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The Effect of H2S Consumption on SSC Susceptibility of High Strength Wire Grades for Flexible Pipes

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Flexible pipes are widely used in offshore installations especially in Brazil to produce and export oil and gas. In order to guarantee safety operation in pre-salt fields without super estimate the field corrosiveness many efforts are dedicate to study the actual H2S concentration in the pipe annulus. The data presented in the literature point to a considerably lower H2S concentrations than those predicted based on traditional annulus permeation models. The consequences of these studies have direct impact on flexible pipes design suggesting the possibility to use lighter lower cost with good performance structures. However to specify correctly the armor wires is crucial to evaluate the corrosion resistance on annulus condition especially the stress corrosion cracking. The present study goal is to evaluate the corrosion performance of a range of flexible pipes armor wires considering susceptibility of SCC and HIC on laboratory simulated annulus condition. The laboratory tests were conceived to measure and compare pH H2S and Fe2+ concentration during the whole test period. The H2S flowrate was constant at 4 mL/min. All experiments were carried out at 30 oC 0.1 MPa using a gas mixture with 1% H2S in CO2. It was also controlled the ratio of liquid phase (synthetic seawater) and steel surface area (0.7 mL/cm2). The studied flow rate was 10-6 mL/min/cm2. A blank test without steel wires revealed that pH was stable at 4.9 and the maximal H2S concentration recorded was 20 ppm both after 150 h of test. On test simulating the annulus condition the pH was around 6.1 from 20 h of immersion until the end of the test (2000 h). The H2S concentration was always lower than 1 ppm and the Fe2+ concentration increased with immersion time up to 1200 ppm. Among the three high strength wire grades tested by four point bend test only one was susceptible to SCC under the conditions tested. Further studies will be presented concerning the HIC and microstructure influence on corrosion performance. The aim of this test program is to provide technical data to support future modifications on flexible pipe design with guarantee of good corrosion performance of armor wires.

Product Number: 51319-13483-SG
Author: Merlin Bandeira
Publication Date: 2019
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Picture for HIC Resistance Testing of Flexible Pipes Pressure Armor in H2S-Containing Environments
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HIC Resistance Testing of Flexible Pipes Pressure Armor in H2S-Containing Environments

Product Number: 51319-13368-SG
Author: Jonas Sá
Publication Date: 2019
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Flexible pipes may be exposed to high pressures during deep-water operation. Pressure armor layer is designed to withstand the hoop stress that is caused by the inner fluid pressure. It is wound around an internal polymer sheath to isolate it from production gases and fluids. However wear and tear damage or even diffusion may cause gas or fluid buildup in the annulus exposing the pressure armor to environments that may contain H2S. Depending on the mechanical properties and microstructure of the steel the absorption of hydrogen generated by corrosion in a H2S-containing environment can lead to the failure of these layers. Hydrogen induced cracking (HIC) tests will be carried out in H2S-containing environments in order to assess the resistance to HIC of two pressure armor wires (with distinct microstructures). The tests will be carried out for different durations in order to investigate crack growth. After each test the specimens will be examined by visual and ultrasonic inspection followed by sectioning of the specimens at any suspicious regions with subsequent metallographic examination of the cut faces.
Picture for Adaptation of NACE Standard TM0177 C-Ring Test for Flexible Pipes Carcass
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51313-02680-Adaptation of NACE Standard TM0177 C-Ring Test for Flexible Pipes Carcass

Product Number: 51313-02680-SG
ISBN: 02680 2013 CP
Author: Merlin Bandeira
Publication Date: 2013
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Picture for Corrosion of Armour Wire Steel in the Annulus of Flexible Pipes at Near Neutral pH
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Corrosion of Armour Wire Steel in the Annulus of Flexible Pipes at Near Neutral pH

Product Number: 51319-13302-SG
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Stress corrosion cracking (SCC) of carbon steel may occur at near neutral pH in deoxygenated solutions with high bicarbonate and iron (II) ion concentration. Similar conditions are encountered in the annular space of flexible pipes when it is filled with CO2 containing condensed water or seawater. The CO2 diffuses from the bore into the annulus through the polymer sheets. Corrosion of the armour wires in contact with the water will result in high levels of Fe2+ and bicarbonate (HCO3-). Several studies have indicated that SCC may occur in solutions with HCO3- concentration by the production of elemental hydrogen from the corrosion reaction. The objective of the present study was to investigate if such conditions can persist for sufficiently long time to induce SCC of armour wire steel. It implies that the nucleation and growth of siderite (FeCO3) is so slow that the solution remains supersaturated for days to weeks. At temperature lower than 40 °C at CO2 partial pressure 0.1-0.05 bar saturation ratios (SR) of siderite can remain much higher than 1 and maintain a near-neutral pH for long time. The conditions that may induce SCC in the annulus flexible pipes are thus possible but the corrosion rate of armour wire steel at these conditions was low at room temperature. Siderite precipitated at the steel surface and the cathodic reaction rate became diffusion controlled. However the pores present in the siderite layer may be suitable sites for the H+ reduction and adsorption of hydrogen in the material.
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