Steven Shu
Randy Seehausen
Steve Bledsoe
Tim Powell
Jacobs Engineering Group Inc.
Over the past decade, several advances have been made with regard to offshore production riser systems, both in terms of their engineering design and materials. These advances have become increasingly important as the oil and gas industry has moved farther offshore, where water depths place intense pressures on production facilities. In particular, advances have been made with regard to riser pipe diameter, design tools, component manufacturing, system integration, construction and installation. Some of the more notable and market-ready innovations are described below, and additional information is available on the Deepwater Production Riser Systems and Components poster included in this issue.
Riser selection
Deepwater riser selection is one of the major drivers in the evaluation of the technical and commercial feasibilities of a project, along with the subsea architecture and floating production platform. Selection of these three elements is a highly coupled process.
A preliminary field layout and floating platform type are selected based on reservoir, drilling and environmental conditions. A riser selection is interdependent on the field layout and the selected floating platform. A floating platform with proper motion performance will offer reliable riser behavior, whereas a robust riser configuration imposes less design constraints on platform design and project execution.
Two new ideas that have been utilized in recent years are Free Standing Hybrid Riser (FSHR) and Offset Steel Catenary Risers (OSCR). Both offer following significant improvements in performance in deepwater environment:
- Less dynamic fatigue loading on the risers
- Less load transferred to the floating platform
- Construction and installation schedule is independent of the floating platform.
Both have beneficial applications in the following circumstances:
- Harsh metocean environmental conditions
- Severe motions with non-heave optimized floating platforms
- Sour services that requires riser pipe with high fatigue resistance
- Requirements for larger number of risers tied-back to existing floating platforms with challenges of vessel payload, seafloor layout and riser attachment locations
- Disconnectable systems in active storm areas.
The original FSHR took the form of a bundled arrangement. The hybrid bundle consists of a single vertical tower, containing various numbers of production, export, water injection and service lines enclosed within thermal insulation and syntactic foam buoyancy. The hybrid bundle has been used in fields including Green Canyon 29 and Garden Banks 388 in the Gulf of Mexico, and Girassol, Rosa and Greater Plutonio in West Africa. An alternate FSHR arrangement is the single line offset riser (SLOR). The SLOR utilizes a single steel riser, and has been used in a number of fields including Exxons’s Kizomba A and B fields and BP’s block 31 NE in West Africa, Petrobras’ P-52 in Brazil, and Cascade/Chinook in the Gulf of Mexico.
A new hybrid riser design known as the Offset Steel Catenary Riser (OSCR) system consists of an SCR hanging off from a Submerged Access Module (SAM) that is supported by a submerged buoyancy module above, and a flexible jumper attached to the SAM, and connected to a floating platform. The OSCR system is tethered to the sea floor using a polyester mooring line and gravity weight/suction pile assembly. This allows the system to take advantage of the best attributes of both the SCR and the flexible pipe; an SCR that can go deep and a flexible pipe that can accommodate a vessel's severe motions. It avoids the limitations of both, while capitalizing on the advantages of each.
Riser design
If motion induced fatigue is not a concern, a steel catenary riser is a good candidate for wet tree applications, since it is more economical and can go deeper than a flexible riser. An SCR’s water depth capability is limited by its own weight and an installation vessel’s tension capability when the wall thickness increases to resist an external pressure.
A riser’s internal pressure resistance capability is a key consideration. Note that a riser pipe is not efficiently utilized especially with high internal pressure, since the design is usually governed at the top of a riser where the differential internal pressure is the highest. This penalty is getting worse for an ultra high pressure field, not only on a riser wall thickness sizing, but also on topside equipment and personal safety. In such a case, a design with HIPPS may be a solution. A flexible riser can accommodate more motions than a steel catenary riser.
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