A sleeve procedure (re-pad, saddle, split tee, etc) might be needed for stress purposes, to spread the stress of an attachment over a larger area.
Also, sleeve procedures might be needed because the location where the Cosasco is needed has corrosion or nearing its min. wall limitation. Upstream/downstream 1'-00" each way might be good metal so welding a sleeve at that location will resolve the corrosion issue and place the Cosasco where it was intended to be.
I think the RP is that the carrier pipe must be 2X the diameter of the Cosasco fitting or it needs a repad or split tee, etc. I might be wrong on that…Site specifications may differ.
You can put a 2" Cosasco on a 6" pipe without a repad but not on a 3" pipe.
If you have good sound metal and no sizing issues, a weld-o-let for the Cosasco is all that you need. Of course, the Engineer may need to be involved in there are other concerns.
They have a WPS qualified to appendix B API 1104 20th edition or later for sleeves (circumferential fillets) and want to apply that procedure to a branch connection welded in-service? Answer would be no. Table B-1 clearly separates the testing criteria (as clear as API gets at least). Although the type and number of tests are identical, the results of such testing can vary significantly with regards to hardness measurements. Pass sequence and root opening are important variables in an in-service application.