Top of the Line Corrosion Mechanism for Multiphase Sour Gas Subsea Pipeline

The development of an offshore industry is directly related to the development of subsea pipelines as well. Top of the Line Corrosion (TLC) is a phenomenon of global importance in the subsea pipelines. TLC is driven by the condensation of water enriched with corrosive gases. TLC occurs in a multiphase flow when water vapor condenses at the top and the sides of the pipeline, leading to a severe corrosion attack. The condensation happens due to the heat exchange occurring between the pipe and surroundings seawater. TLC corrosion can be dominated by either the sweet corrosion or sour corrosion mechanism. Where sweet mechanisms dominate, the mechanism is dependent upon having a high condensation rate in excess of 0.25g/m⁼²s to maintain corrosion, as at lower rates, corrosion is stifled by the corrosion products within the condensing area. For the sour TLC mechanism, there is no minimum condensation rate beneath which TLC is stifled. In fact, there is little knowledge about the sour TLC in the literature. This study was conducted to determine the TLC risk for new sour gas pipeline network. Experiments were performed in order to determine the effect of temperature, water condensation rate, and organic acid on TLC corrosion and the formation of iron sulfide (FeS) scale. The results confirmed that TLC corrosion rates in sour environments are mainly dependent on iron sulfide scale characteristics which are a function of temperature. The weight loss maximum corrosion rate was 0.28 mm/y at gas temperature of 67 °C. The coupons surfaces were entirely covered by a dense iron sulfide corrosion product layer that seemed to have provided some natural corrosion protection. However, at gas temperature of 40 °C, the maximum corrosion rate was 1.02 mm/y. The entire surface was covered by a 50-100 μm thick iron sulfide corrosion product layer that seemed to have provided very little corrosion protection. In order to maintain the system integrity, the internal coating supplemented by V-jet batch inhibitor injection has been selected to protect against TLC.