Tank structures and fluid displacement devices

A tank structure, comprising:

• a bi-level tank including a transfer tank with a transfer port configured to open and close and a return tank with an upper displacement line port and extending upright from the transfer tank to and beyond the upper displacement line port, the transfer port between the transfer tank and the return tank;
• a displacement device mounted for displacement in a displacement tank with a lower displacement line port;
• a displacement line coupled between the upper displacement line port above the transfer port and the lower displacement line port below the transfer port fluidly connects the return tank to the displacement tank;
• a balance line below the transfer port fluidly connects the transfer tank to the displacement tank;
• and
• the displacement device operatively coupled to the lower displacement line port, wherein displacement of the displacement device in the displacement tank effectuates fluid transfer between the displacement tank and the return tank via the displacement line.