Method is for operating fluid-processing plant configured to generate and store pressurized fluid, and spaced apart from body of water. Method includes: (A) positioning variable-buoyancy assembly in body of water in such way that buoyancy force urges variable-buoyancy assembly to move toward surface of body of water; (B) positionally anchoring, at least in part, non-collapsible fluid-line assembly underground in such way that non-collapsible fluid-line assembly extends, at least in part, into body of water; (C) fluidly connecting, via non-collapsible fluid-line assembly, fluid-processing plant and variable-buoyancy assembly together in such way that non-collapsible fluid-line assembly conveys pressurized fluid between fluid-processing plant and variable-buoyancy assembly; and (D) transmitting an anchoring force, via non-collapsible fluid-line assembly, from ground to variable-buoyancy assembly in such way that anchoring force substantially counteracts buoyancy force acting on non-collapsible fluid-line assembly, and anchoring force substantially urges variable-buoyancy assembly to remain below surface of body of water.