Hog & Sag..........

Dear all,
We are doing topside design for an FPSO in which my client has asked us to consider HOG&SAG in our topside design. Though they have given the values....... im not aware of how they achieve those values & im very eager to learn the same.

Can any one suggest me how to give input for HOG /SAG values in both CAESAR & PV Elite...........

More over i could not understand the concept of this HOG & SAG. Where can i learn this in detail???????????

Ur valuable suggestions will help me a lot....

thanks in advance

"Sagging" is when the bow and stern of the ship are both supported by a wave crest and midships (the middle of the ship) is in the wave trough - thus unsupported. This causes the middle of the ship to "sag".

"Hogging" is just the opposite, the middle of the ship is supported by the wave crest while the bow and stern are unsupported over the adjacent troughs.

Hogging and sagging are typically approximated by predefined displacements. Wave motion is typically approximated by applied accelerations (G factors).

In PV Elite you can go to the Seismic tab and specify the wave loading as 3 components of acceleration (gx, gy, gz). Remember to run 2 cases with PV Elite, with +ve gy and -ve gy.

Mandeep,

While considering this +ve & -ve Gy's how does it considers in calculation.


what my understanding is shown below:

For eg: we know, F=m * a

in this eqn a = 9.81

if we consider G's does this value adds up with 9.81 and provides us higher / lower force

In one case the additional acceleration adds to the g and increases the effective weight of the vessel.

In the other case the effective weight of the vessel is reduced but the tensile load on the bolt increases.

So, you have to consider both +gy and -gy case in PV Elite.

Hello piping stress experts,

Further question on how the hogging & Sagging values can be input correctly in caesar, the shipbuilder informs us that the hog and sag values are +0.79mm/m and -0.65mm/m respectively.

Then how I can to input these values in the Caesar?

Thanks,

Tubero Ako

Use "pre-defined" displacements at your supports locations.

Thank you Mr. Ay for your answer,

Does this mean that in every support location there is a pre-defined displacement? But how can I get the exact value of the pre-defined displacement for every support at different location?

A hogging value of +0.79mm/m, can I use the middle of the ship as my point of reference?

For example, a support#1 located 3000mm from middle of the ship, then +0.79mm/m x 3m = 2.37mm (pre-defined displacement at support#1 due to hogging). Is this correct?

And for sagging value of -0.65mm/m, does my point of reference is also at the middle of the ship?

then the support#1 pre-defined displacement due to sagging is -0.65mm/m x 3m = -1.95mm. Is this correct?

This mean that I need to calculate the displacement(pre-defined) for each supports because they are in different locations?

Kindly please give me more enlightenment. Thank you.

Tubero Ako

I've never done a "hogging & sagging" analysis, so I'm not sure how to answer most of your questions. I'd suggest asking the person that gave you that 0.79mm/m value.

But yes, you'll have to compute a displacment at each support.

Hi!
I've just visited this forum. Happy to get acquainted with you. Thanks!