Predict System Description

The figure below shows a flow chart delineating the hierarchical reasoning structure of the Predict system.



Based on data specified for different parameters, the system will instantaneously display the following results:

The user can specify data for any of the parameters and watch the effect of that parameter on the corrosion rate in the system instantaneously. The system starts with a set of default values and calculates a corrosion rate based on any changes to the displayed values. A typical consultation will involve the following five steps:

Specification of pH related data: At the outset, the system determines a corrosion rate only if the operating environment is acidic or has aeration. If the specified environment has no acid gases or there is sufficient buffering to produce a pH higher than 7.0, the system will predict zero or very low corrosion rates, except under conditions of aeration. So, the first step in consulting the system involves specification of the acid gas (H2S and CO2) partial pressures as well as the bicarbonate and acetate content of the environment.

Corrosion Profile: It is essential to perform corrosion analysis over a pipe length since many factors like change in temperature/pressure occur along the length. These changes affect many parameters including the water phase behavior which in turn affects corrosion rates. In some cases the system may show very high corrosion rates at certain points downstream due to the condensation of water. The systems enhanced calculation rules for predicting the water of natural gas and dew point calculations enable accurate predictions for water condensation ans thus corrosion rates. A glance at the corrosion profile informs the user of such problem spots in the pipe system where there is a high possibility of water condensation and thus very high corrosion rates.

Temperature/Gas-Water ratios: Temperature has a significant impact on corrosion rates since precipitation of corrosion products and scaling are functions of temperatures. Corrosion rates typically increase with increasing temperature, though in CO2 dominated systems, FeCO3 scaling at higher temperatures can produce significant protection against further corrosion. If the Gas to Oil Ratio indicates gas dominated conditions (as opposed to an oil dominated system) the system uses the water to gas ratio and the dew point as means to determine availability of an aqueous medium to measure corrosion. So, depending on the value entered for the Gas to Oil Ratio, the system will let you specify the relevant water-related parameters. If the Gas to Oil Ratio is less than 5000 scf/bbl (which denotes an oil well), the system uses the water cut and oil persistency to determine the wetness effect.

Chlorides/oxygen/ sulfur: Chlorides and sulfur typically make corrosion worse if the process has been initiated by the presence of acid gases. Their role, while not as critical as that of H2S or CO2, is significant because these parameters can significantly increase corrosion rates in mildly corrosive systems. Presence of oxygen beyond 20 ppb even in mildly acidic systems can lead to significant corrosion rates, especially with high chlorides and high flow rates at elevated temperatures.

Velocity/Type of flow: Flow parameters are very critical in both determining and controlling corrosion effects. Erosion corrosion as well as the protection (or the lack of it) from corrosion films is very much a function of fluid velocity. Velocity has a significant impact on mass transport with in the corrosion boundary layer and also impacts a corroding system's ability to form protective scales. Inhibition/corrosion allowance: Inhibition choices in the system allow the user to select applicable methods of inhibition for vertical or horizontal flow and determine the extent of corrosion mitigation. In some cases, the system might provide no protection due to inhibition because of high velocities or chloride concentrations. The system's rules assess the appropriateness of method of inhibition delivery for a given set of conditions.