Subsea separation and produced water reinjection (PWRI) or discharge comprise an integral part of the subsea processing strategy that can bring many benefits, including economic, operational, and environmental, for the oil and gas industry. The importance of subsea separation and PWRI to economics has been demonstrated through work done by Statoil, which installed the world's first full-scale subsea separation system at its Tordis field in the North Sea. Statoil estimated that the system's installation would enable the company to achieve an additional total field oil recovery of 6%, which is equivalent to an extra 26 million bbl of oil. At a price of USD 50/bbl, this would lead to more than USD 1 billion of extra revenue. The most economic means of implementing subsea separation and PWRI or discharge operations is to use continuous online subsea water-quality measurement devices.