We installed a 3rd boiler in a VLCC we were converting into an FPSO. After doing some rather painstaking drawings, the boiler company decided that they could get all the bits in via a relatively small hole in the upper deck of the boiler flat. Once cut and a few things stuck out of the way, a jig was lowered in bits to the flat and bolted together. The next two parts lowered in were the water drum and steam drum, which were accurately placed in the jig. The rest was installing the waterwall and other watertube piping, the boiler casing, mountings, cladding etc., with the removal of the jig somewhere in amongst it all. On seeing the boiler all done and dusted, anyone who saw it didn't believe that it wasn't there when the ship was built!
Shipyard riggers are pretty good all in all - on another ship they removed 3 generators ashore and replaced them in situ. Pretty good job.