Fiber Bragg Grating Sensors and Signal Based Detection Method for Failure Detection of an Offshore Wind Turbine Grouted Connection
- Nathalie Müller (Fraunhofer Institut für Windenergie und Energiesystemtechnik IWES, Nantes University) | Peter Kraemer (Bochum University of Applied Sciences) | Dominique Leduc (Nantes University) | Franck Schoefs (Nantes University)
- Document ID
- International Society of Offshore and Polar Engineers
- The 27th International Ocean and Polar Engineering Conference, 25-30 June, San Francisco, California, USA
- Publication Date
- Document Type
- Conference Paper
- 2017. International Society of Offshore and Polar Engineers
- Offshore Wind Turbine, Structural Health Monitoring, Fiber Bragg Grating, Grouted Connection, Signal analysis, Wigner-Ville Distribution
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A fatigue test has been conducted on a large scale offshore wind turbine grouted connection specimen at the Leibniz University of Hannover. For detecting the damage occurrence and severity into the grouted joint, a Structural Health Monitoring (SHM) system based on fiber optic sensors type Fiber Bragg Grating (FBG) has been implemented; and the features of the FBG signal responses have been then extracted using a time-frequency analysis called Wigner- Ville Distribution (WVD). Marginal properties of the WVD (such as the Energy Spectral Density) have also been investigated. The results show that the WVD can indicate the presence of structural damages in an efficient manner. This work is the first stage of investigations about the damage detection of offshore wind turbine grouted connections.
The grouted connection consists in the high performance grout-filled space between the two structural steel components of respectively the sleeve and the pile of offshore wind turbines (OWTs). It is located either in the transition area between the tower and the substructure of monopile OWTs, or between substructure and foundation pile of jacket and tripod OWTs. While grouted joints for monopiles are exposed to bending moments, grouted joints for lattice structures (tripods and jackets) are exposed to predominant axial loadings and low torsional moments [Schaumann and Böker 2005]. It is a critical structural part of OWTs. In 2009-2010 engineers reported grouted connection failures causing slight and progressive settlement of turbines. The problem affected about 600 of the 988 monopile wind turbines in the North Sea, requiring further investigations concerning the design of the grouted connection [Gail Rajgor 2012]. Since, two grouted connection designs reducing the axial forces in this area are recommended by Det Norske Veritas [Det Norske Veritas 2014]: using a conical grouted-connection or a tubular connection with shear keys.
With the 1st design, permanent vertical displacements due to axial loading will be limited with compressive contact stresses between the steel and grout thanks to the conical design. Small cone angles in the range 1° to 3° are recommended.
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