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Corrosion fatigue of welded C-Mn steel risers for deepwater applications: a state of the art review

S J Maddox, R J Pargeter and P Woollin
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 thefatigue 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 attentionhas been turned to establishing the fatigue performance in the internal environment, which may contain water, CO 2 , H 2 S 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 andoutside 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 containvarious 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 NorthSea [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 wereperformed 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°Cand 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 intemperature 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, therebeing 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/mm 2 ) 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 notedearlier, 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 HSEGuidance 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 H 2 S 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.

 

spsjmjune2005f5.gif

Fig.5. Influence of sour seawater on fatigue crack growth in C-Mn steels

As a whole, it will be evident from Fig.5 that seawater with H 2 S 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 H 2 S. 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 H 2 S 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.

spsjmjune2005f6.gif

Fig.6. Effect of cycling frequency on fatigue crack growth in a sour saltwater containing H 2 S at 0.035 bar partial pressure [20] .

Effect of CO 2

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% CaCl 2 fed with CO 2 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 ofthe 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 thepaper). 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 CO 2 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 .

spsjmjune2005f7.gif
 

Fig.7. Influence of hot 'sweet' saltwater and CO 2 solution on fatigue crack growth in C-Mn steel [21] .

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 CO 2 solution was higher than that for free corrosion in seawater above about ΔK = 500N/mm 3/2 . However, for lower values of ΔK the CO 2 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 CO 2 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 intests 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 CO 2 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 CO 2 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 H 2 S, 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 H 2 S or H 2 S 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 H 2 S 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 H 2 S 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 2 S 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 CO 2 , 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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