TECH SPOTLIGHT
Understanding and Mitigating High Frequency Torsional Oscillation (HFTO)/Torsional Resonance
Here, James Crowley, Director at Rockatek Ltd, discusses how Rockatek are leading the way helping their clients overcome damaging downhole dynamics and how they are continuing their own R&D efforts to develope cutting edge technology for the future.
If you are aware of the abbreviation HFTO, you are probably very aware of how seriously it must be taken for all operators of downhole drilling systems.
High Frequency Torsional Oscillation – HFTO (or Torsional Resonance as it is often referred to) is not a new phenomenon, but over the last few years, many service companies and operators have started to acknowledge the presence of HFTO and are becoming increasingly aware of the extreme power dissipated within downhole tools when exposed to HFTO.
However, the understanding of HFTO fundamentals does not (yet) appear to have grown amongst the wider industry. It is still a partly misunderstood issue.
In its most basic form, HFTO is a type of resonance. All systems have resonant frequencies and in the case of HFTO we are effectively talking about a torsional resonance at one or more system harmonics.
However, as with everything operating in the downhole environment, the scenario becomes vastly more complicated.
• What is the difference between the theoretical natural frequency and the
real-life damped natural frequency of the system? • Is HFTO is a natural frequency event or a forced frequency problem? • What effect do the drilling BHA, formation, drilling mud and drilling parameters play? • What types of downhole event can excite HFTO? • How does HFTO interact with other drilling dysfunctions?
Rockatek is leading the way and helping its clients develop technology, systems, and best practices to begin to mitigate HFTO in real world applications. We are able to do this because we have been studying HFTO for the last 15 years. We understand the underlying theory and physics, we understand the destructive power in drilling environments and we have a great deal of experience in avoiding and mitigating it.
Using our various in house HFTO test rigs, we are physically simulating HFTO on a component and full assembly basis.
• Rockatek’s TR1 medium size HFTO rig can simultaneously apply constant rpm of up to 200 rpm and HFTO of up to 200Hz to medium sized test pieces and assemblies.
• Rockatek’s TR2 scale HFTO test rig can test scale models of complete BHA’s at HFTO frequencies up to 150Hz.
• Rockatek’s 10M rig has the capability to test full scale drilling tools up to 60ft in length at HFTO frequencies up to 100Hz.
How this behaviour affects working systems and how damaging it can be
to structural parts is astonishing.
Our real life test simulations not only demonstrate first-hand how HFTO occurs, what affects its severity and what can be done to mitigate it, but it allows Rockatek to continue to advance its understanding and research into such an important phenomenon in the downhole drilling sector. This R&D effort continues to help Rockatek serve its clients and to continue its own learning and developments.
With an ever-increasing emphasis on reducing costs, there is absolute necessity to increase rate of penetration (ROP – how fast drilling occurs) and minimise non-productive time (NPT).
Rockatek’s experience, expertise and ongoing research and development into downhole dynamics supports our longstanding belief that HFTO is a primary cause of downhole drilling tool failures and associated NPT.
With this valuable information in hand, new technology can be developed and drilling practices can be altered to help operators and service companies drill faster and more efficiently with lower repair and maintenance costs of downhole equipment and with less NPT during the downhole operations.
q +44 1452 379844 +44 7503 474599
E
enquiries@rockatek.com H
www.rockatek.com
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www.sosmagazine.biz October 2020
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