Dear Stress Enthusiasts,
Good day to you.
I would like to solicit expert advice on the problem faced between two stress analysts involved in performing stress analysis for an offshore bridge piping. The two stress systems between lines connecting on bridge and offshore platform are to be separated by anchors.

[img]http://desmond.imageshack.us/Himg221/scaled.php?server=221&filename=detailsie.png&res=medium[/img]

[img]http://desmond.imageshack.us/Himg263/scaled.php?server=263&filename=stressanchor1.png&res=medium[/img]

[img]http://desmond.imageshack.us/Himg141/scaled.php?server=141&filename=stressanchor2.png&res=medium[/img]

[img]http://desmond.imageshack.us/Himg37/scaled.php?server=37&filename=displacements.png&res=medium[/img]


Bridge displacements are present in tie-in support. Now one of the stress analysts says the support at tie-in is a physical anchor and the forces and moments are to be given to structural. If we consider Rx,Ry, Rz as zero for line with bridge displacements, the moments are huge…(say 40 ton in My, 2o ton in Mx). The other stress analyst says that a three way stop will be sufficient and the loads in three way stop can be given to structural.

The problem here is designing the anchor support / three way stop. I kindly solicit expert advice on the correct approach.

Regards,
SAM.

Hi,
Can someone throw an opinion on these topic, please, as it will help in getting clarity and understanding the criticality of such Analysis.

Dave & Richard, would like to know your thoughts on this topic.

Thanks & warmest regards,
SAM

Looking at the diagrams, I can see no reason to make this a "full" anchor, I would do it as a support / guide / limit stop (no rotational constraint).

Dear Clarke,
Thanks for the details.
Would be helpful if an explanation is given on the use of three way stop. The explanation given by stress analyst was that a physical anchor at tie-in will prevent the moment from transferring from bridge piping to offshore platform piping,... Is that reasoning correct.... Kindly correct if the approach is wrong....
Regards,
SAM PSA.

SAM PSA, yes a three directional stop will not create moments at the support location. The reason is that the pipe is allowed to rotate in the three axes, but it is not allowed to move very far.

A pipe anchor could involve the pipe being welded to a pipe shoe, and the shoe being welded to the support steel. So it is a full anchor. This will produce moments at the support.

If you are using shoes, you could guide the shoe at the location, and install axial stops also, both with 1/16" or 1/8" gaps. The shoe is not welded to the support steel. Therefore the pipe centerline will stay in place but there will be rotations. CAESARII will output only FX, FY, and FZ at this support location.

Make sense?

Hi Devin,
Thank you for the details and sharing of knowledge.

Well the three way stop will arrest the displacements in three directions, but my question involves the following points.

1.whether the rotations which are transferred to adjacent piping will cause any problem in the stress analysis of adjacent piping system?

2.whether physical anchor support only to be used between two adjacent stress systems or a three way stop can be used for segregation?

3.Can we check in CAESAR II how much rotations are transferred through displacement summary in the CII Output processor?

Regards,
SAM.

Hi devin,

Any comment or updates on supporting methodology....

Thanks & Regards,
SAM.

You want to split systems at places of known displacement. With a 3 way stop and a guide on the same piping run, the displacements at the 3 way stop will be basically known - minimal translation and rotation.

Your answers are then

1. No, because they are small, and the piping on each side looks somewhat flexible. You will confirm by doing the analysis.

2. Under appropriate conditions, a 3 way stop can be used for segregation, but the piping should be well guided on each side. A 3 way stop by itself is not enough to accurately split up a system.

3. Yes, if the entire system is modelled together.

Dear Clarke,
Good day to you and many thanks for knowledge sharing.

I would like to have your views on this topic"A 3 way stop by itself is not enough to accurately split up a system".

System breakup before tie in is an important area of concern for stress analysts when two parties are involved in the analysis of the entire system.

Many believe an physical anchor before the tie-in is the best but some have the view that a three way stop is enough Again not a clear explanation available for the way forward..

Also there is an allowable tie in load given by client... The tie ins are generally flanges and only anchor with Cnode at the flange will give the loads at tie-in..Can such a procedure used to verify the allowable loads at tie-in.

Correct me if the concept and approach is wrong...

Regards,
SAM

It is a case by case judgement. A phisical three way restraint is enough if you know that the pipe will not rotate. If you have doubts about this, you should extend your calculation in a way you do not have anymore external influence.

Regarding the second question.

It is an aproximation of the fact that it will be an anchor point near the flange. Normally it is enough and you can stop your calculation at that flange. Sometimes it is not enough and you should extend your calculation up to a pint you know that you can stop.

I can see that there are two system interfaces at a tie-in point. In case the two system analysed by two different companies you need to define the allowable forces and moments at the tie-in connection and displacements to be able to communicate in the analysis issues. Mainly the displacement issue is solved by using 6 directional anchors or translation restraints only.

If you have the ability to solve the two system in one analysis, and if the system is passing with/without the interface area supports you are OK. However, if you do not have suitable support at the interface you may have other difficulties in the construction of the piping or in operation if the tie-in is to be made many times during the on/off operations.

You know the system and the operation. Therefore you are the best decision maker for the support issue.

There are some other discussions either in this forum or in the www.eng-tips.com on the tie-in issues.

Hope it helps.

Ibrahim Demir