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Annular CO2 Corrosion in Flexible Pipes

TWI Core Research Project 1151/2021

Overview

Although not usually severe, CO2 corrosion of annular elements has previously been reported (Taylor et al., 2002; Wood, 2017). Furthermore, unusual instances of cracking in CO2 rich environments in the Brazilian pre- pre-salt have been reported (De Motte et al., 2022). The environmental conditions of the annulus are not straightforward to predict or measure due to the confinement of the wires and the supply of corrosive components via permeation from the bore (see Figure 1). Temperature, partial pressure of acid gases and the degree of occlusion all influence the corrosion rate.

A study was undertaken at TWI in order to determine the annular pH and corrosion rates to flexible pipe environments.  Confined conditions approaching those in the annulus of a flexible pipe were created using CO2 partial pressures at the extreme end of those expected in service.  A high strength steel wire grade (>1000MPa Yield) commonly used in flexible pipes was tested.

 

Objectives

  • Develop capability for simulating flexible pipe annular environments relevant to service conditions
  • Determine the effects of temperature and CO2 partial pressure on the pH and corrosion rate, relevant to flexible pipe annular environments

 

Approach

A specialised test facility was commissioned with appropriate instrumentation (see Figure 2).  Three autoclaves were operated in parallel at different temperatures, to allow data for three different temperatures to be collected simultaneously.  The occlusion ratio (V/A) of the aqueous environment, gas flow rate and partial pressures were defined and continuously monitored.

Figure 1. Evolution of annular environment through permeation
Figure 1. Evolution of annular environment through permeation
Figure 2. Laboratory for simulated annulus testing
Figure 2. Laboratory for simulated annulus testing
  • The following variables were explored within the experiments: temperature 30-60°C; partial pressure 1-40 barg; confinement ratio 0.3-1ml/cm2; duration 58-124 days
  • A 3.5wt% NaCl solution and volumetric CO2 flow rate controlled at 0.001ml/min/cm2, mimicking permeation flux, was maintained
  • In situ pH and dissolved iron [Fe2+] measurements were monitored, and values were compared with commercial water chemistry model predictions
  • Test specimens were subject to visual inspection, metallography and scanning electron microscopy
  • Corrosion rates were calculated from weight loss measurements
  • Profilometry was carried out in order to assess the extent of localised corrosion

 

Results – pH and Dissolved Fe

The long term nature of environmental evolution under nominally constant conditions was reflected in the results (see Figure 3). The pH was seen to stabilise after approximately 30 days, whereas the iron concentrations that developed to peak values within the first few days continued to reduce beyond the 30 day mark.  Indeed, the effect of temperature and pressure was more pronounced in the absolute Fe concentrations than for the pH values, although trends between them were similar.  Water chemistry models agreed well with the pH measurements when the actual Fe2+ concentrations were inputted.

Over the range investigated, increasing the CO2 partial pressure increased [Fe2+] and reduced the pH (see Figure 4).  The effect of temperature on pH was not consistent at all pressures but overall, an increase in temperature related to a reduced pH.  The effect of temperature was more evident in the [Fe2+], with higher concentrations at lower temperatures.

For a given test temperature and CO2 partial pressure, the pH increased with the concentration of dissolved iron concentration.  The test solution remained supersaturated with iron throughout the duration of the experiments.  The variation of [Fe2+] could be visualised in the aliquots removed (see Figure 5).

The dissolved iron concentration is associated with the CO2 corrosion mechanism of steel (Nešić, 2007, Nyborg, 2002). Dissolution of CO2 acidifies the solution and accelerates the anodic dissolution of steel. The solubility of CO2 increases with pressure, explaining the more rapid acidification of solution and dissolution of Fe at 40barg CO2 partial pressure. However, the pH values appeared to substantially converge towards the end of the tests, whereas differences in dissolved iron concentration vs. temperature persisted. The stabilisation in pH values for all test temperatures at 40barg CO2 and 20barg CO2 over the long term can be attributed to the stability, structure and thickness of the scales formed.

Results – Corrosion Rate

It was shown that the corrosion rate decreased as CO2 pressure and temperature increased (see Figure 6).  This is in keeping with trends reported for carbon steels at partial pressures between 15-80barg CO2, 50-65°C (Choi, 2011; Bai, 2018).  However, in contrast to these studies, where corrosion rates were reported to vary between 1 and 20mm/year, the annular corrosion rates in the present study were found to be an order of magnitude lower, at circa 0.1mm/year.  The low corrosion rates are associated with the V/A, i.e., the degree of occlusion of the annulus and the supersaturated conditions.

Results – Corrosion Product

Low corrosion rates were associated with protective iron carbonate scale formed on the surface of the test specimens. At 60°C, the scale was thickest. Low V/A ratios, and higher partial pressures both seemed to promote scale (Mitzithra, 2020). At 30°C, more through thickness discontinuities were observed, and these may be implicated in the slightly higher corrosion rates observed. Formation of scales at such low temperatures are not often observed in more open systems, and may be somewhat related to the fluid flow, and increased solubility of siderite at lower temperatures (Anderko, 2000).

Outlook

This project has demonstrated that corrosion in confined annular environments is relatively reduced compared to those in more open systems. Although effects of temperature and pressure were evident, they are relatively inconsequential on an engineering level. However, disruption of the protective scale through upset conditions and fatigue warrants further investigation.

The [Fe2+] and pH were shown to vary significantly over time and this could have a consequence for cracking mechanisms. In particular, the reduction of these values are below those reported in some other short term studies. This highlights a potential issue, for example on the prediction of sulphide stress cracking (SSC) in mild sour annular environments, as it is dependent on both the H2S partial pressure and the pH.

The CO2 stress corrosion cracking cited in the literature remains a concern and should be studied further. However, incidences of this issue in the field seem to be rare and no incidents of cracking were identified in this study.

Acknowledgements

We would like to thank Petrobras for their guidance and support of the work, and the TWI laboratory technicians and engineers involved in carrying out this study.

For a list of references, please email: crp@twi.co.uk.

 

This project was funded by TWI’s Core Research Programme.

Figure 3. Solution pH and Fe concentration over time
Figure 3. Solution pH and Fe concentration over time
Figure 4. Effect of CO2 pressure on pH and [Fe2+]
Figure 4. Effect of CO2 pressure on pH and [Fe2+]
Figure 5. Visual indication of [Fe2+] seen in aliquots extracted over the test duration
Figure 5. Visual indication of [Fe2+] seen in aliquots extracted over the test duration
Figure 6. Corrosion rates calculated from mass loss
Figure 6. Corrosion rates calculated from mass loss
Avatar John Rothwell Principal Project Leader - Materials

John is a Principal Project Leader working in the Materials and Structural Integrity Group at TWI. He specialises in cracking phenomena as seen in ferritic steels and welds, and has many years of experience studying the degradation mechanisms in flexible pipes, particularly for mild sour environments offshore. Before joining TWI, John worked with Corus, the steel company, and also within the micro-silicon industry. John is a qualified welding engineer (EWE), and he received an MEng in Material Science and Engineering from The University of Sheffield.

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