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Robust Identification of Modal Parameters of Nonlinear and Time Variant Systems

The identification of modal parameters of systems under ambient or operational vibrations is termed Operational Modal Analysis (OMA). One of the basic assumptions in OMA is that the system being analysed is linear, and time invariant. This limits the applicability of OMA.

The Centre for Oil and Gas – DTU is working on extending the life time of the off-shore oil and gas platforms in the North Sea, through structural health monitoring. This is done by monitoring the vibrations of the platforms. 

One of the analysis steps in structural health monitoring is Operational Modal Analysis. The off-shore platforms are generally considered nonlinear systems. The main concern is the nonlinearity that come from the friction in the sliding bridge bearings on the bridges connecting the platforms.

The topside weight of the platforms is constantly varying due to emptying and filling of storage tanks. This means the platforms cannot be considered time invariant, which is another limitation regarding the application of OMA. These challenges in applying OMA on these structures gives reason to improving the existing OMA tools, that allow a robust identification of modal parameter, when the system includes nonlinearities and is not considered time invariant. 

The main objective of this PhD project is to improve existing OMA techniques such that the modal parameters of the platforms can be identified under the above-mentioned conditions.   

Anders Brandt
Karsten Krautwald Vesterholm

Last Updated 21.02.2022