There was a documentary about this exact type of job on the uk channels about 15 years or more back, it was in one of the tyneside yards, hawthorns or swan hunters, a ship came into the dry dock to be chopped in half and a length added in. Not the sort of job you do in a weekend with a halfords mig and a 4 1/2" grinder.
I've seen it done with a narrowboat (took three days, which I thought was some going), and I've seen a combine harvester chopped in half down the middle to be widened two feet and I thought that was pretty good too, but that is on another planet. Fantastic.
I did a little work for a firm wich operated the RRS DiscoveryIII which is/was a oceanagrapgic research vessel operated by the national environmental research council. She had ten meters added to her length back in the 90's which was impressive but nowhere near as impressive as that time lapse! That's a cool clip
At least it all fitted, we wanted 2 transfer lines modifying (by the original Dutch manufacturer) so they built all the new parts from their drawings and arrived to fit it during a summer shutdown, there were a few red faces when they found the extension steelwork was 1 metre too short!