INVESTIGATION OF A CORROSION FAILURE IN AN OFFSHORE GAS COMPRESSION SYSTEM: THE ROLE OF H2S SCAVENGER AND ORGANIC ACIDS

摘要

A corrosion failure was experienced in the gas compression module of the Gorm C platform in the Danish sector of the North Sea. It was found that the carbon steel discharge pipe from IP compressor A had ruptured causing gas to leak to the module. This paper describes the subsequent investigation into the corrosion mechanism responsible. The pattern of corrosion seen in the ruptured Gorm C pipe shows clearly that the corrosion is related to the injection of H_2S scavenger into the dry gas phase, 5 percent relative humidity (RH) at 135 deg C. The corrosion attack is seen as a gradual increase in metal loss from just downstream of the scavenger injection point to the point of rupture immediately after the first bend. The metal loss is seen where a continuous phase of liquid scavenger is expected to be present. Corrosion tests carried out on the commercial scavenger product containing the active component 1,3,5-tri(2-hydroxyethyl)-(2H,4H,6H)- hexahydro-1,3,5-Triazine show clearly that the corrosivity as a function of temperature increases dramatically at a temperature of 80-100 deg C. This is attributed to a thermal breakdown of the product reverting to the reactants of formaldehyde and monoethanolamine (MEA), and, in turn, in the formation of formic acid, which reacts with MEA to form the salt. This is supported by the finding of ester /carboxylic acid, proportional to the metal loss, in the surface layer of the pipe. Inclusion of CO_2 in the corrosion tests results in a further increase in corrosivity, probably due to the absence of a protective iron carbonate layer that would be expected to form on carbon steel in the presence of CO_2 at the given test conditions.