Comprehensive Studies on the Factors Affecting Sucker · PDF fileComprehensive Studies on the Factors Affecting Sucker Rod-Coupling Make ... field applications? ... shall be included

Gas Well Deliquification WorkshopSheraton Hotel, Denver, Colorado

February 23 - 26, 2009

Comprehensive Studies on the Factors Affecting Sucker

Rod-Coupling Make up

Erik Tietz, P.E.Arun Sriraman

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Why did UPCO perform this project?

• Marketing research has revealed the fact that there has been a lot of make-up failures in this industry

• Some of the questions which were raised and led us into this project are as follows:

– Are the current displacement values adequate for a proper make up?

– What type of lubrication (dry or wet face) is appropriate for field applications?

– How many times can a sucker rod be made up without compromising the load carrying capabilities?

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Goals and plan of action

• Phase 1 - Testing of thread lubricants

• Phase 2 - Verifying the current displacement values (analysis of rod body design strength ratings and achievable load carrying capabilities of a made up rod string)

• Phase 3 – Analyzing the effects of dry and wet face make up

• Phase 4 – Studying the decay of sucker rods (decrease in the load carrying capability of a sucker rod after multiple make ups)

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Engineering concepts used in this project

The core Engineering concepts used in this project are as follows:

A. Circumferential Displacement

B. Strain (Normal and Shear Strain)

C. Types of strain in a make up process

D. Relation between stress and strain (Modulus of Elasticity)

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Circumferential Displacement

Minimum Displacement value

Maximum Displacement value

Circumferential displacement:API states that circumferential displacement is the distance measured after makeup, between the displaced parts of a vertical line scribed across the external surfaces of the box and pin when they are in a shouldered hand tight relationship prior to makeup.

Hand tight level (No displacement)

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Types of strain in a make up process

A

A

A - Normal strain (straight pull) acting in a direction parallel to the axis of the rod body. Normal forces are a very desirable attribute in a sucker rod- coupling make up process

B - Shear strain (twist) acting on the pin which is a detrimental attribute in a sucker rod-coupling make up process.

BStrain is the amount of stretch of the sucker rod pin when it is subjected to a load.

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Strain Gauges

• The pictures below show examples of shear and normal strain gauges (both positive and negative).

Shear Normal

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Relation between Stress and Strain

In the elastic region of the curve,Stress / Strain = MOE (Modulus of Elasticity)

• Units = PSI

Plastic Region

Elastic Region

Measured from the test

Known

Unknown

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Testing Apparatus (Displacement Testing Machine (DTM)

How was the test performed?

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Displacement Testing Machine (DTM)

Displacement testing machine – A brief overview• The machine can produce sufficient amount of torque to

shear a 1 1/8 ” sucker rod pin.

• The gear ratio is slow enough to perform a test in 30 seconds.

• It has a 1 HP DC drive with variable drive capability.

• The machine is coupled with a data acquisition system which has the capability to sample 10000 data points per second.

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Measurements

• The three key measurements from the tests are as follows:– Torque applied– Shear Strain– Normal Strain

• The torque applied (positive & negative) during the displacement process was measured using a load cell with a 12” moment arm.

Load cell set up with a 12”moment arm for measuring

the torque

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Sample output from the test

Normal force curve

Shear force curve

Torque curve

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First phase of project

• A total of 13 tests (12 thread lubricant test + 1 no lube test) were conducted under laboratory conditions.

• Strain gauge testing was used in establishing the following three groups of lubricant.

– Best Performing lubricants.– Medium performing lubricants– Low performing lubricant

• The best performing thread lubricant was used in performing phases 2, 3, & 4 of the project.

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(Phase 2) – Flow chart

Conduct Test (Make up sucker rod and coupling to the respective

displacement values)

Calculate the preload stress (clamping force) in the undercut OD

Compare the preload stress (PSI) with the design yield of the sucker rod

Compute the values in % (effectiveness of the

design strength ratings of sucker rod).

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Results –Sucker Rod grades

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Results –Sucker Rod size

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Results based on design yield

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Data Analysis

• Average design rating on all new run displacement values is 91.17 %.

• Average design rating on all re-run displacement values is 81.83 %.

Run % Deviation from design strength ratings

Average New Run 9 %

Average Re-run 18%

• Do we compensate for this deviation in our rod string design?No

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Perspective as an end user

What are the common causes of failures in our industry?

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The phenomenon (% deviation of displacement values from design strength ratings of the sucker rod) occurs at the pin.

Is it Value added to compensate the service factor of the sucker rods for the deviations in the current displacement values?

Yes, due to the following reasons

- Increase in the efficiency of rod string design.- Address one of the very common causes of failures in our industry (pin breaks – 60%).

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Phase 3 – Wet and dry face make up

• Dry Face:The thread lubricant was applied only on the threads of the sucker rods (Scenario 1).

• Wet Face:The thread lubricant was applied on the threads and faces of sucker rod and coupling. (Scenario 2).

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• A set of 7/8” HS (High Strength) sucker rods was used for scenarios 1 & 2 of this phase.

• The torque, normal and shear strain values were measured at various stages of the displacement test.

• The test was conducted with displacement values in increments of 0.125” .

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CD #5 – Min. displacement

CD #7 – Max. displacement

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Typical output from the test

Normal Strain

Torque

Shear Strain

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Graph of dry vs wet faceDry face Wet face

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Pros and cons of dry and wet face

Dry Face Wet Face1 The achievable torque was 40.77 %

higher than wet face.

2 The applied torque goes to friction fit of the joint.

The applied torque goes in shearing the pin (Shear strain 23.62 % higher).

3 Sucker rod pin is not yielded and is in good serviceable condition for future make ups.

The sucker rod pin is permanently yielded during the first make up. What happens to the load carrying capabilities of the sucker rod for the subsequent runs?

4 The displacement values / cards are no longer valid because of the permanent twist during the first run of the sucker rod.

5 Normal strain is 9% higher due to actual shear of the pin during the first run.

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• The achievable torque on the dry face test was 40.77 % higher than the wet face test.

• Normal strain remained 9 % higher for wet face tests.

• Shear strain was significantly elevated in the wet face tests (23.62 %) when compared to dry face.

• Conclusion:Considering the facts established from the test results, dry face make up is the best and only option for sucker rod coupling make up.

Results of dry and wet face make ups

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Phase 4 – Decay of Sucker Rods

• Decay can be defined as the decrease in the load carrying capability of the sucker rod after multiple make ups.

• How was the test performed?

A sample of 3/4”, 7/8” and 1” sucker rods was tested for repeated make ups.

The samples was tested for 16 consecutive make upsor until failure.

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Run # Scenario Displacement Value

1 New Rod-New Coupling New Run displacement

2 Old Rod – Old coupling Re-run displacement



5 Old Rod – New Coupling Re-run displacement




9 Old Rod – New Coupling Re-run displacement

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– The coupling was changed after every 4 tests.

– The new run displacement values were used for the first test and the re-run displacement values were for all the subsequent make ups / tests.

– The torque, normal and shear strain values from each test were used for establishing the decay curves.


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Outputs from the test – Torque (1”)

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Perspective as an end user• Results of decay curve analysis:

An average increase in torque values by 22.98 % (13.63 % - 32.32 %)

What does this mean to end users in this industry?• There is an average variation of 23 % in torque based upon the

usage of sucker rods.

• Note: There is a 74 % variation in torque values in the choice of thread lubricant selected.

• Conclusion: Due to the in-built variation of torque in this process (sucker rod-coupling make up), the following practice is strictly recommended to maximize the Mean Time between Failures (MTBF).

• Torque providing devices and methods should not be adjusted based upon torque values. Instead, the sucker rod and coupling should be made up using circumferential displacement.

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Normal Strain – 1”

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• A decrease in Normal Strain (load carrying capability of the sucker rod) by 14.06% (7.22 % - 20.90 %).

• The sucker rod thread burnish and smoothen out in the first few make ups.

• The threads tend to yield after the first few make ups because of the saturation level in the burnishing effect of the threads.

• This is a justification for using two different types of displacement cards for new and re-run make ups.

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Shear Strain – 1”

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Perspective as an end userWhat is the optimal number of make ups on a sucker rod?

Size No of make ups or runs 3/4” 167/8” 151” 11

There was extreme face deformation and thread galling after 10 and 11th make up on all samples.

Considering all the facts and phenomena, the optimal number of make ups on a sucker rod was experimentally determined to be 9.

From a practical standpoint, the load carrying capability of the rod string will be enhanced if there are processes and procedures to keep track on the usage of sucker rods in the field.

37Feb. 23 - 26, 2009 2009 Gas Well Deliquification Workshop Denver, Colorado

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38Feb. 23 - 26, 2009 2009 Gas Well Deliquification Workshop Denver, Colorado

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Comprehensive Studies on the Factors Affecting Sucker · PDF fileComprehensive Studies on the Factors Affecting Sucker Rod-Coupling Make ... field applications? ... shall be included

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