Maintenance Offshore Wind

Abstract

In the transition from a fossil fuel driven economy to renewable solutions, the activities in offshore wind energy are growing. Operation and maintenance costs of offshore wind turbines are high, therefore alternative methods should be developed to reduce these costs. The replacement of heavy components, like a gearbox or a generator, is a frequently occurring operation that can be improved.
Currently the replacement of these components is performed by a jack-up crane vessel which has two downsides: (1) long mobilization time and (2) high operational costs. The development of systems that can replace heavy components can contribute to reduce the maintenance costs of offshore wind turbines.

Therefore in the first part of this thesis, developments and techniques were investigated which can do so. It was observed that different types of solutions are already available for onshore maintenance purposes like the crane developed by Liftra and the Gamesa flexifit. Furthermore new concepts are being developed by the wind turbine manufacturer Vestas and Anson, a Chinese crane builder. These developments are addressing the problem of reducing the maintenance costs, but still two important downsides are present: (1) insufficient lifting capacity for gearbox replacement and (2) most systems are designed to be used on specific turbines.

The current developments and available techniques were studied to generate concepts that can address these shortcomings. This resulted in a promising solution where a relatively small crane is installed on the wind turbine tower from a floating vessel. A great advantage over currently used method is that no specialised maintenance vessel is required for a gearbox replacement and thereby costs and time can be saved.

This crane is attached to the tower structure by use of a clamping mechanism and is kept in position by generating frictional force. To generate this force, hydraulic cylinders are pressing the pads (contact surfaces of the clamp) against the tower structure. For the applicability of this concept it is of major importance that the structural integrity of the tower is maintained, while sufficient frictional force is generated to avoid the crane from slipping down during lifting operations. Therefore, in the second part of this thesis a feasibility study is done on the application of this clamping mechanism on wind turbine towers. By use of finite element modelling a workable contact surface configuration of this clamping mechanism was found.

The following four variables were studied: (1) elasticity of the contact layer, (2) number of pads used, (3) width and (4) height of the pads. The latter three relate to the contact surface area and the elasticity of the contact layer influences the distribution of the forces from the clamp to the tower structure.

Investigating these variables, it was concluded that; the proposed solution should have a contact layer where the elasticity modulus is higher than 1200 Mpa. By adding more pads, the loading capacity increases. However, it also results in a more complex structure and therefore it is advised to reduce the number of pads and instead widen them. Considering this trade off, a four pad configuration is selected to determine the required width and height of the pad. For this set-up it is advised to use a pad width of eighty degrees and pad height of three meter to meet the requirement of replacing the gearbox, the heaviest component of the wind turbine powertrain.

Furthermore, to assure this clamping mechanism works on towers of different dimensions (i.e. diameter and wall thickness combinations), the loading capacity was determined for a variety of tower dimensions. By doing so, insights are gathered on the application of this clamping mechanism on different turbines. To make the data useful for further application, tables are included which indicate the loading capacity for investigated wind turbine towers.