Dear all,

I recently came across a FSRU project, in which hogging and sagging was considered for cargo lines on hull deck but purpusefully not considered for Topsides. it was client's requirement.
Even though we followed client's requirement, i still have a doubt why the hogging and sagging is not applicable for topsides.
topsides is a part of ship only. when the ship undergoes hogging or sagging, the deflection should affect the lines on topside too as it affects lines on deck.
All that i was able to presume was: since the topside is elevated from deck and the lines are also running at certain elevation above the deck, the deflections due to hogging and sagging may get minimised and shall be negligible for topsides.
this is just my assumption. i am not sure about it.
If anyone knows the exact justification for not considering Hog & Sag in topsides, please share your knowledge.

Also, if any of you strongly feel that we have to consider hog & sag for topsides too, please pen down your views also. i am trying out to get a clear cut picture about it.

Moreover, i wanna discuss about innertial displacements in FPSO after getting a clear picture of this Hog & sag.

Looking for a healthy discussion.

-shan.

There is a longitudinal neutral axis along any ship where no axial change in length will be experienced under hogging and sagging. Logically it will be somewhere between keel and top deck. As you move above that neutral axis, longitudinal tensile and compressive strain will increase under hogging and sagging.

Now consider topsides piping in an FPSO. Where do we put piping on an FPSO ? Frequently along a rack, high above the deck, then we connect it to equipment fixed to the deck at opposing ends of the ship, again elevated above the deck, and perhaps add a few line stops for good measure.

To neglect the axial strain caused by hogging and sagging, the consequent fatigue inducing stresses and loads in such piping is to invite rapid and easily avoidable failure. I think you Client is either mis-informed or misunderstood.

Hi Moverz
thanks for your views. I was mentioning that we didnt consider hog & sag for topsides piping. But Actually, this portion is a module at the FORE end (not topside rack or above the deck).
I got a clarification from my colleague that this module is at the FORE end and the stiffness here is higher as the frames are more & closeby. so it is assumed that the FORE and STERN ends are stiff enough. thats why we didnt consider hog & sag in this module. i am now quite clear about it.
Anyhow, thanks for your quick reply. and welcome any other views or suggessions on this.