Hybrid riser opens up subsea production at 3,000-meter depths

Girassol unit first second-generation hybrid

Stephen Hatton
2H Offshore Engineering

Odd Avid Olsen, Richard Hill
Aker Maritime

• Typical configuration of the top end of the offset riser with an air can connected via a flex element to the top of the riser bundle, and jumpers connected immediately below the air can [11,728 bytes].
• Riser base arrangement [11,139 bytes].
• 2H Offshore Engineering's submerged tow - typical vessel/riser configuration [9,377 bytes].

Shallow water risers tend to be viewed as a field proven, off-the-shelf add-on to the production vessel. As water depths increase, however, riser systems achieve a higher profile. The main reason for this increased level of interest is the total installed cost of these systems which is now a significant proportion of the total deepwater field development cost. Interfaces are more complex and there are higher levels of technical and commercial risk.

Numerous other factors make deepwater risers more challenging than shallow water systems:

• Commercial deepwater reservoirs need to be quite large, so the developments require large diameter risers to achieve required flow rates and also to allow export from often remote locations
• Large reservoirs require a large numbers of risers and consequently seabed congestion and riser/flowline corridors can be both design and installation issues
• High levels of thermal insulation are needed on these deepwater fields to prevent hydrate and wax formation. However, thermal insulation is expensive and can cause response degradation, particularly with steel catenary and flexible catenary riser systems
• Many deepwater reservoirs are located in remote areas where there is little operational experience and little environmental data is available. Riser design work must be conducted therefore on the basis of extrapolated current data where maximum values are conservatively estimated and long-term distributions largely unknown
• Deepwater risers push existing installation vessels and methods to the limit and beyond, which means newbuild vessels and new installation techniques must be developed.

Production constraints

For production and export risers there are many different arrangements, but these can be classified broadly as flexible, steel catenary, hybrid, and vertically tensioned free-standing. The latter is used for TLP, Spar, and barge applications where surface wellheads are required.

With regard to floating production semisubmersibles and FPSOs, the hybrid riser system shows considerable potential. 2H and Aker have been working together to offer this technology to the Gulf of Mexico and Brazil. The objective is to combine engineering, project management, and fabrication capabilities to offer EPC/EPIC solutions.

Flexible risers have manufacturing limitations which result in a diameter/depth cut off - typically around 1,500 meters for small diameter sizes and less than 500 meters for 16-in. diameter systems. The hybrid riser uses short lengths of flexible pipes and is thus similarly constrained with respect to diameter. However, the hybrid concept allows water depths up to 3,000 meters to be considered. Furthermore, designs have been developed where large diameter export facilities are provided (20-30-in.) through use of manifolding.

The hybrid riser consists of a vertical bundle of steel pipes supported by syntactic and air can buoyancy. It can be configured in either non-offset or offset arrangements. Short lengths of flexible jumpers are used for the connection between the vessel and the top of the riser.

Original designs were installed from the production vessel like a drilling riser, however, the second generation will be beach fabricated and installed by tow-out. They can be deployed for a wide range of water depths and vessel types.

Although hybrid risers have already been used by Ensearch and Placid in the Gulf of Mexico, Girassol in Angola block 17 will feature the first application of a second generation hybrid. Girassol represents the first of a predicted band of deepwater developments based on FPSOs with subsea completions. A key reason behind the selection of the hybrid riser on Girassol is its ability to offer a U value in the range 0.6 to 0.8 W/sq meters K with a loss of only a few degrees along the entire riser length.

Corrosion protection of the steel lines is also important and must be considered along with the sealing philosophy. Preventing free circulation of water limits corrosion rates, but can also reduce the effectiveness of cathodic protection systems. The need for internal inspection of the central structural member by intelligent pig is another consideration, and this affects the design and specification of bulkheads and the method of terminating the bundle at the top end.

Offset hybrid riser

A typical configuration of the top end of the offset riser involves an air can connected via a flex element to the top of the riser bundle. The jumpers are connected immediately below the air can. Sizing of the air can, offset distance and jumper length are critical and must be determined following analysis, which must in turn confirm a wide range of load conditions and vessel offsets.

An alternative arrangement is to extend the peripheral lines through the upper air can to the top face and to connect the air can to the bundle via a tapered stress joint (as is the case with Girassol). While this is feasible in a mild environment and offers the advantage of reducing the depth at which the jumpers are attached, there are also disadvantages:

• Creates a complex fatigue hot spot at the air can bundle interface
• Complicates the launch, tow-out and upending procedure and results in a smaller installation weather window
• Increases loading on the air can, which means increasing its steel weight and size while reducing its efficiency
• Does not allow removal and replacement of the air can.

Riser base

Various configurations are possible at the riser base. Typically the foundation is provided by a suction pile of 6-7 meters diameter and 18-25 meters long. The riser is attached via an anchor latch connection, providing articulation up to 20 degrees. Peripheral lines are terminated in flow bends, allowing connection of rigid spools. Riser base gas injection can be provided readily by routing 2-in. gas lift lines adjacent to each production line.

The anchor latch has been developed and proven by Oil States Industries for TLP tendon application. On the hybrid, the main difference in application is the lower tensions (order of magnitude) and lower dynamic range.

Design of the bundle is driven largely by the assembly method. Various arrangements are possible depending on the number of lines, diameters and facilities at the selected yard. It is important, however, that the riser is fabricated with a relatively high quality welding and inspection since, unlike a flowline bundle, the riser is a fatigue-sensitive structure. The riser bundle can be fabricated in a single length or in numerous shorter lengths (300-500 meters). This allows a wide range of fabrication sites to be considered.

Riser sections are welded, assembled and tested on the dockside prior to load-out by crane. The riser sections are positively buoyant in the load-out configuration. Following load-out, they are surface towed to a sheltered shallow water location where they are mated and the riser is trimmed for the offshore tow operation.

Numerous configurations can be adopted for the tow-out. Surface and near-surface tows have been considered but may be restricted to narrow weather windows, which may in turn lead to relatively high fatigue damage buildup. With the surface tow method, the riser is positively buoyant while in the near surface tow, the riser is ballasted by flooding a number of lines so that it has a low weight in water. The riser is then suspended from buoyancy modules attached by wires.

2H's submerged depth tow method is similar to the near surface tow technique. However, through careful design of the buoys and tow configuration, a much greater depth of tow can be achieved, resulting in negligible fatigue. Furthermore, tow speed can be higher, the rear vessel can be eliminated (no back tension required) and in the event of engine failure, the riser simply floats back to the surface.

The submerged depth tow works by the action of drag forces on the support buoys. As the buoys deflect, the uplift provided to the riser is reduced. The riser then sinks until an equilibrium position is achieved between drag forces on the buoys and riser and buoyancy forces.

Once at its offshore site, the riser can be upended via numerous different methods, including pull-down using a tensioned wire connecting the base of the riser and the suction pile. Once it has been pulled down to the pile, the riser (which is slightly positively buoyant) is tensioned from the surface, assisting it to free stand. Required tension depends greatly on the current velocity at the time of installation, but is typically between 50 tons and 100 tons.

Following connection to the suction pile the upper air can,which is floated out by barge, is launched and connected to the top end of the riser using a similar anchor latch connection as used at the base. The air can is de-watered by injecting air through a pre-plumbed air up system. The riser can then free stand indefinitely until the FPSO is in place, when the flexible jumpers can be installed.

Hybrid risers may have a relatively high hardware cost compared to steel catenary risers, but their installation cost is low - very competitive, in fact, even disregarding the technical and schedule benefits offered. The hardware cost is approximately 50% of the total for an 1,800 m non-offset configuration.

The technology is cost-effective mainly through use of low cost installation methods, which is particularly important in locations such as West Africa that do not have easy access to high specification vessels. Hybrid risers also offer numerous other technical advantages such as high levels of thermal insulation, which is important for all deepwater developments. The hybrid imposes only small loads on the vessels, which simplifies interface design, and in addition the riser can be pre-installed to maximize schedule flexibility and reduce start-up duration.

Reference:

13th Floating Production Systems conference, organized by IBC UK Conferences in London, December 1998. Papers for this and other topics are available from the organizer.

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