Hi Gilbert - most likely the seam on the pipes in on the top of both objects in your example - if you move the seam (SrfSeam command) to the quad that is away from the intersection - actually anywhere may do but I’d move it all the way out from the ‘operation’ you intend - then Intersect makes clean curves and the Boolean will work.
FInding clean intersections where surfaces come tangent to one another and edges coincide is an ongoing project…
@pascal what if instead of close circles there are 2 arcs, each one tangent to the other? (imagine one 90° arc, mirrored and copy to its end, forming a “V”)