The Sea Store reinforced concrete storage base both provides storage for produced and processed hydrocarbons of approximately 300,000 bbls and can also act as an artificial seabed some 27 metres above the actual seabed. It thus requires a smaller Sea Nova to provide the required surface platform, reducing costs and increasing flexibility in the solution.
Typically the storage facility comprises predominately reinforced gravity structure approximately 56 metres in diameter and 27 metres high. It incorporates 4 ballast cells, which are used during installation. The main part of the structure forms a single oil storage cell and is designed to resist the loads imposed by oil containment together with the loads imposed during inshore movement and subsequent towage to site.
Some pre-stressing is used to accommodate tensile concrete stresses. This is located substantially at the upper sections of the ballast cell domed roofs and at the perimeter of oil containment wall. In general, however, the structure is conventionally reinforced with fibre reinforcement to control cracking.
The structure will be constructed in a temporary graving dock. A flat surface will be prepared over the site and soffit formwork will be placed on a temporary steel grillage between temporary concrete plynths cast into the ground.
The base slab will be poured as a continuous pour following fixing of steelwork and waterbar. Care will be taken to control drying and exothermic reactions.
All joints will use expanding bentonite waterbar rated to 80 metre hydrostatic head.
The ballast cells will be formed once the base slab has been completed. Two construction methods are being considered. They are firstly slip-forming or alternatively glued-segmental construction. The latter option is likely to be selected. In this case completed sections of the cells will be formed and stacked. Each section will have matching ducts, which will subsequently be used to run post-tension cables.
The oil containment wall will be slip-formed using a single set of circular formwork.
Internal division walls will be poured in parallel. Scaffolding and support to soffit formwork for the roof slab will be fitted following completion of all walls and the roof slab will be poured. All pipework penetrations will be installed prior to pouring with ballast and flow control pipework connected to them following completion of concrete work.
Following completion of the base and removal of all formwork all pipework will be cleaned and tested. Valves will be fitted and system commissioned.
During inshore movements the storage facility will float with a draft of approximately 8 meters. No water ballast will be added. Upon completion the gravity dock will be flooded allowing the storage facility to float. The structure will be floated clear of the gravity dock to a quayside to await towage and installation.
All ballasting systems will be commissioned prior to towage. Inshore tests will include lowering and raising the structure by ballasting the four ballast cells. Upon completion of testing the facilities will be made ready for tow.
This is a low cost platform option which Ocean has developed for stranded assets in shallower water depths (up to 140m). The system comprises a subsea pre-stressed, self-installing and re-locatable reinforced concrete Subsea Storage Tank positioned directly over the well positions with a Minimum Facilities Platform, Sea Nova, located on top.
For more details have a read of our Sea Nova page.
Produced fluid is imported via dry trees located on the platform lower deck. Production conductors run from the drilling slots to pre-installed sleeves in both the platform gravity base and the subsea storage unit.
The produced fluid is processed for export by conventional production equipment located as required on the platform topside. Once stabilised for tanker export it is transferred by the production riser to the subsea storage tank where it displaces seawater ballast to a settling cell within the structure. The settling cell has a long residence time and oily water settles at the top of the cell from where it is drawn off and returned to the platform for further clean-up prior to discharge. Ballast water could be expected to be discharged with contamination levels less than 20 ppm.
The stored produced fluid displaces the ballast water until the cell is fully loaded. At this point it must either be exported by shuttle tanker or production must halt. If the stored fluid is heavy (viscous) it can be heated to maintain temperature with a heating medium being pumped and re-circulated through the storage volume from the platform. Export to shuttle tanker can be achieved through a conventional CALM Buoy or, in certain circumstances, via a reel system on the platform itself.
The FPSO Tanker is eliminated thus removing the most expensive system component in terms of both Capex and Opex costs. No mooring, no manning, no excessive HSE costs, no fuel costs, no expensive maintenance! Relocation costs are similar for both systems though platform and storage reinstatement will be considerably lower than for the FPSO.
Contact us to see how Ocean's Storage Solutions can provide you with a robust and cost-effective development plan using proven technology!
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