ABSTRACT
The systematic studies to evaluate the corrosion resistance to Sulfide Stress Cracking (SSC) and crevice corrosion susceptibility of UNS S30400 and S31603 carcass material showed that serviceability limits to these materials can be extended to: 60°C, 1%H2S 99%CO2, pH>4.0, 60,000ppm Cl–. Notwithstanding, the present field conditions at Campos Basin is still less aggressive than the new serviceability limits just mentioned, but the oil production scenarios may change to more aggressive environments in a near future. Therefore, the test program was extended to lean duplex stainless steel, such as UNS S32101 and S32304. The aim of this test program is to provide technical data to support the replacement of austenitic flexible pipe carcass, at Campos Basin, by low cost materials with suitable mechanical properties and corrosion resistance when the field conditions turn too aggressive to carcass materials in operation. According to results presented herein, in the range of 70 to 90 °C, up to 4% H2S, 96%CO2, 120,000 ppm Cl– and pH>3.8 the UNS S32101 and UNS S32304 are not susceptible to SSC or any other type of localized corrosion, pits or crevice corrosion.
INTRODUCTION
Flexible pipes transports oil, gas, water and other fluids in oil and gas production. At Campos Basin in Brazil flexible pipes are widely used in offshore applications. These structures are made of various layers of steel wires for structural reinforcement and several polymers layers to restrain the internal bore fluid and protect the metallic components from the seawater. The carcass main function is to prevent collapse of the inner line when it is subjected to higher pressures1. Since fluid is allowed to leak through the carcass, it is only subjected to external compressive forces. Thus, to test these components at simulated field conditions it is necessary to consider the fluid composition, pH, pressure and the resistance to compressive stresses in that specific environment. Typically, the carcass is made from austenitic stainless steel such as 316L (UNS(1) S31603) and to deep water applications, especially to highly corrosive production fluids, duplex stainless steel like 22%Cr (UNS S32205) has being used2. Therefore, the choice of carcass material is guided by its mechanical properties, corrosion resistance and competitive prices. Lean duplex stainless steel are not contemplated on in ISO(2) 15156 part 3. However, exists a consensus that table A.24 may be used as a guide. In the requirements specified in ISO 15156 part 3, table A.24; the duplex stainless steel shall be in the solution annealed condition3. Once the carcass material has a significant amount of cold work inherent of conformation process, qualification tests must be done in order to verify their performance on field conditions and guarantee safe operation. The qualification tests are even more relevant to lean duplex because not only their PREN (Pitting Resistance Equivalent Number) are lower than 30 but also the molybdenum content is less than 1.5% (as specified on table A.24).
