History of Slim Hole Coupling Derating Factors ...· 8th Annual Sucker Rod Pumping Workshop...
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8th Annual Sucker Rod Pumping Workshop
Renaissance HotelOklahoma City, Oklahoma September 25 - 28, 2012
History of Slim Hole Coupling Derating Factors &
Application to Special High Strength Rods
Norman W. Hein, Jr., P.E.NPS Norris/AOT
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Content Origin of Derating Factors by Mr. F. W. Gipson Modifications by Mr. D. Hermanson Extension of D Grade Rod Strength to Special
High Strength Rods Conclusions and Recommendations
HF Design, selection, clearances table
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Gipson & Swaim Green Book
The first discussion that provides derating factors for slim hole couplings was published in the 1969 by Gipson & Swaim in the Conoco Green Book
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Green Book (first published 1969) Although not discussed was why the derating was
required other than field failures were occurring. Looking back into the production time of the
fields, it is assumed that waterflood response was occurring, wells were able to produce more fluids, and prior rod string installations were inadequate to handle the required, applied loads/stresses.
Normally, if higher loads were being applied, rod body cross sectional area would still be the weak link and the body should fail first.
This may be occurring again w/new, deeper wells09/25-28/2012 2012 Sucker Rod Pumping Workshop 5
Development of Derating Factors T Class Couplings
Gipson first compared full sized cross sectional area to associated rod body area.
Established the following area relationship:
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Nominal Coupling Size (in)
Rod Body Area(in)2
Area)/ Rod Area
5/8 1.0508 0.307 3.42283/4 1.1579 0.442 2.61977/8 1.4398 0.601 2.39571 2.2342 0.785 2.8461
1- 1/8 2.4694 0.994 2.4843
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Based on rod body to coupling areas, with the coupling area 2.4 to 3.4 times associated rod body area, the rod body is the weakest link.
If couplings are properly made up to required connection preload, and then installation properly operated, you should never see a connection or coupling failure
Slim Hole Coupling Area Comparison
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Similar coupling to rod area comparison made. A derating of the slim hole (SH) coupling could
be made for all sizes but response from the field said 1 SH couplings were not failing.
The derating was modified to normalize the factors to account for the non-failures of 1SH couplings
Normalized SH Coupling Deratings
Nominal Coupling Size (in)
Delta Coupling Area (in)2
(Delta Area)/ Rod Body Area
Normalized Derating (prior column)/2.0606
0.3234 1.6471 0.79935/8 0.5109 1.6642 0.807 0.8510 1.9253 0.93437/8 0.9337 1.5536 0.75401 1.6176 2.0606 1.000
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Rod Strength vs. Coupling Strength
Rod Grade Tensile Range (ksi)
Coupling Tensile (ksi)
C &K 90 to 115 90 minAll 3 D 115 to 140 (56 to 62 HRa)
Special HS 140 to 150 (155; 160)
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Dean Hermansons Extension
API Rod Size (in) Normalized Derating Factor5/8 0.81 0.937/8 0.751 1.0
API Rod Size (in)
API Grade K API Grade C API Grade D
5/8 na 0.97 0.77 na na 0.867/8 0.93 0.88 0.691 na na 0.89
Special High Strength Rod T Class Derating Extension for SH & FS
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Gipsons mathematical comparison of areas originally resolved connections failing; but this compared nominal C versus coupling T grade.
Hermanson said that if D grade rod used versus T grade couplings, then SH deratings may need to be further reduced.
Extending strength comparison to special HS rods says deratings need further reduction not only for SH, but also FS.
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Rod string designs need to consider the potential need to apply the SH derating factor or at least provide to the operator that a failure potential may exist
If dont want or cant derate rod string, then operator needs to run the correct sized tubing in the length requiring SH couplings
This can be run as full length or tapered tubing string
Or could consider running special HS couplings09/25-28/2012 2012 Sucker Rod Pumping Workshop 14
Many rod strings have been installed w/o excessive failures even though SH couplings included
Some believe there is not a need to derate; but there has been recent rash of coupling failures high in the string
Operators may want to consider running a pilot string w SH couplings w/o derating to see if there are operational problems/failures then take the risk
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