During recent years, in the exploration for new oil and gas sources, the tendency towards developing offshore fields in more stringent environments has increased. Operators are now developing deep water, high pressure, high temperature, and high CO2 fields. This tendency has created numerous challenges to the project initial cost (CAPEX) and subsequent operational cost (OPEX). The material selection is critical for such fields showing high CO2 content. Although the initial capital cost for using carbon steel may be much lower than that of a corrosion resistant alloy (CRA), the total cost of the carbon steel option could be much higher due to the required corrosion inhibition programs as well as the potential for equipment failures representing losses from not only the cost of repairs, but also the loss of production. This paper addresses the material selection assessment for topside process equipment for production fluids with high CO2 content. A project specific case study is analyzed in terms of: 1) fluid/CO2 partitioning through the topsides production systems, 2) corrosion analysis involving multiphase (oil, gas, and produced water) and single phase streams, and 3) the material selection for associated piping and equipment.
Due to the large demand for hydrocarbons, operators are now developing deep water, high pressure, high temperature, and high CO2 fields. This trend results in numerous challenges to the capital cost (CAPEX) and operating cost (OPEX). The material selection is critical for such fields showing high CO2 content. Due to the complexity of the topsides processes, the piping and equipment selection represents an important part of the overall project costs (CAPEX/OPEX). In general, the topside processes are analyzed as follows: • Multiphase/crude streams: These streams carry the multiphase fluid from topside inlet, passing thru multiphase separators, to the crude storage/offloading facility. The streams involve equipment such as high, medium, and low pressure separators, heat exchangers, pumps, etc. • Gas streams: These streams carry the gas that is removed at the multiphase separators. The gas is generally treated (dehydrated) to be exported or to be injected into the field for maintaining the reservoir pressure. The streams involve equipment such as vapor recovery units, scrubbers, compressors, heat exchangers, TEG dehydration units, etc. • Produced water streams: These streams carry the produced water removed at the multiphase separators. The streams involve equipment such as hydrocyclones, produced water treatment vessels, pumps, etc. Note: Raw seawater streams are also prominent but are not included in the paper as they are not considered produced fluid streams. The material selection can be optimized based on a good understanding of the corrosion mechanisms and the fluid partitioning through the topsides production systems. Stainless Steel Duplex and Super Duplex Piping and Equipment (Solid and Cladding) While stainless steels such as UNS S32205 (duplex), UNS S32750 (super duplex), UNS S31603 (austenitic), and UNS N08904 (austenitic) are resistant to CO2 corrosion, they are susceptible to pitting and chloride stress corrosion cracking (CSCC).
