EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE UNIFORM CORROSION IN THE ANNULUS OF OFFSHORE FLEXIBLE PIPELINES

摘要

Flexible pipelines used in the offshore industry are composed of steel wires enclosed in an annulus formed by inner and outer thermoplastic sheaths. The CO_2 corrosion of the carbon steel wires located in this annulus occurs within restricted volumes of electrolyte. For instance, the typical V/S ratio between the volume of electrolyte and the exposed steel surface is in the order of 0.03 mL.cm~(-2). In such confined environments, corrosion measurements clearly show that the results classically obtained in bulk conditions (infinite electrolyte volume) do not remain valid. Thus, the corrosion rates measured in the annulus conditions are commonly 2 or 3 orders of magnitude lower than those predicted by classical CO_2 corrosion models. Moreover, measured pH is significantly higher than the saturation pH predicted by thermodynamic models such as CORMED. Recently, large progresses have been made in the understanding of the uniform corrosion processes in the annulus of offshore flexible pipelines. These progresses, related to the design of innovative experimental setups, the achievement of an important testing program and the development of new theoretical models are reported in the present paper.