Dear Professionals,

Currently i am performing Stress Analysis for a FPSO. I was told by one of my senior engineer that vertical structural deflection due to Hogging & sagging should not be considered. He said that many professional engineers do not consider it too. I would like to know if such statments are correct? If it is true, what are the reasons not to consider vertical deflection?

We ignore the stiffness of supporting steel because

1. It's conservative (usually).

2. For B31.1, at least, the design rules are from a time when accounting for structural deflections would have been horribly difficult, so the rules are structured according to the analysis capabilities of 70+ years ago. Remember, structures often have different stiffnesses in different directions. This would have been brutal to try to analyze with slide rules and log tables.

3. It reduces the amount of interdiscipline coordination required. Those structural guys WILL stiffen precisely the member you most need to remain really flexible!

4. We often design the piping before the structure is well-defined.

5. Accounting for the deflection of a structural member requires that we keep track of ALL relevant structural loads. Most of us have enough to do as is without trying to keep up with what structural is doing. (Those sparkies WILL change how they hang their cable trays at the very last minute.)

Having said all that, it may well be valuable to use real stiffnesses for structures near piping terminal points. But then, we usually also assume that the steel or foundation supporting the terminal equipment is rigid.

The basic reason is that management usually wants the pipers to be done with their stress analysis in a finite amount of time.

1. I had a case where deflection of the steel structure members was causing big problems in nozzle loading

2. I have a current case where old pipes have excessive stresses in FP (Steam pipe). If I would model steel stifness I most probably would get much lower stresses. I do not do this to make sure that pipe is monitored in the future (NDT, replica). Modelling would never be accurate and would only hide potentially risky area.

3. I know that some stress engineers use sometimes steel modelling to get stresses down in the pipe.

4. I have current case where too flexible long support steel members have excessive deflection and other problems. Design has to be changed.

Conclusion: Have to use some common sense but where possible I would follow Craig's method.

Jouko:

I agree 100%. The first 4 or 5 attempts to meet load limits and Code stress limits should be with rigid steel. Only then, when you have exhausted your bag of piping tricks, should you bring out the bag of "floppy steel" tricks.

denlim

I think CraigB & Jouko have missed the point of your question.

We do stress analysis on FPSOs also and we do consider the vertical displacements from hog and sag since this can be several inches over the length of your vessel.

Since this motion occurs with a large number of cycles, you may want to consider fatigue analysis also.

Dear all,

Thanks for your replies. What if its a Fiberglass piping system ? As you know fiberglass are more flexibile and less stiff than steel pipes.

I was told that we only consider structural deflection due to Hog & Sag in the longditudinal direction and not vertical direction. During severe stormy condition in the sea, strong waves causes bending in the longditudinal axis and because of this, the structure would move in the longditudinal and vertical direction.

Therefore in my opinon, we should consider both vertical and longditudinal deflection.

Maybe you guys can reccommend some books that could explain my queries.

Appreciate all of your response.

These deflections should be considered as a displacement fatiuge type stress... the ship as it "works" in the seaway will bend (or else it will break) (I'll try to post an amazing pictue of a Great Lakes Ore Freighter working through a Novemebr Gale here in the Great Lakes of the U.S.A.)

Anyhow the big question is how many cycles N that should be accounted for. If for instance you want to go down to the endurance limit your allowed stress would be very, very, small.


FPSO is usually designed to Norske Veritas specs FPSO is basically a ship board refining process unit... FPSO stands for Floating Production Storage and Offloading see.... http://en.wikipedia.org/wiki/Floating_Production_Storage_and_Offloading

Fiberglass is flexible but also like anything else has its limits....


About the MV Selkirk... http://www.clydesite.co.uk/clydebuilt/viewship.asp?id=628

Thanks John.

You gave me a better understanding on how structural deflection actually works and what its usage for.

The consideration of including structural deflections in piping stress analysis is all depends on the scope of piping system to be analysed.

For example for FPSO, there will be some piping which is internal to module. If you see the structural deflections of this module due to hogging & sagging of FPSO, then these will be smaller in magnitude and hence does not really affect piping much. As long as pipe is properly supported by structure, it will also move with structure. This is known as rigid body displacement.

It is a matter of concern when one pipe structure supporting one portion of pipe starts moving (deflecting) with respect to the other pipe strutcure supporting the same pipe.

This will occur in FPSO in piping connecting from one module to another or piping which is running along the length of the FPSO. These loading should be categorised as displacement loading, as the displacements are imposed on piping and hence are computed as the displacement stress range. Since the FPSO will be continuosly moving up & down, we have to take fatigue also into consideration.

When doing any pipe stressing on FPSO’s it is important for the stress engineer to fully understand how both the hull and process modules deflect. There are two approaches with the process modules, either a rigid module that does not bend with the hull and has sliding supports, or a compliant module structure that has legs fixed to the hull and bends with the hull. In both cases it can be shown that vertical deflections will not significantly affect MOST piping. Where you have to be careful is if you have a pipe that is supported on different modules or between the hull and a module, with a short length of pipe between. In this case vertical deflections can be significant.
If you where to bend a pipe to the same curvature as the hull, you would never have a stress problem in the pipe. The problem is caused by the differential deflections of the hull and process modules.
Yes most definably consider Fatigue.
John, I love your photos. Don’t forget to include greenwater loads.

"greenwater loads" There is strong evidence to the fact that the freighter Edmund Fitzgerald was lost with all Hands in a gale on Superior because a hatch cover was not dogged down properly. This led to flooding in the hull and when combined with a slide dwn the back of the wave pulled the Fitz to her grave in a matter of a couple of miunutes perhaps. The loss was so fast that the ship did not get a SOS off!

Green water loads therefore are very significant as well as hull integrity... the great lakes are littered with wrecks due to unfortunate things. The early adoption of shielded arc welding occured here in Northern Ohio (Cleveland Oh is the home of Linclon Electric) due to cold temperature fracture of Oxy Acwetylene welds on freighters.... one of which snapped amidships in a gale on Huron and whose crew stayed alive in the aft section for a couple of days in sight of shore but unrecoverable. Finally the aft section went down with a loss of all hands.

My shipboard piping work concerned some N stuff on something that I cannot talk about. I have never actually had the oppurtunity to work on a FPSO project I am sure it would be rewarding albeit tricky.

An excellent source of information and low cost wonderful design books.... James F Lincoln Foundation