Basically, you seem to want to make a cofferdam, extending from the floor of the reservoir to above the level of fluid, open at the top and bottom. The first thing that comes to mind is a large piece of pipe, with flanges top and bottom to keep it round. Face the bottom flange with rubber to seal against the ceramic floor, position it, and pump out the fluid. You probably want to brace it against the shore/tank walls so it won't/can't move from where you place it.
Will it need ventilation so the people don't have to breathe fumes of the process fluid or the exhaust of whatever power tools you put in it? Consider the issue.
It will probably need flanges or similar features at intermediate points also, because it's externally pressurized by the hydrostatic head and therefore unstable, or quasi-stable, meaning that if it's not strong enough, it will not fail in a graceful way, it will buckle and collapse instantly, with probable loss of life, so it needs to be seriously engineered.
It could be rectangular as drawn, but that will require more reinforcement than a round cylinder would.
Is the process fluid more dense than water? That will affect the pressure at depth on the outside, and needs to be accounted for.
If the cylinder were tapered, smaller end up, i.e. a cone not a tube, that would provide some hydrostatic downforce on the seal, at the expense of some added complexity of fabrication. Speaking of which, there may be load limits intrinsic to the floor that you will have to consider.
For a reservoir large enough to hold a dump truck, earthen cofferdams might be more appropriate, but you haven't so far provided a dimension to tell us the scale of the problem.
Mike Halloran
Pembroke Pines, FL, USA
