Flowline bundles, riser towers resolve Girassol flow dilemma

Feb. 1, 2002
Early last December, production comm-enced from the Girassol Field, marking the start of ultra-deepwater field developments in Angola. Girassol, which has been developed by TotalFinaElf, is a 725 MMbbl oil field located in 1,350 meters water depth, 150 km off the coast of Angola.

Early last December, production comm-enced from the Girassol Field, marking the start of ultra-deepwater field developments in Angola. Girassol, which has been developed by TotalFinaElf, is a 725 MMbbl oil field located in 1,350 meters water depth, 150 km off the coast of Angola. The field, which covers an area 45 sq km, was discovered in April 1996. In July 1998 TFE and its partners sanc-tioned the project.

The technical challenges have been immense, with 39 wells spread across the field. Girassol is one of the biggest subsea developments undertaken in this kind of water depth. As with many deepwater fields, the reservoir is located not far beneath the seabed, consequently the reservoir temperature is well below that normally encountered in shallower water reservoirs. This, coupled with the cold water temperature at the seabed (40°C at Girassol), imposes tough requirements on the flowline and riser systems as the oil must be sufficiently hot to prevent formation of wax and hydrates. The delivery temperature of the oil at the floating production storage offloading (FPSO) vessel needs to remain constantly at 400°C, and not fall below 200°C for 16 hours during shut down conditions. Together with the water depth, these requirements proved to be some of the most critical for the whole project, leading to the adoption of techniques hitherto untried in offshore field development.

At the start of the project, TFE initiated a design competition for the flowline and riser systems, which led to various solutions being investigated. The competition was won by Alto Mar Girassol (AMG), a consortium made up of Stolt Offshore (67%) and Bouygues (33%). The winning solution was based on bundled flowlines and riser bundle towers.

AMG proposed transporting the fluids from the subsea wells back to the FPSO location through a series of flowline bundles, connected in turn to one of three riser bundle towers. This appeared to resolve the flow assurance issues by providing the required level of insulation. Bundling the pipes, moreover, would simplify the subsea field layout greatly compared to a more conventional steel catenary riser solution. In addition, the risers and flowlines would be constructed using standard fabrication techniques, which were readily available in Angola, providing employment for the local work force.

Riser Tower Fabrication-welding individual pipelines
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Achieving the required thermal efficiency was the main challenge for AMG during the design of the flowlines and risers. The key was utilizing the insulating properties of syntactic foam buoyancy blocks. Two 8-in. flowlines were bundled together with a 2-in. service line and packed in syntactic foam modules, subsequently encased in a steel carrier pipe to form the flowline bundles. It proved relatively straightforward to achieve the required thermal performance with the horizontal flowlines, however this was not the case with the riser towers.

The thermal behavior of the fluids is much more complex for the vertical riser situation, compared to the horizontal flowline case. This is mainly due to the changing pressure in the fluid as it is transported up the riser, resulting in expansion of the gas within the fluid, in turn causing a significant overall cooling effect. For the Girassol riser towers the production fluids are expected to lose around 9°C over the length of the riser, with 7-8°C resulting purely from the effects of gas expansion.

Substantial development work was performed by AMG with Balmoral, the syntactic foam supplier, to achieve the required level of thermal performance from the foam. Standard syntactic foam would not meet the stringent requirements on Girassol regarding thermal insulation, hydrostatic pressure and abrasion resistance. What resulted was the development of a new type of upgraded syntactic foam by Balmoral now known as "Ultratherm."

Assembly of the riser bundle tower
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The thermal performance of the riser towers was critical to the success of the whole Girassol Project. An extensive series of full-scale tests was performed on a 9 m section of riser to check out the thermal behavior of various riser configurations. This full-scale testing was combined with computer simulations to determine if it was possible to meet the strict requirements for both steady-state flow conditions and shut-in conditions.

Thermal testing of the riser sections produced some unexpected results, which significantly affected the detailed design of the riser towers and which were not predicted by the analysis work. The small gaps between the various sections of the riser tower allowed the transfer of small amounts of water, which caused further loss of heat due to convection currents. This was the subject of great concern during the project as it reduced the thermal performance of the riser towers to below the required level.

Completed riser bundle tower-preparation for tow to the field
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After more extensive testing with the full-scale riser sections, a solution was eventually found and the final riser tower configuration was shown to fulfill the stringent thermal performance requirements. The overall heat transfer coefficient (U) for the riser tower system was confirmed to be around 2.0 W/m2°C. Results from the first weeks of production have since demonstrated that the thermal performance of the riser towers is well within the specified requirements.

The three riser towers, each 1,300 meters long, were fabricated at the yard in Lobito, which is about 450 km south of Luanda. Sonamet, a joint venture between Stolt Offshore and Sonangol, operates the yard. A mechanized production system was set up at Lobito by Stolt Offshore specifically for the Girassol riser tower fabrication.

The individual lines (8-in. production, 3-in. gas lift, and 2-in. service) were first welded and then brought together at the main assembly line where the syntactic foam elements were attached forming the bundled section. Finally the Serviwrap was applied to the outer surface of the riser bundle. As the construction progressed, the riser bundle was fed out into the water of the bay until the whole riser was complete. The final stage was then to attach the steel buoyancy tank to the top of the riser tower, which provides the required 500 metric tons of tension to the system.

Fitting the foam buoyancy elements to the riser tower
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The buoyancy tanks, which were fabricated in St. Nazaire in France and transported to Lobito, were attached to the riser bundle by ballasting the tank down to the same level as the main bundle followed by a complex welding procedure. The riser bundle towers were then hydrostatically tested before preparations for the tow to the Girassol field.

Each riser bundle tower was towed just below the surface from Lobito to the Girassol field using a large tug as the leading tug, with a small trailing tug providing 30 metric tons of back tension. The towing route (around 600 km) was selected to minimize fatigue loading experienced by the towers, with consideration also given to the prevailing wind, swell, and wave conditions.

This was the subject of much analysis and discussions prior to the towing operations, and resulted in monitoring equipment being installed for the first bundle tow. The results proved to be within the predicted values, and all the bundle tows were performed successfully without problems.

Completed riser tower-preparations for launch
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Of all the complex marine operations to be performed in such deep water, it was always the upending of the riser bundle towers that provided the greatest cause for concern. Consequently considerable analysis, testing, and planning were put into these operations, which had never previously been performed, at any water depth, let alone the immense depths at Girassol. In the event, all the careful planning paid off with all three riser bundle towers successfully installed within 25 days without problem.

The 8-in. flowline bundles, which have a total combined length of 18 km, were fabricated at the yard at Soyo, 325 km north of Luanda, in lengths varying from 800 meters to 2.9 km. Petromar, a joint venture between Bouygues (90%) and Sonangol (10%), runs the Soyo yard. Each bundle consists of two 8-in. flowlines, which are packed around with syntactic foam modules to provide the required level of thermal insulation. A 2-in. service line is also packed with the production flowlines and enclosed within a 30-in. carrier pipe.

Each flowline bundle was equipped with a towhead structure at each end. This provided the termination points for the individual pipes within the bundle and the connection points for the tie-in spools, as well as towing rigging connection points. Each bundle was towed 220 km to the field using the on-bottom tow method, which has been successfully adopted for pipeline bundles in the Gulf of Mexico. In this case the carrier pipe is used to provide protection to the inner pipes and insulation during the towing operation.

As with the riser bundle towers, planning of the installation operations was extremely important to the AMG project team. A towing trial was performed in February 1999 to fully investigate the on-bottom towing operations and set all the required parameters. All the flowline bundles were successfully towed and installed at the field during spring 2001, using the Seaway Explorer as the leading tug.

The final part of this deepwater piping system was to join the various parts of the system together. For the first time in such deep water the tie-in work was performed using spool pieces fitted with standard flange connectors. In the past standard flanges have been used extensively for shallow water tie-in work, where the flanges have been assembled manually using divers.

Until recently it has not been possible to use standard flange joints for deepwater applications, despite their reliability and industry acceptance. However, the tie-in work at Girassol was performed successfully by Stolt Offshore using its Modular Advanced Tie-In System (MATIS) - this was developed specifically to align and connect flanges remotely in water depths down to 3,000 m.

The tie-in work is being performed in two phases. Phase 1 of the installation program included the connection of 38 flanges in the horizontal position, located on top of the flowline sleds and 13 flanges at the base of the riser bundle towers, which were in the vertical position. Two sizes of flange were used, 9-in. and 13 5/8-in., both standard API flanges with a 5,000 psi pressure rating. All of the spools were pre-installed, the coarse alignment being achieved with a set of simple flange catchers welded on the flowline sled.

The Deep MATIS unit, which has been designed to be neutrally buoyant, is lowered to the seabed in a launch frame. Its success on Girassol has established the viability of standard flange connections for deepwater pipelines. This should have a significant impact on future deepwater developments. The cost savings of a standard flange over a remote connector system are immense, and by far outweigh the extra installation costs associated with the remote connection of flanges.

The whole issue of flow assurance is not something specific to Girassol - for most ultra-deepwater fields it is the main issue driving the selection of the subsea facilities. The riser bundle towers and insulated flowline solution used at Girassol have provided an extremely efficient way of coping with these issues.

The success of this project has demonstrated that the offshore industry is ready for the challenges of ultra-deepwater developments. The bundle tower and flowline concepts are particularly well suited for developments down to 3,000 m. It is significant that Girassol has shown that a major subsea infrastructure project in ultra-deepwater can be successfully installed using cost effective construction vessels and equipment, without the need to resort to the colossal new pipelay vessels and equipment predicted by some industry observers.