Esben Orlowitz

Operational Modal Analysis for Dynamic Characterization of a Ro-Lo Ship

The dynamic characteristics of ship structures are becoming more important as the flexibility of modern ships increases, for example, to predict reliable design life. This requires an accurate dyna ...

Modal test results of a ship under operational conditions

Sea vessels are exposed to a complex vibration environment, influenced by the sea as well as by operational conditions. Particularly, the hydrodynamic load effects are difficult to estimate analytically. Experimental results are therefore important to verify the analytical models. In the present paper preliminary results from a full-scale modal test of a Ro-Lo vessel carried out for three different operating conditions are presented. Since little full-scale modal testing seems to have been conducted on vessels in operation, the experimental setup together with preliminary modal parameters extracted from the measurements are presented. This preliminary study is focusing on investigating the data with respect to operational conditions and shows a significant variation of the modal damping of the vessel in operation, with approximately 400 %, 200 % and 400 % difference in the first three global vertical bending modes, respectively.

Influence of Noise in Correlation Function Estimates for Operational Modal Analysis

The modal parameters in Operational Modal Analysis (OMA) are often estimated based on non-parametric signatures of the structure’s dynamic response. For time domain OMA methods the non-parametric signatures are often correlation functions (CFs) and the pre-processing step for these methods is thus the estimation of CFs. The present paper demonstrates how measurement noise from sensors and measurement equipment affects the estimated CFs. Furthermore, the influence of the measurement noise on the modal parameter estimates is discussed. It is shown how effects of this noise can easily be avoided by ignoring the first part of the CFs when estimating the modal parameters. This is demonstrated by a theoretical review and on simulated and experimental data. The paper also addresses how to add noise to simulated data, so that it resembles a real-life scenario.

Producing Simulated Time Data for Operational Modal Analysis

Producing simulated test data from models of mechanical structures is often important for validation of analysis methods, for example parameter identification methods. Whereas investigations of test procedures and identification methods for operational modal analysis (OMA) are widely reported based on simulations, little attention is often given to the actual procedures to produce simulated response signals. For example the influence of the excitation strategy in a simulation on the following identified properties. In the present work different excitation strategies are applied to simulate responses of a modal model of a Plexiglas plate. OMA parameter identification is performed on the simulated responses and the results are compared to see the influence of chosen excitation strategy. Furthermore OMA results from a lab measurement of the real physical Plexiglas plate are presented and compared to the simulation results.