Trunnion Design for Existing Vessel

[ColinPearson]

In order to design a trunnion or lug to pick up a vessel with, one obvious parameter is the shell thickness. I was hoping to solicit a bit of advice with regard to trunnions welded to existing vessels and how to deal with the possible corrosion.

My thoughts on designating a wall thickness for calculation purposes are to call out a UT tech, take a couple shots in the areas where the trunnions are to be welded, make sure they are pretty consistent with each other and finally make sure they do not fall below where the corrosion allowance spec says the vessel should be for how far into its service life it is.

Unless the corrosion was really minimal, I would then probably take the low end of the corrosion allowance as the shell thickness and proceed with the design.


I guess my question is - How reasonable does this approach sound? The practical effect that I anticipate is a repad that is possibly larger than necessary, but I don't see a huge monetary or time impact (and no safety or mechanical integrity impact) if I tell the craft guys to weld on a 1/2"x20"OD repad as opposed to a 3/16"x16"OD repad. Anyone have any light to shed or opinion to voice?

Many thanks for the input.

 

[gr2vessels]

You should review API 510 for your vessel modifications.
Cheers,
gr2vessels

 

[DSB123]

Welding onto the pressure membrane - probably need to re-hydrotest the vessel.

 

[bernoullies123]

Your approach is spot on. Done this a couple of times to remove and replace the top half of a couple of vessels. Did the same as you proposed, and all went well.

 

[Duwe6]

Approach is valid, well done sir. Two additions: Put at least a 2-inch radius on the repad corners, and put in a 1/4 to 3/8-inch weep hole in it also. Weep hold is to allow detection of thru-wall leakage under the repad [in the distant future]

Properly designed and built vessels will generally outlive us.

 

[ColinPearson]

All - thanks for the input.

DSB, as you state, a hydro definitely follows something like this, regardless if the superintendent whines and says something about "temporary attachments".

gr2, I did and I'm not sure that I entirely follow...
5.5.5 Says to check the thickness, but this references determining corrosion rates.
5.7.2.1 Talks about suitable methods and prefers UT
7.4 Has provisions for averaging thickness reading, but this is in regard to evaluations the effects of circumferential stresses.
I've seen a number of your posts gr2 and you've put a lot of helpful stuff up here, so I'm not trying to argue... just proving that I've read what you suggested and I want to make sure I'm not missing anything.

Duwe6 - agreed on the repad corners if the pad isn't round and the weep hole.

Thanks again to all. I've asked three other engineers here at the office what they think and they keep telling me that the original drawings will have the thickness. I guess the difference is that I work turnarounds and they design new vessels. A bunch of smart guys, but they need some sunshine!

 

[Duwe6]

Amen to "sunshine". All designers should go to the field regularly. Reality seldom looks like the concept drawing.

 

[TGS4]

Agree with the above - you are on the right track.

Amen to "sunshine" as well.

This is rather refreshing from a newbie to eng-tips. Welcome, and I hope you stay a long time and contribute more.

 

[ColinPearson]

I've worked in the field for a couple years; still trying to wash this "green" off though! I worked civil for a bit out of school, got laid off when the econ started going south (and took land developers' loans with it) and finally fell in with an industrial contractor.

It blew my mind the first job I went on where we'd work up calcs/dwgs in the trailer and then walk 200 yards to an operating unit and see just how different real life could look from the paper/screen. I feel pretty lucky to have gotten the field experience I have had so far.

Thanks for the welcome and the thoughts. I'll contribute what I can and listen the rest of the time!

 

[gr2vessels]

Colin,
I am calling for caution in excessive enthusiasm, particularly when selecting reo pad thickness in a forum. The API 510 gives you guidance on inspection and testing requirements, requirements for proper design of this type of alteration. Are you confident that two UT shots for thickness measurements will give you the entire picture of internal fine cracks under the proposed location of that trunnion? It could be OK, but I suggest to calculate and properly design the trunnions and the required reinforcement, based on complete engineering facts. The risk of failure could be high, if not proper engineering is applied. That costs money, but is a lot cheaper than the defence council in court.
Cheers,
gr2vessels

 

[ColinPearson]

Thanks gr2, that was the reason for my question. I asked some of other engineers and did not feel comfortable with their answers.

I wanted to know more about how many UT shots were required/suggested... Really I was trying to get a gauge on what constitutes a complete set of engineering facts. I looked again at API 510 but don't yet see guidelines specific to an acceptable practice for determining an 'as is' thickness wrt analysis of new imposed loads. What I see is wrt circumferential stresses, etc.

As above, I'm here to get better, and only asking for clarification of your suggestion, not being argumentative. I agree completely with your sentiment. I don't care at all about the cost difference between a 12std and a 20xs trunnion... My pm may complain but a couple hundred bucks is nothing, as you state, compared to court and/or getting someone killed.

Many thanks to all here so far.

 

[saplanti]

ColinPearson,

Can I ask what exactly the purpose of the lifting the vessel?

1. Is it for demolotion of the vessel to replace with the new one?
2. Is it a vertical vessel or horizontal?

If it is a vertical/horizontal vessel with a reasonable diameter, and the purpose is an alteration you have got the answers above.

If you have different purpose, you may have several options depending on the diameter. In case the purpose is to demolish the vessel and the diameter is reasonable small, you may penetrate the walls both side and attache a pipe or other section trunnion. Lifting locations for the actions, and the beam (the trunnion) may be checked structurally. The connections of course should be at the thickest wall as expressed above. If the lifting is going to be vertical-to-vertical only you may not need reinforcing plates if the welds and wall thicknesses adequate for the load transfer. If the lifting vertical-to-horizontal for vertical vessesl, or horizontal-to-horizontal for the horizontal vessels you may need additional reinforcing plate depending on the wall thickness left.

So, I am guessing you know what you are doing, and answers in the posts above might be satisfactory. I have just wanted to warn you if you have different case and consideration. That is all.

Regards,

Ibeahim Demir

 

[ColinPearson]

Appreciated... the vessel was vertical and needed to be transported horizontally, then set back into service.

 

[r6155]

ColinPearson

Use the same procedure as in the original erection

Regards

r6155

 

[normet]

Colin,

If the vessel material and repads are thicker than 1/2", you need to consider the risk of lamellar tearing as you will be developing significant through-wall stress. If this is the case, do a complete area UT scan (need a machine with a CRT) to check for laminar indications. Also, before and after welding, do MT for surface linear indications and remove any. Interior surface profile should also be evaluated for roughness (crack-like profile).

Material toughness of the repad and lug is also important, so make sure that it at least matches that of the vessel material.

The standard FOS for lifting gear is 10. If this is a temporary attachment to the vessel you might get away with a FOS of 6, but nothing less!

Again, if the attachments are permanent, you may want to consider locating them a distance of 2 root RT away from any other major structural discontinuities.

As you will probably be attaching a pad with fillet welds that will be carrying significant load, you will want to ensure that the fillets are fully penetrated. Welders who mainly make groove welds frequently do not give a fully penetrated fileet and therefore I always like to do a fillet weld test if this is the case (see ASME IX for this simple test - you will be surprised how many welders fail this test)

 

[ColinPearson]

Normet - Thanks for the specific recommendations RE lamellar tearing and MT. Agreed on locating the lugs as far as possible from any discontinuities to prevent large stress concentrations. If you've got a minute, can you elaborate on why this would only be a concern for plate thickness > 1/2".

Also, I had not thought about weld testing aspect of this. We usually have a lot of confidence in our welders (at least our pipe folks and anyone that would be welding on a lifting point or vessel) and I know that their certs are up to par with what is req'd, but paying for the test and paying the guy half a day to go burn through a fillet test is (as gr2 alluded to) as small price compared to ending up in court or out in a graveyard with someones family. I'm definitely going to pass this to our QC folks to ensure that any welder that may weld this type of attachment have current certs.

Thanks!

 

[ColinPearson]

@suplanti - RE: trunnions vs through pipe... I think our guys have a bit more fun (if the vessel IS demo) when they get to burn a hole through the side rather than worry about the quality of design and execution of a lug/trunnion!

And, of course, it is faster.

 

[normet]

Colin,

Lamellar tearing occurs in thicker plate sections because during rolling most of the compression takes place near the surface (redundant deformation), so imperfections in the material are fragmented more severely at the surface than in the core of the material. With thinner material, the compression extends through the thickness and we get smaller imperfections. It is larger imperfections that are more likely to propagate and thus thicker materials with larger imperfections are more prone to lamellar tearing.

The fillet weld test shouldn't take too long - it is just a 2" long fillet bead joining two plates that are a right angles. You simply break open the weld and visual the root for full penetration.

 

[ColinPearson]

Thanks, that makes sense.

 

[Duwe6]

Good catch on Lamellar Tearing. 1970's and earlier A-36 steel in thick sections was essentially 'piecrust' due to the layering. The slabcast stuff we have been getting in the last 20 years have caused me to forget the 'allowables' on laminations found during the beveling for a weld prep.

And I don't miss the "Good Old Days" a bit.