OFFSHORE NICKEL ALLOY TUBING HANGER AND DUPLEX STAINLESS STEEL PIPING FAILURE INVESTIGATIONS

S. Huizinga, B. McLoughlin, W.E. Liek and J.G. de Jong

Shell Global Solutions International

P.O.Box 38000

1030 BN Amsterdam

The Netherlands

ABSTRACT

In the offshore oil and gas industry, extensive use is made of corrosion resistant alloys. Two failure cases are described, one with the Cr and Mo containing nickel base alloy UNS N07718 and one with the 22%Cr duplex stainless steel UNS S31803.

A UNS N07718 nickel alloy tubing hanger failed at the box end of the hanger at one of the upper threads, where stress was at a maximum. The cause was found to be the presence of delta phase, which rendered the material sensitive to hydrogen embrittlement, as was shown in a laboratory study. Hydrogen could originate from the manufacturing process, in which case it could be prevented, or from in situ corrosion, which may be enhanced by galvanic coupling to carbon steel. UNS N07718 accessories should therefore be produced in such a way as to minimize the formation of delta phase and evaluation of the presence of delta phase should be part of the qualification procedure.

In a UNS S31803 duplex stainless steel hot condensate piping system, a leak was detected. It was found to have resulted from cracking from the inside at a location where pressure drop could lead to water evaporation. A laboratory study confirmed that at elevated temperature (140 °C) chloride stress corrosion cracking occurs, even in essentially oxygen free conditions, when the steel is exposed to the highly concentrated brines with reduced pH that result from evaporation. The role of oxygen appears to be an acceleration of the cracking process. In cases where brine concentration cannot be avoided, construction materials need to be upgraded.

Keywords: nickel alloy, tubing hanger, duplex stainless steel, piping, embrittlement, corrosion, cracking

INTRODUCTION

In the offshore oil and gas industry, extensive use is made of corrosion resistant alloys. These alloys are selected to perform without weight loss corrosion or corrosion cracking under expected operating conditions. Nevertheless, failures do sometimes occur, even with the “work horse” alloys of the industry. Two examples will be described, one for a Cr and Mo containing nickel base alloy (UNS N07718) and one for a 22%Cr duplex stainless steel (UNS S31803). The first case concerns a tubing hanger and the second topside condensate piping. Typical compositions of the alloys are given in Table 1.

The failures occurred in a gas and condensate production operation with CO2 partial pressures in the range of 2 to 10 bar and formation water with chloride levels up to 170,000 ppm (as chloride ions). The materials used are also suitable for mildly sour conditions, but till today, virtually sweet (H2S-free) fluids have been produced.

The described cases are different in nature in that the in the nickel alloy case, the failure turned out to be related to an improper microstructure of the alloy used, while in the duplex case, conditions were found more aggressive then expected at the time of selection of the steel.

NICKEL ALLOY UNS N07718 TUBING HANGER FAILURE

General description

The nickel alloy UNS N07718 case concerns a tubing hanger, which served to connect 25%Cr duplex stainless steel tubing to the platform. The Cr-Mo nickel alloy was selected for its corrosion resistance and mechanical properties. The casing material was C110 steel.

The operators detected communication between the production tubing and the annulus between tubing and casing. When the tubing was pulled, it was found that the box end of the hanger had failed at one of the upper threads, where stress was at a maximum. The lower part of the box with the tubing attached had parted from the upper end.

The failed component was analyzed at TWI (Cambridge, UK). The mechanical properties (tensile strength and Charpy impact properties) were according to the specification. The fracture surface had a brittle appearance, which varied from almost purely crystalline, predominantly intergranular fracture in the area of the crack initiation and early propagation through a mixed appearance with some evidence of ductility in the section of final propagation and ultimate fracture. It was also found that delta phase precipitations were present associated with grain boundaries. In addition, bulk hydrogen levels of 3 to 4 ppm (by mass) were measured.

A laboratory investigation was carried out to reproduce and explain the failure and to check if other alloy UNS N07718 components would also be susceptible to cracking. The work included the effect of hydrogen loading from galvanic coupling to carbon steel and the effect of temperature.

Laboratory investigation

For the laboratory testing, several specimens were available, including part of the failed tubing hanger. The failed material had a strength of 135 kpsi (0.2% proof stress). Additional material in a range of strength levels was also available.

The samples were checked for the presence of delta phase. This was only found in the failed hanger material, not in the other specimens. Figure 1 shows the typical appearance of this delta phase.

These precipitations are most likely the result of incorrect thermo-mechanical processing when strengthening the alloy to the required level.

From the tubing hanger part, tensile specimens (NACE sub size) were cut and subjected to Slow Strain Rate Testing (SSRT).

The test environments were representative of annulus fluid at ambient temperature (A in Table 2) and formation water at downhole temperature (B in Table 2). The gas cap was the same in both environments, assuming that as a worst case a leak from tubing to annulus allowed produced acid gases (CO2 and H2S) to dissolve in the annular fluid.

The results of SSRT of the failed alloy UNS N07718 specimens are summarized in Table 3.

The sample in test 1, performed in air, passed the specification tensile requirements. It was, however, noted that the fracture face exhibited some “brittle “ features and some fine secondary cracks were observed (see Figure 2, top). These facts alone would have been sufficient to consider the material to have failed the SSRT had it been in a pre-qualification program. The brittle appearance is thought to be a consequence of the retained 4 ppm of hydrogen that was subsequently measured in the specimen.

The specimen in test 2 that was galvanically coupled to a coupon of C110 steel, typical for the casing material, with equal surface area showed an elongation of 7.9% and Reduction of Area (ROA) of 8.5% in the SSRT, indicating significant loss of ductility. Numerous secondary cracks were observed (see Figure 2, middle). Although the fracture appearance did not fully reproduce all features of the actual failed hanger, it showed a brittle appearance, and more clearly so than in the blank test 1. The hydrogen level measured after the test was 10 ppm, confirming that the galvanic coupling had resulted in an increase in hydrogen in the alloy UNS N07718.

The specimen in test 3 was not galvanically coupled and SSRT resulted in an elongation of 22.8% and an ROA of 16.2%, to be compared to the blank reference of test 1 that showed 25.7% and 20.6%, respectively. This suggested some loss of ductility, but the fracture appearance was similar to the sample that was tested in air and did not duplicate the failure. The hydrogen level was confirmed as 4 ppm after the test, indicating very little, if any, hydrogen ingress during the test.

The specimen in test 5, exposing the material to brine at elevated temperature, showed an Elongation of 11.9% and an ROA of 9.8%, to be compared to values of 26.5% and 19% obtained in the blank reference test 4. These numbers again indicated significant loss of ductility and susceptibility to cracking. A few very fine secondary cracks were also observed and 2 ppm hydrogen was subsequently measured in the fractured specimens. A significant feature was evidence of corrosion in the form of darkening of part of the fracture face, indicating that exposure of tubing hanger material to produced fluids might give a risk of failure by a stress corrosion cracking mechanism rather than hydrogen embrittlement. (It should, however, be recognized that the SSRT is a severe test and further cyclic loading SSRT would normally be performed to qualify the material).

Separate tests on the specimens free of delta phase did not reveal cracking as found on the specimens from the failed hanger.

Discussion

The SSRT results indicate that hydrogen plays an important role in embrittlement of the delta phase containing material. While the source of hydrogen in the SSRT was a galvanic couple to C110 steel in simulated annulus water, the failed hanger may not actually have been exposed to such water. While in service, the hanger in all likelihood contained higher levels of hydrogen at the fracture than measured in

the present work in the samples from the retrieved hanger. A possible source of this hydrogen is the copper plating process.

The observation of the darkened fracture face in test 5 is significant as it confirms that exposure to aggressive produced fluids was not a contributory factor to the actual tubing hanger failure. Had it been so, the fracture face on the failed hanger would have been similarly discolored, which was not the case.

It is, therefore, highly likely that the hanger failed in the high stress area of the upper threads of the box due to the presence of delta phase, which made the material susceptible to the effect of hydrogen.

22%CR DUPLEX STAINLESS STEEL PIPING FAILURE

General description

For an offshore condensate piping system, the UNS S31803 22%Cr duplex stainless steel (DSS) was selected as a material with resistance against corrosion and stress corrosion cracking (SCC) under the expected conditions of about 2-10 bar CO2, low water content with chlorides up to 170,000 ppm (as chloride ions), negligible H2S and at temperatures of around 140 °C. Nevertheless, a condensate leak was detected, which had resulted from cracking, starting from the inside of a pipe section at locations where external welding had been applied.

An analysis of the failure carried out at TWI (Cambridge, UK) revealed that the DSS was of proper quality and that the welding had not caused detrimental effects in the microstructure of the steel. The role of the welding in the failure was most likely an increased stress level. The cracking was partly branched and mostly transgranular in nature. Only very little, if any, corrosive attack was found associated with the cracks.

Some precipitated salt was found near the failed location, suggesting that concentrations could have been higher than expected. In fact, the failure occurred at a widening section of the duplex pipe after a level control valve, i.e. a location where considerable pressure drop occurs and water flashing could take place, causing salt concentration to possibly very high chloride levels.

In classic chloride SCC, oxygen plays an important role as the oxidizer that drives the corrosion. In the present case, although it cannot be unambiguously proven that no oxygen has been present, the system is essentially free of oxygen.

Laboratory investigation

To understand the problem and define remedial measures, laboratory work was initiated in which DSS strips were strained to yield in 4-point-bend (4PB) jigs, while exposed to brine at 140 °C (see Table 4). Complementary long duration C-ring testing was performed at Sintef (Trondheim, Norway). The composition of the brine was varied from chloride contents of 200,000 ppm (6 M Cl-) to 460,000 ppm (9.9 M Cl-) to simulate degrees of evaporation from 50 to 90%. Calculations show that during evaporation, NaCl precipitates but MgCl2 concentrates and hydrolysis effects cause a drop in pH to values of around 3.4. The brine was made up from various salts to reproduce this effect. (Note that in pure concentrated MgCl2 as used in standard SCC testing, pH values drop significantly further). For the gas cap, both oxygen-free CO2 and a mixture of oxygen and CO2 were used, with CO2 kept at 2 bar.

The 4PB tests, summarized in Table 4, revealed that at the high end of the chloride concentration range, the DSS failed in a mostly transgranular mode, as was the case with the actual pipe work. In test 1, the gas cap was essentially free of oxygen, with less than 1 ppb dissolved in the water phase. Nevertheless, failure occurred within a few days. Test 2 shows that with additional oxygen failure occurs more rapidly and test 3 with intermediate oxygen content confirms this behavior. Figure 3 shows the typical appearance with branched cracking of a failed 4PB specimen. There is very little, if any, indication of corrosive attack associated with the cracks. Figure 4 shows a detail of the duplex microstructure and reveals the transgranular nature of the cracking.

Test 4 aimed at investigating a lower degree of evaporation while increasing the oxygen content from 1 to 20%. Within the total duration of the test (216 hours) failure did not occur, but sectioning and microscopy revealed some micro cracking. Test 5, simulating 50% evaporation of the brine, did not lead to failure within 960 hours.

Some corrosive attack with deposits developed in tests 4 and 5. This is not unexpected given the chloride concentration and oxygen level and the test duration.

While the occurrence of failure depends on oxygen content, the role of chloride concentration coupled with pH (which drops with increasing evaporation to a calculated value of about 3.4) is also evident from the test results. It would appear that more than 6 M chloride, i.e. more than 50% evaporation, is required for cracking of 22%Cr DSS to occur in the studied formation brine.

The 4PB tests provided a relatively rapid means of assessing a range of conditions. In parallel, some long duration C-ring testing was performed to confirm the 4PB results. Indeed, testing in a brine simulating 90% evaporation at 140 °C with 20% oxygen in the gas led to failure of triplicate C-ring specimens within 90 hours. Figure 5 shows one the failed specimens and also reveals secondary cracking. An experiment in which the gas cap was essentially free of oxygen (corresponding to less than 1 ppb dissolved in the brine) led to failure of triplicate specimens within 920 hours. Again, very little if any corrosive attack was found associated with the cracks.

Discussion

Perhaps the most significant result of the test work is that even under oxygen-free conditions (meaning less the 1 ppb dissolved), cracking can occur, thus reproducing and confirming the actual failure in the condensate piping. The nature of the cracking is mostly transgranular and partly branched, but whereas classic chloride SCC is usually accompanied by visible localized attack, no such attack is seen in the present failure and laboratory tests.

It can be concluded that the combination of very high chloride content, reduced pH, elevated temperature and stress can give rise to SCC even in the absence of oxygen. The present work gives an indication of the boundary values for the chloride level at 140 °C, but more work is needed to provide a safe window of application for the 22%Cr DSS based on combinations of parameters.

In those cases where concentration effects of brines cannot be avoided, construction materials would need to be upgraded.

CONCLUSIONS

Nickel alloy UNS N07718 tubing hanger

Even when alloy UNS N07718 conforms to requirements for mechanical properties, the presence of delta phase can render the material sensitive to hydrogen embrittlement, leading to failure of components. Hydrogen could originate from the manufacturing process, in which case it could be prevented, or from in situ corrosion, which may be enhanced by galvanic coupling to carbon steel.

Components from alloy UNS N07718 should therefore be produced in such a way as to minimize the formation of delta phase. Evaluation of the presence of delta phase should be part of the qualification procedure.

22%Cr duplex stainless steel piping

22%Cr duplex stainless steel (UNS S31803) may suffer from chloride stress corrosion cracking, even in essentially oxygen free conditions, when subjected to highly concentrated brines at elevated temperature. Such brines can form upon evaporation of water from formation brine, at the same time causing a drop in pH due to the presence of Mg ions. The role of oxygen appears to be an acceleration of the cracking process.

In cases where brine evaporation cannot be avoided, construction materials need to be upgraded.

ACKNOWLEDGEMENTS

TWI in Cambridge, UK (Dr. Paul Woollin), performed the initial analysis of the fractured alloy UNS N07718 tubing hanger and the failed 22%Cr piping section.

Experience of Shell Exploration and Production Company in Houston, USA (Dr. R.D. Mack), was indispensable in assessing the role of microstructure in the alloy UNS N07718 tubing hanger failure mechanism.

The C-ring SCC testing of 22%Cr duplex specimens was performed at SINTEF in Trondheim, Norway (Dr. Trond Rogne).

TABLE 1

NOMINAL CHEMICAL COMPOSITION OF ALLOYS UNS N07718 AND S31803

alloy type UNS code composition in % by mass

Ni Cr Mo Fe

Cr-Mo Nickel Alloy

N07718 50-55 17-21 2.8-3.3 balance

22%Cr Duplex Stainless Steel

S31803 4.5-6.5 21-23 2.5-3.5 balance

TABLE 2

ENVIRONMENTS FOR SSRT OF ALLOY UNS N07718

# environment test temperature pCO2 pH2S chloride

°C bar mbar ppm (by mass)

A annulus fluid 20 7 10 1,000

B formation water 150 7 10 100,000

TABLE 3

SSRT RESULTS OBTAINED ON ALLOY UNS N07718 FROM FAILED TUBING HANGER

test #

strain rate

environ-ment

UTS ROA elongation coupled to C110

steel

H content

result

s-1 MPa % % Yes/No ppm

1 3x10-6 Air 1145 20.6 25.7 N 4 -

2 1x10-6 A 1003 8.5 7.9 Y 10 cracks

3 1x10-6 A 1130 22.8 16.2 N 2 small cracks

4 2x10-5 Oil 1109 19 26.5 N n.d. -

5 1x10-6 B 1025 9.8 11.9 N 2 very small cracks

TABLE 4

4-POINT-BEND TESTING OF 22%CR DUPLEX STAINLESS STEEL IN BRINES AT 140 °C

test #

simulated degree of evaporation

(chloride concentration)

oxygen in gas cap

cracking time to failure remarks

% (M Cl-) volume fraction Yes/No hours

1 90 (9.9) 10-15 ppm Y <96

2 90 (9.9) 1-1.5 % Y 40

3 90 (9.9) 0.1-0.15 % Y 72

4 78 (7.4) 1 to 20 % N (test terminated after 216 hrs)

some microcracks and attack with deposits

5 49 (6.0) 0.5 % N (test terminated after 960 hrs)

some attack with deposits

FIGURE 1 – Delta phase precipitates, visible on etched surface of an alloy UNS N07718 specimen, taken from failed tubing hanger.

FIGURE 2 – Appearance of tensile specimens from failed alloy UNS N07718 hanger after SSRT according to Table 3; - top: test 1 in air; - middle: test 2 in simulated annulus water with coupling to carbon steel; - bottom: test 3 in simulated annulus water.

FIGURE 3 – Cracked 22%Cr DSS 4-point-bend specimen; exposed to simulated 90% evaporated brine at 140 °C (Table 4, test 3).

FIGURE 4 – Detail of branched transgranular cracking found in 22%Cr DSS specimen; exposed to simulated 90% evaporated brine at 140 °C (etched to reveal duplex structure).

FIGURE 5 – Cracked 22%Cr DSS C-ring with secondary cracking visible; exposed to simulated 90% evaporated brine at 140 °C with oxygen.