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Belly Spar - An Ultra Deep Water Solution
Henrik Hannus, Vice President, Aker Engineering and Technology Aker Solutions’ Belly Spar is an exclusive new Spar platform design. Through the FEED contract for the Spar substructure at the Aasta Hansteen field in the Norwegian Sea, awarded by Statoil, this ultra deepwater solution will be technically verified. The Aasta Hansteen Spar platform will hold several records, including the world’s largest Spar platform and the first with condensate storage capacity.

With a total hull length of about 190 meters and a draft of about 170 meters, the Aasta Hansteen Spar Platform will be the largest Spar platform in the world, and in addition the worlds first Spar platform with condensate storage capacity. The Belly-Spar concept is an exclusive Aker Solutions design, where the ‘belly’ refers to the increased diameter on part of the circular shaped hull, which is also the location of the condensate storage tanks. This gives the Aker Solutions Belly-Spar its characteristic shape.

The use of the Spar structure for oil and gas production was conceived by Edward Horton in the eighties. As the first project was realized, Aker Solutions got involved in the design and delivery. With subsequent projects came experiences and new design variants were developed, the truss Spar was the “next generation” and the cell Spar has been another variant. The use of the Spar has been limited to the Gulf of Mexico (except for one in Malaysia), with certain metocean conditions, mostly mild weather, but occasionally loop currents or hurricane events.
During a hurricane, the platform is shut down and the personnel are evacuated. When considering applications in other parts of the world, the local conditions need to be considered, which may yield changes to the design.
Aker Solutions’ Belly Spar development has been carried out over the last two years, with par ticular emphasis on the motion response and the structural arrangement.

With the Belly Spar concept, Aker Solutions is re-entering the Spar market, after being away for more than a decade. Aker Solutions divested its Spar business in 2000. The Belly Spar hull consists of a spar hard tank, midsection and soft tank. The hard tank has a wide lower section (belly) and a narrow upper section (neck). The combination of the neck/belly results in low heave excitation forces and a high natural heave period. The midsection and soft tank have a smaller diameter reducing the hull steel weight.
The hull has a flooded center-well housing the risers. The center well goes all the way from the top of spar to the bottom of the hard tank. The hull compartments are divided by decks, radial bulkheads and by circumferential bulkheads. For an oil storage application, storage tanks can be arranged in the lower part of the hard tank with a favorable vertical center of gravity (VCG). Water ballast tanks for compensation of oil cargo and for trimming are located outboard of the oil tanks. The upper portion of the hard tank consists of void spaces providing the required buoyancy.

A key driver in the design of the belly spar concept is to achieve very small heave response typical for a spar while avoiding Mathieu instability due to parametric heave-pitch coupling, resulting in large amplitude resonant motions.
Mathieu instability of a spar is particularly an issue for long period waves as seen in the North Atlantic. Wave spectrum peak periods for extreme conditions is above 20 seconds and with wave lengths close to twice as long than for previous spar designs, which are mainly in the Gulf of Mexico. Robustness against Mathieu instability is achieved by a combination of the smaller water plane area and damping in heave caused by the belly top and bottom.
In comparison, a classic spar with constant hull diameter would need a draft of approximately 250 meter to achieve the same heave period as the belly spar with a draft of about 170 meters and it would have considerably less damping in heave. The classic spar concept would therefore require a substantially heavier and costlier hull to obtain the same robustness as the Belly Spar. The favorable motion characteristics of the Belly Spar have been extensively documented by model test and fully coupled time domain analyses.

For the Aasta Hansteen field the water depth is approximately 1300 meters. This gives some challenges to both the mooring system as well as the risers.
Polyester lines are chosen for mooring the installation to the sea bed. Polyester lines are lighter than a steel chains-wire system, and will form a straight line between the hull and the anchor. This limits the horizontal motions on the floater. A chain-wire system will, due to its self-weight, form a a catenary with less stiffness and larger horizontal excursisons. The particular application considered utilizes subsea wells and Steel Catenary Risers (SCR). Riser analyses have confirmed good performance and extreme response utilization well within acceptable. SCR fatigue is found to be acceptable and, alleviating the need for fatigue reducing measures such as weighting, special welds or connections or lazy wave configuration. Depending on the persistence of currents, strakes on the hull may or may are evaluated.
A conventional flexible riser system is not feasible in 1300 meters with large diameter risers. The steel catenary risers are feasible but require a low motion floater.

The Aker Solutions Belly Spar development has been targeted for the severe North Atlantic environment. The design development has focused on verifying the performance under metocean loads as well as the structural integrity. The motion analyses have been conducted with state of the art commercial software (DNV Sesam program suite) and in-house standard methodology. With the Spar response being a combination of wave frequency and low frequency response as well as non-linear effects, time domain simulations are extensive. A benefit from applying commercial software is the benefit from the quality assurance. The analyses show that the Belly Spar is well performing the in the long period extreme storms, without attracting any non-linear or chaotic responses. In a 10,000 yr return period sea state, the most likely maximum heave is 5 to 6 meters and the maximum pitch is less than 10 degrees. The motion response has been verified by model testing. The correlation between the model tests and the analysis is very good, demonstrating the good performance as well as the ability of the analysis to capture the non-linear effects.

Apart from being the world’s largest Spar platform, and the first with condensate storage capacity, the Aasta Hansten platform will hold even more 'top of the list' positions. "It will be the first production platform on the Norwegian Continental Shelf (NCS) with steel catenary risers. With a water depth of 1300 meters, this riser technology meets the challenges on the Aasta Hansteen field", says Henrik Hannus, Vice President for Deepwater and Arctic Solutions in Aker Solutions.
“The mooring system consisting of a set of polyester lines is also unique. At this time there are no production facilities with polyester mooring on the NCS," Henrik Hannus says. Aasta Hansteen will represent a step change in water depth, compared to previous installations.
For a year and a half Aker Solutions has been running a pre-FEED on the Aasta Hansteen development. "This work has given us valuable insight to this quite unique project. Key personnel from the pre-FEED have continued their work also in the Aasta Hansteen substructure FEED project organization," says Henrik Hannus.