Corrosion Fatigue of Welded C-Mn Steel Risers

TWI

Proceedings of OMAE 2005: 24th International Conference on Mechanics and Arctic Engineering(OMAE 2005) Halkidiki Greece, 12-16 June 2005, OMAE2005-67499

Abstract

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the fatigue behaviour of welded carbon-manganese steel for risers in relevant environments. A substantial body of data exists relating to the performance of girth welds in seawater with cathodic protection and consequently recent attention has been turned to establishing the fatigue performance in the internal environment, which may contain water, CO₂ , H₂S and chloride and bicarbonate ions.

Introduction

Steel catenary and top tension risers for deepwater applications are subject to fatigue loading from wave and tidal motion and vortex induced vibrations. They also experience potentially corrosive media on both the inside and outside surfaces. The fatigue life of such structures is controlled by the behaviour of the girth welds and there is a need to quantify the effect of the internal and external environments on the fatigue behaviour of these girth welds. The corrosive effect of the external seawater environment is typically controlled by application of cathodic protection, although this leads to hydrogen generation on the steel surface, whilst the internal produced fluids contain various aggressive salts and may contain carbon dioxide and hydrogen sulphide which are both acid gases and contribute to the corrosivity of the environment.

Fatigue endurance tests

Effect of seawater

Corrosion fatigue endurance data were obtained from welded joints in carbon steels over a period of several years as part of the European efforts to provide fatigue design data relevant to offshore structures operating in the North Sea ^[1-3] . Since this research was directed principally at tubular steel jacket structures, the endurance data were obtained mainly from full-penetration welded cruciform joints, simulating the brace to chord connections in tubularstructures, or actual tubular joints. However, there is no reason to suppose that the relative effect of the environment on fatigue endurance would be significantly different in the case of girth butt-welds in pipes. The tests were performed either in artificial seawater (3% NaCl solution or to the ASTM specification), or sometimes in actual seawater, under conditions relevant to wave loaded structures in the North Sea. This meant a temperature of around 5°C and a load cycling frequency of between 0.15 and 0.5 Hz. This limits the applicability of the findings to other circumstances since, in general, it is found that the detrimental effect of the environment increases with increase in temperature and decrease in cycling frequency.

Evaluation of the results from the various European research projects has led to the widely accepted design recommendations contained in the UK HSE Offshore Guidance Notes ^[2] , as adopted by DNV ^[4] and ISO ^[5] . Briefly, for steels freely corroding in seawater, the fatigue life is reduced by a factor of about 3 and the fatigue limit disappears. In air performance is restored by cathodic protection, but only at low stresses, there being no benefit at high stresses. Indeed, cathodic protection might even be more harmful (from the fatigue viewpoint) than free corrosion at high applied stresses due to hydrogen embrittlement, especially in high-strength steels. Apart from the fatigue endurance data, account was taken of fatigue crack growth data (discussed below) when formulating these design recommendations. Examples of the resulting design S-N curves for a specific design category (Class E, widely adopted for riser girth welds) are shown in Fig.1. High-strength steels (yield strength > 500 N/mm² ) are excluded unless they can be qualified by reference to relevant data or by performing special tests. However, no limitations are given regarding temperature or loading frequency, even though, as noted earlier, the background data were obtained under relatively narrow temperature and frequency conditions. In fact, the effect of seawater temperature was addressed in a later phase of the European project ^[1] . Tests showed that the rate of growth of fatigue cracks could be doubled, or the fatigue life halved, as a result of an increase from 5 to 20°C. However, this was not taken into consideration in the drafting of the HSE Guidance Notes ^[2] .

√in (345 N/mm^-3/2 ), at frequencies between 0.01 and 10Hz. The sour environment was exactly the same as that used for fatigue endurance tests on strips cut from girth welded pipes discussed earlier ^[6] , saltwater containing H₂S at 0.035 bar partial pressure. The results are summarised in Fig.6. There was no clear evidence of saturation in the frequency effect, which is seen to increase steadily with decrease in frequency. The magnitude of the increase in da/dN due to the sour environment is consistent with the other published data obtained under similar conditions and at the same ΔK. The authors conclude that their crack growth data could be used to correct fatigue endurance data obtained at a higher frequency than required. However, the dependence of the detrimental effect of a sour environment on ΔK noted earlier ^[19] indicates that this could be far too conservative, particularly if, in practice, the near-threshold crack growth regime is relevant.

 

Effect of H₂S

The product in a riser pipeline can also influence the fatigue performance. Of particular concern are the 'sour' conditions arising when the product contains H₂S and saltwater. Only one reference could be found to comparative endurance fatigue tests on welded joints in air and a sour environment ^[6] and these show a very significant reduction in fatigue performance due to the environment. The specimens consisted of strips cut from girth welded steel pipes. For testing, they were immersed in a sodium chloride solution containing H₂S (NACE TM0177 Solution B) and cyclically loaded at a frequency of 0.33 Hz under positive stress ratios (not specified). Some of the specimens were extracted from relatively thick API X 80 pipe, 12.75" diameter by 1.625" wall thickness (324mm dia. x 41.3mm wt), and tested axially in seawater containing H₂S at a partial pressure of 0.5 psi (0.035 bar). The remainder were from 6.25" diameter by 0.625" wall thickness (158mm dia. x 15.8mm wt) X 65 pipe and these were tested in bending in seawater containing H₂S at a partial pressure of 1 psi (0.07 bar). The results are shown in comparison with published data obtained in air from fatigue tests on similar full size girth-welded pipes ^[7] in Fig.2. The strip test results obtained in air in bending are comparatively high compared with the database for full-scale pipes, suggesting an exceptionally high fatigue endurance limit of around 180 N/mm² . This may be a reflection of the mode of loading, bending tending to be more favourable in fatigue than axial loading. However, the sour environment has reduced that fatigue performance considerably, giving results that are all below the database for full-scale pipes. The results obtained from strips cut from thicker pipes in axial tension tend to lie near the lower bound to the database for full-scale pipes, again possibly due to the loading mode but also the thick section, both factors that tend to reduce fatigue performance. However, again the sour environment has reduced their fatigue performance. Since the only difference between the tests on the two pipe sizes was the partial pressure of the H₂S in the sour environment, it seems from these rather limited data that doubling the partial pressure of the H₂S from 0.035 to 0.07 bar doubles the reduction in fatigue life from a factor of around 10 to 20. However, the authors draw attention to the run-out obtained in the lowest stress test in the sour environment, suggesting that the effect of H₂S may be much less at low applied stresses, which would be highly significant to riser design since most service loads produce relatively small cyclic stresses. Clearly, far more test data are required to substantiate this.

Effect of CO₂

Another potentially harmful environment arises in the product carried by some pipelines under so-called 'sweet' conditions, from the presence of CO₂ and saltwater. However, no published fatigue endurance data obtained from welded C-Mn steels could be found.

Fatigue crack growth tests

Effect of seawater

The European research projects referred to earlier also included extensive study of fatigue crack growth in marine environments. However, again the testing conditions tended to be confined to water temperatures of around 5°C and cycling frequencies of 0.15 to 0.5 Hz. Briefly, key findings were:

• Free corrosion produces around a 3-fold increase in da/dN compared with air.
• The use of cathodic protection (correct or over-protection) restores in-air behaviour for ΔK values up to 144N/mm^3/2 and applied load ratio R > 0.5 (representative of the conditions experienced by welded joints containing high tensile residual stress) or 315N/mm ^3/2 for R < 0.5, but da/dN then increases at higher values.
• Cathodic over-protection (-1100 mV) is particularly detrimental at high ΔK and K_max due to the generation of hydrogen.
• Increasing the water temperature from 5 to 20°C increases da/dN by a factor of 2.

The data generated, and other relevant published data, were collected and analyzed ^[8] to produce the recommended fatigue crack growth curves given in BS 7910 ^[9] . These were set two standard deviations of log ( da/dN) above the estimated mean to embody safety comparable with that associated with design S-N curves. Examples are shown in Fig.3.

More recent work ^[10-12] on steels in seawater showed similar fatigue crack growth characteristics for high-strength steels, weld metals and heat-affected zones (HAZs), up to 900 N/mm² yield, but also confirmed the detrimental effect of cathodic over-protection. This was particularly evident in tests on simulated hard HAZs, above 350 HV, which were found to display higher crack growth rates at high K_max , particularly at very low frequency, due to the occurrence of inter-granular crack growth ^[13] . This was attributed to a combination of fatigue and hydrogen embrittlement. The low testing frequency was actually achieved by applying a saw-tooth shaped loading waveform, with the load applied very slowly and then released quickly. The aim was to maximize the proportion of the cycle during which the environmental action would be most pronounced. In the extreme case of 120 seconds per cycle, da/dN was increased by 1-2 orders of magnitude compared with crack growth at around 0.15Hz. However, even 30 seconds cycle duration produced significantly higher crack growth rates.

Effect of H₂S

Rather more information is available from fatigue crack growth than endurance tests on steels in sour conditions, although not as much as might be expected in view of the fact that the potentially harmful effect of sour service conditions has been recognised for decades. The available information is separated into that which considers the influence of H₂S (either dry or in oil) and that which considers the effect of H₂S and saltwater. Pioneering work performed in the 1970s by Vosikovsky ^[14] considered pipeline steel immersed in crude oil containing H₂S. The steel was from API X65 pipe and the tests were performed at R = 0.05 and a cycling frequency of 0.1Hz. The H₂S concentration was varied between 30 and 5,000ppm and, using the results obtained, Vosikovsky proposed a fatigue crack growth law of the form:

where A ₁ , A ₂ and m are constants.

Watanabe et al ^[15] also tested pipeline steel in crude oil containing 400 ppm H₂S. The tests were carried out with R = 0.03 at a frequency of 0.17Hz. The results from the two investigations involving steel in crude oil with H₂S are summarised in Fig.4. As will be seen, both the air data and those obtained in oil with around 400ppm H₂S from the two investigations are very similar. Also shown is the mean curve for C-Mn steels from BS 7910 ^[9] , which again is in good agreement with the air data from Ref. ^[13] and ^[14] . For the lower concentration (~400ppm) it will be seen that da/dN could be increased by up to 15 times, while the factor is up to 25 times for 5000ppm H₂S (saturation).

A third reference that is relevant included fatigue crack growth tests on steel in dry H₂S ^[16] . This was part of a comprehensive study of the effect of a sour environment on fatigue crack growth in steels, directed mainly at sour service from marine fouling. The results obtained in dry H₂S and the corresponding results obtained in air are included in Fig.4. The material was a cast steel similar to Grade 50 structural steel and the H₂S pressure was 1bar (equivalent to 3000ppm H₂S). The tests were performed at R = 0.05 and 0.17Hz. As will be seen from Fig.4, the results are consistent with those from Vosikovsky for crude oil and a high concentration of H₂S. Thus, again they show that da/dN can be increased significantly, by up to 35 times in this case, as a result of the presence of H₂S.

Webster et al ^[16] also performed tests at R = 0.7 and obtained even higher crack growth rates in H₂S. However, da/dN in the steel was particularly sensitive to applied mean stress and the resulting difference between air and H₂S was less than that seen at R = 0.05. Tests were also included at a higher frequency (50Hz) and, as expected in the light of corrosion fatigue behaviour in general, the environment had little effect on da/dN.

It will be evident from Fig.4 that the air and H₂S results tend to converge at low ΔK, implying the same threshold. Reference ^[16] considered that indeed the presence of H₂S was not detrimental with regard to the threshold or near-threshold fatigue crack growth behaviour.

Effect of H₂S in Saltwater

The important difference between H₂S in oil as a dry gas and H₂S in saltwater is that the saltwater is also 'corrosive' and hydrogen generated by corrosion is absorbed at the crack-tip. Thus, an even greater effect on fatigue crack growth can be expected, and this proves to be the case.

Early exploratory tests on structural C-Mn steels were performed by Bristoll and Roeleveld ^[17] in plain artificial seawater and saturated (3000ppm) with H₂S. The tests were performed at R = 0.6 or higher and a cycling frequency of 0.2Hz. In all cases the results extended down to the fatigue crack growth threshold. The effect of each environment increased with increase in ΔK, as seen in Fig.5. Thus, the typical increase in the da/dN due to seawater by a factor of 2 to 3 was found, while seawater with H₂S increased da/dN by up to 50 times for the range of testing conditions considered. It will be evident from Fig.5 that there is a strong indication that the threshold stress intensity factor was unchanged as a result of the environment, as appeared to be the case with dry H₂S and oil containing H₂S. It will also be recalled that the limited endurance data ^[6] from welded joints tested in salt-water with H₂S suggested that the effect of the environment decreased towards the fatigue limit. The consistency of those S-N data and the fatigue crack growth data could be checked by calculating the fatigue lives of the welded specimens using fracture mechanics, as detailed in BS 7910 ^[9] .

Webster et al ^[16] also included fatigue crack growth tests in artificial seawater saturated with H₂S. As well as the tests on steel freely corroding in seawater, with and without H₂S, tests were also included on specimens in seawater saturated with H₂S with cathodic protection applied. The results obtained under conditions closest to those used by Bristoll and Roeleveld are included in Fig.5. As will be seen, they cover a different range of ΔK values, extending to K _max values approaching the static fracture toughness of the steel in the relevant environment. However, the results for air, seawater and seawater with H₂S from the two investigations merged and therefore suggest very similar behaviour. A difference is that the apparent threshold ΔK values are considerably higher in the case of Ref. ^[16] . Indeed, they are high compared with published threshold data in general, as reflected in the values recommended in BS 7910 (10). For R = 0.7, as used in Ref. ^[16] , published data for steels suggest that the threshold lies between 63 and 160N/mm ^3/2 , whereas the results from Ref. ^[16] imply a value of around 250N/mm ^3/2 . However, it is interesting to see that a third set of fatigue test results for steel in seawater with H₂S tend to support this value. These come from Eadie and Szklarz ^[18,19] who tested medium strength low alloy steel in brine with H₂S. In this case, the environment was pressurised to produce H₂S partial pressures of 0.2, 2 and 16.5 bar. The tests were performed at R = 0.3 and at frequencies of 0.01, 0.1 and 1Hz.

The results included in Fig.5 are those obtained in air, seawater and seawater with the highest H₂S pressure investigated. The high threshold and convergence of the results in the near-threshold regime were attributed to beneficial crack closure resulting from corrosion products on the fatigue fracture surfaces. However, Ref. ^[16] claimed that there was no evidence of such crack closure and, further, that none would be expected at very high R. In this respect, it may have been significant that Eadie and Szklarz tested at R = 0.3.

Comparison of the fatigue crack growth rates measured by Eadie and Szklarz ^[19] at different frequencies confirmed that, as expected, rates increased with reduction in frequency. However, this was only the case at relatively high ΔK, such that there was no significant frequency effect between 0.01 and 5Hz in the highly significant near-threshold regime. Recently, Buitrago et al ^[20] focused specifically on the frequency effect with the hope of identifying a saturation frequency below which there would be negligible further increase in crack growth rate. The approach was to measure crack growth rates in compact tension specimens at R=0.7 and constant ΔK, 10ksi in (345 N/mm ^-3/2 ), at frequencies between 0.01 and 10Hz. The sour environment was exactly the same as that used for fatigue endurance tests on strips cut from girth welded pipes discussed earlier ^[6] , saltwater containing H₂S at 0.035 bar partial pressure. The results are summarised in Fig.6. There was no clear evidence of saturation in the frequency effect, which is seen to increase steadily with decrease in frequency. The magnitude of the increase in da/dN due to the sour environment is consistent with the other published data obtained under similar conditions and at the same ΔK. The authors conclude that their crack growth data could be used to correct fatigue endurance data obtained at a higher frequency than required. However, the dependence of the detrimental effect of a sour environment on ΔK noted earlier ^[19] indicates that this could be far too conservative, particularly if, in practice, the near-threshold crack growth regime is relevant.

As a whole, it will be evident from Fig.5 that seawater with H₂S has a significant effect on fatigue, particularly at high ΔK (or K _max ), where da/dN was increased by more than two orders of magnitude. Painted steel locally exposed to the environment gave the highest effect noted in Ref.16, an increase in da/dN by 570 times. In this particular case, the detrimental effect of the environment was partly due to the use of cathodic protection. As seen in Fig.5, rather than being beneficial this further increases da/dN in seawater with H₂S. Other tests ^[16] showed that cathodic over-protection (-1050mV) was even more harmful. This was attributed to increased availability of hydrogen at the crack tip.

Additional points to emerge were:

• da/dN increases with increase in H₂S partial pressure (0.2 to 16.5bar), but even the lowest pressure can be highly detrimental.
• da/dN increases with reduction in cycling frequency (between 0.01 and 10Hz).
• Data presented indicate that increasing the temperature of the sour environment from 30-90°C has no significant effect on fatigue crack growth rate.

However, many of the observations in Ref. ^[18] and ^[19] related to the effect of crack closure and corrosion deposits on the fracture surfaces. Such effects were less at high ΔK values and they would be expected to be less, or even absent ^[16] , at high R. Since it is expected that welded joints in real welded structures will contain high tensile residual stresses, the latter is more relevant and conclusions drawn in Ref. ^[18] and ^[19] should be treated with caution.

Effect of CO₂

Only one reference could be found to fatigue tests on steel in 'sweet service' conditions ^[21] . Fatigue crack growth tests were performed on pipeline steel in a pressurized solution containing 10% NaCl and 10% CaCl₂ fed with CO₂ at 3 bar pressure. The temperature of the environment was 95°C. The aim was to reproduce the environment in a gas pipeline and the work specifically addressed assessment of fatigue damage from the inside of the pipe under the high thermal strains that arise during shut-downs. Tests were performed at cycling frequencies from 0.04 to 5Hz. The applied load ratio was 0.15 (private communication with the author since it was not stated in the paper). The greatest effect of the environment was seen at the lowest frequency, 0.04Hz. Below around ΔK = 400N /mm^3/2 the crack grew at a rate of between 10 and 100 times faster than in air, independently of ΔK. This was attributed to crack tip dissolution rather than mechanical growth. At higher ΔK, da/dN was independent of frequency and all the results fell within the scatter-band shown in Fig.7. At low ΔK, da/dN was similar to that in air, but then at around ΔK = 500N/mm ^3/2 it increased rapidly to produce crack growth rates more than an order of magnitude higher than those in air at the highest ΔK reached in the tests. The fact that the detrimental effect of the hot CO₂ solution increased with increase in ΔK implies that the effect could be even greater at high R due to the higher values of K_max .

To put the results into perspective, they are compared with the mean crack growth laws recommended in BS 7910 ^[9] for air and seawater (free corrosion) for low R (<0.5), noting that Szklarz's air data were slightly lower than the BS 7910 mean. As seen, compared with crack growth in air the increase in da/dN in hot CO₂ solution was higher than that for free corrosion in seawater above about ΔK = 500N/mm^3/2 . However, for lower values of ΔK the CO₂ solution seems to be less harmful than seawater alone, and indeed slightly beneficial compared with air. This probably reflects the lack of oxygen in the CO₂ solution, but the authors note that the build up of corrosion product on the crack faces often inhibited crack growth, causing crack closure and hence reduced growth rates. The same effect has been observed in tests in seawater with cathodic protection, but it is ignored in BS 7910 on the basis that such build-ups cannot be relied upon to remain effective in service. It would be prudent to adopt the same view here.

Szklarz ^[21] also performed tests with 500ppm inhibitor added, as is commonly used to control CO₂ related corrosion. This was effective in reducing the effect of corrosion on fatigue, even at the very low frequency, but only at ΔK values below about 1000N/mm^3/2 . At higher values, the 'free corrosion' crack growth rate seen in Fig.7 was restored. Furthermore, introduction of the inhibitor after a fatigue crack had started to propagate was of no significant benefit.

The mechanism by which the CO₂ environment increases da/dN is not known. Szklarz ^[21] suggests that sulphides in the steel dissolve at the surface in the corrosive environment and produce localized concentrations of H₂S, thus giving rise to hydrogen embrittlement. However, he admits this is speculation.

Summary

• Extensive fatigue endurance and crack growth data from carbon steels in seawater have led to comprehensive design guidance. However, most experience is confined to North Sea wave loading conditions, that is a water temperature of around 5°C and a cycling frequency of 0.15 to 0.5Hz.
• The fatigue lives of welded joints are reduced in seawater, with or without cathodic protection at high stresses, typically by a factor of 3 but more at higher temperatures.
• The detrimental effect of seawater with cathodic protection can be even greater in vulnerable microstructures, such as in some high-strength steels and hard HAZs, due to combined fatigue and hydrogen embrittlement crack growth.
• There are few published fatigue data for welded joints in sour environments and none in sweet environments.
• The presence of hydrogen sulphide can be highly detrimental to fatigue performance, due to hydrogen embrittlement.
• However, hydrogen sulphide does not appear to have any harmful effect on fatigue crack growth rate at or near the fatigue crack growth threshold. In contrast, any harmful effect increases with increase in ΔK, or more probably K _max .
• Dry H₂S or H₂S in oil can increase the fatigue crack growth rate by around 35 times at saturation level; the effect is less at lower concentrations.
• Produced water containing H₂S is far more harmful, increases in fatigue crack growth rate by more than two orders of magnitude having been reported. The effect can be even greater if cathodic protection is applied.
• The detrimental effect of H₂S increases with increase in applied stress ratio (probably partly because K _max also increases, but also because crack closure is inhibited) and decrease in cycling frequency.
• Limited evidence suggests that H ₂S in seawater is no more harmful at 90°C than at room temperature. This is consistent with the fact that susceptibility to hydrogen embrittlement is greatest at around room temperature and lower at elevated temperature.
• Only one publication dealing with the effect of a sweet environment, saltwater with CO₂ , was located. This showed that the environment, at 95°C, could be highly corrosive in the absence of an inhibitor. Under very slow cycling (0.04Hz) the crack growth mechanism was essentially by crack tip dissolution, leading to an increase in da/dN by 10 to 100 times compared with da/dN in air. At lower frequencies, the environment was not detrimental in the near-threshold regime but above around ΔK = 500N/mm ^3/2 the rate of crack growth increased by up to an order of magnitude with increase in ΔK, or more probably increase in K _max . The use of an inhibitor eliminated the effect of the sweet environment, but only at low ΔK. However, it was important to ensure that the inhibitor was introduced at the start of life, since it was of little value if it was introduced after a fatigue crack had started to propagate.

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