Large diameter subsea pipelines are being used across the world for wet gas transportation over significant distances from offshore fields to onshore facilities. The ability to predict and control corrosion is necessary to ensure long term integrity of these pipelines. Large diameter wet gas lines operating in stratified flow pose unique challenges for top of the line corrosion which occurs when corrosive gases are found dissolved in condensed water at the top of the pipe. This paper will discuss a study undertaken by ExxonMobil1 and RasGas to establish a mechanistic understanding of sour top of the line corrosion and the operational guidelines developed to control its occurrence.
Top-of-the Line (TOL) corrosion can occur in wet gas pipelines operating in stratified flow when low pH water--devoid of inhibitors that are usually present in the bottom of the line fluids--condenses on the upper half of the pipeline causing severe corrosion.
One of the first reported TOL corrosion occurrences dates back to the 1960's at the sour Lacq field in Francei. It occurred in low velocity lines with stratified or stratified-wavy flow regimes. The second reported case was in the sour Crossfield pipelines where again TOL corrosion occurred in lines with low velocities and stratified-wavy flow regimeii. Although the first reported cases of TOL corrosion were in sour fields, sour TOL corrosion has historically been treated as a sweet corrosion phenomenon. Sweet TOL corrosion has been investigated extensively in the literature and several models have been developed to predict its occurrence iii, iv, v, vi, vii. These models are based on the formation of a protective FeCO3 film at the TOL. Sweet TOL corrosion is therefore limited by the amount of iron that can be dissolved in the condensing water. The initial pH of the solution at the TOL will depend on the partial pressure of CO2 and the presence of volatile organic acids in the gas stream. As iron is dissolved in the condensing phase, the pH increases and leads to the possibility of protective iron carbonate formation. At low condensation rate, the accumulation of iron in the TOL solution will increase the pH and result in the formation of protective iron carbonate scale. At high condensation rates, the dissolved iron is continuously removed. This leads to a lower TOL solution pH and instability of iron carbonate scale, resulting in higher TOL corrosion rate. Thus, in sweet systems a critical condensation rate can be predicted below which TOL corrosion should not be significant. The critical condensation rate has been estimated to be between 0.15 g/m2s and 0.25 g/m2s viii.