The importance of welding quality in ship construction

Paper presented at Analysis and design of Marine Structures 2nd International Conference on Marine Structures (MARSTRUCT 2009) March 16- 18, 2009 Lisbon, Portugal.

Abstract:

Welding is one of the most critical operations within ship construction. When welds fail, often the whole structure fails. Fortunately, over sixty years of research and development in the field of welding has provided current ship builders with fabrication processes that are readily automated, can produce consistent welds reliably, and/or can weld thick sections in a single pass for controlled distortion. The expectation of weld quality has never been higher. This paper summarises how consistent weld quality is achieved in practise, through classification society rules, welding procedure qualification, welder certification and weld monitoring and control. However, no matter how rigorous the quality control, sometimes things go wrong. There are also a number of methods that can help to remedy the effects of poorer quality welding. For shipbuilding, weld repair is usually the main strategy, and the importance of repair welding will be discussed. However, it may not always be necessary to cut out the weld and start again. Methods such as fitness-for-service assessment and fatigue improvement techniques can be applied to justify whether imperfect welds might still be adequate.

1 Introduction

Welding is one of the most critical operations within ship construction. When welds fail, often the whole structure fails. ^{[Apps et al. 2002]} Over the past sixty years of developments in welding technology, the current expectation of weld quality has never been higher. Ensuring good quality welding forms an important part of all ship classification societies' rules. Rules for shipbuilding are all written with the expectation of achieving safe shipping, including Lloyds Register ^[1999], Det Norske Veritas ^{[DNV 2008]}, the American Bureau of Shipping ^{[ABS 1997]}, and also the International Association of Classification Societies, IACS ^[2008]. The approach is similar to that for many safety critical construction applications - using qualified welders and welding procedures.

2 The problems with welding

2.1 Distortion

Shipbuilders may consider the main problem with welding is the resultant distortion that occurs. Any process that uses a localised heat source, such as welding, is likely to result in some distortion. However, distortion can be minimised in welds that use low heat input and avoid excessive weld bead sizes. Using jigs and fixtures or pre-setting the components to offset the eventual distortion can also help. Most distortion is corrected after welding using localized flame spot heating to restore the required dimensions.

Distortion associated with welds may cause problems for the ship design strength and stiffness, and for the appearance of the finished vessel, and preventing and remedying distortion can be a major cost of shipbuilding. Despite it being a major subject in itself, distortion is associated with even the best welds and is not a weld quality issue as such, so will only be touched on briefly in this paper.

2.2 Weld flaws

Typical welding flaws can be classified into three categories to assess their significance; planar flaws (cracks, lack of fusion or penetration, undercut or overlap); volumetric flaws (porosity and slag inclusions); and weld shape flaws (misalignment, or incorrect profile). The least severe are gas and slag inclusions, and weld shape flaws. In terms of structural integrity, the most critical flaws are cracks, lack of fusion or penetration, and undercut. The sharp profile of these flaws makes them strong stress concentrators, and it is these flaws that most weld quality codes and standards are seeking to avoid.

3 Getting weld quality right

3.1 Approved welding procedures

The weld procedure specification (or WPS) is the 'recipe' for carrying out a particular weld. It specifies the welding parameters that must be adhered to for compliance to that welding procedure. For arc welding these 'essential variables' include;

1. Welding process(es).
2. Voltage and current ranges, and polarity.
3. Welding travel speed and/or heat input.
4. Welding consumables (wire, rods and gases). Sometimes lists of approved welding consumables are issued by the Classification Society (such as ABS, Lloyds, DNV and RINA).
5. Parent materials type, grade and thickness range.
6. Joint design, welding sequence and welding position.
7. Preheat and inter-pass temperatures, and postweld heat treatment (if required).

The ship classification societies all require approved welding procedures to be used. For example, DNV allows welding procedures to be approved in one of three different ways. Firstly, if the WPS is based on another approved welding procedure, secondly, from verification of documentation showing successful application of the WPS over a prolonged period of time. Thirdly, and most commonly, weld procedure approval is done through review of weld procedure qualification records (WPQRs), collected from the assessment of a test weld made using the WPS. IACS shipbuilding and repair quality standard ^[1999] also requires that a welding procedure should be supported by such a welding procedure qualification record.

3.2 Weld procedure qualification testing

Qualification of a welding procedure through testing generally consists of firstly preparing the written preliminary weld procedure specification (pWPS), then welding up a test piece following that pWPS. The test piece is then subjected to inspection and a range of non-destructive testing (NDT) and mechanical tests to demonstrate the strength, ductility, toughness, and the presence or absence of defects. If the results of the NDT and all the mechanical tests are satisfactory then the welding procedure specification (WPS) is qualified. Documentation such as the record of welding parameters used for the test weld, the NDT and mechanical testing certificates, and the signed weld procedure approval certificate, collectively form the weld procedure qualification record (WPQR). It is usually the responsibility of the ship builder to ensure that the welding procedures (and the welders) are qualified.

The NDT used on the test weld includes visual examination, a surface flaw detection method such as dye penetrant or magnetic particle (MPI) testing, and for butt welds, a method to detect buried flaws, usually radiography or ultrasonic testing. The standard mechanical tests for qualification of a test weld piece are:

1. Macro and hardness test to show the weld shape, and the maximum hardness of the weld and heat affected zone (HAZ).
2. Cross-weld tensile test to demonstrate that the weld is overmatching to the parent metal.
3. Guided bend tests; face bend and root bend, or two side bends (depending on material thickness), to demonstrate the weld ductility, and to open any flaws present for detection, shown in Figure 1.
4. All-weld metal round tensile to test the weld metal strength.
5. ABS also requires fillet weld fracture tests for WPS qualification.
6. Sometimes Charpy vee notch tests are carried out, notched in weld metal, HAZ fusion line, and HAZ+2mm, HAZ +5mm, to determine the impact toughness of different regions of the weld and HAZ. Charpy tests are carried out at a temperature dependant on the grade of steel being used, e.g. -10°C for Grade D and -40°C for Grade E ^{[ABS 1997]}. The required impact energy is stated, usually between 27J and 47J at the given temperature.

Generally the Classification Societies require that the weld procedure qualification, including the welding of the test piece, the inspection, and subsequent mechanical testing is witnessed by an approved surveyor - this is a requirement of Lloyds Register for new welding procedures. It is uncommon for a Classification Society to specify an external standard to which the weld procedure needs to be qualified; ABS and DNV lay out their own requirements for test piece qualification. The most common general standards for qualification of welding procedures are ISO 15614-1 ^[2004a], ASME IX ^[2007], and ANSI/AWS D1.1 ^[2008] for arc welds, and ISO 15609-4 ^[2004b] for laser welds.

3.3 Welder qualification

The qualification of welders or weld operators often goes hand-in-hand with the weld procedure qualification. It is common for the welder who produced the test weld to also be approved for that particular welding procedure as well. All the Classification Societies require that welders and weld operators are properly skilled and qualified for the job; welders are normally required to be certified.

Qualification (and/or certification) of a welder is obtained following satisfactory assessment of a test weld, the production of which is usually witnessed by an examiner, surveyor or test body. The approval test examines a welder's skill and ability to produce a test weld of satisfactory quality. Limits are given on flaws associated with the shape of the weld bead such as, for example, excess weld metal, concavity, excess throat thickness and excess penetration. In a welder qualification, these features reveal the welder's competence and skill (Fig.2). The welder's test piece is subjected to visual examination, possibly NDT (penetrant testing, MPI and radiography), and some mechanical testing which may include a macro-section, fillet break tests (where the fillet weld is fractured through the root and examined for defects), cross weld tensile tests, and butt weld fracture or bend tests (possibly using shallow notches cut into the weld metal of the side bend specimens).

DNV ^[2008] and IACS ^[1999] require that welders shall be qualified to a standard recognised by the society (i.e. EN 287, ISO 9606, ASME Section IX, ANSI/AWS D1.1). Lloyds Register requires that shipbuilders test the welders and weld operators to a suitable National standard, but is not specific, while ABS outlines its own programme of welder qualification testing. BS EN 287 ^[2004] is specifically for fusion welding of steels, while EN ISO 9606 ^[2004c] covers fusion welding of aluminium alloys. The challenges for manual arc welding of steel and aluminium are rather different, and hence it is important for welders to be qualified and trained specifically for the material that they will be welding. Within the remit of EN 287 there are also different categories of steels, depending on their composition and hence their likelihood of cracking during welding. Welding operators using fully mechanised or automated equipment are not usually subject to approval testing. However, they are usually required (e.g. by DNV and IACS) to have records of proficiency showing that operators are receiving regular training in setting, programming and operating the equipment.

3.4 Range of approval

Both weld procedure approval and welder approvals are not limited solely to fabrications using the exact welding parameters used for welding the test piece, but come with a given range of approval.

The objective of setting this range is to allow a weld procedure to also cover welds expected to give similar (or better) mechanical properties. For instance, although the material type cannot change (as it is an essential variable), the plate thickness for which the WPS is approved might range from half up to twice the test weld plate thickness. Another example is that DNV qualification allows welding of grades of steel with lower toughness requirements, but not those with higher toughness requirements.

The range of approval for the welder qualification is also intended to cover welds that are considered 'easier' to weld, so a welder that is qualified for butt welds may also weld fillet welds, but not vice versa; positional welding will qualify welding in the flat position but not vice versa. It is important to check the permitted range of approval before conducting the test welding as it might be possible to cover all the production welding with fewer weld procedure qualification tests given careful selection of the test weld parent material thickness, welding position, access to one or both sides etc.

3.5 Welding co-ordination

Often the requirement for qualification within welding ends with the welding procedure and the welders. However, it is also important that the staff who write the welding procedures and supervise the fabrication are competent and qualified for their roles. DNV is one of the few Classification Societies to refer to this specifically, and gives a note that it bases quality requirements for welding on the EN 719 and 729 series of documents. ISO 3834 ^[1994] 'Quality requirements for welding' has now superseded EN 729 and ISO 14731 ^[2006] 'Welding co-ordination' has superseded EN 719. ISO 14731 is about people and responsibilities, and ISO 3834 is about companies implementing systems for qualified weld procedures. These two standards require companies to show that all their welding operations are under appropriate and technically competent control. The company must show that people with welding responsibilities possess relevant competence. The qualifications of the International Institute of Welding (i.e. International Welding Specialist/Technologist/Engineer diplomas amongst others) are mentioned as examples of such suitable qualifications.

4 Maintaining weld quality during fabrication

4.1 Production welding

Many Classification Societies provide recommendations on measures that can also be taken during welding to help improve the quality of welded structures, for example by minimising distortion. Welding from the centre outwards of a seam and selecting an appropriate sequence for welding different joints can minimise the distortion when welding stiffened panels. Fit-up using clamps, jigs, and tack welds (provided they are in accordance with the WPS) can also be used to ensure good alignment or to minimise residual stresses and distortions. Run-on and run-off plates can help to avoid welding defects and end craters in the actual structure.

Sometimes weld quality is affected by the adverse conditions of the environment during welding on site. Construction of vessel hulls is usually done in stages involving assembly and fabrication of sub-assemblies which are then brought together in the dry dock for final erection and welding. This final stage of fabrication is potentially more exposed to the weather, so where possible the welding environment should be kept free from moisture and draughts. The controlled welding consumables need to be kept dry and clean. A study of weld quality in a new-build FPSO was carried out to address concerns that welding quality was poorer when inspection was not required or if inspection equipment was unavailable ^{[Still et al. 2004]}. The marine piping systems of the FPSO were not required to be inspected by the Class Rules, but the operator radiographed them for the study, and found 60% of the butt welds examined contained lack of root penetration defects. It seemed that when there was no expectation that the work would be inspected, and/or that the Class surveyor's workload is such that they may not have adequate equipment or the time to carry out a thorough inspection, the weld quality reduced. This illustrates the importance of quality control by welding supervisors, since even an approved welding procedure can produce poor welding if not followed correctly during fabrication.

4.2 Joint or seam tracking

For mechanised and automated welding processes, including laser welding, joint tracking offers the potential for improved quality assurance by allowing on-line adjustment of the welding head position with respect to the workpiece to compensate for small variations in joint position and fit-up. Real-time joint tracking is performed by a sensor that first detects the position of the joint and then guides the automatic welding equipment. The sensor communicates with the welding head to send trajectory corrections, maintaining the tool centre point at the optimum position in the joint. The high degree of accuracy achieved while welding improves productivity by significantly decreasing the amount of operator monitoring and intervention required, increasing the welding travel speed, and reducing rework costs.

Vision-based sensors are currently used for the majority of joint tracking systems for both arc and laser welding applications, due to their high accuracy and signal update rate. ^{[Shi et al. 2007]} Some other tracking systems used for arc welding processes such as tactile sensors or through-arc sensors are not suitable for laser welding applications due to their general lack of accuracy and low signal update rate. Faster data handling is required for laser welding due to the high welding speed, which increases the system response and processing speed needed. Vision-based sensors also use an additional low-power infra-red or visible laser which is focused to form a line on the workpiece surface, which detects the joint profile and gap.

Limitations of seam tracking come from trying to detect abrupt changes in direction whilst welding at high speeds, and detecting closely butting edges for laser welded joints, as these may not produce sufficient reflected signals to allow the seam to be detected. The effect is worse on highly reflective materials such as stainless steel or aluminium.

4.3 Laser welding and mechanised welding processes

The reproducibility of automated welding such as laser welding means that once welding parameters are set correctly, high quality welds can be reliably produced time and again. Laser beam welding (Fig.3), as a substitute for arc welding, provides advantages in terms of high productivity (in terms of speed and/or thickness) and low heat input (and thus low distortion). Weld procedure qualification can also be used for laser processes, and is covered by ISO 15609-4 ^{[ISO 2004b]}. Ship building is one of the main industries where laser welding is applied, and although welding lasers are not commonplace in shipyards, the process is mentioned specifically with respect to qualification of laser welding operators by DNV and IACS. The main issue is to ensure that the tight tolerances on joint fit-up required for laser welds are met, which can be difficult to achieve in shipyards. The solution is either to use a welding technique or procedure that is more tolerant to joint fit-up and/or include process control measures to allow accurate seam tracking to be integrated into automated welding systems to maintain weld quality. The combination of an electric arc with the laser beam in the same weld pool in a hybrid process (Fig.4) can significantly improve the gap bridging capability and tolerance to misalignment of the laser process alone.

For in-process monitoring of laser welding, most current techniques employ a variety of sensors to monitor electromagnetic signals from the molten pool during welding, with the objective of correlating the output from the sensor to features such as weld penetration, weld pores, and the weld shape.

5 What can be done when things go wrong?

5.1 Repair welding

Repairs to welds can be very expensive; up to 10 times the cost of getting it right the first time. Most classification societies, and the IACS shipbuilding repair quality standard ^[1999], do not allow any weld cracks to be left unrepaired irrespective of size. For shallow surface flaws it may be possible to grind out the flaw without any further weld repair. Methods of flaw removal include grinding or machining a smooth profiled groove, and then inspection to ensure the flaw has been entirely removed. Before repair welding it is important to know why the defects occurred, so that they can be avoided in the repair weld. Many problems are caused by either the welder, or from issues with the fit up. Therefore it might be necessary to re-train and re-certify the welder, and/or improve the fit-up of the joint. Repair welding must be done using the same quality controls as the original welding (i.e. approved WPS, qualified welder and certified consumables), but if following the welding parameters of the original WPS has resulted in defects, the WPS might need to be revised.

There is usually a limit to the number of times it is permitted to re-weld, for example DNV allows only two cycles of repair. The repair weld will usually be inspected using the same NDT methods as originally applied. Some additional difficulties for repair welding which are not usually experienced during production welding are the provisions required for proper access, lighting and ventilation, while being sheltered from the weather or seawater (for hull repairs).

5.2 Fitness-for-service assessment

Sometimes repairing a flaw might not be necessary with respect to ensuring structural integrity, e.g. a weld containing a small flaw could still be fit for service. This is where a fitness-for-service assessment can be invaluable. Most welding fabrication codes and classification society rules specify maximum tolerable flaw sizes based on good workmanship, i.e. what can reasonably be expected within normal working practices. These requirements tend to be somewhat arbitrary, and failure to achieve them does not necessarily mean that the structure is at risk of failure.

A fitness for service (FFS) assessment (also called engineering critical assessment or ECA) is an analysis, based on fracture mechanics principles, of whether or not a given flaw is safe from brittle fracture, fatigue or plastic collapse under specified loading conditions. An overview is shown in Figure 5. FFS assessment procedures are issued in a number of national standards. The results allow decisions to be made so that a flaw that is tolerable may be left safely in service, whereas an unacceptable flaw needs to be removed and/or repaired.

Although FFS assessment can be used to assess the significance of fabrication defects that have been found to be unacceptable to a given code, its greatest potential within shipping is for assessing flaws such as fatigue cracks that have grown during service. It can also be used to justify life extension of a vessel's service for instance. The FFS assessment concept is widely accepted by a range of engineering industries such as oil and gas, and power generation. However, it has yet to find wide acceptance by classification societies, despite its long track record in preventing unnecessary repairs, cost and delay while ensuring safety in service.

5.3 Fatigue improvement

Fatigue cracking during service is a major concern for ongoing structural integrity of welds in ships. Fatigue failures usually initiate at changes in cross section; machined grooves, bolt holes, sharp flaws or at welds. The sharper the notch, the higher the stress concentration, and the greater the limitation on fatigue life. The risk of fatigue can be reduced in structures under cyclic duty, by either reducing the loading on the structure, or by reducing the local stress concentrations by improved design. Misalignment and distortion of welded joints will increase the applied stress further, which reduces the expected fatigue life. A poorly shaped weld cap with a sharp transition between the weld and the parent metal will also have an adverse effect on fatigue performance. In addition to these geometrical features affecting fatigue life there is also a small intrusion at the weld toe. In a welded component these intrusions act as initiators for fatigue and hence the bulk of the fatigue life is spent in propagating the fatigue crack without any need for a period of time to initiate the starter crack.

There are fatigue improvement techniques that can be applied to welds. The dual objectives of these are the removal of the intrusions at the weld toe and the achievement of a smooth transition between weld metal and parent plate, shown in Figure 6. Different welded joint configurations are given a classification according to their fatigue performance ^{[BSI 1993]}. Full penetration butt welds made from both sides can be promoted from fatigue class D or E (depending on the welding procedure) to class B or C (depending on whether they are longitudinal or transverse welds) by machining the excess weld metal flush with the surface. Distortion of the joint can make this treatment difficult to apply. For fillet welds it is possible to dress the weld toes by grinding away material along the weld toe to remove the toe intrusion while maintaining a smooth weld profile. The toes may be dressed by the careful use of a disc grinder, making sure any machining marks are parallel to the axis of the main stress, to avoid forming initiation sites for further fatigue. For best results the toe should be machined with a fine rotary burr, even though this is slower. Toe grinding needs to be done with care to prevent too much metal being removed and hence thinning the component below its minimum design thickness. Ideally the dressing should remove no more than 0.5 mm depth of material for full burr grinding and 0.8 mm for disc grinding.

6 Conclusions

The shipbuilding industry has a history of using Classification Societies Rules for ensuring good quality ship fabrication. The qualification of welders and weld procedures is an effective way to ensure these quality requirements are met. However, supervision is needed to ensure that the procedures are followed during production welding. When flaws are found in welds, the usual requirement by Classification Societies is for removal and/or repair. However, it can be desirable for unnecessary repair welding to be avoided if possible, and the Classification Societies may wish to exploit the proven abilities of fitness-for-service assessment procedures for justifying this. In-service fatigue damage at welds can be prevented or remedied by a number of fatigue improvement techniques.

7 References

ABS. 1997. Rules for Building and Classing Steel Vessels, 1997. Part 2, Section 3, Welding and Fabrication. American Bureau of Shipping, 1997.

Apps, B., Crossland, B., Fenn, R. and Evans, C. 2002. 'Killer consequences of defective welds - a plan for prevention', TWI Bulletin, January/February 2002.

ASME. 2007. Boiler and Pressure Vessel Code, Section IX: Welding and Brazing Qualifications.

AWS. 2008. D1.1/D1.1M:2008 Structural Welding Code - steel, American Welding Society.

BSI. 2004. BS EN 287-1:2004 Qualification test of welders - fusion welding - Part 1: steels, British Standards Institution.

BSI. 1993. BS 7608:1993 Code of practice for fatigue design and assessment of steel structures.

DNV. 2008. Rules for the Classification of Ships/High Speed, Light Craft and Naval Surface Craft. Newbuilding, January 2005, incorporating amendments 2008. Materials and Welding, Part 2 Chapter 3, Fabrication and Testing of Structures. Det Norske Veritas.

IACS. 2008. Common Structural Rules for Bulk Carriers, Consolidated edition of 1 July 2008.

IACS. 1999. Shipbuilding and Repair Quality Standard, Part A: Shipbuilding and Repair Quality Standard for New Construction, & Part B: Repair Quality Standard for Existing Ships. No.47, Rev. 1, August 1999.

ISO. 1994. ISO 3834-1:1994 Quality requirements for welding - fusion welding of metallic materials.

ISO. 2004a. BS EN ISO 15614-1:2004 Specification and qualification of welding procedures for metallic materials - welding procedure test. Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys (Supersedes EN 288 Part 3).

ISO. 2004b. BS EN ISO 15609-4:2004 Specification and qualification of welding procedures for metallic materials - Welding procedure specification. Part 4: Laser beam welding (Supersedes ISO 9956).

ISO. 2004c. BS EN ISO 9606-2:2004 Qualification test of welders. Fusion welding - Part 2: aluminium and aluminium alloys.

ISO. 2006. BS EN ISO 14731:2006 Welding coordination - Tasks and responsibilities (Supercedes BS EN 719:1994).

Lloyds. 1999. Rules and Regulations for the Classification of Ships, July 1999. Part 3, Ship Structures, Chapter 10, Welding and Structural Details. Lloyds Register.

Shi, G., Hilton, P. & Verhaeghe, G. 2007. 'In-process weld quality monitoring of laser and hybrid laser-arc fillet welds in 6-12mm C-Mn steel' in Proceedings of the Fourth International WLT-Conference on Lasers in Manufacturing 2007 (LIM2007), 18 - 22 June 2007. Munich, Germany.

Still, J., Speck, J. and Pereira, M. 2004. 'Quality requirements for an FPSO hull and marine piping fabrication', in Proceedings of OMAE-FPSO 2004, OMAE Specialty Symposium on FPSO Integrity, 30 Aug - 2 Sept 2004, Houston, USA.