Subsea installation and method for treatment of seawater

A water treatment subsea installation adapted for treatment of raw seawater into process water suitable for use in subsea hydrocarbon production, the water treatment installation comprising:

• a seawater inlet to a primary filtration unit wherein a filtration membrane separates a receiving chamber from a permeate chamber having an outlet for treated water;
• a pump in fluid flow communication with the treated water outlet;
• a recirculation loop to feed a portion of the treated water via a subsea electro-chlorinator back to the water stream upstream or downstream of the membrane of the primary filtration unit;
• and
• a secondary filtration unit installed upstream of the electro-chlorinator, the secondary filtration unit comprising:
• a nano-filtration unit having a nano-filtration membrane effective for sulphate removal having an inlet and outlets for permeate and reject water;
• and
• a reverse osmosis unit having an inlet for receipt of the treated water and outlets for permeate and reject water;
• wherein the reject water outlet of the reverse osmosis unit feed a reverse osmosis reject that is rich in NaCl and high in divalent ions to the nano-filtration unit via the nano-filtration unit inlet;
• and
• wherein the permeate water outlet of the nano-filtration unit feed a nano-filtration permeate rich in NaCl and low in divalent ions to the electro-chlorinator.