Stiffener Beam - Tailing Lugs 1

[lcmechengr]

When analyzing a tailing lug connected to the skirt of a tower, compress says to use a stiffening beam when the maximum stress in base ring is exceeded. Where is this stiffening ring suppossed to be attachd? Also, I am assuming the only parameters for beam size is the cross sectoinal area provided in the calculations. Thanks for any advise.

 

[TomBarsh]

COMPRESS considers the weight of the vessel carried by the tailing lug as resulting from the tangential shear stress in the skirt. This is treated in Case 18 of Table 17 of "Formulas for Stress and Stain", 5th Edition, by Roark and Young. When the tailing beam is added the total tailing lug load will be carried at two locations on the ring: one at each end of the "beam" (actually a "strut" since this is considered a tension-only member). The distribution of the forces is found by equating the deflection of the ring due to tangential shear stress and the vertical loads at top and bottom of the ring to the beam elongation.

The COMPRESS method described above is consistent with some published methods of design of tailing lugs and with standards we have seen from a number of prominent engineering firms.

A consequence of this mathematical model is that the beam cannot be subjected to more than 50% of the total tailing lug load. In fact, for the beam to carry 50% of the load requires an 'infinite' area, which of course is not possible.

Currently, COMPRESS attempts to apply ~about~ 1/2 of the total force to each end of the beam. The sub-routine actually will stop with a final beam size selection once the portion of the load carried by the beam exceeds 45% of the total load. This % was an arbitrary choice and in some cases may even impose a penalty in larger required beam area if the base ring is only "slightly" overstressed without the beam.

A new feature expected to be available in the next build of COMPRESS will allow the user to enter the beam area. COMPRESS will determine the loads carried by the ring at the top and bottom (through the beam) and calculate the stresses in each. If an overstress results, the user will need to enter a larger area (the larger area will "stiffen" the beam, reducing the total elongation, this has the effect of limiting the change in diameter of the ring, which then reduces the bending moment in the ring).

Tom Barsh
Codeware Technical Support

 

[TomBarsh]

Answering your specific questions:

"Where is this stiffening ring suppossed to be attached?"

Theoretically it should be attached at the shear center of the base ring-skirt effective cross-sectional area to minimize the effect of torsion acting on the ring section. Currrenty COMPRESS does not provide this dimension but it may be considered for inclusion.

In practice, I would attach it behind the tailing lug on the skirt (dropped down a bit towards the base ring). If there is a continuous top ring (double base plate) then I would even more so preferentially drop it down. But certainly you need to minimize the effect of some local "crunching" through the skirt at the tailing lug.


"I am assuming the only parameters for beam size is the cross sectional area provided in the calculations."

This is true. The beam is treated as a tension-only member. Any suitable section can be specified.


Tom Barsh
Codeware Technical Support

 

[TomBarsh]

Guidance for some typical construction details of tailing lugs and beams are provided in "Pressure Vessel Design Manual", 3rd Edition, by Dennis Moss (

http://www.brownbookshop.com/detail.aspx?ID=5632).

By the way, this book shows the beams attached at the bottom of the base ring. No consideration seems to be given to the effect of the eccentricity of the load from the shear center.

Tom Barsh
Codeware Technical Support

 

[jte]

It's actually a bit unusual to have tailing beams welded inside a skirt. Most of the time I've seen owner/users want the tailing beam to be easily removable, thus it is bolted on to the bottom of the baseplate and removed while the vessel is on the hook in the vertical position.

I think the torsion issue may be overcome by noting that the baseplate to tailing beam connection is essentially two flat plates in contact. Thus, the beam tends to stiffen the base assby against torsion. The anchor bolt chairs (gussets) tend to also stiffen the base assby against torsion. I suppose it wouldn't hurt to add some gussets on the inside of the skirt, though.

jt

 

[TomBarsh]

jte, Very good points. Thank you.

I agree that in practice the torsion is likely a very minimal issue. Personally I like the method of bolting the brace inside the skirt; removing the beam from underneath the base ring while suspended seems rather hazardous. Of course, the drawback to my method is there needs to be a good size opening in the skirt to remove the beam! (Unless it's left in place.)

Tom Barsh
Codeware Technical Support

 

[jte]

...there needs to be a good size opening in the skirt to remove the beam!

Yup, and you don't want to be around and advertising yourself as "the engineer" when the poor slobs have to get their confined space entry permit and go in and unbolt, or more likely, torch cut the beam loose and manhandle the beastie out of the skirt accessway!

Erecting the vessel to a vertical position, then lowering it so the tailing beam is ~1" from grade and having a crew attack the studs/nuts, then repositioning the vessel on the anchor bolts keeps the crew happier.

jt

 

[TomBarsh]

jt, Sad to say, I have probably done such things (making life difficult for others) more than once in my career!

Tom Barsh
Codeware Technical Support