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Decay tests for obtaining eigenfrequency of a floating barge
Hi,
You suggested that I should compare my barge eigenfrequency which we discussed in the hydrodynamics thread with the literature results. I have used this day to study decay tests and eigenfrequency and I found from other posts that the basic idea might be to remove waves (as you said earlier), offset the structure little bit from its neutral position (I moved and rotated in every DoF), and then after the simulation use “All Data PSD Graph” (?). I imagine that then the translations to X,Y and Z (surge,sway,heave) and roll,pitch,yaw are plotted with frequency on the X-axis (might be wrong, please correct me). The problem which I couldn’t solve with my own research is that I have my barge’s natural frequency given to me which is 0,57Hz. Because I only have that value I don’t know which graphs or variables I should look/plot to compare my eigenfrequency to the one given. And should I look for peaks in the graphs or what exactly when analyzing the graph (this is what I did now)? This feels like basic knowledge but I couldn’t find the information from anywhere so maybe this will help others as well since some posts discussed this a little bit but not thoroughly. Thank you!
(added my graphs below for reference..)
Br,
Niila
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Hi Niila,
the purpose of decay tests is twofold:
- Eigenfrequencies Determination: Decay tests are used to determine the eigenfrequencies of a floater for each of the six degrees of freedom (DoF)—surge, sway, heave, pitch, roll, and yaw.
- Damping Characteristics Analysis: These tests also help in identifying the linear and quadratic damping characteristics for each DoF. This information is important for aligning the model with experimental or field data.
To setup a decay test, a separate simulation should be conducted for each DoF. In these simulations, the floater should be initially displaced or rotated along the respective DoF. The simulation duration must be sufficient to observe the decay of the motion. By postprocessing the motion time series for each DoF, both the eigenfrequency and the damping characteristics can be derived (refer to the literature for specific methods).
It’s important to note that some DoFs are commonly coupled, such as surge and pitch or sway and roll. As a result, a displacement along a single DoF may produce multiple peaks in the frequency spectrum. The largest peak is typically associated with the initially displaced DoF.
In your case, you should identify which DoF corresponds to the frequency of 0.57 Hz for comparative analysis.
BR,
David
