Evaluation of The Effects of CO2 Partial Pressure on The Corrosion Fatigue Behavior of Flexible Pipes Tensile Armour Wires

Abstract

Abstract The upcoming developments in the pre-salt area, offshore Brazil, will require the expansion of the current qualification envelope for the materials used in subsea equipment. It is expected that such fields present a higher concentration of CO2 when compared to the ultra deepwater developments carried out to date, thus leading to more aggressive environments when compared to those already known to the offshore industry. Due to the gas permeation behavior inherent in the flexible pipe's materials, any increase in the CO2 concentration in the conveyed fluid will be reflected as an increase in the CO2 concentration in the pipe annulus, subjecting its metallic armor layers to a more aggressive environment. Based on this scenario, Wellstream has established a research and development programme, in partnership with CEFET-RJ, to evaluate the effects of the CO2 partial pressure on the corrosion fatigue behavior of tensile armor wires used in flexible pipes. The scope of the research programme focused on the design of a corrosion fatigue test fixture for tensile armor wires in environments up to 15 bar CO2, and the execution of corrosion fatigue tests using different partial pressures of CO2 whilst maintaining the consistency of the other test variables, such as test setup, test frequency, test media, tensile armor wire supplier and material batch. Upon completion of the test programme, it was possible to quantify the effects of the CO2 partial pressure on the fatigue service life of tensile armor wires in flexible pipes, which generated valuable test data to support flexible pipe design for such harsh environments. Introduction Unbonded dynamic flexible risers represent a key integrating technology for offshore developments, playing a major role in connecting subsea structures with surface floating production units. The versatility of flexible pipes provide a wide range of applications due to their unique structural characteristics, which comprise the use of helically wound metallic wires and tapes, and extruded thermoplastics, to form a composite pipe structure designed to address field specific requirements. Figure 1 presents a typical rough bore flexible pipe structure.