Offshore Bridge Piping Pipe Stress Analysis

Hi all,

Right now I'm doing pipe stress analysis on Offshore Bridge Piping.I want to ask the correct way to model platform displacement in Caesar II.

Displacement on platform A:

Dx = -33.43,33.43
Dy = -66.56,66.56
Dz = -33.63,33.63

Displacement on platform A:

Dx = -17.42,17.42
Dy = -37.08,37.08
Dz = -15.57,15.57

vector
Dx =
Dy =
Dz =
Rx =
Ry =
Rz =

Thanks in advance.

I presume you mean Platform A and Platform B?

You could put your negative displacements in Vector 1 and your positive displacements in Vector 2. Then you could put these vectors (D1 and D2) in the Static Load Cases as necessary.

Thanks Richard for the quick reply. Yes, platform A and B.I just want to know whether my friend approach in modelling displacement on those two platform is correct or not.Like this :

Displacement on platform A:

Dx = -33.43,33.43
Dy = -66.56,66.56
Dz = -33.63,33.63

Displacement on platform B:

Dx = -17.42,17.42
Dy = -37.08,37.08
Dz = -15.57,15.57


On platform A:

vector1 vector2 vector3 vector4
Dx = 50.8500 -50.8500 0.0000 0.0000
Dy = 29.4800 -29.4800 0.0000 0.0000
Dz = 49.2000 -49.2000 0.0000 0.0000
Rx = 0.0000 0.0000 0.0000 0.0000
Ry = 0.0000 0.0000 0.0000 0.0000
Rz = 0.0000 0.0000 0.0000 0.0000

On platform B:

vector1 vector2 vector3 vector4
Dx = 0.0000 0.0000 50.8500 -50.8500
Dy = 0.0000 0.0000 29.4800 -29.4800
Dz = 0.0000 0.0000 49.2000 -49.2000
Rx = 0.0000 0.0000 0.0000 0.0000
Ry = 0.0000 0.0000 0.0000 0.0000
Rz = 0.0000 0.0000 0.0000 0.0000

He is using relative method.For example :

the platform A move -33.43 in negative X direction
The platform B move +17.42 in positive X direction

So the relative direction that platform A is moving is 50.85 to the positive and negative x-direction with respect to the platform B.

Then i will have D1,D2,D3 and D4 n the Static Load Cases.

My question is this approach correct?Or can suggest the correct way to do it.Thanks

I'm afraid that is more complex than this and will require a lot of work in order to set the load cases.

I think that 6 sets of displacements would be more accurate, D1, D2, D3 for A platform displacements (on X only, on Y only and Z only) while B platform displacements are 0 and likewise for D4, D5, D6 B platform displacements while A platform displacements are 0. Then you can play with displacements as you wish, changing the signs and possible combinations.

Maybe also relative displacements can be used but on less complex system.

Another difficult decision will be to choose the linear, non-linear approach.

Regards,

The first thing is to look at bridge bearing design at both ends. Most of the cases one end of the bridge will be fixed with the platform and other end will be sliding end on the platform. The bearing designs will show the gap axial and lateral.
Next the pipe runs between two platform linking the bridge.
The pipework on fixed end is normally anchored and relative displacement as advised by DANB is applied. A proper flexibility with loops are provided on sliding end of platform.
A fatigue check is required based on the no of deterministic wave cycles at different wave heights.

I hope to Know, The diplacement vectors here are considered secondary or primary [Occ]stresses.

best regards

I try to keep my mouth shut.

I thought that by code is secondary, but there are so many opposite opinions that I think is better to wait others.

Regards,

What will you do with a vertical column nozzle displacement? OR with any other thermaly displace nozzle. Why this will be different?

I agree with what dan's opinion which is more accurate and to build a less complex load cases...

Displacements are secondary load...

Waiting, waiting..........

Strain (displacement) that does not produce collapse would be considered "secondary". The stress they produce is limited to yield. The mode of failure is fatigue.
It gets sticky though when this stress component aggrevates other stress caused by force-based loads (primary stress).

Perfect. The first step is done.

Now, can we identify which are the displacements that produce collapse? (apart of the faulted supports, which are outside normal design)

Note: Remember that B31.3 identify diplacements as secondary.

Dan
B31.3 also advises to consider this total displacement range. If we check the total displacement range and also check the allowable fatigue cycle, hope we have ensured the system will not collapse due tp strain (secondary).

Perfect(and thank you for the help). Everybody agree?

Regards,

Perfect. The second step is done.

Now, can we identify which are the displacements that DO NOT produce collapse?

Regards,

Ok. Maybe you all know or you all do not care.

The reason I insist is the following:

You obtain the system you design for.

If you design a system for primary stress you will have a stiff one, if you design for secondary stress you will have a flexible one. One is detrimental for the other. So careful what are you're doing.

Regards,