Cost Reduction with Improved Process Control in Butt Joint of Cladded Spool Fabrication

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L&T Hydrocarbon, MFF- Hazira,
Surat-Gujarat

  1. ABSTRACT:

In Today’s Oil & Petro chemical industries, weld overlay of Nickle alloys on carbon steel is widely being used due to its corrosion resistance and cheap alternative compared to solid Inconel. Due to uncertainty involved in the quality of weld overlay, digital control is very useful to control weld parameters, hence resulted into desired properties. Improved process control has been achieved with new generation arc welding equipment through digital control, giving the benefits of reduced heat input, improved arc stability and spatter-free welding, hence better consistency in quality. Effort is made in this study to show challenges faced during SAW welding in full weld joint with solid Inconel wire along with benefits of Inconel 625 weld overlay on commercial aspects, compared to full joint by Inconel welding for high thickness joints. In this process CRA layer from Inconel cladded pipes was removed up to Carbon steel on both sides and joint was welded with Carbon steel consumable. Later cut back CRA layer was restored with solid Inconel wire using Hot Wire-Pulsed Gas tungsten arc welding machine. By this process better control on heat input, %Fe dilution, & good weld finish was achieved. Also, it was beneficial on commercial aspects with respect to welding full joint with 100% solid Inconel wire.

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Keywords: Productivity, Automation, Quality, Cost

Project Objective/Aim: Improving Productivity with Quality and Cost Control by using Automation

  1. Introduction

The Structure of Multi Million Projects is designed to produce million barrels of Oil which ignites lives of Human-beings. The Design, Manufacturing and Installation of these structures is the biggest challenge and requires remarkable team efforts. L&T MFF is one of the leading players in South East Asia for manufacturing of offshore modules. Welding is the core manufacturing process used in fabrication of these offshore structures.

To cut down on the overall project fabrication cost, some of the welding variables can be optimized.  The aim is not only to minimize the overall cost but also to enhance the overall final product quality. Welding variables are tactically maneuvered to Increase process efficiency, optimizing cycle time and thereby reducing overall cost without compromising with final product quality.For any given application, some processes are more economical than others. This paper presents some of the initiatives taken by L&T Modular Fabrication Facility for improving productivity, managing time & control of welding costs.

  1. Challenges:
    1. High weld metal deposition due to high thickness (up-to 104 mm)
    2. Client requirements and selection of welding process
      1. Getting required weld chemistry at 3 mm overlay height
      2. Welder training and qualification for automated process
  • Restriction of welding process due to joint geometry
  1. PQR Qualification

High weld metal deposition

For international project (Client: Saudi Aramco), in CS cladded with Inconel 625 spools fabrication there was high thickness varying from48.34mm to 104.45 mm. Due to this high thickness, high amount of weld deposition was required. With existing WPS and welding process, welding shall be done with solid Inconel wire only. Nickle based solid Inconel wire is costlier than CS and was affecting overall cost of project. Brain storming exercise was necessary to reduce welding consumable cost with faster weld metal deposition. Joint Thickness and Diameter wise detailed exercise was carried out for total scope as per below:

Client requirements and selection of welding process

For corrosion resistance overlay welding, client has specific and stringent requirements to be followed like;

  1. If there is access for welding from the ID side, it is acceptable to cut back the CRA layer and carry out girth welding using a carbon steel electrode. CRA layer can subsequently be deposited by automatic weld overlay. Automatic welding is mandatory for the overlay.
  2. Weld overlay shall be carried out using automatic GTAW / GMAW / SAW process only.
  • Welding heads shall be equipped with automatic voltage control (AVC). The direction of weld overlay deposit shall be circumferential irrespective of the diameter of the component.
  1. Overlay thickness shall be 3.0 mm minimum and a minimum of two layers shall be applied.
  2. For PQR – Chemical analysis shall be carried out at a height of 2.0 mm from the base metal. The chemistry shall be within the range specified in ASME SEC II Part C or AWS classification. In the case of alloy N06625, the iron content shall not exceed 10%.
  3. PQR shall be qualified with same make and model of actual job welding to be done.

Selection of overlay welding process was needed to be in-lined with these requirements.

Why hot wire GTAW-Pulsed ID overlay?

Choosing the right welding process and welding technique is the key to achieve required weld chemistry and sound weld metal in Inconel 625 weld overlay.

For Inconel 625 weld overlay, good appearance and sound and high weld deposition will only be achieved if welding process falls into the category of “Hot” such as SAW, GTAW, Plasma transfer arc, etc,. On the other hand, with any “Hot” process high dilution comes as a normal factor.

An ideal balance to be found for lowest possible dilution with optimum deposition for better weld chemistry, and process efficiency. One of the most successful candidates in achieving those variables is Hot wire GTAW-Pulsed. Although the hot wire GTAW-Pulsed process, commonly known as hot wire TIG-Pulsed, was introduced in 1966, its development and application have been slow to take hold. However, recently there has been considerable renewed interest in the process which initially promised “MIG speed with TIG quality.” While there are some limitations, the hot wire gas tungsten arc welding process has found increasing use over a wide range of groove welding, buildup and cladding operations. The development of automated and specialized hot wire equipment has driven application of the process in industries including nuclear, power generation, pressure vessel and offshore oil which all require high productivity-high quality welds.

GTAW-Pulsed was only option, as It was not possible to get required chemistry at 2.0 mm height with GTAW process without pulsing. With a proper welding technique and control over welding parameters, hot wire GTAW-Pulsed will show following advantages.

  • Optimum deposition with lower dilution
  • Near zero weld defects in overlay
  • Perfect weld finish
  • Less consumable wastage due to automated process
  • Very good weld chemistry with consistency
  • Equipment flexibility for welding of small bore pipe and fittings
  • Low cost of labor

Due to these all benefits, Hot wire ID overlay is the suitable process for this type of applications.

Setting welding parameters for ID overlay station

After finalizing welding process for weld overlay, next challenges were setting welding parameters suitable to meet stringent client requirements, and training welding operator for ‘4-axial autowelder machine’. Welding operator was needed to train for programming of power source and ‘4-axial autowelder machine’. Welding Complexity of programming and feeding multiple data in power source as well as programming of ‘4-axial autowelder machine’ was making welder training difficult.

Key requirements for setting welding parameters

  • Weld chemistry of Inconel 625 at 2.0 mm overlay height with iron content less than 10%
  • Lowest possible dilution with base material
  • Max acceptable overlay height – 4.0 mm, with minimum of 2 layers
  • Heat input control with optimum pulsing
  • Zero repair as manual repair was not allowed
  • Best possible weld finish as weld visual was not possible from inside due to less diameter.

After commencing multiple trials, following parameters were established and PQR was qualified meeting above requirements.

  • Peak current – 190 A
  • Base current – 140 A
  • Hot wire current & Voltage – 40 A and 5 V
  • Wire feed rate – 1000 mm/min to 1200 mm/min
  • Voltage – 12 V to 16 V for Barrier layer, 13 V to 17 V for subsequent layers
  • Travel speed – 190 mm / min for Barrier layer, 210 mm / min for subsequent layers
  • Job rotational direction – Clockwise
  • Torch direction – Reverse
  • Bead Overlap – 40% for first layer, 60% for second layer
  • Circumferential Step back Size – 3.5 mm every 360 Deg. rotation
  • Start Mode – High frequency mode (HF mode)
  • AVC start – 0 sec
  • Gas pre-purge – 3 sec
  • At start Initial current – 50% of main current
  • Current up time – 3 sec
  • At start wire feed delay – 2 sec
  • At start rotation delay – 3 sec
  • At stop wire feed delay – 2 sec
  • At stop current down time – 5 sec
  • Current at stop – 50% of main current
  • Gas post purge – 5 sec
  1. JOB EXECUTION

After successful completion of PQR and satisfactory testing results, it was time to execute the same on job. PQR qualification, Welding operator qualification is not enough to assure meet client requirements. Key sequencing, proper methodology and quality checks need to ensure quality during production welding. After Brain storming and past project learning following sequencing and quality check prepared and same was approved from client.

ID OVERLAY WELDING MACHINE SETUP

Step 1: Visual and dimensional inspection was carried out of CRA cladded pipe, CRA cladded Transition piece and CRA cladded Flange. After completion of inspection compound beveling as per fig 1 prepared on Inconel 625 clad pipe and fitting.

Step 2: CRA cladded pipe, CRA cladded Transition piece and CRA cladded Flange, which are to be welded shall be machined from ID side, to remove CRA layer up to 15 mm to 25 mm length from the edge.

Step 3: CRA layer shall be removed completely and shall be verified by using Copper sulphate (CuS04) for identifying presence of ferrous material on existing Inconel Clad/Overlay.

Step 4: It shall be ensured that removal of CRA layer shall be more than 3 mm, by using universal gauge or bridge cam gauge or any suitable method.

Step 5: After completion of above stage, Joint to be fit-up shall be done by bridge tack using carbon steel welding consumables.

Step 6: Girth welding shall be carried out using a carbon steel welding consumable. Butt Welding with GTAW ER80SNi1 + SAW F8P8-ENi5 as per Applicable Approved WPS

Step 7: After completion of welding, Root penetration shall be ground off and surface shall be smooth grinding.

Step 8: Perform Visual Inspection and then perform Volumetric NDE (Radiography Test).

Step 9: Weld repair of carbon steel welding can also be done as per Applicable Approved WPS.

Step 10: Perform PWHT as per approved PWHT procedure

Step 11: Weld Overlay (Clad Restoration) on ID of girth weld with ERNiCrMo-3 as per Approved WPS.

                     EXECUTION OF ID OVERLAY BY HOT WIRE GTAW PROCESS

Step 12: Thickness of restored overlay shall be equal to the surface of existing overlay. Final restoration layer shall have smooth transition with existing CRA layer.

Step 13: Automatic GTAW-Pulsed Overlay welding shall be executed by International Client approved welding operator / welder.

Step 14: Perform Visual Inspection and NDE (PT) on Weld Overlay (Clad Restoration).

             ZERO INDICATION IN DPT – 100% RIGHT FIRST TIME ACCEPTANCE

Step 15: If any repair activity after completion of CRA restoration, shall be done by Approved WPS and followed by NDE (PT).

Step – 5 : ID Weld Overlay with Inconel Filler by Automatic Hot Wire GTAW

Step 16: Final Radiographic testing shall be carried out after completion of the weld overlay.

Step 17: Other stages shall be as per approved ITP

ID overlay finished products photographs

  1. Cost comparison and cost saving

Due to typical joint and spool geometry, ID overlay by automatic process was not possible from inside. Joints were identified out of available scope, where ID overlay by hot wire GTAW-Pulsed from inside was possible. Average cost for single joint is given below for cost comparison;

Total welding cost for welding with solid Inconel – X

Total welding cost with Hot wire ID overlay – 0.34X

Cost saving for welding consumable – 66%

As we can see in following figure, instead of full joint welding with solid Inconel, very less amount of Inconel welding was required, and rest joint can be welded with Carbon Steel welding consumable.

                             Outcome of GTAW hot wire ID overlay welding

  1. Synopsis

For Turn-key projects in Oil and Gas industry, cost and time is a key factor for project success. To survive in this competitive market and for business sustainability demands, new process developments with latest technology is very much required. Effort is made in this study for development of new process, with overall benefit of cost control and reduction in spool cycle time without compromising quality. With this complete exercise, new overlay technique was added in production line.

  • A 100% Sound weld metal and weld chemistry can be achieved with automated overlay welding process.
  • ID overlay is economically obvious choice rather than welding full joint with solid Inconel for substantial savings regardless of the size of the project.
  • With new Specialized weld overlaying equipment’s, it is possible to overcome stringent customer requirements with improved quality.
  • Overall spool cycle time can be reduced with help of replacing traditional method with automatic and digital technology.

This is the first step towards success and goes a long way towards closing the gap in productivity and quality with upcoming projects. This study will be encouraging to take more initiatives and will play an important role for each organization, wants to take steps towards overall cost reduction of project.

  1. References
  2. Dan Capitanescu, “Alloy 625 weld overlays for offshore and onshore structures”, 1991, Alberta, Canada
  3. Keepsake Automation LLP, keepsakeautomation.com
  4. Fronius International, fronius.com
  5. Barbara K. Henon (Arc Machines Inc.), “Advances in Automatic Hot Wire GTAW (TIG) Welding”,
  6. Cladtek, cladtek.com

 

 

 

 

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