WELDING NEEDS SPECIFIED FOR X-80 OFFSHORE LINE PIPE

Jeremy C. Price
Brown & Root Inc.
Houston

High-quality, defect-free welds can be deposited in API Grade 5L X-80 line pipe with pulsed gas-metal-arc welding (GMAW) and shielded metal-arc welding (SMAW) processes.

The newly developed Grade X-80 combines higher yield-strength pipe with thinner walls to reduce fabrication costs and improve some projects' economics.

Use of X-80 pipe can yield as much as 7.5% cost savings over construction with X-65 steel.

Increased demand of natural gas has prompted development of large gas fields which will require large-diameter pipelines at higher operating pressures.

API 5L X-80 line pipe could, therefore, become commonplace by the end of the decade if welding technology can be developed to match mechanical properties without affecting productivity.

Manual welding techniques, such as traditional stick-electrode welding, cannot consistently achieve required weld properties. As a result, more emphasis will be placed on use of mechanized pulsed GMAW equipment, such as that developed by CRC-Evans Automatic Welding, Houston.

Welding procedures have been developed to match the mechanical properties of X-80, such as toughness, and to maximize productivity with various filler-wire alloy systems shielding gases and pulsed GMAW parameters.

The effects of steel metallurgy on properties and the economics of using X-80 steels are also discussed.

LARGE-DIAMETER PROJECTS

Recent completion of several large-diameter, long-distance offshore pipelines in the North Sea and anticipation of more by 2000, particularly in the Far East, have prompted development of API 5L X-80 line pipe by means of a plate rolling technology called thermo-mechanical controlled processing.

Future pipelines could be 64 in. OD and operate at 100 bar (1,450 psi). This increase in yield strength, provided by X-80 steels over present X-65 steels, will enable lower wall thicknesses (25% reduction over 42-in. X-60) and make possible lower construction costs.

The micro-alloy design and compositional control of such steels are critical for achieving the desired properties, such as satisfactory weldment toughness and weldability, to avoid problems such as heat-affected zone (HAZ) cracking.

Additionally, an important consideration for sour-gas service is maximum resistance to hydrogen-induced cracking (HIC), otherwise known as stepwise cracking (SWC) or hydrogen pressure cracking (HPC), sulfide-stress corrosion cracking (SSCC), or stress-orientated, hydrogen-induced cracking.

The typical composition of an X-80 steel is shown in Table 1.

WELDING PROCESSES

Mechanized GMAW has become standard throughout the world for the welding of pipelines longer than 20 miles. The CRC-Evans automatic-welding system has been used by many contractors since 1989 to lay more than 18,000 miles.

It consists of the following:

• A pipe-facing machine used to produce the unique, multi-faceted bevel

• A combination lineup clamp and internal welder to deposit the root pass on the inside of the pipe

• The external mechanized welders used to deposit all subsequent passes To achieve the highest quality weld possible in X-80 material, pulsed GMAW has been utilized to date in hot, fill, and cap passes, whereas short-circuiting GMAW has been used with an internal welder for the root pass because of the former's susceptibility to the effects of residual pipe magnetism.

The pulsed GMAW process has been able consistently to produce discontinuity-free welds at a joining rate equivalent to that of conventional mechanized GMAW.1

The pulsed GMAW power supply typically used is a control-drop-transfer (CDT) synergic machine, rated at 350 amp at 100% duty cycle. The power supply is a secondary SCR-switching power supply and provides an approximate square-wave-output pulse (SCR = selenium controlled rectifier).

The machine has full synergic control of pulse parameters including pulse-peak currents, pulse "on time," pulse "off time," pulse frequency, background current, and average voltage as a function of wire-feed speed.

The CDT can store up to 12 different weld programs in an internal E-PROM. Each weld schedule is dedicated to a specific wire type, diameter, and shielding gas.

The power supply incorporates a patented arc-sensing-control method for compensation of electrode extensions.

Use of the SCR secondary-switching circuitry allows optimization of pulse shape while allowing minimum power dissipation in the weld pool. This allows the CDT to perform out-of-position pulse-arc welding with minimal distortion to the work piece.

By having individual programs that are defined by wire type, wire diameter, and gas, the CDT can optimize the synergic-control algorithms for the broadest dynamic range of welding conditions.

The CDT incorporates a microprocessor, which is used to implement the synergic-control algorithms. It also implements the closed-loop-control functions necessary to ensure optimum pulse parameters under all welding conditions.

By using closed-loop adaptive-feedback-control methodology, the CDT can provide optimum pulse conditions and material transfer while reducing the heat input into the work piece, thereby reducing weld spatter and thermal distortion.

Even with developments in mechanized GMAW, manual SMAW remains important in pipeline construction. Satisfactory tie-ins, repairs, and future maintenance depend on the flexibility of this process.

The development of SMAW procedures for X-80 pipelines is a greater challenge than for GMAW because of the difficulty in selecting consumables to match strength and toughness while maintaining good productivity and acceptable weld quality.

GMAW FILLER WIRES

The commercial filler wires that have been evaluated with the pulsed GMAW process are detailed in Table 2.

In the development of weld procedures, the preferred filler wire was selected based on welding characteristics and mechanical properties of the deposited welds. These welds were radiographed.

The mechanical tests included transverse-weld tension, face and root bead, nick-break, Charpy V-notch impact, micro-hardness, and micro-structural examination.

The operational characteristics of the C-Mn-Ni-Ti wire were poorer than those of the C-Mn-Ti wire because the C-Mn-Ni-Ti wire had greater molten, weld-pool fluidity. This high fluidity produced convex bead shapes in the overhead position.

Although not a technical consideration, the C-Mn-Ni-Ti wire is more costly than the C-Mn-Ti wire because of the nickel content.

The C-Mn-Ni-Ti filler wire had the highest Charpy V-notch toughness (117 ft-lb at 14 F., -10 C. followed by the C-Mn-Ti wire (93 ft-lb at 14 F., -10 C.). The C-Mn and C-Mn-Mo-Ti wires have considerably less notch toughness.

The microstructures of the C-Mn-Ni-Ti and C-Mn-Ti weld wire were predominantly acicular ferrite. Although the C-Mn-Ni-Ti weld had a less than optimum micro structure with large proportions of polygonal ferrite and aligned, second-phase particles, the low temperature toughness was relatively good.

The C-Mn-Ti filler wire was selected for further testing based on the operational and mechanical property results.

The next step was the development of optimum welding parameters.

With the pulsed GMAW process, the welding current and voltage are pulsed from a peak current and voltage, which melts the filler wire to a lower background current and voltage that sustains the arc.

The wire-feed speed varies with the pulsing frequency. With the synergic CDT power supply, each of these parameters can be independently controlled.

Fine tuning of the welding procedure involves the selection of each of these parameters. A series of welds was deposited to determine these optimum parameters.

GMAW SHIELDING GAS

The standard CRC-Evans welding system uses the GMAW process with 100% CO2 as the shield gas. This has been the standard for welding X-60, X-65, and X-70 pipe steels during the past 23 years.

The pulsed GMAW process, however, requires the use of principally inert gas shielding.

Use of inert gas rather than CO2 greatly increases the notch and fracture toughnesses of the weld metal in addition to reducing weld spatter, provides a more stabilized arc, and virtually eliminates lack-of-sidewall fusion defects.

Following are the shielding gases that have been evaluated for the pulsed GMAW process:

• 95 argon, 5 vol % CO2

• 80 argon, 20 vol % CO2

• 82.5 argon, 12.5 vol % CO2, 5 vol % helium.

Operationally, there were considerable differences in the shielding gases because of the very narrow joint design.

In the 95 argon-3 vol % CO2 mixture, the arc deflected to the sidewall during the first and second fill passes limiting penetration and increasing the possibility of lack-of-interpass-fusion defects.

The 80 argon-20 vol % CO2 gas exhibited less deflection and arc wander. But the arc lengths with both of these gases were longer than with the tri-mix (argon/CO2/helium) gas, accentuating the arc-deflection problem.

The helium addition to the tri-mix gas effectively reduces the arc length for a given arc voltage. Additionally, helium has a higher thermal conductivity, so that more heat is produced at any given current than argon or CO2, thus increasing penetration.

Consequently, the tri-mix gas has been adopted as the standard shielding gas for pulsed GMAW welding.

NOVA Corp. of Alberta, Calgary, in conjunction with CRC, used the tri-mix and a Thyssen C-Mn-Si-Ti wire to develop weld procedures for the company's X-80 land line in Alberta but used 87.5 argon-12.5 vol % CO2 for cap passes to overcome external undercut difficulties.2

SMAW FILLER WIRES

The problem with shield ed metal-arc-welded filler wires arises in matching weld metal strength.

Studies have shown that overmatching weld metal strength causes gross section yielding in the pipe.

Undermatching weld metal strength will cause straining of the weld. Higher levels of toughness will be required to prevent fracture initiation from a preexisting defect.

For X-80 pipe, this is a significant consideration because of toughness limitations of cellulosic electrodes. The use of E8010/E9010 consumables is unlikely to achieve overmatching yield strengths, which was demonstrated by Nova, where even matching was difficult consistently to achieve.

This problem has been overcome by use of E7010-G for the root and hot pass and a low cap hydrogen downhill E9018-G for fill and passes.

It was found that use of a hollow electrode tip and arc starting compound avoided porosity. This had been a problem with previous low-hydrogen downhill electrodes.

This combination achieved matching weld strength, and procedures were qualified.2

HARDNESS, WELDABILITY

An important consideration with X-80 materials is softening in the heat-affected zone (HAZ).

Fig. 1 details a hardness traverse of the E7010/E9010 combination with 100 C. preheat which shows that the OD region slightly undermatches, and the HAZ shows increased softening.

This narrow band is difficult to interpret because of the beneficial effect of the surrounding material. In the root, there is no such beneficial effect because the weld grossly undermatches.

Fig. 2 details a typical pulsed GMA micro-hardness traverse which shows that the weld metal matches or overmatches and that there is less HAZ softening than with the shielded metal-arc welding procedure.

The hardnesses shown are obviously a function of the PCM value and heat input (PCM = parameter for crack measurement).

Fig. 3 shows the trend of increasing PCM value and an increase in maximum hardness. For a heat input typical of pulsed GMAW, maximum HAZ hardness is considered to depend principally, on the base metal carbon content.3

Fig. 4 shows the effect of increasing hardness with carbon content (with and without boron). If PCM is 0.2 maximum and carbon 0.1%, maximum hardness (HV) is 350 without preheat.'

The hardness levels must be controlled to avoid the possibility of hydrogen cracking. Y-groove cracking tests have shown that for low-hydrogen electrodes, it is necessary to decrease the PCM value to less than 0.2 to decrease the critical preheat temperature to less than 0 C.

For cellulosic electrodes, however, preheats of 100-125 C. are required for a PCM of about 0.17, depending on whether the hot pass is welded 5 min after the root pass. This is shown in Fig. 5.3

TOUGHNESS; CORROSION
RESISTANCE

Standard toughness requirements for X-80 weldments are urgently needed. These can be achieved from correlations between Charpy and full-scale tests.4

Maximizing toughness in X-80 weldments requires adding titanium and boron to refine the weld metal micro structure by accelerating formation of acicular ferrite, or by retardation of grain boundary ferrite, or by the reduction of solute nitrogen caused by the precipitation of boron nitride.

For weld metals containing 28-30 ppm boron and 290-330 ppm oxygen, a 0.03% titanium addition gives an optimum level of toughness at the lowest level of nitrogen.

With regard to HAZ toughness, carbon, boron, and titanium also have an influence. Specifically, titanium contents should be restricted to 0.06% and satisfactory toughness (50% fracture appearance transition temperature; FATT) is achieved at - 40 C. with a boron-free steel of 0.07% carbon (Fig. 6).3

Recent toughness tests on pulsed gas-metal-arc welds have given Charpy impact properties of 100 ft-lb at - 30 F. and CTOD values of 0.016 in. (CTOD = cracking tip opening displacement).

To meet requirements for sour service according to NACE MR-01-75, it is unlikely that hardnesses in the weldment can be controlled below Rc22 or 248 HV even with post-weld heat treatment. Typical hardnesses have been detailed in Figs. 1 and 2.

Therefore, for verification that X-80 material has acceptable resistance to either HIC, sulfide-stress corrosion cracking, or stress-oriented HIC in sour service, fullscale tests must be carried out.

Tests of this description have already been developed in the U.K. with the full-ring test which demonstrates the residual stress effects that a full-scale pressure test would also produce.3

ECONOMICS

To assess the economic impact of using X-80 steel, it is useful to compare installation costs of X-65 vs. X-80 and note where differences will result.

The following aspects of offshore pipeline construction all have effect on the final cost:

• Material cost for pipe and welding consumables

• Transportation cost

• Welding lay rate

• Non-destructive testing costs

• Heat-treatment costs

• Alignment and pull times.

If we consider a 500-mile, 42-in. pipeline operating at 2,600 psi, the data shown in Table 3 are applicable to X-65 and X-80 pipelines installed with conventional eight-station lay barges.

WHAT'S BEEN LEARNED

Developing procedures and economic considerations for welding X-80 line pipe yielded the following conclusions:

• High-quality, defect-free welds can be deposited with the pulsed GMAW and shielded metal-arc-welding processes in API 5L X-80.

• Satisfactory pulsed GMAW procedures have been developed with outstanding fracture toughness.

• Low-hydrogen shielded metal-arc-welded electrodes must be used to achieve matching weld metal strength in X-80 material.

• Weldment hardness is very sensitive to small changes in pipe chemistry, and levels of pre-heat must be carefully considered to avoid cracking. Heat-affected zone softening can occur, however.

• Construction cost savings of 7.5% can be achieved by utilizing X-80 steel compared to X-65 steel.

• Full-ring sulfide stress-corrosion cracking tests need to be performed on X-80 weldments to verify acceptance in sour service.

ACKNOWLEDGMENT

The author wishes to acknowledge the assistance of CRC Evans and NOVA in providing welding procedure development data for this article.

REFERENCES

1. Teale, R.A., Smoot, W.T., and Trotter, J.J., "Pulsed Spray Transfer for Automatic Pipeline Welding," presented to the Offshore Technology Conference, May 1987.

2. Dorling, D.V., Lover, A., Russell, A.N., and Thompson, T.S., "Gas Metal Arc Welding Used On Mainline 80 ksi Pipeline in Canada," Welding Journal, May 1992.

3. "Development of API X-80 Grade Line Pipe with Excellent Toughness and Superior Weldability" NKK report, 1990.

4. Denys, R., "Pipeline, Safety, Present and Future," presented to the IPLOCA 26th Annual Convention, Bermuda, 1992.

5. Fowler, C., and Golightly F., "The Influence of Residual Stress On Testing of Pipeline Circumferential Welds for Sour Service," presented to the Pipeline Technology Conference, Oostende, Belgium, October 1990,

Copyright 1993 Oil & Gas Journal. All Rights Reserved.