Life shortening threats to offshore hardware

TWI Bulletin, May - June 2009

Corrosion fatigue behaviour of welded risers and pipelines Part ll-examination of the effects of both sour (H₂S) and sweet (CO₂) environments

Professor Steve Maddox has been involved in a wide range of research projects at TWI related to the fatigue performance of welded components and structures, first as a research engineer, later as Head of the Fatigue Laboratory and, until 1989, Head of the Fatigue Department. In the 1970s, he collaborated with TWI colleagues in the formulation of the UK Government's major research project on fatigue and fracture of steel offshore structures (UKOSRP). During the past 20 years, the main source of research projects has been joint industry projects, and Dr Maddox has been involved in all those related to fatigue, either as Head of Department or Technical Supervisor.

Richard Pargeter joined the Materials Department of The Welding Institute in 1976. He gained a BA (Hons) from the University of Cambridge in that year having specialised in Metallurgy and Materials Science, and this was converted to an MA in 1980. He has been a Member of the Institution of Metallurgists since 1980 and was awarded Chartered Engineer status shortly afterwards. He is also a Fellow of the Welding Institute, a European Welding Engineer, and a Registered Welding Materials Engineer Group 1. He was Section leader of the Ferrous section within the Materials department between 1988 and 1999 and is currently a Technology Fellow.

Dave Baxter recently left TWI. He was a Senior Project Leader within the Structural Assessment section of the Structural Integrity Technology Group. He was primarily responsible for developing and conducting projects concerned with corrosion fatigue of steels and corrosion resistant alloys. He also managed numerous research and development projects for either single clients or joint industry teams within the oil and gas, marine or rail industries. Prior to joining TWI he had a similar role at QinetiQ where he managed Ministry of Defence and collaborative research projects to develop advanced metallic materials for various naval, automotive or aerospace applications.

The fatigue design of pipelines or risers in deepwater oil and gas developments, is often critically dependent on quantifying the extent to which aggressive service environments affect performance. Girth welds in these structures are often exposed to seawater on the external surface, and sweet or sour production fluids on the internal surface. As Steve Maddox, Richard Pargeter and David Baxter reveal, all of these environments can lead to higher rates of fatigue crack growth and lower overall life compared to performance in air. Part one looked at the effect of exposure to a seawater environment. Part two now examines the effects of both sweet (CO₂) and sour (H₂S) service environments.

Sweet service environments

The presence of carbon dioxide can lead to a variety of forms of localised corrosive attack. However, very few data have been published to quantify its effect on corrosion fatigue behaviour. With respect to endurance behaviour, most reports describe a significant reduction in fatigue life, and the loss of endurance limit when corrosion is involved. Mehdizadeh reported a 41% reduction in fatigue life when parent carbon steel was tested in CO₂-containing brine. Other data are less relevant to risers or pipelines. For example, Ebara reported that welded AISI 4330 steel suffered only a slight loss of fatigue strength at 10⁷ cycles in CO₂ gas (80°C, 90% relative humidity) compared with similar tests performed in air. However, the test frequency in this case was fairly high (13Hz), and so a minimal effect of environment might be expected.

Fatigue crack growth rate data are reported by Szklarz, where a series of tests were carried out in a simulated sweet operating environment. Tests were performed at 95°C in a pressurised solution containing 10%NaCl and 10%CaCl₂ fed with CO₂ at 3 bar pressure. At moderate frequencies (0.2Hz to 5Hz) crack growth rate was seen to be independent of frequency, but strongly dependent on Δ K. At low Δ K crack growth rates were similar to those seen in air. However above Δ K=500N/mm^3/2 crack growth rates were up to an order of magnitude higher than in air, as shown in Figure 5. This study also examined the influence of 500ppm inhibitor, which was found to be effective in reducing the crack growth rate only when included from the start of the test, and only at low DK. If inhibitor was added following a period of pre-corrosion the observed fatigue crack growth rate was the same as when no inhibitor was present.

At very low frequency (0.04Hz) crack growth rates at low Δ K were significantly higher than seen at higher frequencies, particularly when crack growth was along the weld fusion line. These data suggested that crack growth rate was independent of Δ K over the range examined. Eadie also performed tests at this frequency in a salt solution saturated with 10%CO₂. Crack growth rates were again significantly higher than in air, and higher than comparable tests carried out at higher frequency.

Sour service environments

Early work examining the effect of H₂S-containing environments was carried out in the 1970s by Vosikovsky. This work examined API 5L X65 pipe (non-welded), and tests were performed in crude oil containing H₂S at a stress ratio R=0.05 and cyclic frequency of 0.1Hz. The H₂S concentration was varied between 20 and 5000ppm. Watanabe et al also tested pipeline steel (welded and non-welded) in crude oil containing H₂S, this time at 400ppm, with R=0.03 and a frequency of 0.17Hz. A summary of the data from these two references is provided in Figure 6, along with the mean curve for C-Mn steels in air from BS 7910 (which is in good agreement with the baseline data). It can be seen that at high stress intensity ranges, crack growth is up to 15 times faster at 400ppm H₂S, and 25 times faster at 5000ppm H₂S.

However, crack growth rates in seawater containing H₂S can be somewhat higher than shown in Figure 6. Early work by Bristoll and Roeleveld looked at crack growth rates in a non-welded plain C-Mn steel, both in seawater and seawater saturated with H₂S (3000ppm).

These tests were carried out at R=0.6 or higher and a frequency of 0.2Hz. In plain seawater, crack growth rates were 2-3 times higher than in air, while in H₂S saturated seawater they were up to 50 times higher than in air. In both cases, the environmental enhancement was dependent on the value of applied stress intensity range, and was much lower at low values of Δ K, as also evident in the work referred to above.

Webster et al also carried out tests in seawater saturated with H₂S, this time with and without applied cathodic protection. Tests were carried out using steel conforming to BS 4360 Grade 50D (non-welded) at stress ratios of 0.05 and 0.7 and a cyclic frequency of 0.17Hz. The range of Δ K examined was slightly higher than Bristoll and Roeleveld, but there was reasonable agreement between comparable sets of data, as shown in Figure 7. At intermediate Δ K crack growth rates were typically 20 times faster than in air, but at high Δ K this factor increased to over 100.

A relatively recent study by Eadie and Szklarz has examined the effect of various mechanical and environmental test parameters on fatigue crack propagation in a medium strength low alloy steel, tested in sour dilute brine. The partial pressure of H₂S was again shown to have a noticeable effect on crack growth rate, as shown in Figure 7. It can be seen that crack growth rates were similar at low Δ K but tended to reach a plateau at high Δ K. This was a feature of all tests carried out within this study, and was attributed to the limiting rate of hydrogen diffusion. A decrease in test frequency (from 1Hz to 0.1Hz) was seen to increase the crack growth rate plateau, and led to a diminished influence of H₂S partial pressure. This study also examined the influence of test temperature, between 30 and 90°C. Despite the higher rate of hydrogen diffusion expected at elevated temperature, the observed crack growth rates were very similar, or slightly lower, than at 30°C. This was attributed to the hindering effect of surface scaling, although the embrittling effect of hydrogen may also be lower at this temperature. The influence of stress ratio was also examined, and although increasing stress ratio had a significant effect on lowering the Δ K threshold, crack growth rates at moderate and high Δ K were comparable over the range examined (R=0.1 to 0.6). Indeed crack growth rates at R=0.6 were slightly lower than at R=0.1 or R=0.3.

In a later study the same authors examined crack growth rates in X70 steel in a brine saturated with a test gas containing 10%CO₂ and 1%H₂S. At a frequency of 0.4Hz crack growth rates were comparable to those seen in earlier work. However, at 0.04Hz the crack growth rate was significantly higher. At low Δ K crack growth rates were comparable to those seen in air, and at high Δ K became limited by a frequency dependent plateau. Interestingly, this meant that the maximum environmental effect (relative to air) was seen at intermediate Δ K, as illustrated in Figure 8.

The influence of frequency was further investigated by Buitrago et al where a frequency scanning technique was used to examine the corrosion fatigue behaviour of welded X80 steel in a sour brine based on NACE TM 0177 Solution B. All tests were carried out at Δ K=348N/mm^3/2. The observed crack growth rate data, summarised in Figure 8, suggest that da/dN increases steadily as the frequency is reduced, although between 0.1 and 1 Hz the crack growth rate appeared to be relatively insensitive to frequency.

With respect to fatigue endurance, unfortunately there are very few published data for girth welded C-Mn steels in sour environments. The only published data relate to tests carried out by Buitrago and Weir, where strips extracted from girth welded API 5L X80 and X65 pipe were tested in NACE TM0177 solution B at H₂S partial pressures of 0.035-0.070 bar. Data for the specimens taken from 15.8mm wall thickness X65 pipe (tested at 25°C, 0.07 bar H₂S, a test frequency of 0.33Hz and an applied mean stress of 150MPa) are reproduced in Figure 9. Fatigue life was reduced by a factor of 10-20 compared to both strip tests and full scale tests carried out in air.

The experimental data reviewed above indicate that environmental and mechanical test conditions can have a significant effect on the observed endurance or crack growth rate. The need to ensure that design data are determined from tests carried out under conditions that realistically simulate those encountered in service cannot therefore be over-emphasised.

In seawater, the influence of applied electrochemical potential is well documented and design guides quantify the effect that this variable has on the expected life or crack growth rate. However, other variables such as cyclic loading frequency can also have a dramatic effect depending on the applied Δ K. At moderate to high Δ K the crack growth at low frequency (<0.01Hz) can be an order of magnitude greater than at higher frequency. Since design curves in standards such as DNV or BS7910 are based on data from tests carried out at frequencies representative of wave loading (0.15-0.5Hz) care should be taken when trying to determine behaviour at much lower frequencies. Both crack growth rate and endurance test data suggest that a factor of 2-3 may still be appropriate at low Δ K or stress range. However, at higher stresses the influence of frequency becomes far more significant and needs to be taken into account.

In sour environments, test frequency is again seen to have a dramatic effect on the crack growth rate, although the extent to which this is the case again depends on the applied Δ K. The concentration of H₂S has also been shown to have a significant effect on the observed crack growth rate. Other factors such as temperature or stress ratio have been shown to have a smaller effect.

It should also be noted, however, that most of the data referred to above relate to fatigue crack growth rate tests carried out to examine particular test variables. There are very few test data to quantify the effect of these variables on endurance behaviour, and while crack growth rate data may be useful in highlighting trends and identifying key variables, it is by no means certain that the same response will be observed in endurance tests. Endurance tests are dominated by the behaviour of relatively short cracks experiencing a low Δ K. Furthermore early crack growth is usually from a weld toe with the crack tip in a heat affected zone close to the weld root or cap. This is in contrast to crack growth rate data, which are typically determined using fracture mechanics specimens containing long cracks where the crack tip may be well advanced into the weld metal. The size of the crack, the applied Δ K and the microstructure being sampled can all affect the extent to which an environment degrades performance. Endurance tests in a simulated sour service environment should therefore be carried out to quantify the observed fatigue life reduction factor.

Practical considerations may also limit the extent to which some variables can be explored. For instance, very few data are available for corrosion fatigue tests carried out under conditions of variable amplitude loading. Similarly, although full-scale testing of girth welded pipe has become routine, comparable corrosion fatigue testing, with either seawater on the outside or sour fluid on the inside, presents significant technical challenges.

Conclusions

In sweet CO₂ containing environments

• At moderate frequencies (0.2-5Hz) crack growth rates at low Δ K are similar to those in air. However at higher Δ K (>500N/mm^3/2) crack growth rates are up to an order of magnitude higher than in air.
  
  
• Crack growth rates can be even higher at very low frequency (0.04Hz) and low Δ K up to 100 times faster than in air being recorded for crack growth along a weld fusion line.
  
  
• There is limited evidence to suggest that the presence of inhibitor reduces crack growth rate. However it is only effective at low Δ K and only if present prior to any significant pre-corrosion.

In sour H₂S containing environments

• In crude oil, the presence of H₂S leads to a marked increase in fatigue crack growth rate, up to 35 times that in air at high Δ K. The effect diminishes with decreasing Δ K and decreasing H₂S concentration.
  
  
• In seawater containing H₂S, crack growth rates can be higher still, in some cases being two orders of magnitude higher than in air. Again, the effect seems to be most noticeable at moderate or high Δ K.
  
  
• Fatigue crack growth rates appear to be no higher at 90°C than at 30°C, indeed under some circumstances it appears that they may be somewhat lower.
  
  
• Stress ratio appears to have only a marginal effect on crack growth rate, although it does have a significant effect on the observed Δ K threshold.
  
  
• Frequency again has a significant effect on the observed fatigue crack growth rate.
  
  
• There are relatively few published sour endurance data. However, in one study fatigue lives were a factor of 10-20 lower than in air.

The unabridged version of this paper, including references, can be found in the proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, OMAE 2007, San Diego, California, 10 -15 June 2007. Paper no. 29360