# Frequency Statistical Analysis (AQWA) - Motion Analysis

Discussion in 'Hydrodynamics and Aerodynamics' started by Umar Farooq, Mar 28, 2024.

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### Umar FarooqJunior Member

I am trying to perform a frequency statistics analysis for a heavy lift vessel with forward speed in Ansys Aqwa using Workbench to calculate the motions and accelerations on cargo and vessel. I have performed for zero speed, got the RAOs in hydrodynamics diffraction and frequency statistics in the hydrodynamic response module for different wave headings, Tp and gamma (using design parameters 0-360 deg with increments of 15 deg). Everything is good for zero speed. As soon as perform for forward speed, some of the design points are solved and I get the results but for some design points I get this error.

I am using non-linear roll damping by defining the bilge parameters. Here are the details:

I am not sure why I am facing this problem when it is solving for all the Hs, Tp and gamma combinations but just not being solved for 5-6 combinations. Fwd Speed used is 6.585 m/s.

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### Umar FarooqJunior Member

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Failure to converge can be for many reasons, but ostensibly, there is something that is modelled or selected that is not "closed".
Like running an FEA without preventing rigid body motion.

Firstly, if you remove the bilge keel, shown above, can you run the analysis as you require?

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### Umar FarooqJunior Member

Hello @Ad Hoc , thank you for your response, Yes I can run for the desired combinations of Tp, Hs, and Gamma without the non-linear roll damping defined using the bilge keel.

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### Umar FarooqJunior Member

Moreover, my hull is open from the top.

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That then suggests the way have modelled the bilge keel, or the analyse option using the model is creating the issue.
Rather like using a model beyond or exceeding its parametric limitations, or simply an element that is incompatible.

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### Umar FarooqJunior Member

I tried chaning the number of sub-spectra and it works with the same bilge keel for some points.

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