Subsea Riser Fatigue Monitoring via Acceleration Transfer Functions
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Solution Overview
Problem
Current methods for monitoring riser fatigue in the oil and gas industry require strain or curvature sensors, time synchronous measurements, and finite element analysis, which are not always feasible or cost-effective, especially in subsea environments.
Innovation Solution
A system and method using motion sensors to measure acceleration along a subsea riser, processing these measurements with a transfer function based on Tensioned Timoshenko Beam theory to determine curvature and stress without the need for strain sensors or finite element analysis, allowing for continuous fatigue monitoring.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If strain or curvature sensors are used for direct measurement, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces mechanical strain or curvature sensors with motion sensors (accelerometers) that measure vibration and motion characteristics. The curvature is then derived through signal processing and transfer function analysis rather than direct mechanical measurement, eliminating the need for complex subsea strain sensor installations while maintaining measurement capability
Solution Approach 2:
The patent introduces motion sensors as an intermediary measurement tool that indirectly captures curvature information through vibration measurements. Transfer functions serve as mathematical intermediaries that convert acceleration spectra to curvature spectra, providing a bridge between easily measurable motion parameters and the desired curvature information
2Measurement precision
If finite element analysis and theoretical mode shapes are used, then measurement precision is improved, but device complexity and computational requirements increase
Solution Approach 1:
The patent employs transfer functions that can be determined once through simple FEA or modal analysis and then reused repeatedly for fatigue assessment without requiring repeated complex simulations. This approach makes the complex analysis method economically viable by amortizing the computational cost over many measurements
Solution Approach 2:
The patent performs preliminary determination of transfer functions between acceleration and curvature through FEA or modal analysis before actual fatigue monitoring begins. These pre-determined transfer functions are then applied to field measurements, eliminating the need for real-time complex simulations and enabling continuous practical fatigue assessment
3Measurement precision
If time synchronous measurements are used, then measurement precision is improved, but ease of operation worsens
Solution Approach 1:
The patent replaces time synchronous measurement systems with continuous spectral analysis of acceleration signals. Instead of requiring precise temporal synchronization between multiple sensors, the method uses power spectral density and transfer functions to derive curvature, significantly simplifying the operational requirements while maintaining fatigue assessment accuracy
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
Enables efficient and reliable continuous monitoring of riser fatigue from accelerations, reducing costs and complexity by eliminating the need for strain sensors and complex analysis methods, while providing accurate fatigue assessment.
Implementation Method 1
each sensor includes at least one accelerometer, and preferably two or three accelerometers for measuring acceleration in up to three orthogonal directions
Implementation Method 2
The transfer function is derived using an analytical method based on tensioned Timoshenko beam theory and wave propagation theory
Data Source
AI summary
A system and method is provided for determining curvature for subsea riser system, including but not limited to drilling risers, steel catenary risers, lazy-wave catenary risers and riser jumpers, comprising the steps of: periodically measuring acceleration in a first lateral direction at said vertical position to obtain a first acceleration timetrace processing said first acceleration timetrace to obtain a first acceleration spectra; applying a transfer function to said first acceleration spectra to obtain a first curvature spectra; and processing said first curvature spectra to obtain a first curvature timetrace. Preferably the transfer function is determined by a method comprising the step of modelling the riser as a Tensioned Timoshenko Beam.The curvature may be used to determine stress and fatigue damage in a structure from motions measured at a single location or a combination of motions measured at a single location with or without tension measurement. The method can be used to determine curvature and hence stress and fatigue damage from any source of excitation, for example the excitation at the tension ring by the top tensioner system, and the vortex induced vibration locked in at any water depth.