Radical new approach to joining - in the pipeline

After graduating with an honours degree in metallurgy from Sheffield University, Chris Punshon joined TWI in 1983 where he is currently Manager of the Electron Beam Applications Section in the Electron Beam, Friction and Forge Processes Department. Recently he has been concerned with the investigation of EB welding process/property relationships for a wide variety of metallic materials and the application of the EB process to large structures and components in a range of industry sectors. He is currently responsible for the co-ordination and technical supervision of activities in the EB Applications Section.

Antonio Belloni has been involved in welding research and development in Saipem for more than 20 years. He is presently managing the development of automatic welding systems based on the metal inert gas and electron beam processes.

Pipe laying in the J-mode demands that pipe welding is carried out at a single station. Particularly for thick wall pipe this precludes many of the arc welding techniques such as shielded metal arc welding (SMAW) and gas metal arc welding (GMAW) which require a multi-station approach to achieve the required productivity. As Chris Punshon of TWI and Antonio Belloni of Saipem report it is for this reason electron beam welding (EBW) is extremely attractive. It is unique capable of producing deep, single pass, high quality welds at high speed.

Electron beam welding is a well established, mature technology used widely in engineering industries where welding is carried out with the workpiece under high vacuum (less than 5 x 10 -2mbar). The need to work at high vacuum has restricted application of the process for very large steel structures, including pipelines. Previous attempts to produce local vacuum systems have been largely unsuccessful because of poor seal reliability, the requirement to achieve a pressure of better than 10-2mbar and the intolerance of the welding system to any fluctuation in the working vacuum pressure.

Following a long period of process development at TWI, Cambridge UK (previously The Welding Institute) a prototype EB welding system has been manufactured which is capable of welding heavy section steel at rough vacuum or reduced pressure (ie ~1mbar).

An experimental program, part funded by the EEC has been carried out to examine the practicality of using this system for laying offshore pipelines in deep water in the J-mode.

Girth welds have been performed in 30" OD, 38mm wall thickness (API-5L X65) and 28" OD, 41mm wall thickness, (API-5L X70) thermo mechanically controlled processed (TMCP) steel line pipe materials.

This paper describes the successful outcome of the project and illustrates the great potential for exploiting this technique in pipe laying applications.

In the late 1970s TOTAL CFP built a prototype pipe welding system designed around the EB process. The equipment was sufficiently robust to withstand the rigours of lay-barge operation but certain technical problems prevented industrial exploitation of the system and further development. These were mainly:

• poor mechanical properties of the welds
• severe vacuum requirements.

To be able to proceed eventually to the construction of an industrial machine it was recognised that it was necessary to overcome the above problems and therefore a preliminary investigation was made to assess the feasibility of performing welds under less critical vacuum conditions than 5 x 10-2mbar. In addition, it was realised that it was essential to examine the quality and properties achievable when welding modern commercially available line pipe steels. For these reasons the project was split into two phases - first development work and then prototype construction and welding trials.

Development work

The development programme addressed three different areas:

• Optimisation of vacuum conditions
• Weld quality and mechanical properties of the welds
• Weld procedure optimisation

Optimisation of vacuum conditions

Tests were carried out over a range of pressures from 5 x 10-2 to 100mbar. These tests were conducted as melt runs (bead on plate) using flat plate material supplied to API-5L X65 using the EB welding equipment which has been developed specifically to allow operation within this pressure range.

The objective was to determine the influence of vacuum pressure on welding performance and so establish the maximum pressure tolerable for producing welds of appropriate quality in terms of first weld integrity and second, surface profile.

The tests were made on line pipe steel plate of 25 and 38mm thickness. The preferred operating pressure range was defined as being in the range 10-1 to 10mbar on the grounds that throughout this coarse vacuum range the welding process was exceptionally tolerant to changes in working distance and operating conditions.

Weld quality and mechanical properties of the welds

The aim of this stage of the programme was to demonstrate that the appropriate weld quality and the mechanical properties requirements detailed in the Table were achievable for electron beam welds in typical modern line pipe steels. Again the tests were made on C-Mn, sour service grade line pipe steel of 25 and 38mm thicknesses to API-5L X65.

Table: Mechanical properties - specification requirements

Weld quality
Weld quality was assessed using a combination of X-radiography and metallographic sectioning. Throughout the programme (which involved more than 200 welds) it was found that the welds were of consistent high quality with no incidence of porosity, solidification cracking or lack of fusion being observed. Furthermore the weld profiles achieved were consistently satisfactory with acceptable weld reinforcement (both cap and root) and no significant undercut.

Mechanical properties
Initially, mechanical tests were performed on autogenous welds. It was established that the welds were acceptable in all respects except for the fracture toughness (as measured by CTOD testing) which was slightly lower than the specified requirement of 0.15mm at -10°C. After further development, however, it was demonstrated that satisfactory fracture toughness could be obtained reliably by introducing filler material into the weld.

It is recognised that when welding conventional TMCP line pipe steels autogenously, it may be difficult consistently to achieve satisfactory fracture toughness properties and therefore a filler addition will be necessary. Nevertheless the use of a filler addition is not necessarily detrimental as it permits welding of a wide range of material compositions and impurity levels which can exist within a given material specification.

Weld procedure optimisation

Having established the optimum vacuum pressure range and having demonstrated that satisfactory mechanical properties were achievable through welding plate material, work was carried out to transfer the acquired information to perform girth welds on full diameter pipe samples.

Three pipe sizes were examined:

30" OD, 38mm w.t. (X65)
28" OD, 41mm w.t. (X70)
20" OD, 25mm w.t. (X65)

Again, these were sour service line pipe materials produced by the TMCP route.

The welds were made in the horizontal position (2G) with the pipe vertical. In this case welding was achieved by rotating the pipe section while keeping the electron beam stationary. An example of a welded pipe sample in the 30" OD, 38mm wall thickness is shown in Fig.1. Details of the cap and root weld beads are depicted in Figs 2 and 3 respectively.

Many welds were made in this way which confirmed the reproducibility of the good results obtained on the plate materials thus encouraging the continuation of the programme to the point of manufacturing a prototype welding system.

Prototype construction and welding trials

The results of the tests in the procedure optimisation stage of the programme were obtained by rotating the pipe section while keeping the electron beam stationary. Clearly this is impractical for a full length pipe and therefore the construction of a prototype welding machine was initiated to demonstrate that the same results could be achieved by rotating the electron gun around a fixed pipe.

Prototype construction
The objectives of this phase of the work were to design, manufacture, assemble and test an electron beam welding system capable of orbital welding of pipe under laboratory conditions.

The outline design of the proposed laboratory system is illustrated in Fig.4 and consists of the following:

• an external EB chamber which is sealed to the pipe using local flexible seals.
• an internal clamp which provides a local vacuum inside the pipe and also aligns and clamps the two pipe sections together for welding.
• a mobile high power electron gun which is incorporated in the outer chamber and can rotate around the pipe circumference.
• a computer numerical control system mounted in a central console which controls and monitors the electron beam parameters, the vacuum system and sequencing, and movement and position of the rotary gun axis.

This system has now been built and fully commissioned and pipe welding trials are in progress.

The external chamber and mobile gun are depicted in Fig.5.

Currently the system is designed to accommodate pipe diameters of up to 30" and total pipe lengths of up to three metres for welding trials.

Welding trials
The current programme was scheduled to continue until July 1997 at which time the system will have been used to develop and optimise welding procedures for one type and size of pipe ( ie 28" diameter to API 5L - X70). Having established appropriate welding procedures, a large number of pipe welds will be carried out to demonstrate the reliability of the system and the reproducibility of the welding process. Weld quality will be assessed by both X-radiography and ultrasonic non-destructive testing methods and finally a series of test pipe welds will be produced to illustrate that consistent mechanical properties are available. Sections from typical welds produced in 25 and 41mm C-Mn steel pipe are shown in Fig.6.

Conclusions

From the results of the development work conducted so far, the following conclusions can be drawn:

• The electron beam process can be used to produce single pass, one shot welds with satisfactory quality and properties in modern TMCP pipeline steels, in wall thicknesses of at least 40mm.
• Rapid cycle times can be achieved by adopting Reduced Pressure (rough vacuum) conditions for EB welding. Operating at this pressure has been made possible by development of a new EB gun technology and permits use of local sealing and simple vacuum engineering which is both practical and exceptionally tolerant to outgassing of the pipe or leakage past the rudimentary seal.
• A laboratory prototype pipe welding machine has been manufactured and commissioned for welding heavy wall pipe of up to 30" diameter. Confirmatory pipe welding trials are currently in progress to examine the consistency of weld quality and properties achievable and the reliability and reproducibility of the welding equipment.