Retrofit lab (dont remember well the structure name) or 3rd dhipyard!
Can you try to put one of the two options to use with the upgrade ship (Dont matter if add more time on jpgrading ship)?
Because sometimes ships not ready to use on events(build,reffit or upgrade)
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It got merged in with the Retrofit Lab.
I get what you're aiming at though in having a separate dock for building hull and one for upgrading them. The HUGGY yard kind of does that but it's only for building conquerors and defenders. All the hull upgrades have to go through the main shipyard and that's a PITA.