No abstract

Wet gas corrosion rates of plain carbon steel at the top and bottom of a high-pressure, 10-cm-diameter, horizontal pipeline were measured under annular flow conditions at 0.45 MPa carbon dioxide (CO 2 ) and 90°C. The corrosive medium consisted of deionized water and a low viscosity hydrocarbon phase at water cuts of 80%. Superficial gas velocities (V sg ) of 15 m/s and 20 m/s were applied at a superficial liquid velocity (V sl ) of 0.05 m/s for 100 h. The corrosion rates measured by electrical resistance (ER) technique advanced in three stages, showing an exponential decay corresponding to the iron carbonate (FeCO 3 ) scale forming process. The general corrosion rates measured by ER were low, which were also confirmed by the coupon weight-loss measurements of average corrosion rates. No flow-induced localized corrosion (FILC) was found for the flow and corrosion conditions applied. Surface analysis techniques such as scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) revealed that the FeCO 3 film forming process was flow dependent and different for scales formed on the top and on the bottom of the pipe, resulting in different crystal sizes. Cross-sectional analysis by use of metallurgical microscope (MM) indicates that a low thickness of the corrosion product film, usually <10 µm, was responsible for the high protection of the scales formed under these conditions. KEY WORDS: annular flow, carbon dioxide corrosion, carbon steel, corrosion rate, iron carbonate film, localized attack, surface analysis, wet gas

A sample iron molybdenum alloy with 3.4 wt.-% (2 at.-%) molybdenum, and pure iron, are exposed to hydrogen sulfide saturated saline solution for up to 56 d. In addition, their behaviour under anodic polarization in the same electrolyte is investigated. The initially fast dissolution of the iron molybdenum alloy slows down significantly over time, while iron cor-1 rodes with a constant rate. The observed slow down of the corrosion rate can be described well with an exponential decay of the instantaneous corrosion rate with a time constant of (0.15 ± 0.03) d −1 , which implies stop in corrosion in practical terms after ≈2 w. Relationships are discussed between the instantaneous corrosion rate, and the time-averaged integral corrosion rate. Dissolution under anodic polarization of the iron molybdenum alloy is slower than for pure iron. While at certain times, pyrite, FeS 2 , is found as corrosion product, the main corrosion product is mackinawite, FeS. The latter likely contains a certain fraction of molybdenum in case of the iron molybdenum alloy. On iron molybdenum, corrosion products forming a sealing layer are observed, which slow down further corrosion. The corrosion products on iron molybdenum show better adhesion to the base material surface.

Over the last decades an increase in the exploration and exploitation of impure oil and gas resources in remote environments under aggravated conditions has become necessary. This led to a growing demand for pipes with resistance to sour service conditions. Salzgitter Mannesmann Line Pipe has enhanced its product range of High-Frequency-Induction (HFI) welded pipes in recent years accordingly. In the process of HFI welding of pipes, forming roles bend steel coil into a pipe which is then welded together without any filler metal. This cold forming results in residual stress, depending on the diameter and wall thickness of the pipe. The current state of technology is based on the perception that this residual stress has an adverse effect on the resistance of line pipes to HIC, because it amplifies — or if it is sufficiently high — even triggers the onset of HIC. Aim of this paper is to study the influence of residual stress on the resistance in HFI welded pipes to HIC with regard to process-specific influencing factors. Four material strengths are selected for the tests. The first three material strengths (API 5L Grade from Grade B up to X65) are intentionally produced from non sour service material in order to obtain sufficient HIC damage. The highest material strength examined is a sour service material alternative to ascertain whether under optimal material conditions HIC indications can result solely from high residual stress. Plate and pipe segments are examined by means of the cross-sectioning method for longitudinal and circumferential residual stress at the process steps that influence the residual stress. A series of experiments under simulated residual stress to determine the HIC resistance of these pipe materials in NACE TM0284 test solution A is carried out using the four-point-bend test according to ASTM G 39, usually applied in sulphide stress cracking tests of line pipe steels. A characteristic HIC value, the crack area ratio CAR, is determined as a function of C and Mn content and residual stresses. To verify the results, FEM was used to model a test bar with the same geometry and to re-calculate the above-mentioned case. The results of these experiments combined with the supporting theoretical considerations and modelling prove that in the case of HFI welded line pipes, the residual stress induced by the process has no negative impact on the resistance of HFI welded pipes to HIC.

The challenging environment appearing in recent and moreover future deep offshore explorations promoted the development of linepipe steel grades with reliable sour service resistance. Severe sour conditions such as the combination of elevated production temperature, increasing pipeline pressures and high stress loads initiated by modern laying methods or introduced during service are leading to increasing corrosion demands. Steel pipelines used for the transport of media containing wet Hydrogen Sulphide (H2S) are faced with the danger of the cracking phenomena HIC (Hydrogen Induced Cracking) and SSC (Sulphide Stress Cracking). To prove resistance to HIC and SSC, test specimens are typically tested according standardised test methods. The exposure of test specimens in a sour test solution to a H2S pressure of 1 bar for 96 h, as described in NACE TM0284 is used to prove HIC resistance. Commonly four-point bend testing as described in EFC publication no. 16 is performed for SSC resistance testing with the appliance of a specific load, typically 80% of the actual yield strength. Within this work HIC testing at test conditions representing higher H2S partial pressures (up to 5 bar) and longer test durations (up to 6 months) have been performed on seamless quenched and tempered line pipe steel of grade X65 and X70 produced by VALLOUREC & MANNESMANN TUBES by plug and continuous mandrel mill process. Beside material in as delivered condition also pre-strained material was tested. SSC four-point bend testing has been performed on specimens which were strained up to 10% of plastic strain in longitudinal direction.