Concrete FPS can succeed in ice-infested environments

The first stage of the CONDEEP concept development was presented in early 2014, and recent work has related to analyzing and verifying the platform motion behavior, as well as designing a combined disconnect/reconnect buoy. The result is a concept well suited for extreme weather conditions in areas such as the Barents Sea and the Flemish Cap.

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Integrated buoy provides quick disconnect/reconnect options

Jan-Diederik Advocaat
Hilde Benedikte Østlund

A concrete floating production and storage platform has the overall functionality needed for the challenging and harsh environment of the Arctic and subarctic. The CONDEEP design Concrete Deep Floater platform has an integrated buoy for quick disconnect and reconnect of risers and mooring systems. The Concrete Deep Floater is suited as a remote hub given its oil storage and topsides payload capacity. The design features include:

  • Ice resistant concrete hull with acceptable hydrodynamic motions in severe weather
  • Risers routed inside the hull, protected from ice exposure
  • Mooring system with lines connected in the lower part of the structure with no ice exposure
  • Quick disconnect for controlled disconnect within 12 hours and emergency/survival disconnect within three hours
  • Reconnect through a combined riser and mooring buoy
  • Large topsides capacity: 30,000-50,000 metric tons (33,069-55,116 tons)
  • Oil storage capacity of 1.4 MMbbl
  • Direct offloading system integrated in the topsides
  • Substantially maintenance free concrete hull
  • Possibility for local substructure construction
  • Possibility for local topside integration with inshore completion.

The first stage of the CONDEEP concept development was presented in early 2014, and recent work has related to analyzing and verifying the platform motion behavior, as well as designing a combined disconnect/reconnect buoy. The result is a concept well suited for in the extreme weather conditions in areas such as the Barents Sea and the Flemish Cap.

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In Arctic and subarctic areas, ice action will always be a critical factor for any offshore structure. Ice conditions in different places may vary significantly. There are a number of ice features (level ice, ice ridges, land-fast ice, rubble fields, rafted ice, icebergs, and bergybits) that may interact with an offshore structure. In some areas, only first-year ice features are present. In others multi-year ice can be encountered and cause higher ice loads. Data concerning ice thickness, ice concentration, drift velocity, and physical and mechanical properties are required for ice load calculations. The final design of any offshore structure, whether fixed or floating, has to consider the actual ice conditions at the specific site.

The Arctic is a diverse region and the different areas require different design solutions. A simplified categorization system (divided into four category concepts) has been introduced for concept development in order to differentiate among the floater concepts suitable for the various environmental conditions.

Category 1 concepts.These concepts are suitable in areas with harsh weather but no ice exposure. A typical region where this is applicable is the Norwegian continental shelf where existing concrete floaters in operation are the Heidrun TLP and the Troll B semisubmersible.

Category 2 concepts. Floater concepts suitable in ice infested areas, provided that the normal ice load does not exceed the mooring system capacity, and is accounted for in the design as an accidental scenario only.

CONDEEP floater.

Developments falling under this category include the CONDEEP Deep Draft Floater, the concrete spar, and the circular concrete FPSO developed by Kvaerner for the Barents Sea. Common for these solutions is that the design accounts for quick disconnect of risers and mooring lines as an accidental condition. However, the design solution does not account for reconnect for continued operation after an accidental event. This philosophy permits a simple disconnect system. The concrete hull structures are robust and designed to operate without maintenance in ice infested waters with a large topsides payload capacity in order to enable operation as a hub in remote areas.

Category 3 concepts. This category is an extension of Category 2. The concepts are designed for a more controlled quick disconnect with the intention of later reconnecting the platform for continued production. These concepts will be suitable where having the ice loading exceeding the mooring system capacity is not only an accidental condition but included as an operation requirement. A disconnect operation will likely be costly and should as far as possible be avoided. Hence it may be recommended to combine these solutions with the possibility for ice management. Similar to the Category 2, the hull should be robust and designed to operate without maintenance in ice infested waters. Also, to reduce the probability of disconnection, the hull should be designed to withstand any ice impact with energy less than the mooring capacity.

Category 4 concepts. On-going development focuses on the areas where the ice loading exceeds the mooring capacity during a significant part of the platform's operational lifetime. Reducing the overall ice loads is the main focus of this development work, looking at the active and passive features of the hull geometry in combination with an active ice management operation.

Concept features

The main idea behind the CONDEEP floater design is to use the excellent features of the existing concrete floaters and concrete gravity based platforms located in harsh environments. Local content opportunities and project execution risks are factors driving a clean split between the topsides with large modules and a substructure that can be built in available graving dock facilities.

Continuous slip-forming is an efficient construction method. The water depths inshore and offshore affect the overall draft of the structure and are factored in when defining the construction methods.

The hull is designed using high-strength concrete and consists of a set of interconnected cylindrical cells capped with spherical domes. The geometry is suited to resist the large ice and wave loads, and external water pressure. Some cells are dedicated to oil storage while others are used for water ballast and buoyancy. The cylindrical cells are divided into compartments using concrete domes to ensure that damaged stability criteria are met. The hull accommodates a large, protected oil storage system of 1.4 MMbbl. An internal protected riser system will be both robust and cost-effective with the option for steel riser systems down to the wellhead. With the oil storage located 77 m (253 ft) below sea level, it is well protected from conflict with surface vessels. The outer diameter of the structure with the cellular layout at the top provides for an efficient topsides design and cost efficient topsides mating operation. No additional topsides steel weight is required for the deck-mating load case.

Topsides weight is critical for floaters, and the robustness to accommodate increased topsides weight is important. The presented floater concept is designed for a topsides weight ranging from 30,000 to 50,000 metric tons. This, combined with a bilge keel arrangement and the selection of stability characteristics, will control the motions from waves and wind loading. The floating draft is kept constant during operation by load compensation tanks. The freeboard to underside of deck can be adjusted to the specific field requirements to reduce challenges with ice spray and green sea on deck.

General view of the hull.

Hydrodynamic motions

The CONDEEP Floater has good characteristics for heave and pitch motions while operating.

Model tests confirm the analysis and serve as the basis for site specific motion analysis and hull configuration optimization. This includes non-linear behavior in extreme wave conditions. Model test waves were based on typical Barents Sea conditions.

The model test confirmed the excellent motion characteristics in the operating conditions. For the 100-year condition the maximum estimated pitch is in the order of 7°. For the 10,000 year return period the maximum estimated pitch was found to be about 11° (Barents Sea conditions).

The heave motion is in general favorable for concrete hull concepts. For the concept, the 100-year return period heave is 4 m (13 ft).

The topsides is designed with a long and narrow rectangular deck frame of 68 m x 50 m (223 ft x 164 ft). The layout provides maximum distance from riser area to the living quarters for safety. Riser routing is from the main manifold down into the center shaft. The internal routing protects the risers from external impacts such as ice and vessel collision.

Riser disconnection will be performed at the topsides as the risers are dropped down for hang-off on the buoy through steel conductors. Reconnection will be through the same conductors, using pick-up wires from the buoy.

For deeper water, a favorable solution could be a rigid steel riser tower combined with flexible risers. The choice of flexible versus steel catenary risers or a riser tower depends on the actual field.

The CONDEEP Floater is designed with a wet oil storage system in the lower part of the hull, using the lower outer cells for storage. The storage volume can be increased by adjusting the geometry of the cells.

Crude oil is stored on a cushion of displacement water. The design has an internal under pressure in the storage cells achieved by connecting the displacement water side of the storage to a control level tank. The control tank has a free water level 25 m (82 ft) below sea level so the under pressure achieved in the storage tank assures no oil spill to sea in the unlikely event of tank leakage. The sea water side of the storage system replaces oil to and from the storage to keep the storage cells at a constant under pressure relative to the surrounding sea. Displaced water during oil production is pumped up to the topsides for control and optional treatment, or reinjection into the reservoir.

Typical buoy geometry.

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