Qualification of High Strength Carbon Steel Wires for Use in Specific Annulus Environment of Flexible Pipes Containing CO2 and H2S

Abstract

Flexible pipes have been used for many years in offshore applications for the transportation of crude oil, gas and water. Such structures are subjected to mechanical loads due to tension, high internal pressure and dynamic motions which are sustained by the use of high strength carbon steel wires. The steel wires are located in the annular space that may contain water and acid gas (CO2, H2S) which can be detrimental for steels. For that reason, risks of Sulfide Stress Corrosion Cracking (SSCC) and Hydrogen Induced Cracking (HIC) shall be considered. Moreover, for dynamic conditions, presence of corrosive environment in the annulus could significantly reduce the fatigue performance of flexible pipe compared to air environment. The annulus composition is calculated using a permeability model that has been validated with medium scale tests, full scale tests and field cases [1]. In parallel, corrosion and corrosion fatigue studies have shown that the annulus of a flexible pipe is a very confined space with over-saturation in iron and no renewal of oxygen. This results in a higher pH, much lower corrosion rates and less HIC and SSCC than one would expect based on normal environments. This paper presents, in a first part, the different results obtained during small scale corrosion and corrosion fatigue tests demonstrating the beneficial specificity of the annular space. In a second part, the paper presents the results of pH measurements and corrosion rates obtained during a full scale dynamic corrosion fatigue test in CO2/H2S environment validating the previous results. This test was conducted on a 6” pipe between 1999 and 2003. In a third part, the paper describes how the annulus specificities should be used today to determine the suitability of carbon steel wires for use in flexible flowlines and risers considering SSCC, HIC, corrosion and corrosion fatigue.