Articulated Pliant Link for Fatigue-Resistant Subsea Risers
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Solution Overview
Problem
Conventional subsea riser systems, particularly free-hanging rigid risers, are susceptible to fatigue-induced failure due to dynamic motions from supporting vessels and seawater, leading to high maintenance costs and potential rupture, especially in challenging environments like high sea states and strong currents.
Innovation Solution
A pliant link with an articulated spine and a pliant pipe is introduced, allowing limited relative motion between upper and lower sections of the riser, decoupling from vessel motion and distributing loads through interconnected rigid segments while maintaining fluid communication, thereby reducing stress and fatigue.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a free-hanging rigid riser is used, then installation is simple and cost-effective, but the riser is susceptible to fatigue-induced failure due to dynamic motions
Solution Approach 1:
The riser is divided into two distinct sections: an upper rigid section for simplicity and a lower flexible section for fatigue resistance. The rigid upper riser maintains installation advantages while the flexible lower riser absorbs dynamic motions and reduces fatigue at the touch-down point, resolving the contradiction between ease of installation and fatigue resistance.
Solution Approach 2:
Different sections of the riser are assigned different material properties tailored to their specific functional requirements. The upper section uses rigid material for structural stability and easy installation, while the lower section uses flexible material to withstand dynamic environmental loads, thereby achieving both installation simplicity and fatigue resistance through localized material optimization.
2Reliability
If a flexible pipe is used, then fatigue resistance is improved, but the minimum bend radius is reduced to 3-6 metres
Solution Approach 1:
The riser system segments the flexible pipe requirement to only the lower section where it contacts the seabed, while the upper section remains rigid with a larger effective bend radius during installation. This allows fatigue resistance where needed without constraining the overall riser geometry during installation operations.
3Strength
If composite pipes are used, then bending strain capacity is increased, but the pipes cannot flex without commensurate increase in bending stress
Solution Approach 1:
The riser combines composite material advantages in the upper section for high bending strain capacity while using traditional flexible pipe in the lower section that can accommodate bending stresses. This segmentation allows the system to leverage the high strength-to-weight ratio of composite pipes without exposing them to excessive bending stresses during dynamic operation.
Data Source
AI summary
A pliant link to mitigate fatigue-inducing motion of a subsea catenary riser has an articulated spine having a longitudinal series of interconnected rigid segments. The spine can be coupled to upper and lower sections of the riser to transmit loads along the riser through the link on a load path that extends through the segments. The link also has a pliant pipe terminating in end fittings that can be joined, respectively, to the upper and lower sections of the riser for fluid communication along the riser through the link.