There are several corrosion assessments based on modeling and incorporation of industry data, perceived issues as they are considered relevant. Additionally material and corrosion matters will be related to mechanical integrity, localized corrosion evaluations, identifying where necessary the need for field or accelerated laboratory testing, as well as inspection and on line monitoring.
Corrosion Modeling Assessment
Corrosion modeling is typically performed using Norsok M506 modeling methodology to estimate the base case uniform CO2 corrosion rate. It is considered by industry to be quite conservative, but offers a good interpretation of general (uniform) corrosion rates.
The M506 model does not give guidance or interpretation for localized corrosion; however, the analyses has much general value, especially regarding extreme cases of high or very low CO2 corrosion susceptibilities.
Generally we can infer that any corrosion resistant alloys (CRAs) selected will not require corrosion inhibition, but for any mixed systems including carbon steels corrosion control will require an inhibition approach incorporating an appropriate level of monitoring, inspection, and maintenance.
The detailed corrosion analyses must look specifically at all localized corrosion threats such as microbial, galvanic, mesa, or crevice corrosion type mechanisms coming into effect in addition to base CO2 corrosion. These will mainly be operative in contact with carbon steel (piping and vessels) and so that interface must be attended, ideally by a combination of coating, monitoring and chemical combination. Other physical reasons for using CRA must be assessed for specific geometries such as dead legs, erosion, corrosion under deposits, external marine atmospheric corrosion, etc.
Steel and Inhibition
Regarding steel based parts, necessitated by higher pressures, certain systems will require corrosion inhibition using vendor recommended chemicals. These may be off the shelf, or specifically tailored to address mixed chemicals (cocktailing) performance. As a rule, the dosages applied will need to be of an adequate residual allowing for optimum efficiency of the corrosion reduction mechanisms. This is a critical matter as the distribution of inhibitor will be subject to non-uniform filming due to changing flow regimes, geometry and layout of piping and vessels etc.
Distribution of corrosion probes and coupons should be used to ensure that all wetted parts of systems are covered to give a wide and accurate measure of the corrosion status of all systems. Prior to the selection of any inhibitor mixture or formulation being used, the chemicals will need to be validated as being fit for purpose for the mechanisms of corrosion predicted.
The periods of inhibitor “loss-age” are dependent on the flow patterns in play at the time of the inhibitor loss of injection. But it is reasonable to assume that most “cathodic” (acting on cathode reaction sites) and filming inhibitors will be sufficiently successful in that regard.
Some dangers exist if the inhibitor mixture is effectively under anodic control (acting on anodic reaction sites alone), and under certain circumstances of low dose-age it is thought that accelerated pitting may occur due to an adverse anodic/cathodic area ratio. If that occurs rapid loss of wall may occur.
Flow Velocity and Shear Stress
The Corrosion Management Strategy must assess water entrainment tendencies for multiphase flows, and derive water droplet entrainment tendencies. Thus re-entrainment propensity will be best defined by some discrete flow assurance predictions (e.g. Olga/Hysis) to determine water wetting areas or sites, as well as areas subject to high fluid shear stresses (beyond the 150Pa as limited by Norsok M506).
It is suggested that once production has started the corrosion modeling routines can be re-visited and actual flow rates, flow regimes, etc. can be re-calibrated against monitoring and inspection data, as well as non-dimensional analyses to help determine corrosion risks as they may occur, and therefore give the company opportunity to respond in a timely manner and address any corrosion issues as early as possible.
Dead legs and stagnation
Most facility piping will have tight layouts, and there will likely be some geometry related to deadleg or laminar flow zones. The role of such deadlegs and semi-stagnant flow regime zones will present a challenge since inhibitors are often known to exhibit a loss of efficiency under such areas as active bacterial species in the formulation are depleted in such occluded areas. Therefore inhibitor distribution and replenishment are of critical importance of for all steel piping. The planned use of frequent fluid sampling will be an important part of the future more detailed Integrity Management process.