Subsea storage of a water miscible storage fluid

A subsea storage system comprising a subsea storage unit storing a water miscible storage fluid, wherein the subsea storage unit comprises a wall defining an inner storage volume for storing fluids, wherein the inner storage volume comprises a top side facing the sea surface, a vertical side and a bottom side facing the seafloor,

• wherein the inner storage volume comprises:
• a barrier fluid as a layer between the storage fluid and seawater extending across an entire horizontal cross section of the inner storage volume for separating the storage fluid from the seawater, the barrier fluid being immiscible with both the storage fluid and seawater;
• an upper section that extends down from the top side to the barrier fluid within the inner storage volume;
• and
• a lower section that extends up from the bottom side to the barrier fluid within the inner storage volume,
• wherein the upper section comprises at least one upper section opening for fluid connection from the inner storage volume for loading and unloading the subsea storage unit with fluid,
• wherein the lower section comprises at least one lower section opening configured to provide a fluid connection from the inner storage volume to the ambient sea,
• wherein the stored storage fluid is positioned above the barrier fluid within the inner storage volume,
• wherein seawater within the inner storage volume is positioned below the barrier fluid,
• wherein the inner storage volume is pressure equalized by the fluid connection to the ambient sea,
• wherein the subsea storage system further comprises a monitoring system configured to monitor at least one fluid layer or fluid interphase within the subsea storage unit, and
• wherein the monitoring system comprises:
• a data processor configured to analyze data;
• a plurality of sensors displaced vertically adjacent to a vertical surface of the inner storage volume and configured to detect characteristics of the at least one fluid layer or fluid interphase within the subsea storage unit;
• and
• at least one transmitter configured to transmit data from the plurality of sensors to the data processor.