The dynamic analysis of the riser config-uration for the Girassol project in Block 17 off Angola is complex due to the combination of a floating production, storage, and offloading (FPSO) vessel on the surface and three individual riser towers extending from the seabed to a position app-roximately 50 meters below the surface.
The Girassol field is located in a water depth of 1,350 meters. Sonangol (Sociedade National de Combustiveis de Angola) is the concessionaire and Elf Exploration Angola, a wholly owned subsidiary of Elf Aquitaine, is the operator.
The FPSO is spread moored at a water depth of 1,360 meters, resulting in relatively large vessel excursions of up to 70 meters, equaling approximately 5% of the water depth. Consequently, the risers were configured to accommodate the transverse movements of the FPSO, as well as the first order motions incurred by waves. Further to this, the riser configuration had to be flexible to allow for movement of the riser towers relative to the position and movement of the FPSO.
Particular concern was placed on the potential occurrence of reverse currents, causing the riser towers to move in opposite direction from the FPSO, creating a situation where the riser towers are very close to the FPSO. This is a much more onerous design case than more conventional riser configurations, where the subsea end of the flexible riser is normally tied to a fixed point at the seabed.
The final riser configuration was a single catenary with the flexible risers free hanging through the I-tubes and bellmouths from the FPSO, extending in a large U-shape below the top end of the submerged riser towers, before upending towards the hang-off clamp at the interface point on the riser towers. In order to avoid clashing between non-identical riser structures, three different catenary shapes and lengths were introduced.
Configuration
The Girassol contract to supply 15 dynamic risers for Elf Exploration Angola's Girassol project was awarded to NKT Flexibles. The contract was awarded by the French consortium AMG (Alto Mar Girassol) on behalf of Elf Exploration Angola.
The order consisted of four 317-meter length 8-in. ID water injection risers (2,900 psi - 55 degrees C), ten 317-meter, 8-in. ID production risers (3,400 psi - 70 degrees C), and one 327-meter length, 8-in. ID gas injection riser (5,000 psi - 90 degrees C).
The contract stipulated a specific temperature requirement for the flexibles, ensuring that the production riser temperature would not drop below a certain minimum after 16 hours of shut-off production. Consequently, the production risers were to be equipped with high performance insulation material.
The insulation layer consisted of a number of tapes wrapped on top of the outermost tensile armor layer. Although the insulation layers do not have any load bearing function, they were terminated individually within the end-fitting.
The flexible dynamic production risers for the project were manufactured in a single length ensuring uniform quality. Upon completion of the manufacturing processes, the flexible pipe was cut into suitable lengths, according to the riser configuration. Recently, the critical extrusion of the inner liner was successfully completed using NKT's PVdF extrusion technology, allowing the polymer layer to be manufactured in a single layer.
A total of 4,500 meters of 8-in. ID PVdF pipe was completed in a single length, making it to what is believed to be longest ever built PVdF riser pipe in the world. Due to the volume and weight of the long pipe section, large turntables were used as storage media during the manufacturing process.
Riser liner
All ten production risers were equipped with an inner liner of non-plasticized PVdF. The major benefit of the non-plasticized PVdF is that it is not susceptible to the shrinking problems the firm believes has occurred on other flexible pipe applications equipped with plastisized PVdF.
The FPSO end of the production risers, the water injection risers, and the combined water/gas injection riser were to pass through a bell-mouth and I-tube arrangement, which is located at the starboard side of the FPSO.
The responsibility for the design of the bellmouth was assumed by NKT Flexibles in order to ensure compliance with API 17J and RP 17B. Other points of concern were the minimum bending radius of the flexible pipe, the contact areas between the flexible pipe and the bellmouth wall, and the crushing loads and fatigue.
Due to the unusual aspects of the FPSO mooring system combined with the minimum bending radius requirements of the flexible pipe, the bellmouth estuary has been uprighted at the positions facing another bellmouth. The I-tubes and bellmouths were analyzed for mechanical strength and fatigue, induced by wave loads induced during the transport from the yard in South Kona to its final destination on the Girassol field.
To design these steel structures, confirm the suitability of the flexible pipe designs, and specify the dynamic bend stiffeners, NKT Flexibles completed a range of dynamic analyses which outlined the applicable design basis.
