Moving forward on development of UOE Clad linepipe.
High-strength low alloy steels (HSLA) such as carbon steel linepipe exhibit corrosion rates of approximately 0.2 mmpy (under environmental conditions such as 10 bar CO2 @ 60[degrees]C), whereas CRA materials would exhibit 0.005 mmpy, under the same conditions. This would mean for a 30-year lifetime of a pipeline, a wall thickness loss of 6 mm would occur for the HSLA steel, whereas the corrosion-resistant alloy (CRA) would lose approximately 0.15 mm.
This difference in corrosion resistance between carbon steel and CRA materials, in addition to the development of oil and gas fields, where the parameters of the line/fluid result in an increase in the "corrosive activity" of chloride ions, hydrogen sulfide, and carbon dioxide, have resulted in increased demand for CRA pipe material. In the past carbon steel pipe material has been used for such applications, and the following corrosion-mitigation measures have been employed during the operation of the pipeline:
* Use of carbon steel pipe material with the injection of inhibitors--can be difficult due to variations in efficiency of the inhibition and downtime of the pipeline.
* Use of carbon steel pipe material after dehydrogenation and desulphurization.
These measures are expensive and difficult to maintain at the required level to ensure minimal or no corrosion.
Corrosion-resistant alloys include austenitic and martensitic stainless steels, nickel-based, and titanium-based alloys. As a guide, materials with [greater than or equal to] 11wt% chromium are required for the environments considered to ensure adequate corrosion resistance. As the corrosivity of the solution increases, the cost and technical issues associated with the first three options also increases. However, corrosion-resistant alloys can exhibit acceptable rates of corrosion, for their use to be considered economically feasible. For offshore application, the use of corrosion-resistant alloys on their own would not provide sufficient strength to resist hydrostatic collapse for the depths considered, or the level of strain induced during the pipelaying procedure.
Therefore, corrosion-resistant alloys are used as the "corrosion-resistant inner sleeve" to a higher strength outer casing, such as carbon steel. It is possible to use solid CRA pipe material, but to achieve the required strength properties, the wall thickness of the pipe would be uneconomically thick.
Clad linepipe is a structure where the CRA is metallurgically bonded to the higher strength material. Corrosion-resistant alloys have been used and are being used where the CRA is mechanically bonded to the higher strength material through deformation, i.e. expansion. This form of CRA linepipe is referred to as "lined."
Due to the increasing market requirements for clad and lined pipe material, Corus Tubes has assessed the feasibility and determined a production route for the manufacture of clad linepipe using the UOE method, which would enable the business to supply large quantities of clad linepipe with the same dimensional and mechanical tolerances that are available for offshore-grade carbon steel material.
Composition. Four plates with a carbon steel thickness of 25.4 mm (WT), and a CRA (316L) thickness of 2.5 mm were sourced, with the composition of the 316L and carbon steel materials as listed in Tables 1 and 2, respectively.
All the elements of the 316L material are within the specification tolerances of ASTM A240.
The production route of clad linepipe material within the 42-inch pipe mill is illustrated in Figure 1. The majority of the route is the same as for CMn steel pipes; however, there are a few exceptions:
* Addition of an offline welding station for the deposition of the CRA layer along the internal weld seam, after completion of the carbon steel longitudinal weld seam, prior to mechanical expansion.
* NDT of the pipes' seam welds after the replacement of the CRA layer, as well as after SAW welding.
* Dye penetrant testing of the CRA layer.
* Endoscopic examination of internal SAW and overlay weld.
* Surface cleaning of the CRA layer.
The pipe material (24-inch OD x 25.4 mm WT (X65), + 2.5 mm thick 316L) was manufactured in accordance with API 5LD and ASTM A240. During the forming trials the surface of some of the plates was protected with matting in the areas where the plate would come into contact with the forming dies.
After the completion of pipe forming and surface preparation, the surfaces of the CRA layer were inspected for any signs of visible damage and compared against the plates where no protection was applied. There was no evidence of degradation on the plates where protection was not used when compared with the plates that were not protected.
[FIGURE 1 OMITTED]
Internal and external SAW welding procedures are the same for clad pipes as they are for standard CMn pipes. However, careful consideration and expertise must be applied to the selection of the welding parameters for the internal SAW weld to ensure a bead height of 0-0.5 mm while at the same time maintaining the correct through-thickness profile of the internal weld bead. This is to ensure that the required mechanical properties within the carbon steel backing material are achieved and that the toe of the internal SAW weld is free of any defects (i.e., undercut).
The selection of the dimensions at the plate edges (Figure 2a) is critical to ensure the optimal welding profile, both of the SAW welds, and of the CRA replacement weld. Corus Tubes utilizes a double V-preparation (Figure 2b) within the carbon steel. In addition, the CRA layer is removed to a pre-determined dimension. The ability to gain the required dimensions along the longitudinal edges of the plate is achievable due to the recent investment in a new plate-milling machine for which bespoke cutting heads can be manufactured for any desired application.
[FIGURE 2 OMITTED]
A large number of techniques were considered in partnership with Air Liquide Welding for the deposition of the CRA layer on top of the internal SAW weld, including MIG, twin SAW, multi-pass SAW, oscillating SAW, and electro-slag. During the assessment of these techniques, the parameters considered included welding time, achievable quality levels (spatter), weld shape (contour, bead height), minimal dilution of SAW weld, availability of consumables for various CRA layers, level of HAZ heat tint, consistency of consumable feed to the welding head and dimensions of the welding head.
Electro-slag welding was chosen as the method for the deposition of the CRA layer, as it was considered capable of achieving the required levels of quality, flexibility and productivity targets. This operation will be an offline operation using a purpose-built facility. For the CRA material (316L) welded in the latest trial work, 309L Mo strip was used as the welding consumable.
was used to identify the presence of any disbondment on the bevel face at the pipe ends between the carbon steel backing material and the CRA layer and also to inspect for any delamination of the CRA surface layer.
Although some linepipe specifications do not specifically state a requirement for ultrasonic inspection of the completed clad weld, Corus Tubes inspected the weld seam after internal and external SAW welding and also after overlay welding.
All clad linepipe specifications request X-ray inspection of the completed weld (SAW + overlay), which was also conducted without any issues being raised.
API 5LD requires the following: "The corrosion-resistant behavior of the CRA layer is adversely affected by poor surface condition. Therefore, blasting, pickling, brushing, or a combination of these methods shall remove scale spatter and heat treatment surface residues of the CRA layer." The maximum specified surface roughness is 12.5 [micro]m. Two methods have been explored--namely, blasting with Met-abrasive SO60 shot and pickling.
Shot Blasting. To assess the suitability of shot-blasting to prepare the surface of the CRA layer to the required standard, a pipe was processed through the facilities at the BSR coating plant, which is on the same site as the 42-inch pipe mill.
Pickling. Test rings were pickled using adherent paste in lieu of liquid solution at a local subcontractor. For comparison purposes, 50% of the internal surface of the ring was pickled for three different periods of time (30, 60 and 90 minutes). The remaining 50% of the internal surface was left untouched. The results show there are no significant differences between pipes pickled for 60 and 90 minutes. However, the difference between no cleaning and pickling was very significant as shown in Figure 4. From this trial it was concluded that pickling was more effective than shot blasting. The average surface profile in the transverse direction was 1.0 [micro]m, and in the longitudinal direction it was 2.2 [micro]m.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The mechanical properties of one of the pipes manufactured are listed in the tables. The transverse weld tensile properties were measured at room temperature and after PWHT. The tensile strength (Rm) was 600 MPa and 612 MPa, respectively. The shear strength of 370 MPa was also determined using the ASTM A264 method, compared to the minimum specified requirement 140 MPa.
For all the mechanical tests mentioned, with the exception of the shear test, the CRA was removed by machining prior to testing, and are therefore the mechanical properties of the carbon steel material itself.
Forward and reverse bend tests (180 [degrees]) were conducted on the carbon steel base material of the pipe, around an 83-mm diameter former. No test samples showed any evidence of cracking.
The hardness values of the SAW and overlay weld region (Figure 5) were measured and are reported in Table 5.
[FIGURE 5 OMITTED]
Tests (ASTM 262 Method E) were conducted in both plate and pipe form. Both passed without issues. Tests are ongoing to assess the resistance of the 316L layer to the ferric chloride test (ASTM G48 Method A).
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Due to the increasing demand for linepipe material resistant to more corrosive environments, Corus Tubes has developed the capability and equipment required for the production of clad linepipe material, manufactured using the UOE process route.
Through consideration of the critical parameters affecting the quality, flexibility, and productivity targets of the deposition of the CRA layer, along the internal weld bead, the electroslag welding process was chosen to be the most suitable method. As a result of the recent development work, Corns Tubes has established a production process capable of producing clad pipe through a volume-based UOE pipe mill in external diameters between 406.4-1,067 mm.
This article is based on a presentation at the Pipeline Technology Conference in Hanover, Germany, in April 2007.
Author: Dr. Mark Fryer is manager of development and technical support for the Corus Tubes Energy Business.
By Dr. Mark Fryer, Corus Tubes Energy Business, UK
Table 1: Chemical composition of 316L layer (wt %). C Si Mn P S Cr Ni Mo N 0.020 0.49 0.88 0.028 0.000 16.97 10.12 2.08 0.05 Table 2: Chemical composition of carbon steel (wt%). C Si Mn P S N 0.065 0.287 1.55 0.010 0.0008 0.0028 Al Cu Mo Ni Cr V 0.041 0.0253 0.025 0.423 0.049 0.001 Nb As Sn Ti B Ca 0.020 0.004 0.001 0.001 0.000 0.0017 Table 3: Tensile properties (90[degree] to the weld). Transverse Rt0.5 Rm Elong. RI0.5 (Mpa) (Mpa) (%) /Rm Acceptance 450 535 18 0.92 Criteria (DNV OS-F101) min min min max Strap 493 596 51 0.83 Strap (PWHT) 465 586 48 0.79 Strap (Proportional 518 597 38 0.87 Gauge Length) Round Bar-Test 1 470 564 31 0.83 Round Bar-Test 2 478 566 29 0.84 Longitudinal Rt0.5 Rm Elong. RI0.5 (Mpa) (Mpa) (%) /Rm Acceptance 450 535 18 0.92 Criteria (DNV OS-F101) min min min max Strap 517 587 48 0.88 Strap (PWHT) 511 571 50 0.89 Strap (Proportional 527 589 38 0.89 Gauge Length) Round Bar-Test 1 493 589 28 0.84 Round Bar-Test 2 523 598 26 0.87 Table 4: Impact toughness properties at -30[degrees]C. Impacts Energy (J) Shear (%) Transverse Body 281 319 219 80 100 90 (Mid-Thickness) Transverse Weld 125 121 125 90 80 90 Centreline (Mid-Thickness) Transverse Fusion 150 233 249 50 70 90 Line (Mid-Thickness) Transverse Body 319 413 412 100 100 100 PWHT (Mid-Thickness) Table 5: Hardness values SAW & overlay weld. Carbon Steel CRA Layer Minimum 182 165 254 (below Maximum overlay weld) 259 (body) Average 221 201
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|Title Annotation:||Tech Notes: Product Development; Corus Tubes Energy Business|
|Comment:||Moving forward on development of UOE Clad linepipe.(Tech Notes: Product Development)(Corus Tubes Energy Business )|
|Publication:||Pipeline & Gas Journal|
|Article Type:||Statistical table|
|Date:||Feb 1, 2008|
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