July - August 1998
Technology transfer
Research & Technology
- Guideline development for pipe reeling
- Microfocus inspection system a winner
- Project AIMS to develop technology
- PhD training at TWI and University of Cambridge
- Development fo girth weld repair assessment procedure for offshore pipelaying
- Crack arrest checks for new LPG storage tank steels
Best practice
- Job knowledge for welders. No.36: Distortion - prevention by fabrication techniques
Training and certification
Lasers help Benetton move up the grid
Formula 1 in action - Jean Alesi at the wheel of Benetton's Renault
Benetton Formula 1 wanted to use a new design for the rear uprights on their cars at the start of the 1997 Formula One season. This change would make a large weight saving and simplify fabrication so TWI was asked for advice on how to achieve this.
The original component was in Cr-Mo steel, manually TIG welded and then heat treated. For the new design, distortion was critical and so an alternative welding technique was needed. TWI suggested laser welding as it is fast, can be automated, and gives much lower distortion than TIG welding.
TWI fabricated test pieces and developed welding procedures before supplying a number of pairs of rear uprights to Benetton throughout the season. After fabrication, the Test House non-destructively tested all parts.
All the laser welded parts gave improved performance over the previous design and at the end of the season, Benetton finished third in the Constructors' Championship, with the weight saving contributing to this success.
More information on all aspects of laser welding may be obtained from Kevin Ayres. E-mail: krayres@twi.co.uk
3-D virtual reality factory planning
TWI's Consultancy Services Group uses 3-D visualisation software for factory planning
What can be done to support teams planning new factories and robotic production facilities? Effective planning and communication within the team are key factors.
Assisting Members to achieve both has been the driving force behind developments at TWI in the use of virtual reality based 3-D factory visualisation software. Used by the Consultancy Services Group, these tools have helped companies to realise the full potential of their site and facilities. Projects carried out have involved developing models to study cell and factory system, feasibility, layout, production capability and trouble shooting. The latest project is the simulation of a fabrication facility to produce steel beams for the building construction industry, part of the EUREKA CIMsteel project.
The project highlights the integration of scheduling, purchasing and production activities which are being driven by the product design process. The factory and robot cell simulation was developed to show how a flexible welding cell would work and how it could be integrated into the manufacturing system as part of computer integrated manufacture.
The software packages used were Deneb Quest and UltraArc, running on a PC based system, utilising a Windows NT workstation.
The facilities at TWI are available for production, research and development work. More information can be received from John Kell or John Weston. E-mail: john.kell@twi.co.uk , john.weston@twi.co.uk
Guideline development for pipe reeling
Pipe being wound onto reel at the onshore spoolbase. Courtesy Coflexip Stena
Pipe reeling is a fast and efficient method of laying offshore pipelines. Girth welds between pipe lengths are fabricated and inspected at an onshore spool base before being wound onto a spool mounted on a reeling vessel. The vessel sails to site, the spool is unwound and the pipe laid. The ramp on the vessel allows the pipe to be laid at angles between S-lay and J-lay.
However, the reeling process imposes repeated high plastic strains on the pipe. To avoid failure during reeling or subsequent service, it is important that the pipe properties, dimensional tolerances and fabrication flaws both in the weld and parent pipe, are controlled.
To define acceptable flaw sizes and material properties and to ensure fitness for service, full-scale tests are possible but expensive and time consuming and only applicable for the materials and conditions examines.
Fracture mechanics-based assessment procedures, used successfully by TWI, are less expensive and have general application. However, they need further development and validation for high strain situations especially where weld metal strength undermatching is possible.
To address these issues and to develop generally applicable fitness-for-service assessment procedures for reeled pipelines TWI and Det Norske Veritas, have launched a joint industry project - Fracture control for installation methods introducing cyclic plastic strains. One of the project aims is to update and improve guidance on the use and assessment of reeled pipe currently given in the DNV Rules for submarine pipeline systems of December 1996. Sponsors for this project are being sought.
For further information, please call Henryk Pisarski at TWI. E-mail: henryk.pisarski@twi.co.uk
Microfocus inspection system a winner
The X-Tek system makes microfocus inspection for hidden defects an easy task
Since its installation in the Microtechnology and Reliability Centre, the X-Tek benchtop microfocus X-ray inspection system has become a valuable asset to many companies in a wide range of technology areas.
The 160 kV X-ray gun with definition/resolution of between 5-10mm and immediate photo print facilities allows hidden defects to be quickly identified. The five-axis workstage allows the component to be manipulated until the optimum view of the
defect is found.
Recent applications have included:
- ball grid arrays; alignment accuracy, shorts, voiding, solder ball shape and quality
- wirebonding: broken, unattached or misplaced Au bonds have been detected
- solder joints and conductive adhesives: detection of voiding, alignment accuracy and quality
- sensors structures: internal failure of encapsulated devices
- cracking: in a wide range of materials and components.
If you would like to discuss microfocus X-ray analysis or microtechnology testing options in more detail, contact Jan Cole in the Microtechnology and Reliability Centre. E-mail: jecole@twi.co.uk
Project AIMS to develop technology
Adhesives use in the marine industry, which inclues oil and gas, and shipbuilding, has been aided through a TWI led project - Adhesives for Minor Attachments to Marine Structures (AIMS).
The programme objective was to develop adhesive bonding technology for attaching minor structures to ships, offshore and other marine constructions and to evaluate the applicability of this technology.
This was delivered through:
- design rules and guidelines
- procedure specifications.
- code of practice for Quality Assurance Procedures.
The design guidelines provide a framework for component classification and risk assessment, as well as presenting a structured procedure for selecting the right adhesive for the job.
The guidelines also describe fabrication procedures, the influence of environmental conditions on joint performance and flaw identification techniques.
The Quality Assurance documents provide a code of practice for ensuring that the strict control principles needed for adhesive bonding technology can be successfully implemented.
Organisations taking part in the project included Shell UK Exploration and Production, British Gas plc, MoD, Vosper Thornycroft UK Ltd, Total Exploration and Production, BARMAC, Kvaerner Masa-Yards Inc and the Cooperative Research Centre (Australia). The involvement of Lloyd's Register, HSE and DnV in the project made it possible for the guidelines to be implemented by the user with confidence.
Under a new work package, TWI intend to build on these guidelines by evaluating the use of adhesives for structural components.
The objective is to generate design guidelines, methodologies and optimised joint designs for adhesives use in load-bearing applications, leading to significant cost savings and performance advantage.
For further information about both projects, contact Andy Bush or Greg Thomas. E-mail: andy.bush@twi.co.uk or gthomas@twi.co.uk
PhD training at TWI and University of Cambridge
Recruiting is in progress to select well qualified graduates to join the University of Cambridge/TWI Postgraduate Training Partnership (PTP) to undertake a research project leading to a PhD degree.
Projects include experimental and modelling approaches for all engineering materials, and are supervised jointly by the partners.
PTP partnerships are a joint initiative of the Department of Trade and Industry and the Engineering and Physical Sciences Research Council. They are designed to provide for training of postgraduates and introduction of new skills and knowledge to industry.
A PTP studentship offers engineers, materials scientists, metallurgists, physicists and mathematicians:
- PhD training in an industrial environment
- Industrially relevant projects
- Business skills training
- Enhanced career prospects.
Readers of Connect who are interested in gaining a PhD by this route are invited to contact Richard Smith or Stephanie Sutton who will be able to supply them with more information on PTP. E-mail: richard.smith@twi.co.uk , stephanie.sutton@twi.co.uk
Development of girth weld repair assessment procedure for offshore pipelay
Pipeline under test in TWI's Structural Integrity Department laboratory
European Marine Contractors is an offshore pipeline contractor whose operations include pipelay using the S-lay method. Girth welds made on the laybarges are subjected to NDT, any flaws which do not meet the specified acceptance criteria are removed.
Defect removal can either be by in-situ repair or by reversing the barge so that the repair or cut-out is undertaken at a location where the pipe is no longer under load. In-situ repair is desirable to improve productivity and reduce cost. However, since local repairs are conducted under loads, which may be as high as 90% of the pipe yield strength, an analysis has to be undertaken to ensure that the in-situ repair is safe.
An analysis procedure has been developed to predict safe repair groove sizes. The procedure is derived from small scale testing on typical offshore welds, with BSI PD 6493:1991 methods used to perform a ductile tearing instability analysis. The procedure has been validated by a series of three large-scale fracture mechanics tests on 36 inch diameter API X65 linepipe (31mm wall thickness) which simulated the removal of defects during weld repair.
More details on procedures or fracture mechanics testing from TWI's Structural Integrity Department. E-mail: sid@twi.co.uk
Crack arrest checks for new LPG storage tank steels
TWI's large scale test to determine structurally representative crack arrest properties
A TWI member company has been studying the possibility of using alternative materials for new liquefied propane gas (LPG) storage tanks with an operating temperature of -50°C.
Previous TWI group sponsored projects have shown that the crack arrest properties of 1.5% nickel thermomechanically controlled processed (TMCP) steel and their weldments are suitable for LPG applications. However, recent trends in steel making and the high cost of nickel have resulted in development of micro-alloyed TMCP steels with nickel contents of approximately 0.5%. These steels are reported to possess very similar crack arrest properties.
TWI was called in to provide independent data, and to ascertain that the low temperature properties of these steels will not be affected by welding. A work programme was carried out consisting of ten large-scale double-tension crack arrest tests on parent steels, weld metal and HAZ regions to study structurally representative crack arrest behaviour. Charpy, 'Pellini' drop-weight and fracture toughness initiation (CTOD) complemented the test programme.
Results show that these two steels generally have excellent initiation toughness properties at temperatures down to -50°C. Also, the programme illustrates that there are 0.5% nickel TMCP steel grades and associated higher nickel weldments available which will arrest fast running brittle cracks for LPG storage tank operating conditions. However, crack arrest is borderline for other 0.5% nickel TMCP steels and their weldments. The use of 0.5% Nickel steel for LPG storage tank fabrication is hence a possibility, but independent evaluation of properties under structurally realistic conditions is needed before any decision is made.
Further information from Christoph Wiesner. E-mail: christoph.wiesner@twi.co.uk
Job knowledge for welders
Distortion - Prevention by fabrication techniques
Distortion caused by welding a plate at the centre of a thin plate before welding into a bridge girder section. Courtesy John Allen
Assembly techniques
In general, the welder has little influence on the choice of welding procedure but assembly techniques can often be crucial in minimising distortion. The principal assembly techniques are:
- tack welding
- back-to-back assembly
- stiffening
Tack welding
Tack welds are ideal for setting and maintaining the joint gap but can also be used to resist transverse shrinkage. To be effective, thought should be given to the number of tack welds, their length and the distance between them. With too few, there is the risk of the joint progressively closing up as welding proceeds. In a long seam, using MMA or MIG, the joint edges may even overlap. It should be noted that when using the submerged arc process, the joint might open up if not adequately tacked.The tack welding sequence is important to maintain a uniform root gap along the length of the joint. Three alternative tack welding sequences are shown in Fig 1:
- tack weld straight through to the end of the joint (Fig 1a). It is necessary to clamp the plates or to use wedges to maintain the joint gap during tacking
- tack weld one end and then use a back stepping technique for tacking the rest of the joint (Fig 1b)
- tack weld the centre and complete the tack welding by back stepping (Fig 1c).
Fig. 1 Alternative procedures used for tack welding to prevent transverse shrinkage
a) tack weld straight through to end of joint
b) tack weld one end, then use back-step technique for tacking the rest of the joint
c) tack weld the centre, then complete the tack welding by the back-step technique
Directional tacking is a useful technique for controlling the joint gap, for example closing a joint gap which is (or has become) too wide.
When tack welding, it is important that tacks which are to be fused into the main weld are produced to an approved procedure using appropriately qualified welders. The procedure may require preheat and an approved consumable as specified for the main weld. Removal of the tacks also needs careful control to avoid causing defects in the component surface.
Back-to-back assembly
By tack welding or clamping two identical components back-to-back, welding of both components can be balanced around the neutral axis of the combined assembly (Fig 2a). It is recommended that the assembly is stress relieved before separating the components. If stress relieving is not done, it may be necessary to insert wedges between the components (Fig 2b) so when the wedges are removed, the parts will move back to the correct shape or alignment.
Fig. 2 Back-to-back assembly to control distortion when welding two identical components
a) assemblies tacked together before welding
b) use of wedges for components that distort on separation after welding
Stiffening
Fig. 3 Longitudinal stiffeners prevent bowing in butt welded thin plate joints
Longitudinal shrinkage in butt welded seams often results in bowing, especially when fabricating thin plate structures. Longitudinal stiffeners in the form of flats or angles, welded along each side of the seam (Fig 3) are effective in preventing longitudinal bowing. Stiffener location is important: they must be placed at a sufficient distance from the joint so they do not interfere with welding, unless located on the reverse side of a joint welded from one side.Welding procedure
A suitable welding procedure is usually determined by productivity and quality requirements rather than the need to control distortion. Nevertheless, the welding process, technique and sequence do influence the distortion level.Welding process
General rules for selecting a welding process to preven angular distortion are:- deposit the weld metal as quickly as possible
- use the least number of runs to fill the joint
Unfortunately, selecting a suitable welding process based on these rules may increase longitudinal shrinkage resulting in bowing and buckling.
In manual welding, MIG, a high deposition rate process, is preferred to MMA. Weld metal should be deposited using the largest diameter electrode (MMA), or the highest current level (MIG), without causing lack-of-fusion imperfections. As heating is much slower and more diffuse, gas welding normally produces more angular distortion than the arc processes.
Mechanised techniques combining high deposition rates and high welding speeds have the greatest potential for preventing distortion. As the distortion is more consistent, simple techniques such as presetting are more effective in controlling angular distortion.
Welding technique
General rules for preventing distortion are:- keep the weld (fillet) to the minimum specified size
- use balanced welding about the neutral axis
- keep the time between runs to a minimum
Fig. 4 Angular distortion of the joint as determined by the number of runs in the fillet weld
In the absence of restraint, angular distortion in both fillet and butt joints will be a function of the joint geometry, weld size and the number of runs for a given cross section. Angular distortion (measured in degrees) as a function of the
number of runs for a 10mm leg length fillet weld is shown in Fig 4.
If possible, balanced welding around the neutral axis should be done, for example on double sided fillet joints, by two people welding simultaneously. In butt joints, the run order may be crucial in that balanced welding can be used to correct
angular distortion as it develops. 
Fig. 5 Use of welding direction to control distortion
a) Back-step welding
b) Skip welding
Welding sequence
The sequence, or direction, of welding is important and should be towards the free end of the joint. For long welds, the whole of the weld is not completed in one direction. Short runs, for example using the back-step or skip welding technique, are very effective in distortion control (Fig 5).- Back-step welding involves depositing short adjacent weld lengths in the opposite direction to the general progression (Fig.5a).
- Skip welding is laying short weld lengths in a predetermined, evenly spaced, sequence along the seam (Fig 5b). Weld lengths and the spaces between them are generally equal to the natural run-out length of one electrode. The direction of deposit for each electrode is the same, but it is not necessary for the welding direction to be opposite to the direction of general progression.
Best practice
The following fabrication techniques are used to control distortion:- using tack welds to set up and maintain the joint gap
- identical components welded back to back so welding can be balanced about the neutral axis
- attachment of longitudinal stiffeners to prevent longitudinal bowing in butt welds of thin plate structures
- where there is choice of welding procedure, process and technique should aim to deposit the weld metal as quickly as possible; MIG in preference to MMA or gas welding and mechanised rather than manual welding
- in long runs, the whole weld should not be completed in one direction; back-step or skip welding techniques should be used.
Bill Lucas prepared this article in collaboration with Geert Verhaeghe, Rick Leggatt and Gene Mathers.
Engineering Institutions under threat?
Comment by Tim Jessop, Associate Director Professional Affairs
Links between higher education and the engineering profession, and the role of the UK Engineering Council and its Nominated Bodies, are not mentioned in a key consultation document about learning and qualifications recently published by the Department for Education and Employment. This is seen as a serious omission and may signal a threat to the important and widely recognised activities of Engineering Institutions like TWI.
The Green Paper [1] sets out the Government's view of the future structure of vocational learning and qualifications in the UK. The Paper is welcomed in that it recognises the importance of workplace skills and competence to the future economic prosperity of Britain. But it misses the opportunity to build upon some major successes in the engineering sectors in relation to qualifications for Engineers and other technical personnel.
There is no mention either of the Engineering Council and its Register of qualified people, or of the specific, well-recognised and Government- accredited, certification programmes like the Certification Scheme for Welding and Inspection Personnel (CSWIP) and The Welding Register. Furthermore, college-based teaching and qualification programmes like City and Guilds, which have served industry well over many years, are similarly ignored.
The naï ve assumption in the document that other countries will readily endorse any new initiatives adopted by the UK is also worrying. The matter of international recognition of qualifications is too important to be taken this lightly. Companies need it to help them compete in the global marketplace. The Government needs to plan to ensure this kind of recognition, and to take advantage of UK qualification bodies like TWI who have already achieved it. There is no point of developing a new UK scheme for qualifying Welding Engineers, for example, when there is already a world-wide harmonised scheme through the European Welding Federation and the International Institute of Welding.
The Professional Board of TWI will be responding formally to the document and raising the concerns mentioned above. The Engineering Council will also be taking a lead role on behalf of the engineering profession as a whole. Professional Members of TWI, Engineering Council Registrants, EWF Diploma holders, CSWIP certificate holders and Registered Welding Engineers, and members of the Association for Welding Training and Education, etc, may wish to obtain a copy of the document and respond directly on an individual basis.
[1]The Learning Age - a renaissance for a new Britain. Department for Education and Employment.
Reference Cm 3790 February 1998. Price £9.40.
Available from HMSO, or free of charge on the Internet at http://www.lifelonglearning.co.uk. A summary document may be obtained free of charge by telephoning 0345, 474747.
Cambridge technology scoops the world's fastest
Andy Green OBE
Few names in the half-century history of The Welding Institute have pulled a greater audience than that of Andy Green OBE, when TWI's Welding & Joining Society played host recently to a talk by the fastest man on earth.
As the only person ever to drive above the speed of sound he holds the world land speed record of 763.035 miles an hour in the car SSC Thrust.
He spoke for some 90 minutes on the car's design, construction, testing and financing using a stunning array of facts, figures and photographs. But it was riveting video footage taken inside the cockpit of Thrust which had the most impact as it showed Green applying full opposite steering lock at 700 miles an hour to keep the car on track.
Pressed on the cost of the project Green invited guesses from his audience. They topped fifty million pounds. With a wry smile he closed with the words 'We actually managed to do it for just under three million'.
