Corrosion Fatigue of Steel Armours in Flexible Risers Available to Purchase

Fatigue life calculation of flexible risers is normally based on the assumption that the annulus is dry. Experience has, however, shown that the annulus may become water flooded. Carbon dioxide and hydrogen sulphide permeate from the bore and into the annulus, making the annulus environment corrosive. In such conditions the influence of corrosion fatigue on fatigue life has to be taken into account. A test program was performed to investigate the corrosion fatigue performance of steel armour in water containing carbon dioxide. The results showed that the corrosive environment reduced the fatigue life to about 1/10 to 1/50 of that derived in air. The lifetime was significantly shorter in the environment with 0.1 bar compared to 0.01 bar, indicating that corrosion fatigue becomes more pronounced as the corrosivity increases. The work shown that corrosion fatigue has to be considered for flexible risers with annulus that is or may be water flooded.