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 looks at the effect of exposure to a seawater environment. Part two, to be published in a subsequent issue of Bulletin, will examine the effects of both sweet (CO2) and sour (H2S) service environments.
The seawater environment has been studied in some depth, and fatigue design codes provide advice on how steel structures are likely to behave under conditions of either free corrosion or cathodic protection. However, it is important to note that there are limits to how widely these guidelines can be applied, and for more complex environments, such as production fluids which are inevitably project-specific, design guidance is rarely available.
Laboratory testing provides a means of quantifying material behaviour in a simulated service environment, and allows the impact of various environmental variables to be explored. This is important as parameters such as temperature, H2S concentration or loading frequency can have a significant effect on the extent to which performance is affected. This paper provides a review of published information and recent research data, and highlights particular areas where existing data are limited and design challenges remain.
Steel catenary or top tension risers are widely used for deepwater applications and are subject to fatigue damage associated with wave or tidal motion and vortex induced vibration. Fatigue loading is particularly severe at the touchdown area where the near-vertical riser meets the pipeline or flowline. Depending on the adopted design, the latter can also be susceptible to fatigue damage from a variety of sources. For example, high pressure high temperature pipelines laid on the seabed may be susceptible to lateral buckling, where fatigue results from the thermal expansion cycle associated with shutdown-restart sequences.
Risers and pipelines are also exposed to potentially aggressive service environments on both the inside and outside surfaces. The corrosive effect of seawater on the external surface is typically controlled by the application of cathodic protection. However, this can lead to hydrogen generation on the steel surface, and the resulting degradation in fatigue performance needs to be accounted for in design. Sweet or sour produced fluids on the internal surface also have a significant effect on fatigue behaviour. The presence of salts and water, acidified by the presence of carbon dioxide or hydrogen sulphide, all contribute to the aggressiveness of the environment.
Laboratory testing to quantify the corrosion fatigue performance of welds in aggressive service environments can be performed in one of two ways. Endurance data are obtained by testing to failure of actual girth welds usually as narrow sections cut from girth-welded pipe ('strips'), and plotting the results to produce an S-N curve. The effect of environment is then expressed in terms of a fatigue life reduction factor, by comparison with endurance data for tests carried out in air. Standard design curves, such as those given in BS 7608, can then be offset by this factor. By contrast, when fracture mechanics calculations are carried out to determine critical tolerable flaw sizes, fatigue crack growth rate data are required.
Standard specimens can be used to generate fatigue crack growth rate v stress intensity factor range (da/dN-ΔK) data in the environment of interest, and an upper bound curve determined. The determination of suitable endurance or fatigue crack growth rate data is of course dependent on ensuring that tests are carried out under appropriate environmental conditions. There is a need for a mechanistic understanding of the processes involved in environmental attack, and the manner in which they influence fatigue behaviour. Service environments can be complex and it is important to have an appreciation of the environmental variables that can influence the extent to which fatigue performance is degraded. This paper, provides a review of relevant corrosion fatigue data, and discusses the influence of variables such as pH, temperature, H2S concentration and applied electrochemical potential.
Mechanical variables such as stress range, stress ratio (R) or cyclic loading frequency can also have an effect on the extent to which an environment affects fatigue performance. Relevant corrosion fatigue data are again reviewed, and their relevance to particular applications discussed.
Seawater environments
Corrosion fatigue endurance data for welded carbon steels were obtained over a period of years as part of European efforts to provide fatigue design data relevant to offshore structures operating in the North Sea. In general, these tests were carried out in either natural or artificial seawater, under conditions relevant to the North Sea. Typically this meant a temperature of around 5°C and a loading frequency of 0.15-0.5Hz. Evaluation of these data led to the widely accepted design recommendations contained in UK HSE Offshore Guidance Notes, as adopted by DNV and ISO. For steels freely corroding in seawater, the fatigue life was reduced by a factor of about three, and the fatigue limit disappeared. When cathodically protected the reduction factor at high stresses was only marginally lower (approximately 2.5), but at low stresses there is more benefit and lives approach those seen in air. Typical design S-N curves are shown in Figure 1.
Fatigue crack growth rate data were also determined in these research projects, and under free corrosion conditions there was a three-fold increase in da/dN relative to air. At low DK cathodic protection was effective in restoring in-air performance, although at high ΔK the crack growth rate remained high. Cathodic overprotection (-1100mVSCE) was particularly detrimental at high ΔK due to the generation of hydrogen. These and other relevant published data were collated and analysed to produce the recommended fatigue crack growth curves described in BS7910. These are summarised in Figure 2.
It should be noted however that both the S-N curves described in Figure 1 and the crack growth rate curves described in Figure 2 are derived from data generated under the specific test conditions. An increase in temperature from 5°C to 20°C has been seen to double the observed crack growth rate, although any possible effect was lost in experimental scatter during comparable endurance tests.
In the case of cyclic frequency a reduction in test frequency has also been seen to result in an increase in fatigue crack growth rate. Vosikovsky examined the effect of frequency on the corrosion fatigue behaviour of X65 steel in seawater, both freely corroding and cathodically protected. A schematic summary of the results is given in Figure 3. It can be seen that when cathodically protected a series of frequency-dependent plateaux is observed. At 0.01Hz the crack growth rate reached approximately 6x10-3 mm/cycle before da/dN became independent of ΔK, while at 0.1Hz this plateau occurred when the crack growth rate was just under 10-3 mm/cycle. Under free corrosion conditions the shape of the crack growth curves differed, and no distinct plateaux were observed. It can be seen that the observed increase in growth rate (relative to air) is highly dependent on frequency, but also on the applied ΔK.
The occurrence of a plateau, where crack growth rate is independent of ΔK over a specified range, is a common feature of corrosion fatigue data. These plateaux can arise from diffusion limited, time dependent, environmentally assisted cracking processes that cannot be sustained above a critical crack growth rate.
More recently, the effect of very low frequency cycling has been examined by carrying out so-called 'frequency scanning tests'. These are crack growth rate tests carried out under conditions of constant ΔK, where the cyclic frequency is varied in blocks to determine how frequency affects the fatigue crack growth rate, at a particular value of ΔK. By monitoring the crack growth rate over a relatively short crack increment, it is possible to determine da/dN for much lower frequencies than would be possible using conventional test techniques.
Figure 4 shows a typical set of data for X65 parent steel tested in 3.5% NaCl at -1050mVSCE. It can be seen that as the frequency decreases the measured crack growth rate increases until a plateau is reached, where it appears a further decrease in frequency has no further effect. At ΔK=400N/mm 3/2 the plateau occurs at approximately 0.1-1Hz. At a higher value of ΔK however, the observed increase in growth rate was initially similar, but a plateau was not reached until a much lower frequency, at least as low as 0.001Hz.
Unfortunately, corrosion fatigue endurance data for girth welds to quantify the effect of frequency are very scarce. However, one reference confirms that under free corrosion conditions, lives were shorter at 0.01Hz than they were at higher frequencies, most notably at high applied stress range.
From a practical standpoint, one additional factor that may influence the observed corrosion fatigue behaviour is the formation of corrosion products or calcareous deposits. These may form on crack surfaces, and act to wedge open the crack so that it does not experience the full range of applied stress or stress intensity factor. In these cases a higher threshold condition may be observed, although it would be inappropriate to take advantage of this as the presence of similar conditions should not be relied upon during service.
Summary
In seawater
- With or without cathodic protection, fatigue lives at high stress range are a factor of three lower than in air.
- When cathodically protected, fatigue lives at low stress range are comparable to those in air.
- However the majority of data are associated with tests carried out at around 5°C and 0.1-0.5Hz. At lower frequencies particularly at high ΔK and higher temperatures fatigue crack growth rates increase significantly.
End of part one....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.
Part two resumes the corrosion fatigue behaviour story, in a subsequent issue of Bulletin, with the examination of the effects of both sour (H2S) and sweet (CO2) environments.
