Subsea Riser Joint with Negative Stiffness Force Elements
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Solution Overview
Problem
Subsea risers experience cyclic bending stresses due to vessel movements, leading to fatigue issues at the wellhead connection, as existing flex joints and bend restrictors only resist bending but do not effectively reduce bending moments.
Innovation Solution
A joint with flexible means and force elements that provide a 'negative stiffness' to counteract bending moments by applying forces that increase the angle deviation from the neutral position, reducing the overall bending stress on the riser connection to the subsea installation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a flex joint with certain stiffness is used to resist bending, then the joint can force realignment of the riser back to neutral position, but the bending moment is not reduced and cyclic bending stresses cause fatigue problems at the wellhead
Solution Approach 1:
The patent inverts the traditional approach by using force elements that apply forces in the same direction as the displacement, creating negative stiffness. Instead of resisting bending to return to neutral position, the joint is designed to amplify displacement and reduce bending moments, thereby solving the fatigue problem at the wellhead while maintaining bending resistance capability
Solution Approach 2:
The patent changes the stiffness parameter from positive (traditional flex joint) to negative (invention). By configuring force elements to apply forces in the direction of displacement rather than opposing it, the joint exhibits negative stiffness behavior that reduces bending moments and cyclic stresses at the wellhead connection
2Stress or pressure
If a bend restrictor is used to resist bending and avoid point stresses, then point stresses at the connector are avoided, but the bending moment as such is not reduced
Solution Approach 1:
The patent changes the mechanical parameter of stiffness from positive to negative. The force elements are configured to apply forces that increase with displacement in the same direction, creating negative stiffness that simultaneously reduces both point stresses at the connector and the overall bending moment in the riser
3Force
If force elements are configured to apply force in the same direction as displacement (negative stiffness), then bending moments are reduced, but the joint must be designed to handle increased lateral displacement
Solution Approach 1:
The patent inverts the traditional force application approach. Instead of force elements opposing displacement to limit lateral movement, the force elements are configured to apply forces in the same direction as displacement. This negative stiffness approach reduces bending moments while the flexible means accommodate the resulting lateral displacement
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The joint effectively reduces bending moments at the connection point, extending the life of subsea installations by mitigating the cyclic stresses induced by vessel movements, and allowing for relative angular displacement between riser parts.
Implementation Method 1
The force means are configured to add a force to the one end when it is moved out of the neutral position. The force added is applied in the same direction as the direction of the movement of the one end out of the neutral position. The force will try to bend the joint further out from a neutral position.
Implementation Method 2
The joint comprises flexible means allowing a first end of the joint to be lateral displaced relative a second end of the joint.
Data Source
AI summary
The present invention relates to a riser joint for a riser with a joint connecting two parts of a riser where the two parts are allowed angular displacement. According to the invention the riser joint comprises means for connection to the two parts of the riser at a distance from the joint, and means for adding a force between the two parts. The invention also relates to a method for reducing bending moments in a riser at a connection between the riser and a subsea installation.