The role of sand erosion in the integrity of process pipe-work
Sand erosion in process pipe-work is one of the key contributors to hydrocarbon leaks in the offshore industry. The parameters influencing the erosion process are numerous and a review of investigations presented in the literature has classified these in terms of particle, fluid and material properties. A review of the mechanistic and empirical erosion models currently available to predict the amount of erosion is also presented. This review highlighted the need for a quantitative investigation into the effect of surface roughness on the erosion process, as this has not been investigated or included in any of the models presented in the literature. The experimental investigations were twofold. The first set of experiments, referred to as the Surface Morphology experiments, utilised optical and scanning electron microscopy (SEM) to investigate the mechanisms of material removal of ductile target materials, when subjected to erosion by sand particles entrained in a liquid medium. Particle impingement was at impact angles oblique (30°) and normal (90°) to the material surface. The second set of experiments, referred to as the Surface Profile experiments, utilised surface profilometry to monitor the changes in the surface profile of mild steel plate specimens, when subjected to erosion at an oblique (30°) impact angle. Fast Fourier Transforms were used to convert the surface profile scans into wave-number spectra, to facilitate better monitoring of the changes in the material surface over the erosion exposure period. Five specimens with initial average surface roughness Ra in the range of 0.7 microm to 1.14 microm were used. The investigation showed that the surface roughness parameter plays a significant role in the erosion process, particularly in the initial stages. The operating conditions of the impinging jet were associated with an equilibrium condition, both for the surface roughness and the eroded depth. As a result, specimens having an initial surface roughness value greater than the equilibrium Rae value showed a decrease in surface roughness, whereas those having an initial value less than the equilibrium value showed an increase in surface roughness. This trend was confirmed by the changes in the amplitudes of the wave-number spectra for each plate, over the erosion period. The eroded depth was greatest for the plate having the smallest absolute difference between the initial Ra value and the equilibrium Rae value. This subsequently decreased as the absolute difference between the initial and equilibrium surface roughness value increased.