Coiled Tubing Fatigue Service Life Criterion

 The residual strength and fatigue life of coiled tubing are greatly affected by external wall friction and inner wall sand flushing in running, pulling up and sand flushing conditions.

When coiled tubing fails, it is a serious challenge to operate safely and efficiently, resulting in irreparable losses.

In order to accurately predict the residual strength and fatigue life of coiled tubing during service, this paper studies from the following three aspects:

Firstly, the wear degree of coiled tubing outer wall during running and pulling up operations was studied by using the ‘wear-efficiency’ model between drill pipe-casing during drilling.

Workbench was used to analyze the influence of wear rate, outer diameter, wall thickness and other sensitive factors on coiled tubing residual strength.

The theory of elasticity is applied to analyze the theoretical algorithm of coiled tubing residual strength.

By comparing the residual strength of the three wear models under the same conditions, the residual strength calculation method which is relatively close to the actual coiled tubing wear is obtained.

And the wear condition of coiled tubing is predicted.

Secondly, Fluent is used to analyze the influence of mass flow rate, sand particle size, sand ratio and other factors on the sand flushing rate of coiled tubing in reverse circulation sand flushing condition. By analyzing the relationship between sand flushing rate and wall thickness loss, the influence of above  fact or son residual strength of coiled tubing sand flushing wear was studied.

Finally, according to Manson-Coffin model, Miner rule and neutral zone hypothesis, two criteria of coiled tubing fatigue service life including wear, sand flushing and fatigue damage were established. Fatigue performance experiments were carried out to obtain the fatigue life model parameters of CT110 coiled tubing.

And an example analysis of fatigue life and residual strength of coiled tubing under a service log was carried out.

The results show that the residual strength of coiled tubing decreases nonlinearly with the increase of wear rate and outer diameter, and increases nonlinearly with the in crease of wall thickness.

The sensitivity of influencing factors to residual strength of coiled tubing is in the order of wear rate > wall thickness > outer diameter.

When the wear rate is less than15%,eccentric cylinder wear can replace crescent wear.

When the wear rate is more than 15%，uniform wear can replace crescent wear.

The first damaged part of down hole wear coiled tubing is also different with different wear rate.

In general, the sand flushing rate and wall thickness loss increase with the increase of mass flow rate, particle size and sand ratio, while the residual strength decreases.

The effects of sensitive factors on coiled tubing sand flushing rate, average wall thickness loss and residual strength are as follows: sand particle size >sand ratio > sand mass flow rate.

In a service log, the number of theoretical trips for CT110was11.

The more operations, the shorter the coiled tubing life and the lower the cross section limit load.

This study can provide technical guidance for residual strength and fatigue life monitoring of coiled tubing in actual service.

According to the fatigue analysis of the coiled tubing operation above, it can be known that the working positions of the coiled tubing during the tripping operation are: drum-guide frame-injection port-injection port-guide frame-drum, and the process of entering and pulling out each time is called a Complete operating cycle, a total of six fatigue cycles are experienced in each complete operating cycle.

According to the field investigation of coiled tubing operations, there is a certain amount of plastic deformation in these six cycles, and the existing materials show that this deformation belongs to the category of low cycle fatigue.

The fatigue analysis of coiled tubing without initial defects can be realized by conventional plastic low cycle fatigue analysis.

From the perspective of damage mechanics theory, fatigue damage is the process of plastic deformation energy dissipation.

As the number of cycles increases, the strain energy accumulates to a critical point and causes fatigue damage.

According to the material properties of coiled tubing, fatigue test parameters and stress-strain analysis results, the Manson-Coffin formula shown in formula (4-3) is used for analysis:

In the formula, ℇ_a is the strain amplitude, mm;ℇ_p is the plastic strain amplitude, mm; ℇ_e is the elastic strain amplitude, mm; E is the elastic modulus, MPa; N_f  is the failure cycle number; b is the fatigue strength exponent, dimensionless; σ_f is fatigue strength coefficient, dimensionless; c is fatigue ductility index, dimensionless; ℇ_f  is fatigue plasticity coefficient, dimensionless.

After fitting the constants b and c, the ℇ- N curve can be obtained. According to the actual strain level of the coiled tubing, the corresponding load times are obtained from the curve, which is the fatigue life of the coiled tubing.

In the well entry operation, the coiled tubing is straightened from the bending state at the drum, which is the first deformation, and the straightened coiled tubing enters the guide frame, which is the second deformation, and the coiled tubing on the guide frame is bent again.

It is the third deformation, and then enters the oil and gas well; the same is true when the coiled tubing is pulled out, and a total of 6 deformations occur in one tripping operation.

Moreover, due to the different tension and compression states of different parts of the coiled tubing during the deformation process, the first deformation of the outer side is in the compressed state, and the first deformation of the inner side is in the tension state, and then calculated in turn.

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