Just a thought
3 foot diameter pipe is used in drilling for oil - drill crews are used to handling it.
the pipe is not welded together but threaded. Cat recall thickness or weight, but try a search on 36" casing for oil wells.
You could assemlbe your pipe from a drill ship or semi-submersible.
then either get out the way (on a semi-sub at least, if you can drop the pipe low enough) or once the payload has been added, somehow get the derrick out of the way.