You have to consider all the available shear paths on the lugs.
How do you intend to lift the vessel? If your chains act an angle this will increase the load on the lug. Check out my site for information on fillet weld design
Another point to bear in mind is the effect of the localised load on the vessel; it may be prudent if the vessel is large to perform local load calcs using WRC 107 try
The lifting lug design are on the references kstaylor mention, some basics:
-First define a shape and location you find pratical to use on the field. Sometimes the location can lead to unnecesary scratches or paint damage that can be prevented with a propper location. In general nozzles and anchor devices can work fine as lifting lugs, so if they are adecuate people will use them sometimes.
Your design won´t look good if you have designed lugs and in the field people preffer to use other atachments wich are more useful.
-Make the calculations for the lug and the weld.
In general calculations suppose you put all the weight in a single lug. Some calculations vary on the critical angle of the rope.
-Pads are for reducing stresses. Also consider using pads in order to protect the vessel from big welds (you reduce the risk of damaging the thickness of the vessel with the lug welding procedure) in case it is important (small thickness vessel, unknown welder, etc).
-Perform a WRC107 or similar stress analisys on vessel wall if necessary.
