Drill String PI Control for Fundamental and Higher-Mode Stick-Slip
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Solution Overview
Problem
Current methods for damping stick-slip oscillations in drill strings, such as the SOFT TORQUE system and PID control, are inadequate as they either require additional sensors, increase complexity and cost, or fail to effectively inhibit both fundamental and higher mode stick-slip oscillations, particularly in long drill strings where the second natural mode becomes unstable.
Innovation Solution
A PI or PID controller is used to damp stick-slip oscillations by tuning the drilling mechanism to reduce effective inertia and adjust the PI controller settings based on string geometry or real-time monitoring, allowing for selective damping of both fundamental and higher mode oscillations without the need for additional torque or drive torque feedback sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a PI or PID controller is tuned to dampen the fundamental stick-slip oscillation mode, then the fundamental mode is effectively damped, but the second natural mode becomes unstable and grows in amplitude
Solution Approach 1:
The patent applies dynamics by making the damping characteristics of the drilling mechanism adjustable and adaptive. The controller dynamically modifies the damping behavior to target specific oscillation modes (fundamental or higher modes) based on real-time conditions, transforming a static system into a dynamic one that can respond to different vibrational states without requiring additional sensors
Solution Approach 2:
The patent changes parameters by adjusting the damping characteristics through controller tuning (modifying PI or PID parameters) and by reducing effective inertia through gear selection. These parameter changes allow the system to shift its natural frequency and damping properties to match and counteract specific stick-slip modes, enabling selective damping of either fundamental or higher mode oscillations
2Measurement precision
If additional sensors (torque or drive torque feedback) are added to detect and control stick-slip, then measurement precision improves, but device complexity and cost increase
Solution Approach 1:
The patent applies self-service by enabling the drilling mechanism to use its own operational parameters (rotation speed, inertia properties) to detect and control stick-slip oscillations. The system monitors its own performance through existing controllers without requiring external sensors, making the system self-diagnostic and self-regulating regarding stick-slip conditions
Solution Approach 2:
The patent implements feedback using existing controller data (rotation speed measurements) to detect stick-slip oscillations and adjust control parameters in real-time. The feedback loop uses readily available operational data to modulate the damping characteristics, eliminating the need for additional torque sensors while maintaining effective stick-slip control
3Length of moving object
If the drill string length increases beyond 5km, then the fundamental stick-slip period exceeds 5-6s, but higher mode oscillations become unstable and grow in amplitude
Solution Approach 1:
For long drill strings, the patent changes parameters by reducing the effective inertia of the drilling mechanism through gear selection and adjusting the damping characteristics through controller tuning. These parameter changes shift the system's natural frequency and damping properties to counteract the instability of higher mode oscillations that occur in extremely long drill strings
Solution Approach 2:
The patent applies dynamics by making the damping characteristics adjustable in response to drill string length. The system dynamically adapts its control parameters based on the fundamental stick-slip period measurement, enabling it to effectively dampen either fundamental or higher mode oscillations depending on the specific conditions created by the drill string length
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
This approach achieves significant damping of stick-slip oscillations across a wide frequency range, effectively inhibiting both fundamental and higher mode oscillations, even in long drill strings, without increasing complexity or cost, and maintains drilling performance by adjusting the damping characteristics of the drilling mechanism.
Implementation Method 1
tuning the drilling mechanism to reduce effective inertia
Implementation Method 2
damping stick-slip oscillations
Data Source
Figure 1
Figure 2
Figure 3~5
AI summary
A method of damping stick-slip oscillations in a drill string (12), which method comprises the steps of: (a) damping said stick-slip oscillations using a drilling mechanism (30) at the top of said drill string (12); and (b) controlling the speed of rotation of said drilling mechanism (30) using a PI controller; characterised by the steps of (c) tuning said PI controller so that said drilling mechanism (30) absorbs most torsional energy from said drill string (12) at a frequency that is at or near a fundamental frequency of said stick-slip oscillations; and (d) reducing an effective inertia of said drilling mechanism (30), whereby a damping effect of said drilling mechanism is increased for frequencies above said fundamental frequency.