New Developments in Field of Structural Health Monitoring

New Developments in Field of Structural Health Monitoring

The interest in structural health monitoring (SHM) in industry has increased significantly in recent years. Failure of a mechanical structure can indeed occur suddenly and provide a lot of damage. With SHM it is possible to follow up (online or offline) defects with permanently attached sensors.

Damage to mechanical structures may consist of (small) changes in the material properties and / or changes in the geometric integrity of the structure. There are a lot of non-destructive evaluation tools available for conducting inspections, but with SHM-tools, substantial improvements can be achieved. This results in an enhanced security and a better control. In addition, substantial cost reductions can also be realized. For these reasons the interest in SHM has significantly increased in recent years.

State of the art in SHM

On the ninth international SHM workshop at Stanford the current state of the art and emerging new technologies in the domain of SHM and the roadmap for intelligent structures have been discussed.

One of the speakers was Professor Fu-Kuo Chang from the Stanford University. He is a world authority in the field of SHM. The roadmap of the development of SHM can, according to the professor, be divided into the following four sequential steps: detection, identification, quantification and decision. The detection is the lowest level of maturity that SHM technology can or should achieve. Once the detection is confirmed with a high degree of certainty, one can proceed to identify the event in time and place (locate). When this step is completed, one can proceed to the quantification of the event. A good quantification should ultimately lead to a correct decision and efficient solution for the maintenance of the structure.

Offshore applications create additional complexity, because the place where the damage can occur is not known in advance and because the system is also much less accessible to perform measurements or maintenance. One has tried to achieve better results through the application of prior simulation techniques and next, at the most critical places vibration measurements were carried out. For oil platforms this technique has not been entirely successful yet, because there are additional complications (e.g. marine weather, wave, fouling, variable mass of the platform, …).

Wnd turbine loadsIn the offshore wind industry SHM gets more and more attention because the inspection and maintenance of the structural parts are expensive. Because of the difficult weather conditions and because places are sometimes inaccessible (think of the foundation structure), remote monitoring can be a good solution. Compared to the offshore oil & gas industry it is even more important to follow the structural condition of offshore wind turbines. Offshore wind turbines are very dynamic systems with low inertia and damping characteristics in comparison with oil & gas structures. There is a risk of resonance behaviour when structural damage occurs (e.g. by corrosion or fatique problems), with changes to the substrate (e.g. by scour) or due to extreme weather and wave conditions.

New Developments in Field of Structural Health Monitoring (2)Besides the mast and the foundation the technique is also interesting for structural monitoring of the turbine blades. In order to map the dynamic behaviour of the wind turbine and to detect any changes in natural frequencies OWI-Lab, in collaboration with the Vrije Universiteit Brussel (VUB), performs SHM services and conducts research projects on the Belgian offshore wind farms Belwind and Northwind. This research validates the potential of some new techniques. One is using operational modal analysis (OMA) tools in operational conditions to get such insights.

Press release, November 15, 2013; Image: owi-lab