Every coiled tubing string has a limited lifespan because of the damage caused by fatigue accumulation and diameter growth.
What is Coiled Tubing Fatigue?
Every time coiled tubing is cycled a small amount of fatigue is added to the string.
There are two main points where fatigue usually accumulates during the operation: over the coiled tubing reel and over the injector.
Usually, coiled tubing software is used to track fatigue accumulation and predict the amount of life left on a coiled tubing string.
When coiled tubing fatigue reaches a certain limit (for example 80%) the string is retired to avoid possible failure during the operation.
Some companies also use the number of running meters as a retirement criterion for coiled tubing strings but this method is not very accurate.
Mechanical damage and corrosion can decrease coiled tubing life even further and should be taken into account when deciding when the coiled tubing should be taken out of service.
Coiled tubing failures can be very expensive and potentially can result in injuries.
That’s why every coiled tubing company tracks fatigue one way or another.
Related: 18 Most Common Coiled Tubing Applications
What is Coiled Tubing Diameter Growth?
It is also important to keep track of diameter growth which is caused by coiled tubing internal pressure.
Diameter growth will decrease the wall thickness of the coil and in very severe cases can make it challenging to run the coiled tubing through the stuffing box.
It can also impact how the gripper blocks on the injector handle the coil.
How to Increase Coiled Tubing Life?
1. Use higher grade coiled tubing
The amount of life you can get from the coiled tubing string depends on its grade.
However, you need to keep in mind that higher grade pipe doesn’t perform well in sour wells.
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2. Decrease pumping pressures
Whenever coiled tubing is moving, internal pressure has a significant impact on coiled tubing life.
Pumping at high pressures when the coil is stationary doesn’t have an effect on coiled tubing fatigue.
3. Increase the bending radius
Increasing the size of the gooseneck and reel will allow you to get more life from your coil.
A large bending radius is a reason why only an insignificant amount of fatigue is added to the coiled tubing when it bends downhole while going from the vertical and into the horizontal section of the well.
4. Decrease coiled tubing diameter
You can get more life from a smaller size coil.
However, it is important to keep in mind that decreasing the size of the coiled tubing might increase pumping pressure which in turn will have a negative effect on the coil’s life.
5. Increase coiled tubing wall thickness
Coiled tubing fatigue results in small micro-cracks that originate from the inner wall of the coil and with time propagate to the surface of the coiled tubing.
That’s when you get coiled tubing failure.
Increasing coiled tubing wall thickness will delay this process and will give you more life.
Related: Top 5 Chemicals Used for Coiled Tubing Operations
Coiled Tubing Welds and Fatigue
Almost every coiled tubing string is made up of long sections of pipe connected by specially designed bias welds.
Coiled tubing welds are usually the weakest points in the coiled tubing string because they have a slightly different composition than the rest of the pipe.
That’s why welds are usually de-rated when it comes to fatigue.
In other words, welds are limiting factors when it comes to coiled tubing life and everything should be done to avoid cycling coil over the welds.
How About Quench and Temper Technology?
The quench and temper method consists of heating the coiled tubing and then quickly cooling it down.
This creates finer size grains in the metal.
This results in welds that have very similar properties to the rest of the coiled tubing and in theory don’t need derating.
This can significantly increase the amount of life you can get from coiled tubing.
Related: How Coiled Tubing Milling Operations Are Performed?