The inception of OMA in the development of modal testing technology for wind turbines

Abstract

Wind turbines are immense, flexible structures with aerodynamic forces acting on the rotating blades at harmonics of the turbine rotational frequency, which are comparable to the modal frequencies of the structure. Predicting and experimentally measuring the modal frequencies of wind turbines has been important to their successful design and operation. Performing modal tests on wind turbine structures over 100 meters tall is a substantial challenge, which has inspired innovative developments in modal test technology. For wind turbines, a further complication is that the modal frequencies are dependent on the turbine rotation speed. The history and development of a new technique for acquiring the modal parameters using output-only response data, called the Natural Excitation Technique (NExT), will be reviewed, showing historical tests and techniques. The initial attempts at output-only modal testing began in the late 1980's with the development of NExT in the 1990's. NExT was a predecessor to OMA, developed to overcome these challenges of testing immense structures excited with environmental inputs. We will trace the difficulties and successes of wind turbine modal testing from 1982 to the present.

Structural health monitoring of offshore wind turbines using automated operational modal analysis

“…5 Classical experimental modal analysis methods cannot be applied because the input force due to the wind and the waves cannot be measured. For this reason, OMA methods have to be used to identify the modal parameters from the response of a mechanical structure in operation subjected to unknown random perturbations.…”

A retrospective of VAWT technology.

“…At the second IOMAC, Thomas Carne and George James presented the conference's keynote address, a description of the inception of operational modal analysis (NExT) in the development of testing for wind turbines. 30 The fatigue behavior and performance of the Test Bed were analyzed extensively. This analysis led to the realization that the typical aircraft fatigue characterizations of the materials used in wind turbine blades were not adequate.…”

Explaining operational modal analysis with data from an arch bridge

“…Now, it is relevant to point out some works with recent and interesting applications. In [47], it is described the ambient vibration test of the Humber Bridge, the largest suspension bridge in the United Kingdom; Siringoringo and Fujino [48] describe the processing of a database collected in a suspension bridge in Japan (Hakucho Bridge); in [49] several relevant applications on bridges and other special structures are presented; in [50], it is demonstrated the usefulness of ambient vibration tests in the context of the implementation of vibration control devices; Pakzad and Fences [51] present the results obtained with an ambient vibration test performed on the Golden Gate Bridge with wireless sensors; Carne and James III [52] describe the use of OMA in wind turbines testing; reference [53] characterizes the monitoring of historical masonry structures, taking profit from OMA.…”