Hi,
iam working as a Stress Engineer for one of the Offshore org.currently iam facing a problem with the Loads in Trunnion.

I have generally observed that when iam modelling a Double trunnion & checking the same for analysis, i have found that the Two trunnion takes unequal load(i.e one is more & other less ) & this case happen more in operating condition.

I want to know whether its OK if it happen like this or else i have to bring the load equal in both trunnion.

Pls. Help.
Waiting for a Expert comment.

hi
i think ur new in stresss
because when u model double trunnion u have to see the pipe movement as well as vessel movement
if u define support on centre of pipe and devide that loads to both of trunnion for calculation
i think it will halp u

What you find is most likely the case. If you can follow the pipe movement and rotations you may be able to make the both side loads close to equal by rotating both trunnions (they shall have 180 degrees between)around the vertical pipe.

In case you do the same analysis by using two spring hangers with 180 degree angle in between you will get similar result, hangers will have different operating loads and/or may require different springs under the different loads unless they are rotated properly.

You either will accept the result and design/check the trunnions for the loads or rotate around the vertical pipe untill they have the same load.

You can only see this kind of behaviour from the support in case you model them or previously experienced with this problem. Most of the engineers I have met do ignore this effect or do not see it, however I do think they are lucky if anything does not happen.
Some cases the trunnion yields with the development of plastic hinge and equalize the load between the two if the yielding stops at the shared load somehow. Or one of the trunnion takes the load if oversized the other do not even touch the suppoirt structure. Most of the cases this may not be acceptable and may cause fructure depending on material and manufacture.

Our approach has to be seeing the problem area at front and do the design properly at the first place by complying with requirements. I suggest you rotate them properly and equalize the loads and make deflections same for both. Otherwise everyone in the chain will have some kind of difficulty to understand the difference and the result is not going to be the intended.

Kind regard,

Ibrahim Demir

Your problems are almost certainly due to a small amount of rotation of the pipe, unequally loading the trunnions (in theory).

As Ibrahim points out, some yielding of the trunnion might equalise the load, but in the real world, the support structure itself is flexible (as Mr Klein reminds us ... "all the world's a spring"), and the local structure will deform a tiny amount to similarly equalise the loads. You could model in a segment of the surrounding steelwork to demonstrate this.

B31.1 para???? (Cant remember) dictates that riser clamps etc be designed in a manner such that ALL the load can be carried by one side. This recognizes the real world possibility that uneven loading may occur due to construction error.

The situation is more critical and usually justified for larger diameter piping to model the two trunnions to the POS (point of support). The behavior of the supports will most definitely be affected by larger piping rotations for systems with Hinge and Gimbal expansion joints or the like. Also for spring supports/hangers.

Usually for a first pass analysis, most folks will model the support at the pipe centerline and one should go into more detail depending on the criticality and review of the predicted rotations at the pipe centerline.

As mentioned by others, steel flex (and local flex of the trunnion) or play in the riser clamp will affect the actual load distrbution on the two trunnions.

Thanks for the reply,

Mostly i faced this problem when there was a support taken from equipment in form of trunnion near the nozzle. i observed that when both trunnion 180degree apart are taking equal load then there is less load on Nozzle & in case of Unequal load,Nozzle load increases.So i feel Trunnion should carry equal load.But the question is how we can equalise the load?????

Regards,

Jeba Anand