The animation shows a subsea storage unit (SSU) solution, patented by National Oilwell Varco, which enables storage of crude oil, chemicals and produced water on the subsea floor.

The SSU is a gravity based storage unit that employs the new concept of a flexible bag protected by a dome. According to NOV, the system offers a safe, more cost-effective method of developing subsea fields in extreme weather zones or in the Arctic where ice is prevalent.

The seawater outside the protection structure surrounds the flexible bag through free-flow seawater openings in the protecting structure, such that the hydrostatic pressure acts directly on the stored fluid, hence the internal pressure is equal to the external seawater pressure. The oil is stored at a hydrostatic pressure greater than the oil vapour pressure and gas separation is therefore prevented. Once the bag is filled to capacity, the partially stabilized oil can be transported to a tanker through a standard flowline and offloading system. Depending on the type of soil, the SSU is designed to be stable on the seabed using weight piles.

The Subsea Storage System can be applied during well testing and early production start-up with the flexibility to add on storage capacity as the production rate increases and additional structures and multiple SSUs can also be incorporated during the lifetime of a production system. The storage is flexible in size depending on field needs during development, expansion and end of field.

The SSU has a double barrier against leakage to the sea. If the first barrier (flexible bag) should experience a leakage, the integrated leak detection system will detect the leak and alert the operator of the problem. The second barrier (the dome) is capable of containing all oil inside the SSU, preventing leakage to sea. The leaked stored medium can be extracted to a sister SSU or discharged to a shuttle tanker on the surface. The flexible bag can be replaced through the removable hatch on top of the SSU.

NOV’s modelling and simulation software is used to design the complete SSU solution with incorporated control/automation procedures, including filling and offloading operations.

The SSU is qualified with support from Statoil, Lundin, Det Norske and the Norwegian Research Council.

Source: NOV