X80 heavy wall pipe solutions for deep/ultra-deepwater field developments in mild sour environment
Deep and ultra-deepwater field developments require the utilization of thick-walled steel pipes, to withstand the local loads when using conventional X65 grade.
Switching to higher-strength steels, like X80, allows a reduction of wall thickness and pipe weight. This can bring significant advantages, in terms of hang-off and floater design, as well as installation vessel utilization, thanks to the decrease of payload and the reduction of top tension requirements. Furthermore, the reduced tonnage of steel manufactured can contribute to improving the overall CO2 footprint of the projects.
In this context, X80 seamless line pipe material was developed to address the challenges of the ultra-deep offshore projects in mild sour environments, targeting outer diameters up to 406 mm and wall thicknesses up to 40 mm.
BENEFITS OF X80 SEAMLESS PIPES FOR DEEPWATER PROJECTS
Subsea projects in deep waters present several technical challenges. Deepwater operations require the use of heavy walled pipes, to withstand the high hydrostatic pressure. This, combined with the high length of the submerged risers, leads to high load on the FPSO and line pipes.
Additionally, the pipe laying is complex, due to the high payload and high-top tension requirements on the installation vessels. One way to reduce those difficulties is to use high-strength steels, such as X80. A case study has been performed to illustrate the benefits of replacing conventional X65 with X80, assuming a project with a water depth of 2,000 m, a design pressure of 10 ksi and an overall pipeline length of 88 km in a steel lazy wave riser configuration.
Figure 1 shows the wall thickness that would be required for risers and flowlines. The blue bars correspond to the wall thickness with X65 and the orange ones to X80. When comparing X80 and X65 grades for similar water depths, one can see a reduction from 12% to 20% in wall thickness when switching from X65 to X80.
In this case study, replacing X65 with X80 allows savings of about 2,000 tons of steel pipes. The reduction of steel tonnage also has a positive impact on the carbon footprint of the project. Indeed, at Vallourec, the average CO2 emission amounts to an average of 1.8 tons CO2 per ton of tube, as estimated along the entire chain of value (i.e. from the raw material supply to the transport to final destination), according to the EPD International PCR 2012:01 standard (2012). Thus, in this case study, switching from X65 to X80 could allow savings of approximately 3,600 tons of CO2 emissions.
The use of lighter X80 pipes presents other benefits, such as an easier design of the hang-off system and a reduction of the floater payload. Switching to X80 also allows savings of cost and time in installation. The top tension requirements on the installation vessels can be reduced, thanks to lower steel pipe weight, which allows more flexibility in the vessel for the pipe laying. In addition, in the case of steel lazy wave risers, the reduction of pipe weight can generate significant savings on buoyancy modules.
The tests presented in this article were carried out on X80 seamless pipes in dimensions of 273.1 x 40 mm, produced at Vallourec Brazil. The material shows a carbon content of 0.07% and a carbon equivalent Pcm of 0.19%. The steelmaking process used to produce those pipes includes vacuum degassing and continuous casting. Special care was taken to maintain Phosphorus and Sulfur-contents at a low level, to enhance the sour-service resistance of the steel.
The pipes were produced, using the PQF continuous rolling mill. Quenching and tempering were performed to reach the X80 grade.
MECHANICAL & CORROSION RESULTS ON PIPES
Mechanical test results. Table 1 gives the results of mechanical tests performed on the X80 pipes in dimensions of 273.1 x 40 mm.
The tensile tests were performed according to ASTM A370 (2022), using round specimens taken in the longitudinal direction. The mean yield strength was 586 MPa, and the average yield ratio was 0.88.
The hardness measurements were performed at the outer surface (OD), mid-wall and inner surface (ID), as per DNV-ST-F101 (2021). The average hardness values were respectively 243, 216 and 232 at OD, mid-wall and ID. Individual hardness values may have exceeded locally 250 HV10, but were globally under 275 HV10.
The Charpy tests were performed according to ASTM E23 (2018), using full-sized specimens taken both at mid-thickness and at the ID wall, plus 2 mm. Good charpy results were obtained at -30°C, with energy values close to 300 J.
The fracture toughness of the pipes was determined at -10°C, using Single Edge Notch Bend (SENB) B x 2B specimens, as per BS 7448- 1 (1991). Excellent CTOD values—much higher than the minimum threshold of 0.15 mm of DNVGL-ST-F101 (2021) standard—with an average of 1.4 mm, were obtained in the quenching and tempering condition.
CORROSION TEST RESULTS
Several four-point-bend tests, according to ASTM G-39, were performed on the X80 pipes in sour conditions, on the NACE severity diagram in regions 2 and 3. The specimens were taken at the inner diameter of the pipe.
Table 2 shows the results of the tests done on samples in the as-quenched and tempered condition. The magnetic particle inspection and observations of the cross-sections did not reveal evidence of SSC cracks on the specimens after 720 hrs of exposure in the sour environment.
Welding procedure. The welding trials were conducted on the X80 pipes in dimensions of 273.1 x 40 mm. The welding procedure used in this study was the following: 2G position, GMAW-STT (Surface Tension Transfer) process at root, GMAW-Pulsed process to the other passes, 150 °C as preheating temperature and a heat input between 3 and 12 kJ/cm. The welding consumables were ER70S6 for the root pass and ER80S-G for the remaining passes.
The characterization of the mechanized weld included all-weld metal tensile tests, hardness tests, Charpy, SENB and SSC tests.
RESULTS OF X80-WELD
All-weld metal tensile tests. The results of the all-weld metal tensile tests performed on the X80 weld are shown in Fig. 2. According to DNV-ST-F101 (2021), the yield strength of the weld metal should be a minimum 80 MPa above the SMYS of the base material, i.e., above 635 MPa in the present case. The welding strategy used for the X80 weld allows it to reach the required strength, overmatching that in the weld metal.
Impact toughness. Figure 3 shows the results of the impact tests performed at -30 °C on the mechanized weld, according to ASTM E23 (2018). All individual values comply with the requirements from DNV-ST-F101 (2021) standard.
Fracture toughness. Table 3 shows the fracture toughness results. SENB tests were performed at -10 °C, according to BS EN ISO 15653:2018, to assess the fracture toughness of the weld joint at the fusion line and at the weld centerline. The CTOD values were above 0.7 mm at the fusion line and above 0.4 mm in the weld metal, complying with the requirement of DNV-ST-F101.
Macro-hardness evaluations were performed at the five o’clock and seven o’clock positions, according to DNV-ST-F101 (2021), on the mechanized girth weld, using Vickers with a 10-kg load. Figure 4 shows the maximum hardness values that were obtained in the base metal, the heat affect zone and the weld metal. Overall, the individual values were below 290 HV10.
SSC testing. Four-point bend tests were performed on the X80 girth weld in region 2 of the NACE severity diagram, according to NACE TM 0316, in the following conditions: pH 5, 0.14 bar partial pressure of H2S (balance CO2) and applied stress of 80% SMYS. The tests were conducted in NACE TM0177 solution B; the pH was adjusted through addition of sodium bicarbonate. The specimens were tested with weld root left intact. Details of the cross-sections are displayed in Fig. 5. None of the specimens shows evidence of SSC cracks after exposure during 720 hrs in the mild sour environment.
The use of higher-strength steels, such as X80, allows a reduction of wall thickness and pipe weight, compared with the use of a conventional X65 grade. A case study demonstrated a reduction from 12% to 20% of wall thickness when switching from X65 to X80. The resulting pipe weight savings have several advantages, since they have a positive impact on the CO2 footprint of the project and allow significant cost and time-savings in installation.
An X80 steel solution was developed with wall thickness up to 40 mm. A welding procedure relevant for J-lay was tested; it gave good toughness results in the HAZ and in the weld metal. Good SSC results were also achieved in NACE region 2 at pH 5 and with an H2S partial pressure of 0.14 bar.
The overall results highlight the possibility of using X80 heavy-walled seamless pipes for risers in a mild sour environment, using cost-effective welding solutions. As a continuation of this work, it has been considered to check the performance of the X80 in strained and aged conditions, to evaluate the possibility of reel-lay installation.
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