Submerged water desalination system with product water pump cavitation protection

A method for operating a submerged offshore water desalination system, the method comprising the steps of:

• i) producing at least partially desalinated product water and concentrate or brine from a plurality of offshore water separation membrane elements submerged at a depth of at least about 200 meters;
• ii) collecting product water in an offshore product water collector that receives product water from the membrane elements;
• iii) pumping product water from the product water collector through a product water conduit for surface or subsurface use using a variable output submerged motor and pump having a suction side that receives the product water from the product water collector and a discharge side that pumps the product water through the product water conduit, wherein the variable output motor and pump are submerged at a depth of at least about 200 meters, and provide at least a 27 bar pressure drop across the membranes and a recovery ratio of product water volume to feedwater volume that is no greater than 40% during normal pump operation;
• iv) sensing the occurrence or onset of cavitation due to occlusion, membrane fouling or mechanical failure upstream from the pump suction side;
• and
• v) automatically reducing the pump output upon the occurrence or onset of such cavitation, and discouraging or preventing cavitation over a range of product water flow rates from the membrane elements.