|Description / Abstract
||Back to Top
1.1 Project Background
The work presented in this report is the third in a series of annual
projects conducted by the Production Research Advisory Committee
(PRAC) of the American Petroleum Institute to investigate the factors
affecting leak resistance in API connectors. PRAC projects 84-53 and
85-53 investigated leak resistance in 8-Round connectors in 1984 and
1985, and this project 86-53 analyzes the factors affecting leak
resistance of Buttress connectors.
The Technical Advisory Committee established by PRAC to direct the
work consists of the following:
The contract for the work was executed March 19, 1986. Status meetings
of the Technical Advisory Committee were held on April 2, May 13 and
November 19, 1986. Also, presentations were made at the API
Standardization Conference in New Orleans June 23-26, 1986, and API
task group work week in San Antonio December 8-11, 1986.
1.2 Buttress Connector Background
The Buttress connector was patented November 27, 1956 by Mr. Samuel
Webb and assigned to United States Steel Corporation. Statements in
the patent indicate that the goal of the Buttress connector is to
provide joint strength equal to pipe body strength, and that high leak
resistance should not be expected unless the thread clearance on stab
flanks is closed. The patent recommends mismatched leads to accomplish
API design equations for Buttress connectors are based on tests
conducted at U.S. Steel in 1954-5. The tests were conducted by Mr.
Webb, and were interpreted by Mr. Bill Clinedinst to establish API
performance properties at pipe yield for both tension and internal
Despite relatively large thread clearances with nominal dimensions,
early experience with Buttress did not indicate difficulties with leak
resistance. One explanation is that tolerances were not held as close
with early manufacturing techniques and variations in lead caused
clearances to close, resulting in higher leak resistance. However,
more recent manufacturing techniques hold tighter tolerances on lead,
introducing clearances and lowering leak resistance.
One popular approach to achieve leak resistance in Buttress connectors
is to use tin plating. During make-up the tin plating fills the thread
clearances to seal the leak path. However, API performance properties
do not account for the presence or absence of tin plating on threads.
The overall objective of this project is to develop information for
the API Task Group on performance Properties for API Buttress
connectors. Specific objectives are as follows:
1. Verify computer model accuracy by comparison of model predictions
to laboratory test data.
2. Define the contact pressures, thread clearances, and stresses in
Buttress connectors for nominal dimensions in 5-1/2″,
9-5/8″, and 13-3/8″ casing.
3. Establish the changes in contact pressures, thread clearances, and
stresses in Buttress connectors as a result of changes in diameter,
make-up, thread lead, and tension.
1.4 Work Description
Work conducted in this project consists of the following tasks:
1. Simulate conditions for tests conducted by Nippon Steel
Corporation, and compare calculated and measured strains.
2. Generate computer models for 5-1/2″, 9-5/8″, and
13-3/8″ nominal Buttress connectors, and modify the models for
mismatched leads in 9-5/8″ and 13-3/8″.
3. Execute make-up and tension load cases for the connector models
created in step 2.
4. Plot and interpret stresses, strains, crest/root contact pressures,
thread flank clearances, and displacements.
5. Communicate results of project through TAC meetings, presentations
at API conferences, discussions, progress reports, and this final
Conclusions apply to buttress connector performance only, but may be
extended to other connectors in some cases. In view of the lack of a
reliable leak criteria for leak through a thread clearances, results
and conclusions in this project address the factors affecting leak,
but do not predict specific leak pressures.
Some conclusions presented here may be obvious to some readers, but
are included for completeness. Following are conclusions based on the
calculations in this project and interpretations that draw on related
1. Clearances and displacements due to make-up and tension loads are
significantly greater for API BTC connectors when compared to 8-Round.
This requires thread compound to serve the major role in leak
resistance by sealing the larger clearances.
2. Due to relatively large thread clearances in Buttress connectors,
the most likely leak path is through the thread stab flank clearance
3. An important factor influencing leak resistance in Buttress
connectors is the change in thread clearances during make-up and
tension loads. During make-up, radial compression on the pin causes it
to lengthen axially due to Poisson's effect and radial expansion of
the coupling causes it to shorten. The lengthening of the pin and
shortening of the box cause load flanks near the end of pin to open.
Load flank stand-off near the end of pin increases with make-up turns.
Subsequent tension closes load flanks, thereby opening stab flanks and
introducing a leak path.
4. During hand tight make-up, load is transferred from the thread stab
flanks to the load flanks. The initial stab flank contact may cause
thread compound to be wiped off and may affect leak resistance.
5. Mismatched leads, properly applied, will tend to assure load flank
bearing at one end of the full depth threads on the pin and stab flank
bearing at the opposite end during make-up, thereby improving leak
6. Poisson's effect in the threads during make-up results in an
increase in pin crest clearances with make-up turns, thus reducing
leak resistance. During make-up, Poisson's effect on the threads in
minor, but does result in a reduction of crest or root contact
7. Unlike some 8-Round connections, additional make-up turns beyond
nominal may not result in increased leak resistance because of the
effects described in 6 and 3.
1. For improved leak resistance, it is recommended that API
Specifications 5B on machining tolerances for lead be modified so that
thread clearances are reduced or eliminated. Within the existing
tolerance range, stay on the negative side of lead tolerance for the
pin and on the positive side of lead tolerance for the coupling if at
all possible during machining. Do not allow lead to vary in the
opposite direction (positive on pin or negative on coupling). This is
consistent with the original patent on the API Buttress thread form
2. Establish a set of leak criteria for utilization of the results
presented here. Specifically, two leak criteria are needed to better
2.1 Leak past a sealing surface as a function of contact pressure and
2.2 Leak past thread compound in a thread clearance as a function of
clearance dimensions and changes in clearances.
3. Due to the clearances and displacements involved in BTC connectors,
and the unknowns concerning thread compound, Buttress connectors
intended for high pressure service should be tested prior to field
use. Testing should include thermal cycling with axial load and
internal pressure cycling with dimensions in worst case combination.
4. Calculate API pressure performance properties for each diameter,
weight, and grade based on contact pressures, clearances, and changes
in clearances presented here and leak criteria recommended in 2.
Change API performance properties as needed.
|Additional Supplemental Documents