Vibration Mechanisms In Drillstring by SperrySun

Vibration in drillstring is one of major downtime associated with drilling operation. Therefore, it is very important to learn and understand what each vibration mechanism is. This VDO created by Sperry Drilling Services well explains various of vibration modes that is happened downhole with the drillstring. You can watch the VDO here and we also provide full VDO transcription to help learns get more understanding about the content if the learners cannot catch all wordings from this VDO.

Full VDO Transcript

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At Sperry where time is money, improving drilling performance starts with the integrity of the drill string. A recent worldwide survey puts the cost of down time to the industry at $31 billion with down hole failure as a significant percentage. Since vibration at high shock loads are a major factor in down hole tool failure and can also cause ring repair and hole problems, prevention is a very high priority.

When you look at these obvious consequences of drilling vibration and high shock loads, it’s not surprising how much you can boost overall drilling performance when you use a proactive approach to preventing or reducing these destructive forces. Since the first step in solving a problem is giving it a name, let’s focus on how we describe these forces. There are three modes of vibration; axial, lateral and torsional – which are the directions the drill string moves when it runs into trouble down hole.

In the axial mood, the vibration is longitudinal motion along the drill string resulting in varying tension and occasionally compression tension reversals.

In the lateral mode, the vibration is side to side motion that causes flexing or bending of component, again leading to stress reversals where one side of the pipe will be in a different tensional state than the other.

In the torsional mode, the vibration is resistance to rotation resulting in twisting as torque is applied to overcome resistance. So that’s how the drill string reacts if it runs into any of the nine different kinds of trouble we called vibration mechanisms.

The first one called stick-slip is irregular drawstring rotation; the vibration mode is torsional as the bit stops rotating momentarily at regular intervals causing the string to periodically torque up and then spin free at 2 to 15 times the average surface rpm, causing severe damage to the bit and BHA.

The second mechanism is called Bit Bounce; it’s the axial or longitudinal vibration of the drill string that typically happens with free Cone bits and hard rock formations. Bit bounce damages the drill bit cutting structure, bearings and seals, and results in flexing of the drill string causing even more damage from axial and lateral shocks.

Next is Bit Whirl; it occurs when the bit has cut itself a hole larger than its own diameter, allowing it to wander around in the wellbore instead of simply rotating around its natural center. Excessive side cutting creates an over gauge hole that in turn increases the tendency for the bed and BHA to continue whirling.

Number four mechanism is BHA Whirl; the eccentric rotation of the BHA around a point other than its geometric Center in a complex motion generating lateral displacements, shocks and increased friction against the wellbore, occurring as forward or backward whirl is the mechanism that constitutes the main cause of BHA and down hole tool failure. Next, lateral shocks; describes what happens when the vibration mechanisms of bit bounce, BHA Whirl or motor coupling become so extreme. They cause the release of energy builds up in the drill string through several large lateral shock impacts. Unlike BHA Whirl, where the motion settles to a steady state, in this case the lateral shocks occur randomly. Torsional resonance is specifically drill collar torsional resonance and causes of juddering vibration of the drill string. It typically occurs in very hard rock being drilled with the PDC bit.

Parametric Resonance is our name for the severe lateral vibrations generated by axial vibrations, caused by the interaction of the bit with the formation which results in fluctuations of weight on the bit. Bit Chatter is the high-frequency lateral and torsional vibration of the bit and BHA, caused by a slightly eccentric bit rotation where there is cutting interference with the bottom hole cutting pattern.

The cutters right up under the ridge between previously cut grooves and then draw back into the groove. Modal Coupling describes vibration occurring at all three modes; axial, torsional, and lateral simultaneously. It creates axial and torsional oscillations and high lateral shocks along the BHA. The most extreme form of vibration is usually results from failing to control one of the other vibration mechanisms.

Okay, now that we’ve identified the problem, we’re going to do about it? The solution starts with sensors that are able to measure the different modes of vibration and enable us to identify the different mechanisms. Accelerometer can be mounted in tools to measure axial and lateral shock in vibration. Sensors that measure changes in down hole rotary speed and changes in torque at surface can be used to identify torsional vibration. Sperry drilling services extensive experience with our wide range of vibration tools has proven that with all the factors and variations that affect vibration, real time monitoring is the key to managing the problem. We also offer software that provides an automatic recalculation of the natural and harmonic resonance frequencies of the BHA. This is based on change in weight on bit, hole angle, dog legs severity, hole size and mud weight, and reflects the actual drilling conditions encountered. All of the critical rpm values can be displayed at the rig floor, so this form of vibration can be minimized. A vibration monitor analyzes measurement from the down hole tools, can automatically determine the vibration mechanism and provide advice on changing drilling parameters to stop it. of course measuring and responding to the complex array of vibrations while drilling is a real challenge, that’s why Sperry provide specialist from the applied rolling technology service or ADT to help you determine the vibration mode and mechanism that’s causing your problems and come up with ways to mitigate it.

The ADT optimization service will also identify ways of preventing the vibration from occurring on subsequent bit runs or wells. By modeling the expected conditions in making the correct measurements while drilling, we can virtually eliminated vibration. This significant step toward optimizing the   drilling process can save the industry a huge chunk of the 31 billion dollar cost of downtime.For more information, please contact sperry@halliburton.com

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