Is maintenance of offshore wind turbines possible in the field without heavy lift vessels?


Over the years, offshore wind turbines have increased significantly in size and power output. Challenging remote areas and increasingly heavy components create great challenges for the maintenance of offshore wind turbines. Currently, assembly, maintenance, and replacement of parts are time-consuming, expensive, and disruptive. Maintaining wind turbines at a lower overall cost with much less environmental impact reinforces wind energy as a competitive alternative to fossil fuels.

The currently available marine assets will not be able to serve the floating offshore wind market effectively. Wind farm installation vessels are used to install bottom-fixed structures and are based on jack-up technologies. The major maintenance requirements for floating wind turbines cannot be met by jack-up vessels because of the water depth. Alternatively, there are some heavy lift vessels that might have sufficient reach, but they will face difficulties meeting the required horizontal accuracy due to the relative motions between the crane tip and the turbine’s nacelle. Therefore, the increasing heights and weights require a different O&M approach that is less dependent on large crane vessels and jack-up barges.

Conbit became inspired by these challenges and designed a solution that can replace heavy components offshore. As a company ahead of its time, Conbit showed passion to enhance the topic and work on special equipment for offshore wind turbine maintenance and evolve lifting solution and capacity as an alternative to heavy lift vessels and tow-to-port solutions. Conbit’s wind turbine maintenance will make maintenance of floating assets more cost effective and safer.

Since 2020, Conbit has been designing a Temporary Modular Nacelle-Based Lifting Solution for heavy-lift component exchange offshore. Conbit studied both the technical and commercial issues, including logistics and time schedules regarding offshore operations. In this blog post, we will describe this study and share the comparison of it with the tow-to-port approach, which is the only current solution for the floating wind turbine major maintenance.


NACELLE-BASED MODULAR LIFTING SOLUTION OF CONBIT

1.Lifting Device

The methodology of the solution is based on a temporally installed lifting device, a guiding system, and a load transfer principle. The principle is based on a luffing gantry structure which enables lifting any object through the hatch at the top of the nacelle and lowering it at the backside to the nacelle’s base structure. From this location, a set of rotating arms will swing the lifted object towards the tower structure, where it will be connected to the trolley system and lowered to the base of the turbine.

2. Guide System
The guide system required to control the load and avoid resonance during lifting and lowering uses the tower structure to guide a trolley with two guidelines to create continuous pressure against the tower. An alternative solution that is feasible for semi-subs uses two tag-lines controlled by winches located on the two ‘free’ floater columns.

3. Load transfer

The load transfer system used in the design is based on existing marine operations using an offshore construction vessel (OCV). For the semi-sub, a temporary lay-down area is created to create sufficient clearance for the offshore lift operation.

For spar floaters, the construction vessel should be equipped with a clamp system to create a semi-flexible connection between the vessel and floater. This clamp will reduce the relative displacements between the vessel and floater and allow an in-air handshake or tagline operation to transfer the load from the turbine to the vessels' deck.

4. Capacity and Mobilization
Following the bases of design, the equipment is designed for 150t capacity. This corresponds with the assumed weights of a 10MW turbine. Higher and lower capacities are feasible. This might result in higher or lower equipment weights, corresponding costs, and durations. The full equipment set can be packed and shipped in standard ISO container dimensions. It can be installed from an offshore construction vessel and only needs a limited number of interfaces with the floating wind turbine.

TOW TO PORT

Tow-to-port maintenance is currently the only available solution for heavy components for maintaining the largest floating wind turbine. Although tow-to-port is a realistic concept, it requires complex mooring and anchoring systems to make an effective (dis)connection. The marine spread requirement should not be underestimated as it impacts the required weather window.

Based on a hypothetical case study, the turn-key costs for a typical component replacement with our nacelle-based lifting solution have been compared with the ‘tow-to-port’ method for a semi-sub floater type including the required marine spread and quayside crane.

For this specific case, the estimated costs for the modular lifting solution were significantly less than the ‘tow to port’ option. Specifically, the availability of a nearby and suitable quay-side and the floater type can have a huge impact on the ‘tow-to-port’ option and on this calculation. The further away a suitable quay-side is, the greater the cost difference is between the two options.

The following are some of the other advantages of the offshore-based lifting solutions in comparison to the tow-to-port solution:

· Eliminates the additional downtime caused by the difficulty of finding an available crane.
· Delivers cost-effective options by contributing to time efficiency.
· Minimizes disruption with supplied extra special lifting capacity if needed during the maintenance procedure.
· Minimizes the project setbacks occurring because of weather conditions in comparison to the use of floating crane vessels.


INDUSTRY COOPERATION

The solution that we provide is still a work in progress. At Conbit, we found the cooperation of the industry regarding the topic is crucial to evolve our technology and new O&M approaches in general.

Structural interfaces to the turbine structure are required and should be developed in cooperation with industry parties. Close cooperation between stakeholders is required to create sufficient assurance for the required investments necessary for the development of the modular lifting equipment.

This study proves that, for most typical floating wind turbines, the modular lifting technology is technically and commercially feasible. At Conbit, we are eager to respond to the challenges of the offshore wind market to support the transition to the use of renewable energy and improve industry standards and technologies.

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