Drillstring Oscillation Reduction via Open-Loop Speed Pulses
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Solution Overview
Problem
Existing methods for reducing stick-slip oscillations in drillstrings, especially in long drillstrings, are inadequate as they often lead to instability and failure to completely eliminate oscillations due to sensitivity to noise and delay in control signals, and are not effective in preventing re-occurrence of stick-slip.
Innovation Solution
An open-loop controlled speed variation method that applies a predetermined speed pulse to cancel stick-slip oscillations by estimating the dynamic bit speed and matching the pulse phase, which is distinct from continuous torque feedback-based systems, and can be used in conjunction with feedback-based damping systems to enhance stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If continuous torque feedback-based control systems are used to reduce stick-slip oscillations, then oscillation reduction is achieved, but the system becomes sensitive to noise and delay in control signals, leading to instability
Solution Approach 1:
The patent applies periodic speed variations at the natural frequency of the drillstring to counteract stick-slip oscillations. By introducing a speed pulse with amplitude and phase matching the oscillation, the system creates a periodic counter-action that cancels the harmful vibrations without requiring continuous feedback, thereby avoiding sensitivity to noise and delay.
Solution Approach 2:
The patent introduces an intermediary estimation model that predicts bit speed from measured top drive torque and speed. This intermediary approach allows the system to infer oscillation characteristics without directly measuring them at the bit, reducing the need for complex feedback while maintaining effectiveness in counteracting stick-slip.
2Extent of automation
If traditional closed-loop control systems are used, then real-time oscillation control is possible, but the complexity of the system increases and requires direct measurement of drillstring torque
Solution Approach 1:
The patent creates a simplified copy or model of the drillstring dynamics that can be used to estimate bit speed from easily measurable quantities (top drive torque and speed). This copying approach avoids the need for complex direct measurements at the bit while maintaining the ability to control oscillations through speed adjustment.
Solution Approach 2:
The patent replaces the need for complex mechanical measurement systems (direct torque sensors at the bit) with an estimation approach using standard electrical measurements from the top drive. This substitution reduces mechanical complexity while maintaining control capability through mathematical modeling and speed pulse application.
3Reliability
If speed pulses are applied to cancel oscillations, then stick-slip oscillations are reduced, but the timing and amplitude must precisely match the oscillation parameters
Solution Approach 1:
The patent uses feedback from the estimated bit speed (derived from top drive measurements) to continuously adjust the timing and amplitude of speed pulses. This feedback mechanism ensures that the pulses remain synchronized with the oscillation parameters even as they change during drilling, maintaining effectiveness without requiring extremely precise predetermined settings.
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 method effectively reduces or eliminates stick-slip oscillations in drillstrings, including long ones, by using a short-duration speed pulse that matches the oscillation amplitude and phase, reducing the risk of high-frequency instabilities and providing a more stable solution compared to traditional closed-loop control systems.
Implementation Method 1
adding a speed pulse to an operator set speed command to generate an oscillation in the drillstring that cancels the stick-slip oscillation
Implementation Method 2
the drillstring often behaves as a torsional pendulum, i.e. the top of the drillstring rotates with a constant angular velocity, whereas the drill bit performs a rotation with varying angular velocity comprising a constant part and a superimposed torsional vibration
Implementation Method 3
the bit periodically comes to a complete standstill, during which the drillstring is torqued-up until the bit suddenly rotates again and speeds up to an angular velocity exceeding the topdrive speed. This phenomenon is known as stick-slip
Data Source
AI summary
A drillstring with a bit is characterized by controllable variables of vertical and rotational speeds and response variables of axial tension force and torque, referenced to the top of the drillstring. A method of reducing or avoiding at least an axial or a torsional oscillation mode in the drillstring includes:choosing at least one oscillation mode to be controlled;selecting and monitoring a relevant controllable variable and a relevant response variable;determining the oscillation period;estimating from the response variable a dynamic component of bit speed;determining a speed pulse capable of producing a generated oscillation with amplitude substantially equal to the amplitude of the dynamic component of bit speed; andUsing open-loop control to add the speed pulse to an operator set speed command when the dynamic component of bit speed has an amplitude exceeding a threshold level and an anti-phase matching a phase of the generated oscillation.