate the relative horizontal movements between the FOWT and the supply vessel
- The heave-compensated platform must be able to safely close the elevation gap between the quay and the FOWT main deck, which is around 15m
Using a crawler crane on an offshore platform, like an SSCV (semi-sub crane vessel) and a jack-up barge, is not new. However, for a FOWT, the 700t weight of the crawler crane is not negligible – the incurred draft change of a semi-sub floater, with tri-columns of average diameter at 18m, is around 1m. Therefore, detailed engineering is needed for the whole process of crane transfer and operation. Extra pontoons attached to the floater or supporting the crane working platform may be beneficial for easy and safe operations.
Crawler Crane in LCE
We are seeking industrial partners and financial investors to speed up our development