SHM of Floating Offshore Wind Turbines— Challenges and First Solutions

The paper presents an integrated SHM-system for floater, moorings, tower, nacelle and rotor blades. Its core is based on a multivariate SHM-system for rotor blades with three different measuring techniques accompanied by appropriate signal processing approaches:  Acoustic Emission (AE) is used for identification of relative small damages at the whole blade, e.g. bursts of fibres, cracks of bonding, hits of hail and the localization of damages.  Acousto Ultrasonics (AU) provides information about relative small to big damages on the transfer path between emitter and receiver of guided waves (cracks, delamination, damages of the surface).  Operational Modal Analysis (OMA) gives information about large structural modifications e.g. changes of global stiffness, mass and damping ratios in the whole blade. The most important feature of an SHM-system, however, is not the sensor network, but the analysis capability and the decision support system reducing and interpreting measured data. The instrumentation plan of a floating wind turbine off the coast of Spain is presented. INTRODUCTION – THE CHALLENGES OF FLOATING TURBINES The very large wind turbines of the future will have 10 MW and more output power and they are situated in an offshore environment. Due to the geographical situation two construction types exist: _____________ Herbert Friedmann, Woelfel Beratende Ingenieure GmbH + Co. KG Max-Planck-Str. 15, 97204 Hoechberg / Germany 6th European Workshop on Structural Health Monitoring Fr.1.B.2 License: http://creativecommons.org/licenses/by/3.0/ 1 M or e in fo a bo ut th is a rt ic le : ht tp :// w w w .n dt .n et /? id = 14 16 0  Fixed bottom wind turbine technology at the continental shelf with limitations of water depth of ≈ 50 m.  Floating wind turbines in deep water areas with extremely abundant wind resources. Of the latter type only a few research turbines exist worldwide. Due to the harsh offshore conditions and dynamic wind and sea loads SHM is a must on such floating wind turbines. However, there is no experience available with such complex structures. In a European research project HiPRwind such a floating wind turbine will be monitored. SHM is a main research topic at the floater which will be situated off the coast of Spain. A very critical issue for very large offshore wind turbines is the structural integrity of the rotor blades [1], tower and floater or the foundation respectively, and their remote maintenance. On the one hand, the wind turbines of the future will be much bigger than today. Especially the rotor blades which will have a length of 90 m and more are very critical. The probability of structural failure is much higher than with smaller blades. Additional to that, the environmental conditions on the sea are very harsh, that means the loads onto blades and tower are much higher, too. On the other hand, the accessibility of an offshore wind turbine is restricted due to sea and weather conditions and the availability of supply vessels. Some challenges of the project are harsh offshore environment, underwater measurements, a very complex floating structure with varying loads and boundary conditions, a rotating system, interaction of the mechanical dynamics of floating turbine with aero-elasticity and control commands, interaction between different structural components, etc. Therefore, integrated structural health and condition monitoring is a prerequisite of complex remote maintenance strategies for structural parts of a wind energy converter. Structural Health Monitoring, condition-dependent and predictive maintenance combined with long-term planning of repair measures is the key to ensuring the economic viability of very large offshore turbines. Additional to the health monitoring the measurement of the dynamic behaviour of floating wind turbines is of great importance for research purposes. SHM WIND A HEALTH MONITORING SYSTEM FOR ROTOR BLADES To introduce the principles the example of an SHM-system for rotor blades of wind turbines is taken. The development of such an SHM-system was started some years ago in two R&D-projects [2], [3]. Now the system has reached a certain stage of maturity and the transfer of results from research to a product SHM.Blade is planned. At the first stage the upcoming product SHM.Blade will not cover all features mentioned in this contribution which describes the final development stage of the system. Prerequisite – A Sensor Network with optical Data and Energy Transmission The intension of SHM Wind is not only to indicate an upcoming damage, but additionally to deliver information about the position of damage and its extent. To