Annular Corrosion in Flexible Pipes
By M Mitzithra, J Rothwell and S Paul
In order to determine the susceptibility of steel wires in the annulus of a flexible pipe to various degradation mechanisms, it is necessary to understand the environment within it. The susceptibility to Sulphide Stress Cracking (SSC) increases with H2S partial pressure and lower pH values, as indicated in the environmental severity diagram of BS EN ISO 15156-2:2015. Other forms of SCC (e.g. SCC at high CO2 partial pressures) and more general corrosion issues are also of concern. Although, in general, annular corrosion is not found to be particularly severe, there have been instances referred to in the field (JIP Sureflex). In addition, TWI is aware of unusual instances of cracking in nominally CO2 rich (production) environments in the Pre-sal fields. Available corrosion rate data in literature, as relevant to flexible pipe steel wires at high CO2 partial pressures are limited. Prediction of the occluded environment is therefore essential to determining the risk to integrity from such issues. The occluded conditions of the annulus are not simple to predict or measure due to the confinement of the wires and the supply of components via permeation. In particular, temperature, partial pressure of acid gases and the degree of occlusion affect the corrosion rate, dissolved iron concentration and the pH.
Although various commercial software packages can be used to predict the pH of the environment from partial pressures and other factors used as inputs, it is important to experimentally verify such predictions through controlled experiments that simulate occluded annulus conditions. In addition, in order to be able to develop a reliable corrosion rate prediction model relevant to relevant steel wires, the collection of realistic corrosion rates for flexible pipe service conditions is required.
The report herein details a practical study carried out to determine the pH values relevant to flexible pipe environments via in-situ measurements, and the corrosion rates of steel wires subject to the same environments. Confined conditions approaching those in the annulus of a flexible pipe were created and CO2 partial l pressures at the extreme end of those expected in service were used. The resulting data are intended to expand the predictive domain of an empirical pH model and to develop an empirical corrosion rate prediction model.
- A test set up that simulates representative service conditions for flexible pipes has been successfully commissioned and used to study CO2 rich environments.
- The experiments carried out have provided a sensible range of pH values considered to be possible within a flooded flexible pipe annulus and cover the vast majority of conditions associated with CO2 rich service.
- The development of the environment includes the change in iron concentration observed. This presents a challenge to the accurate prediction of the annular environment (pH) using commercial softwares where iron concentration is required as an input.
- For a given test condition, the pH and corrosion behaviour are related to the dissolved iron concentration, iron carbonate precipitation kinetics and scale formation and characteristics.
- With respect to the effects of the experimental variables (temperature and CO2 partial partial pressure for same V/S ratio and CO2 inlet flow rate), the pH and corrosion rate are reduced for an increase in temperature and CO2 partial pressure.
Test set up simulating representative service conditions for flexible pipes
Average general corrosion rates at different CO2 partial pressures and occlusion ratio, for same temperature