A subsea multiphase fluid separation system comprises: a gas separator (22) for separating gas from a wellstream containing oil; a water separation stage downstream of the gas separator, comprising at least one dual pipe separator (24) for separating water from the oil of the wellstream; and a water treatment system (54) for cleaning oil from water that is produced by the water separation stage. On an oil outlet path, downstream of the or each dual pipe separator (24) there is an electrocoalescer (108) and at least one second separator (112) in series.
