Studies on Conventional welding techniques, welding specification requirements followed in upstream oil & gas sectors
1S.Prem kumar, 2 S.T.Selvamani.
1 Trainee Engineer,
QuEST Global Engineering service Pvt. Ltd.
Email: [email protected]: 9972733629
2 Associate professor,
Department of Mechanical Engineering,
Vel Tech Multi Tech Engineering College, Chennai 600 062, Tamilnadu, India.
Email: [email protected]: 9940540221
Oil & Gas industries found to be the major contributors of energy, progresses the growth of the global economy and drive the entire world. Upstream oil & gas fields both in offshore & onshore has lot of complex facilities, piping & process equipment for exploration and production. Joining of these pipelines & structures require high industrial standard practices to be followed for effective and prolonged operation of the asset and to ensure their fit for service condition. The present article describes the technical specification requirements, welding standard practices for carbon & low alloy steel through Shield Metal Arc Welding (SMAW), Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW) procedures. The welding inspection strategy for ferrous & non-ferrous materials in accordance with industrial standards is also elaborated. Finally, safety standards for conventional welding techniques, identification of anomalies and the associated recommendations are also put forward.
Keywords: Upstream Welding Standards, GTAW, SMAW, Anomaly identification, Inspection Strategy.
With rising in the need of energy every day, oil and gas production plants are striving hard to meet the demand requirements. To ensure steady supply, it is necessary to produce the crude and gas supply for long term, hence the desired running life of the plants to satisfy the need for long period is determined during their design phase. Welding place a crucial role in constructing these plant requirements either in upstream, midstream or downstream operational conditions.
Materials which are resistant to corrosion, high temperature, higher pressure bearing are few characteristics which are necessary while involving in selection of materials. The proper selection of welding process varies with thickness and type of material to be welded and other application / environmental factors. Oil and gas production fields commonly employ low and medium carbon steel, stainless steels and HSLA for transportation and process equipment materials. Superior weld characteristics with reduced weld defects is made by proper welding standard process. Hence welding according with standard industry specification is an important consideration to be made.
The conventional welding process followed in the above sectors are SMAW. GTAW, GMAW. Shielded metal arc welding (SMAW) has been the commonly and most widely used welding process during heavily construction requirements due to its simplicity in operations low welding cost. It offers low equipment cost comparing with other welding process. Welding different base material can be adopted easily, it employs faster deposition rate than the manual GTAW. It is widely suited for construction of steel materials.
GMAW process is widely applied on Aluminium and other ferrous materials also it is extended to welding steels. The welding process is faster and provides higher deposition rates with high deposition rates. Wide range of thin materials can be welded with high production factor. GTAW process produce high quality superior welds even while welding thin sections, also welds can be made with or without filler materials, precise control of heat during welding is also made easy through this process. Autogenous welds can be made with wide range of thickness.
WeiweiYu compared the fracture toughness between SMAW and GTAW welded at various locations in a primary coolant piping and found that better comprehensive performance welded joints in GTAW than in SMAW. On welding 316L stainless steel by GTAW process Masoud Sabzi observed that the weldability and the mechanical properties were improved by using electromagnetic vibration.
The particle size distribution due to the Effect of shielding gas temperature is studied by Vishnyakov et.al, the computed results agreed with the experimental results of the particle sizes on the shielding gas temperature. Decrease of vapor-gas mixture cooling rate when the shielding gas temperature increased were observed, this results in increased duration in particles growth which ultimately increases the particle size. ChenxiaoZhu found 90% increase in the tensile strength of welded joint than the base metal with preheating conditions. He analyzed the microstructure characteristics of weld joints at different pre-heat temperature conditions. And investigated the Effect of preheating on the defects and microstructure in NG-GMA welding of 5083 Al-alloy.
From the above literature review it is understood that welding in accordance with defined procedure is importance in improving the weld metal properties. Various industrial standard practices are created and are followed in different countries. For oil and gas, the most preferred standard across the globe is API (American Petroleum of Institute). Welding procedure specification (WPS) is a document followed in the plant for welding procedure technical guideline with respect to the codes. Works related to the detailed studies on relating the Welding specifications requirements given in the standard documents with the various research works available is scanty. Hence the present study focus on explaining the various specification requirements like classification of filler material, choices of electrodes, shielding gas specification, pre heat, post heat specifications which are followed during welding process are described in accordance with the standards followed across globally.
Welding Consumables (Filler Metal and Flux)
Filler metals are a type of consumable electrodes that has been used in most autogeneous welding process like gas welding process with separate wire or strip of filler rod of same base material to be welded. Often these filler rods are copper plated to avoid rusting during long term storage. TIG welding is an electric gas welding process that uses non consumable tungsten electrode to provide the required heat, with the filler rod added manually. These filler rods/metals are specified in WPS by ASME II, Part C/AWS specification and classification.
Filler rods with low hydrogen deposits are used in Groove and fillet welds, in certain conditions cellulose type coated electrodes are allowed they are,
For storage tank API 620 and API 650, fabrication and erection, when the thickness of base metal and the minimum specified tensile strength is less than 1/2 in. and 70 ksi (483 MPa) respectively then cellulose type coated electrodes may be used. Due to high differential thermal expansion, nickel-base filler metals are preferred for temperatures above 600 °F (315 °C).
Pipe welding of ASME P-No. 1, Group 1, carbon steel base metal with the root pass and second pass of single-groove welds not considering the base metal thickness, cellulose type coated electrodes are preferred. For ASME P-No. 1, Group 2, materials cellulose electrodes are used if provided a minimum preheat of 300 °F (149 °C) is used and maintained until the joint is completed. Filler metals designed for ‘single-pass welding’ shall not be used for multiple pass applications and fluxes designated for non-PWHT applications shall not be used for PWHT applications.
When joining dissimilar ferritic steels (P-No. 1 though P-No. 5), the filler metal shall conform to the nominal chemical composition of either base metal or an intermediate composition. When attaching non-pressure parts to pressure parts, the filler metal chemical composition shall match the nominal chemical composition of the pressure part.
When joining ferritic steels to martensitic stainless steels (P-No. 6) or ferritic stainless steels (P-No. 7) or austenitic stainless steels (P-No. 8), the filler metal type 309 and 309L filler rod is used for design temperatures not exceeding 600 °F (315 °C); Stainless Steel belong to P-No. 6, P-No. 7, and P-No. 8 are welded with the electrodes listed in table 1.0
Base Material Type 405 Type 410s Type 410 Type 304 Type 304L Type 304H Type 310 Type 316 Type 316L Type 321
Carbon and low-alloy steel AB AB AB AB AB AB AB AB AB Type 405 ABC ABC ABC AB AB AB AB AB AB AB
Type 410S ABC ABC AB AB AB AB AB AB AB
Type 410 ABC AB AB AB AB AB AB AB
Type 304 D DH DJ A DF DGH DI
Type 304L H DHJ A DF GH HI
Type 304H J A DFJ DGHJ DIJ
Type 310 K AK A A
Type 316 F FG FI
Type 321 I
Table 1.0 Welding electrodes for Stainless Steel Alloys
ASME/AWS SFA/A 5.4, Classifications E309-XX or E309L-XX.
ASME/AWS SFA/A 5.11, Classification ENiCrFe-2 or -3 or ENiCrMo-3
ASME/AWS SFA/A 5.4, Classification E410-XX (0.05 % C max.) (heat treatment at 1400 °F required).
ASME/AWS SFA/A 5.4, Classification E308-XX or E308L-XX.
ASME/AWS SFA/A 5.4, Classification E347-XX.
ASME/AWS SFA/A 5.4, Classification E316-XX.
ASME/AWS SFA/A 5.4, Classification E316L-XX.
ASME/AWS SFA/A 5.4, Classification E308L-XX.
ASME/AWS SFA/A 5.4, Classification E317L-XX.
3.0 Shielding and Purging Gases Requirements
The Welding Procedure Specification (WPS) state the requirements of shielding gas mixture, percent composition and flow rate. Shielding gases should meet the purity requirements of ASME/AWS SFA/A5.32/5.32M. Gas purity should be recorded on the PQR and WPS when a single gas is used. Back purging is required for the GTAW and GMAW processes for welding materials having a nominal chromium content greater than 2-1/4 %.
When a back purge is used, the WPS state the gas used, including composition of the gas mixture and the flow rate. The back purging gas is selected to prevent oxidation or scale formation on the underside of the weld, the purge shall be maintained until at least 1/4 in. (6.5 mm) depth of weld metal has been deposited. For socket, seal, and any other attachment welds on base materials less than 1/4 in. (6.5 mm) thick, the back purging shall be maintained throughout the welding operation.
4.0 Preheating and Interpass Temperature
Preheating applies to all welding, tack welding, and thermal cutting. The requirements of minimum preheat conditions follow the applicable code and recommended practice in Appendix R of ASME BPVC Section VIII Division 1, Table 330.1.1 of ASME B31.3, API 934-A, API 934-C, API 934-E, Any recommendations or requirements for preheat listed in the above relevant code shall be considered mandatory.
The preheat temperature shall be applied for low-alloy steels, maintained until PWHT is completed throughout the entire thickness of the weld and at least 3 in. (75 mm) on each side of the weld. Preheat and interpass temperatures is checked by use of thermocouples, temperature indicating crayons, pyrometers or other suitable methods. For austenitic stainless steels, duplex stainless steels, and nickel alloys, digital hand-held contact thermocouples are preferred over temperature indicating crayons to avoid the potential contamination from tramp elements, such as fluorides, chlorides, and sulfides, which may be contained in the crayons.
The maximum interpass temperature shall be specified in the WPS and PQR for austenitic stainless steels, duplex stainless steels, and non-ferrous alloys and when impact testing is required for carbon and low-alloy steels. When welding high carbon equivalent forgings and fittings, special welding procedures, including preheat and cooling rate control for hardness management, needs to be developed to reduce the risk of hydrogen assisted cracking. Preheat, interpass, and preheat maintenance temperatures shall be measured on the weld metal or on the immediately adjacent base metal. Temperature indicating crayons are not permitted directly on weld metal or on the joint preparation. The maximum Interpass Temperatures is given in Table 2.0,
Material Group Maximum Interpass Temperature
P-No. 1 (carbon steels) 600 °F (315 °C)
P-No. 3, P-No. 4, P-No. 5A, P-No. 5B, 600 °F (315 °C)
P-No. 5C, and P-No. 15E (low-alloy steels) P-No. 6 (Type 410) 600 °F (315 °C)
P-No. 6 (CA6NM) 650 °F (345 °C)
P-No. 7 (Type 405/410S) 500 °F (260 °C)
P-No. 8 (austenitic stainless steel) 350 °F (175 °C)
P-No. 11A, Group 1 350 °F (175 °C)
P-No. 41, P-No. 42 300 °F (150 °C)
P-No. 43, P-No. 44, and P-No. 45 350 °F (175 °C)
Table 2.0 Maximum Interpass Temperatures.
Post-weld Heat Treatment (PWHT)
WPS specifying PWHT should indicate the following: maximum heating rate, holding temperature range, holding time, maximum cooling rate, PWHT of austenitic stainless steel, duplex/super duplex stainless steel, and non-ferrous alloys requires approval by the purchaser. Code exemption of PWHT for P-No. 4 and P-No. 5 materials is not permitted for applications in sour or hydrogen service environment or where the nominal chromium content of the material exceeds 1.25 %. Table 3.0 states the PWHT requirements.
P-No Material Type Nominal thickness at weld (in.) Service condition Holding Temp (°F) Time at Holding Temperature
1 Carbon Steel According to Code Code 1100-1200 1 (min)
1 Carbon Steel all Wet H2S 1150-1200 1
1 Carbon Steel all caustic 1150-1200 1
1 Carbon Steel all amine 1200-1250 1
1 Carbon Steel all carbonates 1150-1200 1
1 Carbon Steel all HF acid 1150-1200 1
1 Carbon Steel all deaerator1150-1200 1
1 Carbon Steel all ethanol 1150-1200 1
3 C-Mn-Mo According to Code Code 1150-1200 1
5B 9Cr-1Mo All All 1375-1425 2
5B 5Cr-1/2Mo All All 1325-1375 2
5C 2 1/2Cr-1Mo-V All Heavy wall pressure vessels for high – temperature, high –pressure hydrogen service 1275 to 1325
9B 3 1/2 Ni Code All 1100-1150 1
Table 3.0 PWHT requirements.
For 9 % Ni, the entire vessel, assembly, or plate must be at the PWHT holding temperature at the same time. The cooling rate from the holding temperature shall not be less than 300 °F (167 °C) per hour down to a temperature of 600 °F (315 °C). A local or partial PWHT cannot be used since this results in portions of the structure being in the embrittlement range of 600 °F to 1000 °F (315 °C to 540 °C) for extended periods of time, thereby impairing material toughness.
6.0 Limitations of Fusion Welding Processes
When used for root pass welding of single-sided joints, GTAW-P shall be performed with the same make and model of equipment using the same program settings as those used in the procedure qualifications. The need to specify the make and model, program, equipment settings, and pulse waveform is based upon the effects these variables have on welding arc performance, especially sidewall fusion and out-of-position welding. Studies have shown considerable variation in arc characteristics when one make or model of welding system is compared to another. This variation can lead to welding defects, some of which may be very difficult to detect by radiography.
The process shall not be used for branch connections, nozzle-to-shell welds, or socket welds. GMAW-S may be used for root pass welding on piping. Root pass welding with GMAW-S for other applications is permitted, provided the root pass is completely removed from the backside. The fill and cap passes for butt or fillet welds may be welded with this process, provided the thickness of any member does not exceed 3/8 in. (9.5 mm) and vertical welding is performed with uphill progression. For vertical welding, the root pass and second pass progression for a material of any thickness may be either uphill or downhill.
Self-shielding FCAW (FCAW-S) may be used only for welding carbon steel structural items. FCAW with external gas shielding (FCAW-G) may be used for either groove or fillet welds for pressure boundary or structural welding.