Development of a Sour Gas Wellhead Welding Procedure

Brian Wilson, P.Eng., Canspec Group Inc.
Peter Cavanagh, P.Eng., Suncor Energy Inc.
Roy Baguley, P.Eng., Metal Engineers International Inc.

Abstract

Suncor Energy had a need to develop a welding procedure that would be capable of joining high-strength low-alloy steel casing and wellhead materials, while maintaining an acceptably low hardness level, suitable for resistance to sulphide stress cracking in a sour service environment. This paper discusses the material properties, weldability issues, laboratory welding trials and key findings that led to the successful completion of Suncor’s sour gas wellhead installation using the developed welding procedure.

Introduction

Suncor Energy has a drilling and gas production program in place for the development of gas reserves along the Alberta Foothills district. These gas reserves contain varying concentrations of hydrogen sulphide, which can pose materials concerns with respect to both corrosion and sulphide stress cracking (SSC). As defined in NACE Standard MR0175, hardness control is a critical factor for the resistance of carbon and low-alloy steels to SSC in sour environments. The maximum specified Rockwell hardness is 22 HRC. The welding guidelines in the publication ” Alberta Recommended Practices For Drilling Sour Wells” reference NACE MR0175, and require hardness controls for welding procedures for all susceptible weldments in sour service.

The Suncor well-completion design for these sour wells calls for fillet welding of slip-on API 6A PSL 3 Class 75K (75,000 psi minimum yield strength) casing heads (“casing bowls”) to API 5CT Grade L80 (80,000 psi minimum yield strength) surface casing. Both the casing and casing-bowl base materials are in the quenched and tempered condition and hardness tested for compliance with NACE MR0175. The choice of metallurgy for the Gr. L80 casing includes a plain carbon-manganese steel and a chromium-molybdenum low-alloy steel. The casing bowl is a forged AISI 4130 material.

The casing bowl is fitted and welded to the casing at the wellsite, after the casing has been installed in the well. Figure 1 is a longitudinal section schematic showing the casing bowl fitted on top of the casing, with the inside and outside fillet welds in place. This circumstance creates the challenge of establishing a fillet-welding procedure for joining the casing to the casing bowl at a field location (often during winter months), which will result in welds that reliably and consistently meet the 22-HRC hardness requirement of NACE MR0175, while still maintaining the necessary strength and integrity of the original high-strength base materials.

Preliminary Assessment of Weldability

Hardness control for steels and low-alloy-steel welds requires control of material chemistry (specifically, carbon equivalent), preheat and post-weld heat treatment (PWHT). The IIW carbon equivalent (CE) formula is the most-common parameter used to define weldability of steels and is defined as follows:

CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Using this formula, it was determined that the conventional C-Mn steel Gr. L80 casing chosen by Suncor had a CE value of about 0.48, while the selected Cr-Mo low-alloy-steel casing had a significantly higher CE value of 0.60. The nominal CE value for the AISI 4130 casing-bowl material was in the range of 0.68. These relatively high CE values require the use of low-hydrogen welding practices and preheat (to avoid the risk of delayed hydrogen cracking), as well as some level of PWHT to control the hardness in the weld and heat-affected zones (HAZ).

For practicality and ease of use in the field, it was established early on in the project that the on-site casing-bowl-attachment welding would be performed using shielded metal arc welding (SMAW), with low-hydrogen electrodes. It was also decided that E7018 electrodes would result in too low a weld strength and that the alloying in E9018 electrodes would be too high for proper weld-hardness control. Therefore, E8018 electrodes were deemed to be an appropriate compromise, providing adequate weld strength and comparable hardenability to the base materials. The particular class of electrode chosen was E8018-B2, which has Cr and Mo alloying similar to the base materials. Nickel-alloyed electrodes (e.g., E8018-C2) were avoided, as NACE MR0175 specifies that nickel should not exceed 1% for steels or welds in sour service.

Bead-on-plate-type testing was performed on samples of the C-Mn and Cr-Mo Gr. L80 casing material, using varying levels of preheat and PWHT as a preliminary evaluation of the effects of preheat and PWHT on hardness. A constant welding heat input of 0.9 kJ/mm was used for all weld deposits. This preliminary testing determined that a minimum preheat of 200° C was necessary to control the as-welded hardness and avoid the risk of delayed hydrogen cracking, while a minimum PWHT temperature of 675° C for one hour was found to be necessary to meet the 22-HRC maximum limit.

Tensile testing of casing-base materials that had been subjected to PWHT thermal cycles ranging from 675° C for one hour to 705° C for three hours revealed that the yield strength of the C-Mn casing dropped below the specified minimum of 80,000 psi for all PWHT cycles, while the Cr-Mo casing had sufficient alloying to maintain yield strength above the specified minimum level for all PWHT cycles. This was because the PWHT cycles were all at temperatures below the original tempering temperature used for the quench and temper heat treatment of the Cr-Mo casing. Due to the need for a minimum 675° C PWHT temperature for hardness control of the 4130 casing-bowl HAZ, the plain C-Mn casing product was ruled out for further evaluation in this program.

Laboratory Welding Simulation Trials

Three welding trials were run in the laboratory using actual API Gr. L80 Cr-Mo casing product and AISI 4130 Class 75K casing bowls. The casing bowl was positioned over the prepared end of the casing, preheated to about 250° C and externally tack welded with E8018-B2 electrodes, and then the internal and external fillet welds were applied using three weld passes for each weld. Welding parameters were measured and recorded for each weld pass, and heat input values were calculated. Electric resistance ceramic heating elements were used to provide the controlled heat necessary for the PWHT cycle, with the heating and cooling cycles controlled in accordance with ASME Section VIII Division 1.

Once completed and cooled to room temperature, internal and external welds were examined by magnetic particle inspection to verify the absence of surface-breaking discontinuities. Metallurgical cross sections were cut from both the internal and external fillet welds and then polished and etched in preparation for Rockwell hardness testing. Figure 2 shows the typical cross section taken from one of the welding simulation trials, which illustrates the fit-up and relative dimensions of the casing, casing bowl and fillet welds.

Figure 2: Typical Cross Section of Weld Simulation Trial

The initial weld simulation trial used a two-hour 675° C PWHT cycle, with externally wrapped heating elements and the temperature being monitored using thermocouples attached to the external weld surface. Hardness testing revealed that the external fillet weld met the NACE sour service criteria of 22-HRC maximum; however, the casing-bowl HAZ for the internal weld was 24 to 27 HRC. It was concluded that this was due to the heavy section thickness (> 30 mm) of the casing bowl, separating the internal weld from the external PWHT heat source.

As a result of this initial weld test, a second weld test was made using identical welding conditions and external wrapped heating elements for PWHT. However, the PWHT cycle was extended to two hours at 675° C followed by one hour at 705° C. Additional thermocouples were attached to the internal fillet weld to monitor the inside temperature during PWHT. Even with the extended PWHT cycle, readings from the internal thermocouples confirmed that the internal fillet weld was still not reaching the specified minimum heat of 675° C over this three-hour period. Subsequent Rockwell testing also confirmed that the internal weld HAZ still exceeded the NACE sour limit of 22 HRC.

These two weld trials, therefore, identified that the only effective manner for achieving the required PWHT cycle for both the internal and external fillet welds for hardness control would be using concurrent application of heating elements on both the internal and external welds. A third test weld was consequently made using internal and external heating elements and thermocouples employed for the PWHT cycle of 675° C for one hour. Figure 3 shows the third casing-bowl weld test after the installation of the internal and external electric resistance heating elements. Rockwell hardness testing of metallographic sections from this third weld trial confirmed that both the internal and external weldments met the 22-HRC maximum criteria.

A tensile specimen was cut from the 4130 casing-bowl material after an extended three-hour PWHT cycle at 675° C. The results of this test were compared with the mill test certificate for the as-supplied casing-bowl material, which confirmed that the PWHT cycle did not have a detrimental effect on the strength of the bowl material.

Implementation of On-Site Casing-Bowl Welding

As a result of the successful laboratory welding trials using actual Grade L80 casing and 4130 casing-bowl materials, a “Casing Bowl Installation Procedure” was written and developed for Suncor Energy’s Drilling Department, providing critical guidelines for the on-site installation of casing bowls. This document was written such that it provided practical, feasible and effective guidelines for the installation of sound welds meeting the hardness criteria for sour service.

This installation procedure includes critical issues to be addressed during the casing and casing-bowl procurement stage (e.g., material grade, alloying and heat treatment), quality control, installation practices (e.g., fit-up, cleaning, preheat, weld procedures and PWHT) and non-destructive inspection requirements. Figure 4 is a photograph showing the installation of the external fillet weld on an actual sour gas wellhead in the field.

By following this installation guideline, the need for in situ hardness testing of the completed welds for verification of compliance with the 22-HRC criteria has been eliminated. This is a significant achievement, as portable hardness testing on the surface of casing-bowl fillet welds at the wellsite is recognized as being inexact and generally unreliable, due to limitations of hardness-testing equipment and inaccessibility to the critical HAZ regions of the welds.

Summary

This research and development work, a collaboration between Suncor Energy, Canspec Group and Metal Engineers International, has resulted in the development of a proven welding practice for the installation of slip-on casing bowls to the top of a casing. The “Casing Bowl Installation Procedure” has been used successfully by Suncor on at least two sour wellsites at this time, and has proven to be a viable and effective means of ensuring the installation of safe casing-bowl connections, resistant to sulphide stress cracking in sour service and compliant with industry and regulatory requirements.

Acknowledgements

This welding development work was carried out under the sponsorship and funding of Suncor Energy Inc. The authors would like to thank Suncor Energy for their support of this project.