The grouted connection is a well-known technology in the offshore sector and notably in the oil and gas sector for the foundations. It consists in the high performance grout-filled space enabling the connection between two concentric steel tubulars.
This technique has also been selected in the offshore wind sector for connecting the wind turbine to its foundation. This one is located either in the transition area between the tower and the substructure in the case of a monopile offshore wind turbines, or between substructure and foundation pile in the case of jacket and tripod offshore wind turbines. Yet, the high dynamic loadings affecting the turbine are really different in comparison with those from offshore platforms. Indeed, the grouted connection represents a real challenge for offshore wind turbine developers. In 2009-2010 engineers reported grouted connection failures causing slight and progressive settlement of around 600 of the 988 offshore wind turbines (with monopile fondations) in the North Sea.
If today partial solutions have been taken notably concerning the design of the grouted connection, the research is undergoing to have a better understanding, on the one hand of the fatigue behavior of the grouted connection, and on the other hand of the influence of water ingress on the grouted joint fatigue performance. The implementation of a structural health monitoring (SHM) system for the grouted connection would enable to follow the damaging process before break, in order to improve models and conception tools and methods.
In the context of the project, the objective is above all to collect information about the damage level of the structure for providing decisional assistance in the maintenance strategy and thus a significant reduction of the operation and maintenance costs. The early detection of damage would enable to reduce expensive and unscheduled repairs because of severe grouted connection damages (corrective/unscheduled maintenance); but also to optimize/reduce the inspection frequency (scheduled maintenance). This project has thus a global perspective of risk analysis (costs/benefits) as preconized in the European action COST TU 1402.
Scientific advances and innovation
The objective of the project is to study how an appropriate damage detection instrumentation and methodology can provide appropriate solutions to offshore wind turbine developers regarding grout issues. Particular emphasis will be placed on the potential of optical fiber strain gauges and the complementarity of information that can be obtained by other means, such as acoustic methods.
From this perspective, different damage methodologies will be exploited and compared. These methods include autoregressive models (AR, ARMA or ARX), methods in the frequency or time-frequency domain, or methods based on DLV (Damage Localization Vector).
To better estimate the limits and performance of these methods, two scales of different sizes of experimental devices will be tested:
- Large scale: As part of the ITN project “OceaNET” and in collaboration with the Fraunhofer Institute IWES and the company Woelfel, Nathalie Müller, whose co-financing of the thesis is requested by the project, participated in Hanover University Leibniz for fatigue testing of ¼-scale grout specimens (GrowUP project). In the spring of 2017, it took part in new tests at the Test Center Support Structures test center in Hannover (QS-M Grout project).
- Small Scale: a test piece of reduced dimensions of about 50 cm radius will be fabricated and instrumented with fiber optic sensors (continuous fibers and Bragg gratings) and acoustic sensors. A fatigue test will be conducted on this specimen. Since acoustic emission is a fairly sensitive measurement technique, a judicious placement of several sensors on the structure will make it possible to detect and track the progress of the damage in real time and will serve as a control. A first objective of the test will be to test the sensitivity of detection and location by optical and electrical strain gauges placed on the surface of the specimen in comparison with the acoustic emission. A second objective will be to assess the suitability of instrumenting the inner tube of the specimen to detect damage that would begin to occur at the shear keys of the inner tube. A third objective will be to test the feasibility and relevance of a distributed measurement of deformations by a mesh of optical fibers to detect abnormal variations of deformations on the surface of the tube that would be caused by damage in the concrete. The interrogation of the optical fiber can be carried out during the fatigue test by a dynamic interrogator dynamic interrogator distributed measurement of deformation (Odisi) that has just acquired the IFSTTAR. It is an innovative measurement equipment that will achieve original results.
Expected technical and economic impact
Development of a new instrumentation method and methodologies on the detection of damage (identification, location and severity of defects) of connection grouts of offshore wind turbines.
Key project milestones
- August 1, 2017 - Start of the project
- August 2017- December 2018 - Analysis of the measurements, development of the method of identification and location of the damages
- September 2018-November 2018 - Implementation of small-scale trials
- May 2019 - Defense of Nathalie Müller's thesis
- June 2019 - End of the project
Small scale grouted connection
For detecting the damage occurrence and severity into the grouted joint, a 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), and one of its marginal properties, the Energy Spectral Density (ESD). A damage indicator – based on the total change of subharmonics in the Energy Spectral Density issued from the fiber optic sensor responses – has been investigated.
The results show that with this method, it is possible to detect the damage status of the grouted connection specimen at a general level, by the identification of the different phases of damage (i.e. Phase I, II and III, with respectively appearance nonlinearly of microcracks, linear increase of these ones, and then appearance/propagation in an unstable manner of macrocracks until complete failure of the specimen). The detection of the final phase has also been done at an early stage in comparison with the first noticeable mean displacements. Some information about the local severity of the damage has also been obtained, such as the fact that the maximum stresses and resulting damages occur at the first loaded shear key.
A modeling work on the damage to the grout bond was carried out. The CDP model was used for the numerical modeling of the grout. Two types of damage were simulated: compression cracking and damage to the steel-cement interface. The crack itself was modeled as a non-propagating joint crack, while the interface failure was simulated by reducing the coefficient of friction between the grout and the steel surfaces. A method based on non-linear harmonic identification in the vibration response of the structure (via frequency domain analysis and energy spectral density calculation) has been proposed and damage indicators have been calculated to monitor damage in the structure. Numerical analysis and calculations of damage indicators confirmed that the method could be used to detect the occurrence of damage, but also to locate and identify the severity of the damage. However, the modeling of the interface failure seems to underestimate the non-linearities of the structure and therefore the corresponding values of the damage index have also been underestimated.
A test piece for small-scale tests has been manufactured. The specimen was equipped with multiple sensors~: 8 acoustic emission sensors, 3 resistive gauges, 3 fiber Bragg gratings and a fiber for distributed deformation measurement running in a coil on the external surfaces of the internal and external cylinders.
The fatigue tests were performed between November 12 and 20, 2018, and the autopsy of the specimen showed that the fatigue test did damage two of the 5 locks. The bias cracking between the inner and outer locks is consistent with the results of the literature. Acoustic emission sensors have highlighted the existence of three phases of significant increase in the number of acoustic events detected. This behaviour describes the classic cyclic creep curve in the literature. A second lesson that can be drawn from acoustic emission is the asymmetric nature of the damage. This asymmetry is confirmed by the distributed measurements by optical fibres.
Publications and papers published
« Damage detection in offshore wind turbine grouted connection by nonlinear harmonic identification », N. Müller, P. Kraemer, D. Leduc, F. Schoefs, Offshore Wind R&D conference 2018, Bremerhaven, 14-16 novembre 2018
« FBG Sensors and Signal-based Detection Method for Failure Detection of an Offshore Wind Turbine Grouted Connection », to be published in International Journal of Offshore and Polar Engineering (IJOPE)
This work is the first stage of investigations about the damage detection of offshore wind turbine grouted connections. The possibility of automating the damage identification process, localizing more precisely the damage depending on the sensor positioning will be investigated with further analysis and new experimental campaigns.