wrc for tailing lug 2

[farzam]

Dear All,

To design one vessel our client asked us to perform wrc for designed tailing lug(normally we design according to Bednar )is it required to check with wrc also?

please let me know your opinion

Thanks in advance

 

[TomBarsh]

Presumably, your client wants the local stresses in the skirt at the tailing lug to be investigated. The WRC-107 method may not be the best approach to this analysis.

Why not? Two reasons:

(1) WRC-107 determines stresses in the "free field" of a shell; the effect of any local shell stiffening is not considered. The tailing lug may be located very near to the skirt base ring, or even attached directly to the base ring, in which case the assumptions of the bulletin are not valid and the analysis may be overly conservative.

(2) The bulletin is limited to rectangular attachments that do not exceed a recangular aspect ratio (eg: length to thickness ratio) or "footprint" on the shell of 4:1. ie: The length of the tailing lug along the skirt cannot be more than 4 times its thickness; most real tailing lugs will be longer than 4 times the thickness. This doesn't mean that the actual lug can't be longer than 4 times its thickness, but the analysis by WRC-107 will be limited to this length. Thus the analysis will likely be very conservative or even overly conservative. Not a problem if it doesn't force you to modify the constuction. But if it shows as overstressed, then it may unnecessarily force you to some more expensive construction.

You might try the EN 13445 analysis for line loads from the European code, this doesn't suffer from limitation (2) above, but does suffer somewhat from limitation (1).

If your tailing lug attaches (welds) directly to the base ring as well as the skirt, then you might just consider the ring bending in the base ring and ignore the local stresses in the skirt. Assuming that the welds to the skirt are not overstressed then the skirt itself will not be overstressed.

If the tailing lug resides between "double base rings" (continuous top ring on anchor bolt chairs), then neither of these two local stress methods are valid at all (too conservative) and you can ignore this effect. Unless you are designing a "major" vessel, very heavy or very costly, and lots of risk is involved, in this case you might consider an FEA, but this is not generally necessary for most vessels.