Investigating welding failures
The failure of a structural weld in a ship, offshore platform or other steel structure has the potential to be catastrophic as well as costly. Often, it fuels a dispute between the asset owner and the constructor as to why the weld failed and who is liable to pay for remedial work and/or loss of operations. As an independent consultancy, we are often asked to investigate and provide expert advice and opinion.
Shipyards and other fabricators should operate their welding activities to a strict set of specifications and parameters and these should be comprehensively documented. As the construction process develops, all major structural welds should be non-destructively tested (NDT) as the asset is being built. This is important as it is not always practical to test all welds once the build is complete. Occasionally, we are asked to witness the NDT process during construction, but more usually we are called in to investigate why a weld has failed.
Our first task is always to review the documentation to check that the initial welding and NDT procedures were adequate. Whilst this gives us a good start, it doesn’t of course, confirm that the procedures were adhered to in practice. But we can identify if there are any gaps in the NDT records which might indicate that the testing was incomplete or in the calibration records to show it wasn’t done properly. We can also assess if the tests themselves were conducted at an appropriate time. Delayed manifestation of hydrogen cracking in welds is a complex issue, dependent on materials, temperature and joint restraint among other things and it is important that the weld NDT is delayed sufficiently to allow for any cracks time to develop.
NDT testing for a weld is more complex than testing flat steel surfaces for corrosion or thinning. A weld is usually prepped with a V or U-shaped bevel to improve root access and side-wall fusion. To achieve a comprehensive ultrasound test, it is necessary to angle the sound waves to capture cracks or discontinuities in a range of orientations from vertical through to horizontal. Whilst yards might believe they are adequately testing their welds, in practice their capabilities sometimes fall short.
To get an accurate picture of the quality of the weld, we would ask for a physical sample to be taken and sent to our in-house laboratory in the UK. Having polished and sectioned the sample, it will then be examined under a sophisticated microscope by our team of trained metallurgists.
We will check for all of the common weld defects - lack of penetration, lack of fusion, undercut, spatter, slag inclusions, cracks and porosity. We will also examine the microstructure to yield information on grain orientation, pass interpenetration and heat affected zone size along with chemical analysis to confirm the consumables used and any dilution effects related to the parent metal. All findings will be checked against the documented weld procedures to confirm their consistency.
Failure to protect welding operations from environmental interference (wind stripping away shielding gases or rain leading to wet consumables) can also cause problems leading to oxide inclusions and porosity and these defects are easily detectable.
There have been some high profile and extremely costly projects that have suffered from poor welding. On closer inspection, the causes can be multiple. Not only are the welds themselves often substandard, but the underlying procedures, oversight and testing might also be inadequate.
Although welding techniques continue to improve, the latest technologies (such as electron beam or laser welding) are not suitable for large steel constructions such as merchant ships. However, advances in non-destructive testing and other investigative techniques mean that it is much easier to fully understand the cause of a welding failure today. At Brookes Bell we always take a holistic view to include a full analysis of the documentation, the management processes, microscopy and NDT. This allows us to gather a comprehensive and fully independent understanding of what went wrong.
- Dr Ian Moody