Hybrid Ultradeepwater-Riser Configuration Saves Costs, Pushes Depths
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The single independent riser (SIR) is a hybrid riser configuration optimized for ultradeepwater field development. The SIR is composed of a flexible jumper and a rigid part vertically tensioned by means of distributed or continuous buoyancy. This configuration features improved dynamic behavior under fatigue and extreme environmental conditions for the rigid riser section thanks to its compliant shape. The design is versatile and can be staggered easily to comply with design constraints or congested layouts.
The SIR is adapted to both turret-moored and spread-moored vessels as well as most other types. The configuration is also suitable for a wide spectrum of environments, from the mild conditions of west Africa to those in the Gulf of Mexico or the North Sea. The configuration can be designed for production risers, whether single-pipe or pipe-in-pipe, and injection and export risers. The strength of this configuration lies in its compliant, fatigue-resistant behavior. Its steep-wave configuration facilitates the mitigation of potential interference; the relative position of the sag and hog of each neighboring riser can be adjusted while reducing the load at the hang-off location.
The SIR features a steep-wave shape. The upper part is a flexible jumper, and the lower part is a rigid, steel-pipe string maintained in a near-vertical position either by means of continuous buoyancy or distributed buoyancy modules. These methods are used instead of a conventional air can in order to provide the upthrust necessary to keep the riser vertical. The use of continuous or distributed buoyancy implies that each section compensates for its own weight, meaning that the concept is applicable to any water depth for which buoyancy materials are qualified.
The transition between the rigid and the flexible parts is achieved with a subsea connector. An overview of the SIR configuration is given in Fig. 1.
The SIR features the following technical advantages:
- The design relies on field-proven technology such as rotolatches to connect the rigid section to its anchor or standard shallow-water flexible-jumper technology to perform the transition between the rigid section and the floater. Though the SIR can be deployed in any water depth, the cost of the flexible jumper limits the suitability of the SIR for water depths below 1000 m, which should be considered the minimum applicable water depth.
- The SIR is compatible with any installation method.
- The fabrication process of the SIR can be reversed fully, meaning the system can be retrieved for relocation or maintenance purposes.
- The SIR features a small dynamic response, thus leading to less-stringent fatigue and welding requirements.
General Design Principle
Design activities related to the SIR are part of a general integrated-building-block approach, meaning that riser design is performed taking into consideration design, specification, procurement, fabrication, installation, and maintenance requirements. The integrated-design approach follows a logic chart that allows the design team to assess all design parameters and constraints properly and develop a suitable configuration rapidly.
Special attention must be paid to the definition of the buoyancy, because it will drive the overall behavior of the system. Tension should be high enough to ensure that the riser remains locked to its anchor but low enough to achieve a compliant configuration and minimize costs.
The properties of the buoyancy modules (diameter, spacing, and density) will define the bottom tension in the SIR. This value can be up to 150 tons depending on project requirements.
In comparison with a single hybrid riser (SHR), the authors find that the SIR provides room for optimization and features relevant synergies with the interfacing components by reducing applied loads and imposed displacements. These synergies positively affect overall project economics.
The SIR can be installed by several installation methods.
- J-lay: The SIR is constructed offshore and directly on site.
- Towing: The riser is assembled onshore and then towed to the site, where it is upended.
- S-lay: The SIR is constructed near its final location and upending.
- Reeling: The rigid section is assembled onshore and spooled on a reel. During the laying process, the required buoyancy modules are added as the pipe is unspooled from the reel.
In the SIR, installation loads are independent of the water depth. Indeed, as each section compensates for its own weight because of the attached buoyancy, the resulting tension decreases as the string is constructed. This is of particular interest in the case of J-lay installation, where the maximum tension in the hang-off clamp usually is the key driver in the selection of the installation vessel.
The compensation of each section for its own weight has important implications. Specific care must be taken to ensure that the system remains heavy enough that it sinks during installation and that no resonance effect emerges from an improper balance between inertia and submerged weight. This is achieved through a clump weight that is attached to the bottom of the riser. Consequently, maximum tension will occur as the first section, typically a quad joint fitted with the bottom assembly, is being installed. During this step, the upthrust provided by the buoyancy modules will be minimal, meaning that the weight of the clump weight must be recovered in full.
The main difference between an SIR and an SHR is the arrangement of the flexible jumper. For an SHR, the flexible jumper is free-hanging, meaning that the configuration can be set easily in a standby configuration in which the flexible jumper hangs along the rigid section. Such a straightforward approach is not possible for the SIR. Because of the configuration, the flexible jumper cannot simply hang from the rigid part, because that would result either in kinking the pipe at the rigid/flexible pipe-transition point or in flapping loose if not properly secured. A dedicated standby configuration has been developed to ensure that the integrity of the flexible jumper is not jeopardized during the standby period. A dry connection at the pipelay-vessel deck is the simplest solution to connect the flexible jumper to the rigid connection. A subsea connection has been developed to allow for decoupling the rigid-part-installation sequence from the flexible-part-installation sequence. Nevertheless, to facilitate the subsea connection of the flexible jumper on the rigid section, a dedicated flexible-jumper-connection tool has been developed to facilitate the subsea connection of the flexible jumper on the rigid section. The installation sequence with the subsea connection of the flexible jumper is detailed in the complete paper.
The SIR pushes the water-depth limit in a cost-efficient manner. This evolution is made possible by structure simplification and a solution that is much less sensitive to water depth. The analyses performed on actual case studies proved the SIR to be a solution well-suited for industry needs. A procedure for installation was developed that covered all options required for installation and maintenance during the whole field life, including a dedicated tool for safe subsea connection and disconnection of the flexible jumper if required. The proposed sequence is quite similar to those used for SHRs while offering the advantage of limited tension-capability requirements.
Hybrid Ultradeepwater-Riser Configuration Saves Costs, Pushes Depths
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