Piggybacked Subsea Riser Assembly for Faster Offshore Installation
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Solution Overview
Problem
The installation of subsea risers is complex and time-consuming, particularly in dynamic environments, leading to high costs and risks due to fatigue and rupture from repetitive motion, and existing methods require extensive vessel time and may miss weather windows.
Innovation Solution
A method involving a subsea riser assembly where the riser conduit is piggybacked onto an elongate support, with fluid communication maintained through a hinge structure, allowing the free end to be detached and lifted to a riser support, simplifying installation and reducing seabed congestion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional free-hanging rigid risers are used, then installation is simpler, but fatigue and rupture risks increase due to repetitive motion in dynamic environments
Solution Approach 1:
The riser system is divided into multiple sections with intermediate support structures (tension legs, buoyancy modules) that segment the continuous riser into manageable sections. This segmentation reduces the span length and minimizes fatigue from repetitive motion while maintaining installation feasibility.
Solution Approach 2:
The riser configuration is made dynamic by incorporating movable tension legs and adjustable buoyancy modules that can adapt to changing sea conditions and operational requirements. This dynamic adjustment capability reduces fatigue stresses while maintaining system reliability.
2Reliability
If complex riser configurations are used to reduce fatigue, then reliability improves, but installation time and vessel time increase significantly
Solution Approach 1:
Riser sections are pre-assembled with tension legs and buoyancy modules attached before deployment. This preliminary preparation reduces on-site assembly time and minimizes vessel time requirements while maintaining the complex configuration needed for fatigue resistance.
Solution Approach 2:
Multiple riser sections and support structures are nested or bundled together during transport and deployment, allowing compact handling and rapid deployment. This nesting approach reduces installation time while maintaining the complex fatigue-resistant configuration.
3Adaptability or versatility
If risers are installed separately from flowlines requiring dedicated bottom structures, then connection flexibility is improved, but installation complexity and cost increase
Solution Approach 1:
The riser installation is merged with flowline installation by integrating the riser deployment process with the existing flowline laying operation. This combined approach eliminates the need for separate bottom structures and reduces overall installation complexity while maintaining connection flexibility.
Solution Approach 2:
The riser system is designed with multi-functional components that can serve both as flowline connectors and as independently deployable units. This universality allows the same structure to fulfill multiple functions, reducing the need for dedicated bottom structures and simplifying installation.
Data Source
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AI summary
A subsea riser is installed by lowering at least one riser conduit to the seabed when piggybacked to an elongate support that comprises at least one flowline. The elongate support may be a pipeline bundle, which may be attached to one or more towheads in a towable bundle unit. The riser conduit may be in fluid communication with the flowline. At the seabed, a free end portion of the riser conduit is detached from the elongate support by releasing subsea-releasable fastenings. Then, with the elongate support and a root end of the riser remaining at the seabed, the detached free end portion of the riser conduit is lifted away from the elongate support to a riser support, such as a platform, an FPSO or a buoy.