Hot Tap Weld Prevents Offshore Piping System ShutdownBY JACK STILL, PETER MACKIE AND PETER HUMMELGAARD
Changes needed to be made to an essential piping system at an offshore oil facility to protect it against barium sulphate scale buildup
Seawater injection is commonly used in offshore facilities to maintain reservoir pressure and sustain oil and gas production. Before the seawater is injected into the reservoir, it is first processed through a deaerator to reduce the oxygen content of the water. High levels of oxygen can promote formation of sulphide-reducing bacteria (SRB) that produce hydrogen sulphide (H2S). Reservoir fluids absorb H2S, which can have a detrimental effect on the processing facilities, particularly if ferritic materials are not procured in accordance with NACE MR-0175-96 (Ref. 1).
Seawater contains sulphates in the form of salts that, if not removed, react with barium in the reservoir to form barium sulphate. This compound is deposited within the production tubing as scale and restricts the flow of hydrocarbons to the wellhead. Removing barium sulphate scale from production tubing is difficult and, in extreme situations, may necessitate abandoning the well. Once a problem with scale has been identified, it is essential to add facilities within the water injection system for its removal.
Fig. 1 - The water injection hot tap.
For the recent North Sea project described in this article, the sulphate removal facilities were erected and fabricated onshore, then delivered offshore as a prefabricated unit a platform crane could lift from a supply vessel. Once installed on the platform, the new equipment was hooked up by attaching a 12-in.-diameter (30.48-cm) branch to the existing 16-in. (40.64-cm) water injection line by welding, using a procedure known as hot tapping - Fig. 1.Materials
The existing water injection pipe system was manufactured from super duplex stainless steel (UNS 32760) (Ref. 2). To maintain material compatibility, the engineering contractor for the new facility selected a similar grade of material. The chemical analysis and mechanical properties of the materials involved are outlined in Table 1.
Welding Procedures
Welding procedures were carried out in accordance with Section IX of the ASME Boiler and Pressure Vessel Code (Ref. 3) using gas tungsten arc welding (GTAW). The welding consumable selected was Metrode Zeron 100X, and all wires were stored in dry condition prior to use both onshore and offshore.
The following procedures were qualified for the welding of the 12-in.-diameter branch to the 16-in.-diameter water injection pipe.
Fig. 2 - Details of the welding procedure for qualifying the welding consumable.
In order to establish the procedure for the offshore weld, it was decided to flow water through the 16-in.-diameter pipe at 15°C (59°F) to simulate the actual water injection process. During the test, a digital thermometer was used to monitor the water temperature during welding of the 12-in.-diameter branch.
To ensure internal gas purge was effective, a nonmetallic baffle was inserted in the 12-in.-diameter branch, as illustrated in Fig. 4. Welding of the simulated hot tap is illustrated in the lead photo. Macrosections of the completed weld are illustrated in Fig. 5A and B.
Weld Metal Properties
Details of the materials selected and the weld metal chemical analyses from the simulated hot tap procedure are outlined in Table 1.
Fig. 3 - Details of the simulated hot tap weld.
Mechanical PropertiesMechanical tests were selected from the weld procedure, and were comprised of cross-tensile tests, Charpy impact and bend tests. Details of the results of the cross-tensile tests and Charpy impact tests are contained in Table 2. The cross-tensile test and Charpy impact tests were carried out in accordance with EN 895/EN 10002 (Refs. 4, 5) and EN 875/EN 10045-1 (Refs. 6, 7), respectively.
Bend tests from the surface and root areas of the weld procedure tests were carried out in accordance with EN 910 (Ref. 8). All test specimens were found to be satisfactory.
Metallography
Samples of the weld metal and HAZ were selected from the 9 o'clock and 12 o'clock positions of the cap and root area of the butt-joint weld procedure tests. Samples were also selected from the simulated hot tap weld at the 3 o'clock and 12 o'clock positions in the 5G welding position. All specimens were electrolytically etched in 40% potassium hydroxide at 78 V (Ref. 9). No evidence of grain boundary precipitates or intermetallic phases were observed in any of the specimens examined. Percentage ferrite was also established for the cap and root weld metal areas of the butt-joint weld and the simulated hot tap weld. Values of 51.8% and 38.3% were recorded for the cap and root areas of the butt-joint weld and 51.8% and 54.7% for the hot tap weld. The values satisfied the contract specification requirements of 4050% ±5%.
Hardness Tests
Hardness surveys using a HV 10-kg load were carried out on the butt-joint weld and the simulated hot tap weld. Hardness specimens were removed from the surface and the root area of the butt-joint weld and the 3 o'clock and 12 o'clock position of the simulated hot tap weld. Details of the hardness results are listed in Table 3. The maximum hardness of the HAZ and weld metal of the procedure test plate recorded values of 283 for both positions. Hardness results from the simulated hot tap weld recorded values of 287 and 299 for the weld metal and HAZ, respectively. The hardness values recorded satisfied the contract requirements of 300 HV 10.
Fig. 4 - Root purging of the hot tap weld.
Corrosion TestsCorrosion test specimens were selected from the root area of the butt-joint weld and simulated hot tap weld at the three and nine o'clock positions. Each test specimen was examined in accordance with ASTM G48-A (Ref. 10), at 35°C (95°F) for 24 hours in the pickled or part-pickled condition. Examination revealed evidence of pitting on the cut surface of each specimen where the weight loss was considered acceptable within the code requirements.
Hot Tapping Offshore
The field weld was carried out in accordance with the weld procedures outlined previously. Beveled shims manufactured from compatible materials were used to maintain the root opening during erection and tack welding of the 12-in.-diameter branch to the 16-in.-diameter water injection pipe. All tack welds were removed during welding of the root run. Precautions taken during welding included ensuring the weld joint area was free from condensation, as any moisture present during welding could result in the absorption of hydrogen into the weld metal. However, the blocking mechanism of austenite in preventing the movement of hydrogen is extremely effective and, provided the correct balance of ferrite and austenite has been achieved, the possibility of cracking is remote (Ref. 11).
Nondestructive examination (NDE) of the completed weld consisted of visual inspection and liquid penetrant inspection (LPI) (Ref. 12). Welding of the saddle and reinforcing plates supporting the 12-in.-diameter branch were carried out in the 5G position, and the nondestructive examination carried out was similar to that of the other welds. To complete the branch connection, a 12-in.-diameter flange section was welded to the 12-in.-diameter branch in the 2G horizontal position. On completion, NDE was carried out using LPI and radiography (Ref. 13) to ensure the weld quality satisfied the contract requirements according to ASME B31.3 (Ref. 14). Figure 6 illustrates the completed 12-in.-diameter branch connected to the 16-in.-diameter water injection system.
Fig. 5 - Examples of the branch-to-pipe weld. A - Macrosection selected from the 3 o¹clock position; B - macrosection selected from the 12 o'clock position.
SummaryHot tapping super duplex stainless steel pipe is not a common practice offshore. Therefore, it is essential to weld a mockup of the pipe joints onshore to ensure the welding process is feasible and the welder designated to execute the work is familiar with the weld position and welding technique to be applied.
Acknowledgments
The authors would like to thank David Baillie for commenting on the preparation of this paper and Kevin Simpson of OIS Aberdeen for use of the metallurgical facilities.
Fig. 6 - The completed weld at the offshore facility containing the saddle and reinforcing plates.
References
1. NACE MR 0175, Sulfide Stress Cracking Resistant Metallic Materials for Oil Field Equipment.
2. UNS S32760.
3. ASME Boiler and Pressure Vessel Code Section IX , Welding and brazing qualifications.
4. BS EN 895, Destructive Tests on Welds in Metallic Materials. Transverse Tensile Test.
5. BS EN 10002, Tensile Testing of Metallic Materials.
6. BS EN 875, Destructive Tests on Welds in Metallic Materials Impact Tests. Test Specimen Location Notch Orientation and Examination.
7. BS EN 10045-1, Charpy Impact Test on Metallic Materials. Test Method (V- and U- notches).
8. BS EN 910, Destructive Tests on Welds in Metallic Materials Bend Tests.
9. TWI document E5632/18/93. 1993. Recommended practice for determining volume fraction of ferrite in duplex stainless steel weldments by systematic point count. Also, IIW documents II-C-942 and IX-H-291-93.
10. ASTM G48, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution.
11. Gunn, R N. Duplex Stainless Steels. Cambridge, U.K.: Abington Publishing.
12. BS 6443, Method for Penetrant Flaw Detection.
13. BS 2910, Methods for Radiographic Examination of Fusion Welded Circumferential Butt Joints in Steel Pipe.
14. ASME B31.3, Petroleum Refinery Piping.
Jack Still is a Welding Engineer based in Scotland. Peter Mackie is HSEQ Coordinator with Brown and Root in Scotland. Peter Hummelgaard is a Welding Engineer with MT Services in Denmark.