Is Offshore Wind Turbine Maintenance in the Field Possible Without Heavy Lift Vessels?

Conbit’s Nacelle-Based Modular Lifting Solution 

1. Lifting Device

The methodology 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. A feasible alternative solution 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). A temporary lay-down area is created for the semi-sub 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 tag-line operation to transfer the load from the turbine to the vessels' deck.

4. Capacity and Mobilization

The equipment is designed for 150t capacity and corresponds with the assumed weights of a 10MW turbine. Higher and lower capacities are feasible. This might result in higher or lower equipment weights, costs, and durations. The complete 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 it 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 quay-side crane.

Cost

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 substantially impact 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 extra special lifting capacity 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 with industry parties. Close cooperation between stakeholders is necessary to create sufficient assurance for the needed investments to build the modular lifting equipment.

This study proves that modular lifting technology is technically and commercially feasible for most typical floating wind turbines. 

At Conbit, we are eager to respond to the challenges of the offshore wind market. We are ready to support the transition to the use of renewable energy and improve industry standards and technologies.

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