INTRODUCTION

ABSTRACT

In 2003, a major integrated oil & gas company initiated a study to define the cost of corrosion throughout the core operations with the objective of focusing plant, engineering, and research investment in corrosion control to the areas that had the largest economic impact on corporate performance. This paper reviews available literature on cost of corrosion, reports the methodology adopted for the study, and highlights the most important problems discovered in gas sweetening and fractionation plants.

For gas sweetening plants, it was found that 25% of the maintenance budget was committed to corrosion control. For gas fractionation plants, 17% of the maintenance budget was due to corrosion. For plants that were capacity limited, deferred production represented the most significant cost. For gas fractionation plants, deferred production costs were estimated to be four times the direct maintenance costs. For a gas sweetening plant, a day of deferred production in one unit at one plant would be on the order of 47% of the annual direct cost of corrosion for that unit.

Common failure mechanisms in amine units included velocity enhanced damage due to operation at throughputs in excess of the original design, selective weld grooving, hydrogen induced cracking (HIC), and on rare occasions, stress corrosion cracking (SCC). Failures in fractionation plants included HIC, sulfide stress cracking (SSC), and caustic SCC. All plants experienced corrosion under insulation, concrete degradation, and utility system corrosion, including firewater and steam generating systems.

Plants with buried pipework experienced severe soil-side corrosion that necessitated daylighting projects. Plants with marine facilities experienced atmospheric corrosion and there has been a major move towards nonmetallic materials for items such as deck grating.

Gas plants are classified as either sweetening or fractionation plants. In the Kingdom of Saudi Arabia, sweetening plants separate and sweeten produced gas, removing hydrogen sulfide and carbon dioxide by absorption in an aqueous amine stream, yielding sales gas, liquefied products, and sulfur. Fractionation plants separate and purify the liquefied products such as ethane, liquid propane, butane, pentane, and hexanes. Most of these products are consumed as petrochemical feed stock. Propane and butane are also export products. The company?s master gas system1 was developed in the mid 1970s and included three sweetening gas plants and two fractionating gas plants. Recent new discoveries have resulted in two new gas sweetening plants being constructed and additional facilities are in the planning stage.

The great majority of the company?s gas sweetening plants use Diglycolamine (DGA). Methyldiethanolame (MDEA) is used in one gas plant. Refinery operations also use gas sweetening processes. A broader range of treatments is used in the refineries, with the preponderance being monoethanolamine (MEA). Diisopropanolamine (DIPA), diethanolamine (DEA), and DGA are also used in refining operations.

In fraction plants, the Merox process is the principal treatment used to clean sulfur species from the product.

LITERATURE

In recent years, there have been several national Cost of Corrosion studies covering a range of countries. Additional papers have dealt with specific industries or projects such as the development of offshore fields. We have been unable to locate studies that specifically address gas sweetening or fractionation plants in any detail.

Figure 1 presents a summary of data from national studies. The average value of the cost of corrosion is 4.2% of the gross national product (GNP) of a country

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