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, and wherein the inner storage volume comprises

• a barrier fluid as a layer between the storage fluid and seawater extending the 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, and
• 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, and
• the lower section comprises at least one lower section opening for fluid connection from the inner storage volume to the ambient sea,
• wherein the stored storage fluid is comprised above the barrier fluid within the inner storage volume, and
• seawater within the inner storage volume is comprised below the barrier fluid, and wherein
• the inner storage volume is pressure equalized by fluid connection to the ambient sea
• wherein
• the subsea storage system further comprises
• a monitoring system comprising
• a data processor for analyzing data, and
• at least one sensor located adjacent to or within the subsea storage unit for detecting characteristics of at least one of a fluid, fluid layer and fluid interphase within the subsea storage unit.