Natural frequency calculation for vessel on lugs

Dear members

can anyone please explain, how does PV-Elite calculates natural frequency of vessel on legs.

There is a big difference in frequency calculated from conventional cantilever beam method and value calculated by PV-Elite

Regards
Prudhvi teja

Hello prubhviteja

That is an interesting question. In the case of a vessel on legs, there are two modes of vibration. They as lateral (side to side) vibration which flex the legs, and there is pendulum or cantilever vibration such as would be experienced by a tall tower supported on a skirt. PV Elite really tries to estimate the cantilever vibration, but the method is not exact as the derivation is very complicated.

Normally vessels on legs are small and vibration is not really. A critical vessel should really be supported on a skirt, or in the case of a horizontal vessel, support on saddles.

Two method are available in PV Elite. They are by an Eigen solver, and the other is by a numeric method from the published paper by C.E. Freese.

This may not fully answer your question, but that is how PV Elite attempts to estimate the vibration for vessels on legs.

Dear Ray

Thanks for your reply.

Yes, I was calculating natural frequency using the numeric method from the published paper by C.E. Freese and PV-Elite was calculating using eigen solver. Now I could confirm my calculation.

As you have correctly mentioned, vibration is actually less in vessels supported on legs, problem is during calculation of seismic force as per ASCE 2010, where natural frequency is the critical parameter for calculating seismic co-efficient.

I understand that, it is generally client's or designer's responsibility to select the method. But, I would like to know whether there is any standard practice followed by industries about selecting method.

Regards
Prudhvi Teja

Hello Prudhvi Teja

The C.E. Freese method is really for a cantilever, which is what a tower on a skirt really is. Doing that calculation by hand is a mission, but, if the tower is divided up into small enough sections, you gat a fairly accurate result. Vessels on legs are very complicated.