Challenges of Hot Tapping Into a Sour Gas Transmission...

2000 International Pipeline Conference — Volume 1 ASME 2000

CHALLENGES OF HOT TAPPING INTO

Ray Goodfellow Chevron Canada Resources

ABSTRACT Chevron Canada Resources recently completed a hot tap on the Simonette high-pressure sour gas transmission line near Grande Prairie, Alberta. The hot tap was required to bring on new production into the Simonette pipeline without shutting in existing production. The hot tap was completed under full line pressure and gas/condenstate flow during the winter with temperatures averaging -20°C.

The design pressure of the 12 " Gr. 359 Cat II pipeline is 9930 kPa and the line operates at 8200 kPa. The gas in the main transmission line is approximately 2% H2S and 4% C02. The gas being brought on through the 4" hot tap tie-in was 21% H2S and 5% C02. At the tie-in point the transmission line temperature was 3°C.

Safely welding on the pipeline under these conditions was a considerable technical challenge. In welding sour service lines it is critical that the final weld hardness be below Vickers 248 micro hardness. This can be very difficult to achieve when welding on a line transporting a quenching medium of gas and condensate. In addition, hydrogen charging of the steel from operation in sour service can lead to hydrogen embrittlement during welding.

Ludwig & Associates developed the hot tap weld procedure and extensively tested the procedure to ensure that suitable weld microhardness was achievable under pipeline operating conditions. As part of the procedure development the welder who would perform the hot tap was tested repeatedly until he could confidently and successfully complete the weld. During fieldwork, the welding was rigorously monitored to ensure procedural compliance thereby minimizing the possibility of elevated hardness zones within the completed weldment.

This paper will detail with the technical development of the hot tap welding procedure and the successful field implementation.

A SOUR GAS TRANSMISSION LINE

Rory Belanger Ludwig and Associates Engineering Ltd.

INTRODUCTION The Simonette pipeline is primarily used to transport Chevron Canada Resources (CCR) Simonette oil battery dehydrated solution gas to the Kaybob South gas plant. The pipeline is also used to transport third-party dehydrated gas and condensate. The typical average acid gas component was 2% H2S and 4% C02 with the average daily production being 670 103m3 of gas and 300 m3 of condensate.

In fall 1998 a third party oil and gas company wished to tie-in a new sour gas well into the Simonette line. This well has an average acid gas component of 21% HiS and 5% C02, and an average daily production of 280 103m3of gas.

To tie-in this new production without shutting in 2000 m3/day of oil and associate solution gas at Simonette it was decided to investigate the possibility of performing a hot tap under operating conditions. There are considerable technical challenges in completing a hot tap on a sour service pipeline. It is not uncommon to hot tap sweet gas systems, however, a review of those procedures showed the final hardness achieved would be unacceptable in a sour system.

The key technical challenges are: 1. Achieving a final weld where all hardness are below 248

HV5oo while welding on a pipeline with a quenching medium of flowing gas and condensate

2. Preventing cracking due to hydrogen charging of the weld from in service interstitial hydrogen

3. Preventing burn through of the pipeline while welding

Before committing to the hot tap CCR had to be certain it was feasible to over come the technical challenges and complete the work without risk to the on site crews. Ludwig and Associates were contracted to develop the weld procedure, lab test the procedure, train the welder and supervise the on site welding.

Copyright © 2000 by ASME

IPC2000-101

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DISCUSSION Development of the Weld Procedure

The Simonette pipeline was constructed of CSAZ245.1 323.9 x 11.3 mm wall Gr. 359 Cat II HIC resistant steel. The mill test documentation for the pipe was available and a joint of pipe from the original construction was available to be used in developing the weld procedure.

A critical aspect of the hot tap weld is the welding sequence. The weld sequence must be carried out such that each pass adequately heat treats or tempers the previous pass. The final cap pass, both inside and out, must be made entirely on the previous weld metal and the branch connection pipe. These final passes must not be welded onto the carrier pipe as the resulting heat affected zone (HAZ) may be un-tempered and unacceptably high hardness could result.

To ensure that the root pass was tempered, the procedure requires the root to be welded from inside the branch first. The first pass is applied to the ID of the stub-on. A second tempering pass is applied to overlap the first pass. It is important that the second pass does not touch the base in-service material. (Figures 1 & 2 show the bead sequence). The root can then be ground back from the outside of the fitting making it easier to weld a defect free root pass. This method can improve the quality of the root pass by reducing the chance of root bead defects. A sharp edged defect in a sour environment can result in stress that leads to SSC failure even with acceptable levels of hardness.

Run Ripe 3/8 in } 3/8 in.

Figure 1- Pass Sequence

Pass 1 is on the inside of the branch connection and two thirds of this pass is applied to the run pipe material. Pass 2 on the inside of the branch connection is welded completely on the first pass or the nozzle material. This will temper Pass 1. Pass 6, the second to last pass has two thirds of the weld applied to the parent material. Pass 7, the last pass on outside of the branch connection has 100 % of the weld on Pass 6 or the nozzle material.

Figure 2 - Completed Wei

It was fortunate that the actual pipe material to be used in the preparation of the procedure qualification record (PQR) was the same material used in the pipeline. This material had a relatively low carbon equivalent and it was anticipated, with some level of confidence, that acceptable procedure qualification would be able to be performed. However, the time constraints associated with procedure qualification required that all of the possible options associated with the coupon be explored in as few PQR preparations as possible. It was expected that welding on the pipe, at the reported lower end temperature of 5°C with flowing water as an internal quench media, could produce unacceptably high heat affected zone hardnesses, even with the relatively low carbon equivalent (CE) pipe material being used for the PQR. For this reason, two procedure qualifications were conducted; each of which employed a different cooling media.

The first PQR welding was conducted on a pressurized length of pipe containing a 50% ethylene glycol/water solution as the cooling media. The choice of this media was based on research, which indicated that cooling rates on the OD of a pressurized pipe OD containing this solution could be reduced relative to those cooling rates associated with flowing water. An internal coupon pressure of 5.2 MPa was chosen to control OD cooling rate to a level higher then that expected on the pipeline by a comfortable margin but lower then that expected with flowing water as a cooling media. Even though preheat was to be administered to both the branch and run materials on the field weldment, preheating was only administered to the branch material during coupon welding. The decision was made to preheat the branch material to minimize the risk of PQR failure on the branch HAZ's due to the relatively high CE of the branch material.

The second PQR welding was conducted on the same coupon but this testing was more conventional as it employed flowing water as a cooling media. A second major difference between the welding conditions was that preheating was to be employed on both the branch and run materials during welding. This was found to be practical because of the relatively thick (11.3 mm)


wall of the pipe material. In the laboratory, a preheating temperature of 100°C was able to be maintained on the branch material. Preheat of 100°C was applied to the run material prior to each new electrode (each pass).

The results of hardness testing conducted on the PQR specimens produced from both quench media were found to fall below the maximum 22 HRC (248 HVjoo) recommended by NACE MR0175. The maximum average HAZ harness levels were found to be higher on the 50% ethylene glycol / water coupon but the actual harness levels were found to be less scattered. This was probably due to the more varied run pipe wall temperature during the course of welding of the continuously re-preheated water filled coupon. The following table presents a summary of the hardness test results:

Table 1 HARDNESS ' 1

Water'-'' 50 % ethylene glycol / water Branch •'"'Run • "Branch'^ • Run •

Hardness Range 172-211 163-228 172-223 188-227

Hardness Average 184 200 195 210

Training of the welder

It is critical that the welder is educated in the correct technique prior to the hot tap. Typically, welders are trained to produced defect free welds and rarely consider weld hardness. The welder must weld with strict adherence to the procedure to attain the hardness levels required. The welder was required to attend a training session to understand the technical aspects of the procedure. The welder was also tested on the same pipe size and orientation as that to be encountered on the hot tap. The welder needed a number of attempts at the procedure to successfully achieve the desired weld quality.

Safety Considerations

The up front technical work gave CCR the confidence that the hot tap could be safely preformed. However, CCR also developed safety procedures to be followed to ensure that risk to the field staff was minimized.

Due to the wall thickness and fracture properties of the pipe there was a very low risk of burn through or cracking to failure during the welding process. The most significant risk of failure would be after the hot tap was completed. The most likely failure scenario would be a crack of the weld root after exposure to sour production resulting in a gas release around the reinforcing saddle.

There was a safety specialist on site at all times during the procedure and an evacuation and emergency shut down plan was in place. Prior to starting the welding operation, both Simonette and Kaybob South were informed and prepared to respond if required.

During the actual hot tap into the pipeline the machine operator was masked up and using supplied breathing air apparatus, H2S detection was in place and a rescue crew was standing by.

Field Welding

The pipeline had to safely exposed and excavated. The bell hole used for access had to be properly prepared and sloped to ensure safety of the crews. (Figure 3)

Prior to welding

At the hot tap location about the area to be welded, a complete Ultrasonic Testing (UT) grid was performed for 600 mm on either side of the weld area. The UT grid scan was used to ensure that no corrosion had occurred and that the wall thickness is uniform and to check for any measurable inclusions or laminations.

In addition to the UT, radiography was used to check the entire circumference of the area to be welded. This was done to ensure uniform wall thickness around the circumference of the pipe. The calculations for the attachment assumed that the pipe wall is in original design condition.

The results from the UT and radiography showed that no corrosion had occurred and no laminations were present in the pipe wall.

The hot tap location was on the up slope of a hill were condensate slugging would occur. To minimize the risk of a condensate slug increasing the quenching rate during welding the pipeline was pigged 4 hours prior to the hot tap.

Although procedure qualification on the glycol/water solution filed coupon did suggest that run pipe preheat did not have to be depended upon to achieve acceptable hardness levels, a minimum base metal temperature of 100°C was attained prior to welding. To increase the amount of heat available and reduce the effect of quenching by the surrounding pipe wall a stress relieving unit was used. Coils were wrapped around the complete circumference of pipe adjacent to the weld and the area below the weld. The stress relief equipment was set up so that it did not impair the welder's access to the weld area or emergency egress.


The circumferential temperature of the carrier pipe had to be less than 120 °C or by CSA Z 662, the MOP of the pipe would be need to reduced. At the time of welding the preheat temperature was found to drop between electrodes so run away interpass temperatures were not a problem.

Prior to preheating for welding, checks of the pipe wall temperature with an infrared heat sensing gun at various times showed that the pipe wall temperature was either 6 to 7°C or 2 to 3°C. The lower temperatures were believed to corresponded to condensate slugs.

After the pig had passed the temperature was checked and found to be 7°C. However condensate built up and slugging was again occurring within hours of the pig passing, resulting in temperatures of 2°C prior to preheating for welding. The stress reliever heating the adjacent area at 100°C was also ineffective at increasing the pipe wall temperature at the weld area. The quenching effect of the gas/condensate was greater that the heat conduction in the pipe walls.

A propane-heating torch could not maintain the required preheat temperature and an oxygen acetylene torch was used. By preheating the pipe immediately prior to welding each electrode, the required preheats could be maintained.

Experienced technical staff was on site to confirm strict adherence to the procedural details such as preheat, rod size, pass sequence, volt and amp settings, travel speed and inter-pass temperatures. If problems occurred during welding or if preheat could not be maintained CCR was prepared to stop the work and shut in the pipeline if necessary.

During welding

Non destructive examination (NDE) was used to ensure that a weld could be produced with no defects that would jeopardize the final weld integrity. After completion of the inside weld and grinding to sound metal from the outside, the root pass of the weld was inspected with the dry powder magnetic particle inspection (MPI) method to check for cracking. (Figure 4). Immediately after the welding was completed the weld was MPI inspected for cracking. The weld was MPI checked again 24 hours after the weld was completed to check for delayed hydrogen cold cracking. (Figure 5)

If cracking had been found on the weld metal or heat affected zone (HAZ) of the branch connection, the extent of cracking would have been reviewed and corrective action taken. If cracking was found in heat affected zone (HAZ) of the carrier pipe the pipeline would have been shut down and depressurized.

If a crack or crack-like indication proved to be a shallow surface flaw, attempts may have been made to remove it by grinding. If this was successful, the pipeline could remain in service. If the crack had extended into the carrier pipe material greater than 25 thousands of an inch and could not be removed by light surface grinding, the pipeline would need to have been shut down, depressurized and the welded section of pipe removed.

The reinforcement saddle was not welded to the carrier pipe wall. The reinforcement saddle is welded to the branch connection and the weld area was then preheated to a minimum of 65°C. The completed weld of the reinforcement saddle to the branch connection was magnetic particle inspected. (Figure 6)

When the welding and inspection was completed an extension was welded on to the stub-on to allow the flanged connection and valve to be located above ground. The connection was back filled and the hot tap completed. (Figure 7)

CONCLUSION Hot tapping on sour gas lines can be completed safely if proper procedure development is carried out. CCR had a technical advantage in this hot tap by having a section of the original pipe, all the line MTR's and HIC resistant steel.

The following is the minimum requirements before attempting a similar hot tap

1. The wall thickness of the pipe must be greater than 4.8 mm to prevent the risk of burn through.

2. The carrier pipe must be un-corroded and free of laminations.

3. The carbon equivalent (CE) of the pipeline material must be known.

4. The weld procedure must be developed using a pipe of equal or greater CE, with the same diameter and wall thickness.

5. The procedure development must duplicate the worst case quenching effects of the carried pipe fluid.

6. Preplanning for emergency is critical to ensure the safety of the personal doing the work.

ACKNOWLEDGMENTS

Frank Langenecker - Ludwig & Associates Albert Van Roodselaar - Chevron Canada Resources Vic Lues - Finlay Inspection


Figure 3 - Bell Hole


Challenges of Hot Tapping Into a Sour Gas Transmission...

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