• Subsea Storage

    The patented Subsea Storage Unit (SSU) solution 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 ensuring long-term competitiveness and continued profitable development of the petroleum resources. 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. 

    • Flexibility – The solution can be placed at any water depth and is adjustable in size 
    • Safety – The solution employs a double barrier against leakage to the sea
    • Efficiency - The SSU enhances the economics of fields with insufficient reserves to support full field development

    How it works

    The SSU consists of a collapsible bag containing the entire stored volume, which eliminates contact between stored medium and sea water. The bag is protected by a protection structure (dome) which accommodates the entire volume. Hence, there is no need to design against pressure and the unit can be deployed at any water depth. 

    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.

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    Description

    Subsea Oil Storage System 
    The Subsea Oil Storage System consists of a structure that employs the concept of a flexible bag protected by a dome for storage on the sea floor, and thereby eliminates problems with emulsion layer and bacteria growth. 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 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 safely 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.

    Subsea Chemical Storage System
    During The SSU development has progressed to include seabed storage of a medium more dense than seawater e.g. Mono Ethylene Glycol (MEG). MEG is used to decrease the hydrate formation temperature below the operating temperature as an important part of developing an economic and attractive solution for remote subsea fields. MEG is typically used during start up and shut down (intermittent) or continuously for inhibition of fluids during transport.
    • Remote storage close to consumer
    • Storage close to platform
     
    Subsea Produced Water System
    Developing new marginal fields and keeping mature fields with declining production profitable requires meeting several challenges. One challenge is subsea separation of water and oil, and by performing the separation subsea the pressure loss in production flowlines is kept low. This enables a lower wellhead pressure that may lead to increased production. Another challenge is meeting increasingly stricter environmental requirements for discharge of produced water.

    The crude oil subsea storage unit is further developed to handle produced water as a large separation module or as a settling tank. Using the SSU as a separation module in a subsea production system allows reducing surface process equipment. Complementing an existing water treatment system with a SSU settling tank for removal of total dissolved solids enables operators to meet environmental requirements for discharged of produced water.
    • Separator/Settling tank
    • Flotation/Buffer tank
    • Settling tank