ABSTRACT

In this work, a case study for the Life Cycle Costing (LCC) analysis to quantify and compare different corrosion mitigation methods based on their effectiveness and make an informed decision on the strategies to minimize the costs due to corrosion is presented. The LCC for corrosion constitutes the costs of corrosion monitoring and control, the equipment replacement costs, and the loss of production due to down time. In this work, the corrosion rates encountered in a gas gathering facility is assessed based on data provided in corrosion monitoring and inspection reports. The effects of employed corrosion control strategies on observed corrosion rate values are analyzed to develop a quantitative estimation of their effectiveness. A model is developed to calculate a resulting corrosion rate based on a selected combination of corrosion control methods. An economic analysis is then performed to include all elements for corrosion costs. The process is repeated for scenarios with better corrosion control and the optimal LCC strategy is proposed by comparison of various scenarios.

INTRODUCTION

Oil and gas facilities such as booster stations, gathering centers and effluent water disposal plants employ multiple strategies to control corrosion. These include addition of chemicals such as corrosion and scale inhibitors for reducing uniform corrosion, and biocides to limit microbial effects on localized corrosion. Coating of tanks, cathodic protection and their maintenance over time is also part of corrosion control strategies. The tradeoff between the cost of these strategies over the reduction of corrosion rate of components is evaluated in this work. Bayesian models are employed to extract the uninhibited corrosion probability distributions based on environmental conditions. The effectiveness of corrosion control strategies is estimated using available data and expert knowledge. The resulting inhibited corrosion rates upon selection of desired corrosion control strategies are then computed using separate Bayesian models. Life cycle corrosion costs are obtained based on annual costs of corrosion control methods employed and the costs of replacement of the equipment in facilities due of corrosion related failure. The analysis provides a way of selecting most cost-effective corrosion control methods for minimizing corrosion and maximizing lifetime of the facilities. The LCC for corrosion constitutes the costs of corrosion monitoring and control, the equipment replacement costs, and the loss of production due to down time. Minimizing the replacements to zero might not be practical due to the highly corrosive conditions in some of the facilities. On the other hand, frequent replacements could result in very high costs especially accounting for loss of production during the facility stoppages for replacements. Reliance on only corrosion control through chemical additions or cathodic protection (CP) may also be expensive. Therefore, a trade-off between the selected corrosion control methods and planned replacement is evaluated for each facility for the optimal overall LCC. The benefits of corrosion control in terms of increased product revenue, etc. are not considered in this analysis.

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