01015 BRITTLE FRACTURE IN AN UPPER TREE CONNECTOR

Prévia do material em texto

B R I T T L E F R A C T U R E IN 
A N U P P E R T R E E C O N N E C T O R S Y S T E M A T M E N S A - 
An Analysis 
Robert Mack 
Shell E&P Technology Applications and Research 
Shell International E&P, Inc. 
Westhollow Technology Center 
3333 Highway 6 South 
Houston, Texas 77082 
Steve Norton 
Shell Offshore, Inc. 
Shell E&P Company 
701 Poydras Street 
New Orleans, Louisiana 70139 
ABSTRACT 
Mensa is a gas field with three subsea wells that are located in 5300 feet (1600 m) of water in 
the Gulf of Mexico. On January 2, 1998, the outer housing (OH) of the upper tree connector of the 
Mensa A-1 wellhead failed in service in a completely brittle manner and directly caused the immediate 
shut-in of the A-1 well. All safety systems worked as designed; therefore, a minimal amount of 
hydrocarbons were released to the environment. 
The fracture initiated at the root of two diametrically opposed keyways in the OH and 
propagated rapidly, rendering the OH into three large pieces. 
The cause of the failure was the extremely poor fracture toughness of the forged, quenched and 
tempered, AISI 4140 OH. The Charpy impact toughness of the material at room temperature was 
determined to be 2.5 to 4.0 ft-lbs (3.4 to 5.4 J); the fracture mode was 100% brittle. Therefore, the flaw 
tolerance of the OH was extremely poor. 
Contrary to the actual properties of the A-1 OH, information in the material certificates 
indicated very good impact properties as measured using quality test coupons (QTC) per API 6A. 
The results of the failure analysis of the OH, and the results from the evaluation of other OH's 
purchased for Mensa and in service at the time of the A-1 failure are presented in this paper. 
The results from this study strongly suggest that "small" QTC samples are inadequate as a 
QA/QC tool to assure the fitness-for-service of large, low alloy steel forgings. Therefore, it is proposed 
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that API Standard 6A be revised to specify better QA/QC processes to determine the quality of all steel 
forgings. Such improvements could prevent failures like the one that occurred at Mensa. 
Keywords: Mensa, subsea, wellhead, connector, Charpy impact, brittle, failure, gas, production, well, 
heat treatment, API, QTC, quality control. 
BACKGROUND 
Mensa is a field of three subsea, sweet gas wells that is located in the Gulf of Mexico in 5300 
feet (1600 m) of water. The gas from these wells flows in separate gathering lines to a manifold that is 
offset by five miles (3 km) from the wells. The gas flows from the manifold through a single flowline 
to a fixed platform in shallower water approximately 63 miles (100 km) from the manifold. 
On January 2, 1998, the A-1 well at the Mensa subsea field shut-in automatically. It had been in 
service for 6 months and was producing gas at a rate in excess of 100 MMSCF/D (3 MMSCM/D) at the 
time of the shut-in. Information available to the operators of the Mensa wells at the host platform 
indicated that hydraulic pressure to tree had been lost. 
A few days later, an ROV revealed that the upper tree connector (UTC) was unlocked, the upper 
tree was damaged, and a 4" (10 cm) FX gasket for the production bore was in the water and lying 
against a part of the tree. Also, gas hydrates were observed at various locations of the upper tree. 
Finally, by this time, the operators had also determined that the lower tree master, the annular master, 
and the subsurface safety valves were closed. Consequently, all safety systems functioned properly, so 
there was minimal release of hydrocarbons to the environment. 
The information from the ROV meant that the upper tree had to be recovered for a complete 
evaluation and determination of the cause of the incident. 
As soon as the upper tree was recovered, it was inspected in the "moon bay" of the recovery 
ship the George Richardson. During that inspection, cracks were seen in the outer housing (OH) of the 
UTC. Immediately after the inspection, the entire upper tree was then sent to the vendor for 
disassembly in the presence of representatives of both the operating company and the vendor. It arrived 
at the vendor on January 18, 2000. 
At the time of the failure of the A-1 OH, another well, the A-3, was also producing at 
approximately the same rate as the A-1 well. The A-2 well had not yet been installed, but its wellhead 
was nearing completion for shipment to Mensa. An unused OH was available for examination. Each of 
the three wellhead trees was identical. 
A schematic of the subsea wellhead tree for the Mensa wells is Figure 1. 
LABORATORY EVALUATION 
Visual Examination 
The UTC is a marine connector of a proprietary design with an inside diameter of 
13-5/8" (34.6 cm). An outer housing (OH) and retaining ring contain the UTC. See Figure 2 for a view 
of the A-1 OH, retaining ring, and associated valve after its recovery from the well and during 
disassembly at the vendor. 
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Disassembly of the A-1 UTC after retrieval provided the opportunity to view the fractured OH. 
Figure 3 is a view of the OH after removal of the valve and during the removal of the upper section of 
the OH, which included the keyways. The upper section was removed in a manner intended to 
minimize damage to the fracture surfaces. Figure 4 includes a close-up view of the retaining ring, guide 
bolts, and a portion of the crack path in the OH. Figure 5 is a view of the OH showing the two 
opposing keyways in which two cracks initiated at the roots of these keyways; this photo was taken 
prior to the removal of the fractured upper section (see Figure 3). 
Figure 6 shows the OH after removal of the upper section; the path of fracture can be seen. 
Figure 7 is one half of the upper section of the OH that has been inverted with respect to its orientation 
in Figure 5; this fracture surface mates that seen in Figure 6. 
Metallographic Examination 
Metallographic samples were prepared from broken Charpy bars of the as-received material 
from the failed A-1 OH and from a boat section from the A-3 OH. Two orientations were prepared for 
examination. The microstructure of one location in the upper OH from A-1 and A-3 wells are shown in 
Fiugres 8 and 9. 
Fractographic Examination 
Scanning electron microscopic (SEM) examination of broken Charpy bars and of the A-1 OH 
fracture surface were conducted. 
Chemical Analysis 
Chemical analysis was performed on samples from the A-1 OH. 
Mechanical Testing 
Charpy Impact. Standard size Charpy test bars were machined from the as-received UTX from 
the region that was immediately adjacent to the intersection of the vertical and horizontal fracture 
planes. The specimens were machined so that two orientations within this region could be evaluated 
near both the inner and outer regions of the upper section of the OH as well as the bottom section 
immediately adjacent to the fracture surface. Their locations are depicted in their outlines in Figure 7. 
Considering the brittle nature of the OH fracture surface, the initial tests were performed at 
room temperature, as it appeared that lower temperatures would not prove worse. After these initial 
tests, tests were performed at higher temperatures in search of the transition temperature. 
Later in the study, an evaluation of an unused OH, the one to be delivered for use at A-2, and 
the one in service at A-3 was made. In each case, a "boat" section was obtained from a thick section of 
the UTX; this material removal did not affect the suitability of the OH for service in the event the 
results proved to be acceptable. An example of a "boat" section removal is shown in Figures 10 and 
1 lb where one was removed from the upper OH recovered from the A-3 well. 
Tensile Testing. Standard, round tensile bars wereremoved from the A-1 failed OH. Room 
temperature mechanical properties were obtained at locations near the fracture and for longitudinal 
(near the ID and near the OD of the upper section of the failed OH) and from a horizontal orientation in 
the OH. All tests were performed in accordance with ASTM E23 Ill 
Heat Treatment Studies. Some low alloy steels are susceptible to temper embrittlement. This 
phenomenon is caused by a segregation of certain embrittling elements (e.g., As, P, Bi, Sb) to grain 
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boundaries in certain low alloy steels; it occurs when tempering in, or slow cooling after tempering 
through, the temperature range of 1100 ° to 850°F (593 ° to 454°C). It is reversible after re-heating to 
1150°F (621°C), or so, for a few minutes followed by a rapid cooling to room temperature. The OH 
had been tempered at 950°F (516°C) and at the time of this stage of the study it had not been 
determined whether or not the material was as specified. 
Charpy bars were heat treated to remove any effects of temper embrittlement, a "de-embrittling" 
treatment. The specimens were subject to one of two treatments; either should have worked if the 
material had been temper embrittled, i.e, the Charpy impact toughness would increase dramatically. 
These heat treatments were as follows: 
Step I: 1100°F (593°C) for 20 minutes, then air cooled to room temperature. 
Step II: 1100°F (593°C) for 20 minutes, then air cooled to room temperature, then 
1150°F (621 °C) for 20 minutes, then air cooled to room temperature 
The second treatment was used in case the first temperature was not high enough to dissolve the 
embrittling elements. 
Tensile bars and Charpy bars were also re-heat treated in accordance with the reported heat 
treatment of the forging. The purpose of this was to determine if the material could be heat treated 
properly and had the proper hardenability. This was not performed as a comparison to the properties of 
a forging with a mass many times greater ( ~ 15,000 times) than these small specimens, i.e., the 
machined OH forging. These specimens were heat treated in our laboratory as follows: 
Austenitizing: 
Tempering: 
1600°F (871 °C) for 1 hour at temperature, then oil quenched, then 
960°F (516°C) for 45 minutes at temperature, then air cooled. 
If this heat of AISI 4140 was susceptible to temper embrittlement because of the temperature of 
the tempering, a low impact toughness value would be obtained. 
Hardness Testing. Hardness measurements were taken on Charpy bar specimens at locations 
adjacent to the fracture plane of the Charpy bar. Measurements were taken on as-received and heat 
treated specimens. 
All tests were performed at room temperature and in accordance with ASTM E8 [21 
RESULTS 
Visual Examinat ion 
Visual examination of the OH revealed that it had fractured into three large pieces, as seen in 
Figures 4-6. A retaining ring and several alignment bolts prevented the segments from separating 
completely. During this event, the entire upper tree, weighing 117,000 lbs (53,200 kg) in air, was lifted 
by the produced gas pressure thereby severing the hydraulic connections to the tree. The SSSV and 
other valves closed and shut-in the well; the OH was guided back into its place by the alignment bolts 
with minor damage to other seal surfaces. 
The large chevron marks on the fracture surfaces indicated that the fracture had initiated at two 
opposing keyways at the top of the UTX followed by the fracture proceeding downward to the thicker 
section then around the entire circumference of the UTX. The keyways had very sharp root radii. An 
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example of this can be seen in a close-up of the keyway in Figure 1 la. Visual evidence indicated that 
the fracture was rapid, brittle, and had no signs of ductility. 
Metallographie Examination 
Metallographic examination showed the forging to be composed of martensite, bainite, and 
blocky ferrite. See Figures 8 and 9. 
Fractographic Examination 
Scanning electron microscopic (SEM) examination (conducted in-house) of broken Charpy bars 
and of the A-1 OH fracture surface revealed the same information. The fracture surfaces in each case 
were created by cleavage, representative of brittle fracture. No intergranular fracture was observed. 
Photographs of these fracture surfaces are Figures 12 and 13. 
Chemical Analysis 
The chemical analysis of samples obtained from the A-1 OH failure were analyzed by ICP, AA, 
OES, X-ray, Leco, and Kjedahl and showed that the material was AISI 4140 as prescribed. The exact 
analysis is provided in Table 1 where it can be compared to the specifications for AISI 4140. 
Mechanical Testing 
The results of all of the mechanical testing are summarized in Tables 2 and 3. 
Charpy Impact. The as-received material from the A-1 OH had Charpy impact absorbed 
energies of 2.5 to 4.0 ft-lbs (3.4 to 5.4 J) at room temperature. In all cases the fracture surfaces were 
100% brittle in nature; lateral expansion for each Charpy bar was also quite low (less than 5 mils, 0.13 
mm); this is consistent with the extremely low impact toughness and the completely brittle fracture. 
Heat treatments designed to reverse temper embrittlement, if present, showed a slight increase in 
impact toughness at room temperature to 9 ft-lbs (12 J) for Step I and to 13.5 ft-lbs (18 J) for Step H 
treatments. The increase in toughness (and concomitant reduction in hardness) is a result of tempering 
the microstructure at a higher temperature than given to the forging. 
The small specimens that were heat treated in our laboratory and in accordance with the heat 
treating certificates provided by the sub-vendor, which heat treated the OH's for the project, showed 
Charpy impact toughness values of 32 ft-lbs (43 J) at room temperature and 29-32 ft-lbs (39-43 J) at 
32°F (0°C); in each case the fractures were 100% ductile. 
All Charpy specimens removed from an unused OH (at the vendor's site), the OH to be 
delivered for use at the Mensa A-2 well, and the OH recovered from service at the A-3 well showed 
very low impact toughness in room temperature testing (13 ft-lbs, 18 J) and 10 ft-lbs and less (14 J) at 
32°F (0°C). In each case, the fracture was 90-100% brittle (0-10% shear fracture). 
After these tests showed that each OH could experience the same failure in service as the one at 
the A-1 well, the A-3 well was shut-in until replacement OH's could be prepared for all three wells. 
Hardness Testing. The as-received hardness of the material from the A-1 OH was found to be 
HRC 28-34. 
Heat treatments designed to reverse temper embrittlement, if present, showed a decrease in 
hardness to HRC 21-24. 
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The small specimens that were re-heat treated in our laboratory in accordance with the heat 
treating certifications provided by the sub-vendor that heat treated the OH's for the project showed 
hardnesses of HRC 32-35. 
Tensile Testing. The results of the tensile tests of specimens removed from the A-1 OH near the 
fracture surface showed yield strengths of 108-114 ksi (746-786 MPa) and very low ductility (as low as 
3% reduction in area and 3% elongation). Strengths were independent of specimen orientation 
(horizontal or longitudinal) or location (near ID or OD) within the A-1 UTX. 
The small specimens that were re-heat treated at Shell in accordance with the heat treating 
certificates provided by the sub-vendor project showed higher yield strengths and considerably 
improved ductility compared to the as-received A-1 OH properties (20 to 30% reduction in area and 10 
to 13% elongation). 
DISCUSSION 
The visual evidence indicated that the OH at the A-1Mensa well failed in a highly brittle 
manner. The cracks initiated at two opposing keyways in the upper OH of the UTC and rapidly 
propagated to failure rendering the OH into three large pieces. All safety systems functioned properly; 
therefore, there was minimal release of hydrocarbons to the environment. 
Chemical analysis showed that the OH was made from the material chosen, AISI 4140. 
Tensile and Charpy testing of specimens from the A-1 OH, and Charpy testing of specimens 
removed from other OH's (A-2 and A-3) demonstrated that the material had very low tensile ductility 
and impact toughness. 
Metallographic and SEM evidence obtained from the failure and broken Charpy bars removed 
adjacent to the fracture surface of the A-1 OH show the fracture is 100% cleavage fracture. 
Hydrogen embrittlement was advanced as a possible cause of the failure by the vendor, partly 
because there were several grains that had failed intergranularly at or near an initiation site at one of the 
keyways (the remainder of the fracture surface was cleavage). The OH was under cathodic protection 
(CP) for six months prior to the failure. Hydrogen embrittlement cannot be detected in low alloy steels 
in Charpy impact testing; the strain rate is too high. Step ! and Step II "de-embrittling" treatments 
would have baked any hydrogen out of the specimens, yet the toughness values remained low. The 
failed A-1 OH, the OH intended for the A-2 well, another OH, and the upper OH in service at the A-3 
well all showed poor toughness. Therefore, it can be concluded that, even if the A-1 OH picked up 
some hydrogen because of the CP in service, hydrogen did not play a role of any significance in the 
failure of the OH in the A-1 well at Mensa. 
Experiments designed to determine if temper embrittlement had played a role in the failure were 
conducted before an SEM examination of the fracture surface and chemical analysis of the material 
could be made. Results from the "de-embrittling" treatment study noted here demonstrated that temper 
embrittlement was not involved. Further, SEM examination showed that the fracture surface was 
entirely cleavage, whereas a temper embrittled steel would have had a very high degree of intergranular 
fracture. 
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A re-heat treatment of samples from the A-1 OH showed that this heat of AISI 4140 could be 
heat treated to produce acceptable toughness in small sections. However, this is not a demonstration 
that the 5170 lb (2350 kg) forging could have been quenched and tempered to produce acceptable 
properties. Instead, it is a demonstration that a small quality test coupons (QTC) of this heat of AISI 
4140, which had received a much higher forging ratio than the OH forging, might show acceptable 
properties. It appears incongruous that the QTC was heat treated to a yield strength of 132 ksi (910 
MPa) with a Charpy impact as high as 53 ft-lbs (72 J) at -18°C (0°F) as noted by the certificate from 
the sub-vendor. 
An investigation of the manufacturing procedures and requirements revealed that Charpy impact 
testing had not been required for the OH, as the vendor considered it to be non-critical to containment 
of pressure. This is clearly not the case as its failure raised the entire upper tree and disconnected the 
hydraulic lines to the tree. Charpy impact testing was performed, however, but only for information 
purposes. 
Study of the manufacturing process showed that the OH was forged from AISI 4140 that had 
been electric furnace melted, followed by vacuum degassing. The original forging was a round 38" in 
diameter x 43" long and weighed 13,000 lbs (590 kg). It was subjected to a 3:1 hot forging reduction 
and drawn to a length of 61-%'. After this operation, it was hot pierced. It was reported that there was 
no temperature monitoring during the forging. After forging, it was cooled in sand for several days. 
The primary vendor of the OH reported that a QTC was used to qualify the machined forging, 
which weighed 5170 lbs (2350 kg). The QTC measured 4" x 4" x 8" (10 x 10 x 20 cm) in accordance 
with API 6A PSL-3 131. The forging ratio for the QTC was approximately 20:1. The machined forging 
and the QTC were heat treated in accordance with the same procedures, but not necessarily at the same 
time. The heat treatment was reported to be 1600°F (871 °C) for 4 hours then oil quenched, followed by 
a single temper at 950°F (510°C) for 8 hours. The reported oil quench tank beginning and ending 
temperatures were 112°F (44°C) and 126°F (52°C), respectively. Considering the size of the tank (as 
reported to us) and the size of the forging, the monitoring temperatures could be in error, e.g., taken at 
the wrong location. 
Per API 6A, the QTC and forging are not required to receive the same hot forging ratios 
(reduction ratios). Neither are they required to be heat treated at the same time or even in the same 
furnace as the forging. Therefore, the mechanical properties of the 36 lb (16 kg) QTC were used to 
qualify a 5170 lb (2350 kg) forging of AISI 4140 low alloy steel. The values from the QTC, as noted in 
Table 2are 47-53 ft-lbs (64 to 72 J), not the actual OH properties of 2.5 to 4.0 ft-lbs (3.4 to 5.4 J) ! 
As noted, the cracks initiated at the root of two diametrically opposed keyways within the OH. 
The root radius of these keyways was quite small. A generous radius of curvature in such locations is 
very desirable to minimize stress concentration and is considered a "best practice" to ensure this. These 
keyways were sites of stress concentration. 
Calculations using empirical correlations between the Charpy V-notch test results and KID and 
KIC 14"61, show that the A-1 OH could have had a Kic as low as 13 ksi~/in (14 Mpa~/m) or a KID as low as 
17 ksix/in (19 Mpa~/m) at room temperature. The OH was exposed to temperatures as low as 36°F 
(2°C); hence, the fracture toughness could have been lower still. In either case, assuming a single edge 
notch defect, the allowable (critical) flaw for a Kic of 13 ksi~/in (14 Mpa~/m) would have been 0.006, 
0.012, and 0.026 inches (0.15, 0.30, and 0.65 mm) if the net cross-section stresses imposed by static 
loading were 0.8, 0.6, and 0.4 of the yield stress of the material, respectively. Defects of this size 
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would not have been cause for rejection of the component, if they could have been detected at all, 
which is very unlikely. 
While the extremely low fracture toughness of this material was the root cause of the failure, the 
sharp root radii of these keyways was a contributing factor and made the OH more susceptible to 
failure. Subsequent to this failure analysis, the vendor has modified its design to incorporate a radius of 
curvature of 0.060 inches (1.5 mm) or greater in the keyways and has changed the material for the OH 
to one with better hardenability. 
The root cause of the failure was the inadequate toughness of the AISI 4140 forging, a direct 
result of the heat treatment and inadequate hardenability of the AISI 4140. 
SUMMARY AND CONCLUSIONS 
On the failure 
The failure of the OH at Mensa was caused by rapid, brittle fracture of the AISI 4140 forging. 
The OH cracks initiated at two opposing key-ways; each had a sharp root radius, which is not 
considered "best practice" design. 
The failure of the OH caused a loss of hydraulic pressure to the tree; therefore, the well 
automatically shut-in at the time of the failure with a minimal amount of hydrocarbons were released to 
the environment. 
The AISI 4140 OH was not heat treated to provide sufficient toughness for the application. The 
Charpy impact toughness was measured to be between 2.5 and 4 ft-lbs (3.4 and 5.4 J) at room 
temperature with a 100% brittle fracture mode. The other OH's tested were also found tohave 
unacceptably low toughness. The A-3 well was shut-in and the wellhead pulled. Replacements for all 
OH's were ordered for rapid replacement. 
Neither hydrogen embrittlement from CP of the wellhead nor temper embrittlement was a factor 
in the failure. 
The QTC was in accordance with API 6A but was not representative of the 5,170 lb (2350 kg) 
forging. 
Sufficient toughness of the final product was not assured by the quality control program in place 
by the vendor. 
The vendor did not consider the OH to a critical component per API 6A. However, the OH was 
critical to the containment of fluids and pressure. 
The replacement OH material was of better hardenability. For the replacement OH's, QTC's 
were not used. Prolongations for each forging were specified by us. Onsite monitoring of the forging 
and the heat treatment of each forging was required. Properties of the prolongation for the 
replacements were acceptable. 
Significant loss of production from three wells occurred during the period of OH's 
replacements. Consequence of failure of deepwater, subsea wellhead components can be significantly 
greater than either at a platform or onshore. 
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Industry related 
The implications from this study strongly suggest that "small" QTC samples are inadequate as a 
QA/QC tool to assure the fitness-for-service of large, low alloy steel forgings. Therefore, API Standard 
6A should be revised to specify better QA/QC processes to determine the quality of all steel forgings. 
Such improvements could prevent failures like the one that occurred at Mensa. 
If the size of the QTC permits a correct comparison of component and QTC mechanical 
properties, then it should be heat treated with the component and experience all of the 
thermomechanical treatment, including the same hot forging temperatures and reduction ratios. 
The definition of "critical component" should be studied carefully by user and vendor. 
Prolongations are required for critical components when a QTC cannot be made that will represent the 
properties of the component. 
An extensive QA/QC program is required for large projects. This program should be designed 
to determine the criticality of the component by including an assessment of the consequences of 
component failure, alloy system, size of component, track record of all, then qualifying and using only 
qualified involved vendors and sub-vendors. 
Users should invest more effort in changing, creating, and monitoring applicable industry 
standards. Users should exert more influence to make changes to industry standards occur faster. 
Consequence of failure of deepwater, subsea wellhead components can be significantly greater 
than either at a platform or onshore. 
RECOMMENDATIONS 
Users should work closely with suppliers and their vendors and sub-vendors, if necessary, to 
ensure that a proper program of quality assurance and quality control are in place. 
Qualification of all suppliers and their vendors and sub-vendors should be treated as critical in 
the success of a QA/QC program. 
Users should establish their own set of guidelines for monitoring and inspection by classifying 
components with respect to criticality by using alloy class, size of component, consequence of failure, 
and opportunity for error. An extensive QA/QC program that includes these guidelines is required for 
large sub-sea projects. 
Users should work hard to ensure that changes are made to API standards that will prevent 
irrelevant QA/QC methods being used to establish the quality of a product, e.g., small QTC specimens 
being used to qualify components that are 150 times the mass of the QTC. Prolongations should be 
considered in lieu of QTC for qualifying and checking the mechanical properties of large carbon and 
low alloy steels such as AISI 4140 and martensitic stainless steels such as AISI 410. 
Users and vendors should work closely to ensure that failures are prevented. Creation of an 
open and informal industry forum to share deepwater experience in material performance, QA/QC, 
vendor performance, etc. should be considered. Participation by both users and vendors should be 
permitted and encouraged. 
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ACKNOWLEDGEMENTS 
The authors acknowledge Jeff Stockman and Paul Schmidt (Shell E&P Technology 
Applications Research, Westhollow Technology Center) who performed some of the laboratory work 
described in this paper. We thank Joel Russo for help during the course of this failure investigation. 
Cooperation by the vendor was greatly appreciated during the analysis of this failure and the during the 
replacement phase of each the outer housings during the reinstallation of the trees for each of the Mensa 
wells. Finally, we thank Shell Offshore, Inc., Shell E&P Company, and Shell International E&P, Inc. 
for financial support of and permission to publish this work. 
CONTACTING THE AUTHORS 
The authors may be contacted by E-mail at the following addresses: RDMack@shellus.com or 
SJNorton@shellus.com. 
REFERENCES 
[1] 
[2] 
[3] 
[4] 
[5] 
[6] 
ASTM Designation E23-82, "Standard Methods for Notched Bar Impact Testing of Metallic 
Materials", Vol. 3.01, Annual Book of ASTM Standards, American Society for Testing and 
Materials, Philadelphia, (1984), pp. 210-233. 
ASTM Designation E8-83, "Standard Methods of Tension Testing of Metallic Materials "', Vol. 
3.01, Annual Book of ASTM Standards, American Society for Testing and Materials, 
Philadelphia, (1984), pp. 13 0-150. 
API Specification 6A, "Specification for Wellhead and Christmas Tree Equipment", 
Seventeenth Edition, American Petroleum Institute, Washington, D. C., (February 1, 1996). 
J. M. Barsom, "The Development of AASHTO Fracture Toughness Requirements for Bridge 
Steels ", Engineering Fracture Mechanics, Vol. 7, No. 3, September, (1975), pp. 605-618. 
J. M. Barsom and S. T. Rolfe, "Correlations Between Kic and Charpy V-Notch Test Results in 
the Transition Temperature Range ", Impact Testing of Materials, ASTM STP 466, American 
Society for Testing and Materials, Philadelphia, PA, (1979), pp. 281-302. 
R. H. Sailors and H. T. Corten, "Relationship Between Material Fracture Toughness Using 
Fracture Mechanics and Transition Temperature Tests", Proeeedings of the 1971 National 
Symposium on Fracture Mechanics, ASTM STP 514, Part II, American Society for Testing 
and Materials, Philadelphia, PA, (1972), pp. 164-191. 
Table 1: Composition of A-1 OH - Actual and Specification Limits 
Composition, Elements by wt.% (bal. Fe) 
Material C Cr Ni Mo Mn Si P S Cu Al Ti V Sn 
(I) A-1 OH Compositions 
InternalAnalysis 0.45 1.04 0.11 0.23 0.92 0.27 0.010 0.019 0.15 0.01 
MillReport 0.41 0.96 0.11 0.20 0.85 0.24 0.009 0.013 0.15 --- 
(II) Specifications per UNS (UNS G41400) 
0.38- 0.80- 0.15- 0.75- 0.15- 0.00- 0.00- 
AISI 4140 . . . . . . . . . . . . . . . 
0 .43 1.10 0.25 1.00 0.35 0 .035 0 .040 
: i ai i i l gmai ali iy is oi £i 6i:i ai o i naic iedl = < 0010 ; B = < 0.0 ii i ............... i 
Nb < ........................................................................... 0.005; As = 0.0007; Sb = 0.003; Bi = 0.0002; W = < 0.020 i 
0.03 0.006 0.012 
--- 0.006 --- 
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T a b l e 2: M e c h a n i c a l P r o p e r t y D a t a f o r M e n s a A - 1 O H 
Room Temperature Propert ies Full Size Charpy Impact Test Data 
Tensile Bar O¥ s Outs Elong. ILA. 
Orientation [ksi, (Mpa)l [ksi, (Mpa)l (%) (%) 
(I) A-1 We l l O H F a i l u r e (upper section) 
As-ReceivedCondition 
H 109 752 132 907 3 3 
H 114 786 145 1003 11 19 
H 108 745 136 935 14 32 
IrmerV 109 752 140 962 9 14 
Outer V 109 752 139 957 9 16 
Test Absorbed 
Charpy Bar TemperatureE n e r g y Shear Hardness 
Orientation (°C) (°F) (ft-lbs) (J) (%) (HRC) 
H 22 72 2.5 3.4 0 29-31 
V 22 72 2.7 3.7 0 29 
V 22 72 3.5 4.7 0 29 
H 22 72 3.7 5.0 0 25-28 
V 22 72 3.7 5.0 0 30-31 
H 22 72 4.0 5.4 0 28-34 
V 128 262 18.5 25.1 50 --- 
H 140 284 27.5 37.3 65 --- 
V 165 329 23.5 31.8 70 --- 
H 180 356 44.5 60.3 100 --- 
After "De-Embrittling" Treatment in our laboratory: (Two Heat Treatments) 
Step I: 20 rain. at IIO0°F (593°C) , AC 21 70 9.0 12.2 25 21-22 
Stepll: 20min. at1100°F, AC, then 20min. at l150°F (621°c),AC 21 70 13.5 18.3 30 21.5-24.0 
After "Re"-Heat Treated in our laboratory (1600°F {szt*c/, I Hr, OQ + 960°F {sl6°cl, 45 rain, AC) 
146 1009 162 1117 10 20 22 72 32.0 43.4 100 35.0 
147 1011 162 1114 13 30 0 32 28.5 38.6 100 34.5 
0 32 31.5 42.7 100 31.5 
( I I ) Q T C for the O H for the A-1 Wel l : QTC 4" x 4" x 8"; Values and treatments reported by vendor 
QTC and OH Forging Heat Treatment: (1600°F 1871°c}, 4 Hrs, OQ + 950°F {slo*cl, 8 Hrs, AC) 
132 910 151 1041 18 54 -18 0 47-53 64-72 40 BHN321 
( I I I ) V e n d o r R e q u i r e m e n t s for the O H 
> 110 > 758 . . . . . . . . . None for Impact Testing 
• Notes: V = Vertical; H = Horizontal; Inner and Outer refer to location on OH. " 
AC = Air Cool; OQ = Oil Quenched 
T a b l e 3 : C h a r p y I m p a c t D a t a f o r M e n s a A - 2 , A - 3 , & O t h e r O H ' s 
Full Size Charpy Impact Test Data 
Test A b s o r b e d 
T e m p e r a t u r e Ene rgy Shea r 
(°C) (°F) (ft-lbs) (J) (%) 
(I) A-2 OH (boat coupon from OH intended for service) 
21 7U 13.2 17.9 U 
0 32 9.5 12.9 0 
Lateral 
Expans ion H a r d n e s s 
(mils) (HRC) 
5 32 
2 27-28 
(II) An OH Intended for Service at A-2 (data from Vendor on 2/2/98) 
-18 0 10-18 22-24 0 . . . . . 
-18 0 12-13 16-18 0 . . . . . 
(III) A-3 OH Removed from Service (boat coupon; see Figures 10 and 1 lb) 
23 73 14.0 19.0 5-10 9 29-31 
0 32 10.5 14.2 0 6 29-31 
-16 4 7.5 10.2 0 7 29-31 
Brad Harrison - Invoice 44808 downloaded on 6/28/2018 11:23:25 AM Single-user licence only, copying/networking prohibited
Figure 1: Schematic of a Subsea Wellhead 
Tree at Mensa. Ou 
Figure 2: Outer Housing and Valve After 
Recovery From Mensa A-1 Well. 
Figure 3: Outer Housing During Removal 
of Upper Fractured Section That 
Contained Four Keyways. 
Figure 4: Two Side Views of Outer Housing With the Retaining Ring Raised Above Its Normal 
Position After Recovery from the A-1 Well and Before Removal of Fractured Section 
Containing the Keyways (Cf. Figures 3 & 5). Inset: Close-up of a crack in the outer housing. 
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Figu re 5: Cracked Outer Housing From the 
Mensa A-1 Well. 
Note: Vertical crack nearest viewer is outlined by a 
white business card inserted into it. Opposite, 
a second crack can be seen at a keyway root. 
F igu re 6: Outer Housing After Removal 
of U p p e r F r a c t u r e d Sect ion 
T h a t C o n t a i n e d the F o u r 
Keyways . Cf. Figures 2-4. 
Figure 8: Microstructure of Outer Housing 
from the A-1 Wel l at Mensa N e a r 
One of the T h r e e F rac tu re s . 
Note: Shown at ~ 300X. 
F i g u r e 7: One of Three Sections from the 
A1-Wei l O u t e r Housing. 
Note: Locations and orientations of test samples 
that were removed from it. Cf. Figure 5. 
Figure 9: Microstructure of"Boat Sample" 
Outer Housing from the A-3 Well 
at Mensa Near F r a c t u r e 
Note: Shown at ~ 700X. 
Brad Harrison - Invoice 44808 downloaded on 6/28/2018 11:23:25 AM Single-user licence only, copying/networking prohibited
Figure 10: Outer Housing from Upper Tree Connector 
for the A-3 Well at Mensa After Its Recovery. 
Figure 11: Close-up Views of the Upper Tree Connector 
After Removal From the Mensa A-3 Well. 
Brad Harrison - Invoice 44808 downloaded on 6/28/2018 11:23:25 AM Single-user licence only, copying/networking prohibited
Figure 12: SEM Views of Fracture Surface from a Frac tured Charpy Bar Removed 
From Failed Outer Housing from the Upper Tree of the Mensa A-1 Well. 
Note: Fracture is brittle cleavage at all locations. Charpy bar was broken at room temperature. 
Figure 13: SEM Views of Frac ture Surface of the Failed Outer Housing 
from the Mensa A-1 Well. 
Note: Shown at ~, 375X. Fracture is brittle cleavage. Cf. Figure 12. 
Brad Harrison - Invoice 44808 downloaded on 6/28/2018 11:23:25 AM Single-user licence only, copying/networking prohibited
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