Riser Stress Reduction via Active Force Control
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Solution Overview
Problem
Subsea risers experience cyclic bending, tension, compression, and torsion stresses due to vessel movements, leading to fatigue issues at wellheads and other connection points, despite existing flex joints and heave compensators, which fail to maintain constant stress conditions under real offshore conditions.
Innovation Solution
A system comprising real-time stress monitoring sensors and an actuating system that applies forces to counteract angular and axial displacements of riser parts, using a control system to calculate and adjust forces based on current stress data, thereby reducing bending moments and other stresses at desired positions along the riser.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a flex joint is used to allow angular displacement, then the riser can accommodate vessel movements, but bending moments are created at the wellhead due to the lateral component of angular deviation
Solution Approach 1:
The system applies a counteracting force through the actuating system that opposes the bending moment created by angular displacement. The force is calculated based on the angular deviation and applied in the opposite direction to neutralize the bending moment at the wellhead, preventing fatigue damage while maintaining angular adaptability.
Solution Approach 2:
The control system continuously monitors the angular position of the flex joint and adjusts the actuating force in real-time based on the measured deviation from the neutral position. This closed-loop feedback ensures that the counteracting force dynamically compensates for vessel movements, maintaining zero bending moment at the wellhead.
2Stress or pressure
If an actuating system applies force to counteract angular displacement, then bending moments are reduced, but the system complexity increases
Solution Approach 1:
The actuating system is designed to perform multiple functions: it counteracts bending moments, maintains riser tension, and provides positional stabilization. By integrating these functions into a single system, the patent reduces overall complexity compared to having separate systems for each function.
Solution Approach 2:
The control system acts as an intermediary that processes sensor data and translates it into appropriate actuating forces. This intermediary layer simplifies the connection between the sensor and actuator, providing intelligent force calculation and adjustment without requiring complex mechanical linkages.
3Measurement precision
If real-time stress monitoring is implemented, then stress reduction accuracy is improved, but the device complexity increases
Solution Approach 1:
Real-time stress sensors provide continuous feedback to the control system, enabling precise measurement of the bending moment at the wellhead. The control system uses this feedback to dynamically adjust the actuating force, ensuring accurate stress reduction while maintaining system simplicity through automated control.
Solution Approach 2:
The system uses its own stress measurements to automatically regulate its performance. The control system calculates the required counteracting force based on real-time sensor data and adjusts the actuating system accordingly, enabling the system to self-regulate without external intervention and reducing the need for complex external control mechanisms.
Data Source
AI summary
Stress reducing system and associated method for reducing stresses at a desired position in an offshore production or drilling system, the offshore production or drilling system comprising: a seabed structure, a floating structure and a riser (24) extending there between, the riser being tensioned, the riser (24) comprising at least a first part (45) and a second part (46), which second part (46) is connected to the first part (45) via a flexible connection (20) allowing an axial, angular and/or rotational movement between the first and second parts (45, 46), said stress reducing system comprises:—a first sensor (41) for real-time monitoring of stresses at the desired position, positioned at or close to the desired position (20),—an actuating system (42) arranged at the flexible connection (20, the actuating system (42) being connected to said first and second parts (45, 46), and wherein the actuating system (45, 46) is configured to apply a force to the first or second part (45, 46) when the first and second parts (45, 46) are moved out of a neutral position,—a control system (40) adapted to receive monitoring data from the first sensor (41), wherein the control system (40) is connected to the actuating system (42) and is able of providing instruction signals to the actuating system (42), wherein the control system (40), based on said monitoring data from the first sensor (41), is able to calculate a real-time set of data for control of the applied force of the actuating system (42) and instructing the actuating system (42) to act accordingly, such as to reduce the stress at said desired position.