[femap][analysis of stiff rigid body under dynamic motions]

[rgr16]

Hi!
I wonder is it resonable to use following analysis (below) for rigid body dynamics:
- analysis version 1: normal modes/eigenvalue --> frequency/harmonic response
- analysis version 2: transient dynamic/time history

for stiff rigid body in dynamic analysis. In example for crane on ship on wave. Ship is rotating and translating so attached to it deck crane (with rigid elements) is under influence of vibrations. If i understand it properly: speed of vibrations in crane ( = of sound) is faster than oscilations acting on crane, so crane is vibrating under influence of that oscilations.

In mine analysis i:
- modeled crane similar as in example 17 of femap.
- movedby it to some point where it woulde be on ship deck
- prepared some displacement/rotations functions [m]/[rad] vs (ship COG motions on wave) and "rotations" of gravity vectors around point 0,0,0 [m/s^2] vs - in time domain and then transformed it with FFT to DISPLACEMENT/ROTATIONS/GRAVITY POWER SPECTRAL DENSITIES [m^2]/[rad^2]/[[m/s^2]^2] vs [deg/s]
- made analysis version 1: for functions in freq domain
- made analysis version 2: for functions in time domain

So i wonder, are these analysis performed properly?
Do i understand good, that:
- analysis version 1 - shows how rigid body with calculated "normal modes" acts under such PSD - i mean how strong its resonanting?
- analysis version 2 - shows how rigid body behave in time domain under long term excitations?

 

[rob768]

Assuming correct behaviour of rigid body moves is only allowable when it is stiff. A crane is too flexible. Also, ship's motions are in the order of tenths of a Hz. The natural frequencies of interest will be much higher and are not influenced by that.

 

[rgr16]

Thank you.

 

[rgr16]

Hmm but ship RAOs, irregular waves JONSWAP PSD and SRAOs=RAOs*PSD are in range of 0.2-3 [rad/s]. Its not tens of Hz. Speed of vibrations spread in steel structure is speed of sound. So vibrations will appear during ship oscillations. Theres no such force that would stop them, xD i think. Mine preliminary crane has first modes at small pulsation, smaller that 1/4 of ship period oscillations, so resonanting will appear and its not "tens of Hz".

I have next problem that with t.step = 0.02 displacements are freaq (like big resonation) and with bigger t.step = 0.1 they are smaller (like smaller resonation), so i wonder whats the problem here.

Could someone explain it simply or in way that he likes?

Robert Grzelka

 

[rob768]

Irregular waves may be of a higher frequency, the main problem will be the cause an impact lke excitation, exciting all resonances affected by the impact. A crane, no matter how stiff, can not be considered a rigid body in this.

 

[rgr16]

damn ;/, i dont know how to understand that part : "the main problem will be the cause an impact lke excitation, exciting all resonances affected by the impact".

At calculations of ship motions i assumed that ship motions are not affected with crane presence and ship-crane system during that calculations were rigid body, but only at that stage of calculations.

Then i go to crane analysis. Crane i assumed then as stiff body connected at bottom edge with RBE2 elements to ship COG node. Crane is at proper distance from COG node as it would be on ship. Time varing displacements/rotations are aplied to that node which is fixed.

Then i perform "response spectrum analysis". "Frequency harmonic response" were not good, cos its for steady state oscillations only, so in best way i would need to perform it for all frequencies in range of SRAOs if i would stick to that solver.

Also i think that impact is sinusoidally growing without some big growt between time steps, so it could be not thinked as sudden appearing force during inclination. Ship is freely oscilating (trying to resonante in that way but under damping). Ship in seas is not bumping from one side to another like in example hitting oposite trees with rotating crane. It goes to one burt with loosing velocity, slowly stoping and then going to other burt.

Robert Grzelka

 

[rob768]

slow motion regular waves will most likely not affect crane resonance. Irregular waves cause some more "sudden" moves, which resemble impacts.
An impact is a very short burst of energy, theoretically composed of an infinite range of frequencies. Thus, an object excited by an impact will be excited at its natural frequencies, regardless of the actual frequency. The good news is that it is not periodic. It will usually dampen away quickly, provided there is damping.