Learning Forum

[Nikhil Mathew]

I am trying to perform a frequency statistics analysis for a heavy lift vessel with forward speed in Ansys Aqwa using Workbench. 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. 

And when I see the solver, there I see this error. 

Has anyone faced this issue? How this issue can be resolved? 

Moreover, I am trying to get the acceleration acting on the cargo and the vessel COG. For zero speed, the accelerations I get on the vessel COG are in-line when compared to the accelerations on vessel COG got from another software (Octopus Office). But the acceleration acting on the cargo point (defined as a connection point in AQWA), the accelerations are totally different. There is a huge difference. The selected accelerations on the cargo are about the structures local axis. which is on the water line. But in octopus, the accelerations are about the local structure axis which lies at the APP at keel level. Is there anyway to get the accelerations relative to a different coordinate system in aqwa? 

I am not sure if this is the issue, because only the cargo point accelerations are different and the ones at Vessel COG are the same, RAOs are same, GM and displacement are more or less the same. What can be the issue here ?

[Shuangxing Du]

For the case of "The selected accelerations on the cargo are about the structures local axis. which is on the water line. But in octopus, the accelerations are about the local structure axis which lies at the APP at keel level.", if the axis directions of the structure local axes of Aqwa and octopus are the same, the accelerations on the cargo should be the same.

Have you define any the hull drag coefficients/Morison elements in the frequency statistic analysis? The linearization of the nonlinear drags may cause convergent problem if the drags are not defined properly.

[Nikhil Mathew]

Hi sxdu, thanks for your response. 

Yes, you are right, but the thing is the axis direction is the same in both octopus and aqwa, just the location is different. The accelerations on the vessel COG are the same, and the normal accelerations on the cargo are also the same, but the issue is with the effective accelerations. The effective accelerations are very different. What can be the reason here? (I am assuming that the effective accelerations are the ones including the static component of gravity, so I am comparing the effective accelerations from AQWA with the "accelerations+g" from Octopus.)

Moreover, I have not defined any drag or Morison coefficients. I have used non-linear roll damping using the bilge keel properties. 

[Shuangxing Du]

Please read Aqwa theory manual, 14.1 Effective Nodal Acceleration RAOs for the effective acceleration definition in Aqwa.  "accelerations+g" is not the effective accelerations from AQWA.

Please check your defnition of non-linear roll damping using the bilge keel properties, which may cause the linearization iteration failure.