How exactly can you have a dwarf survive digging a ocean drain? I'd really like to be able to do that, I couldn't think of a way at all
If your problem is high-pressure water chasing and catching your miner, put in a diagonal before the point of puncture. It'll take out the pressure. Or just make a longish tunnel with a raising bridge before the point of puncture (upward ramp) and raise it immediately after the miner punctures. If done right, he'll end up in less than 7/7 water draining out of the tunnel. If done wrong, he's mercifully atom-smashed.
Methods:
A. Dig a hall to the drain location, as well as to the drain (caverns). Build a door at the far end. Link it to a lever. Ensure that it is closed. Dig a ramp in the wall adjacent to the door, such that water can only enter at a diagonal (depressurizer). Dwarf easily outruns depressurized water back to safety. Open the linked door to allow high-pressure ocean water to flow to the drain.
B. (Exploit method). Dig a hall to the drain location, as well as to the drain. Dig a "safe" access hallway underneath the "ocean" end of the first. Channel a hole in the floor between the two hallways, and cover it with a permanently-shut bridge. A dwarf standing under the bridge on one of the ramp tiles can safely dig an upwards ramp through the closed bridge on the z-level above. The drain hallway floods with pressurized water, but the access hall remains dry because of the bridge.
...Hmm, the burning bin is interesting but I would be worried about it setting something important on fire later. ...
The bins don't burn forever, just for several months. You could avoid it, and by the time your construction is finished it would be cold.
Construction speed of anything: limited by surface area for masons to stand and the number of masons that you have, plus the amount there is to build. Building a ring requires fewer blocks than building a large square area.
But you only need floor on one level, you need walls on all levels. So it depends how deep your sea is, and how roughly circular your block is.
You need to drop the ring at the end anyways in order to sever the connection to the sea floor.
Right but that means building more gantries from scratch so it's a separate cost.
...
Just so we are on the same page, this is a cross-section of the ring I am proposing to build:
_█_█_
The central underscore is a bridge that drops the magma ring when full. One of the walls that retains the magma in the ring is on the outside of the cast obsidian wall, so I can drop the ring at the end to sever the ground. The ring is only 1-z-level high because I drop the magma into the ocean in stages to cast the wall.