Repairs are generally carried out by removing a non-acceptable welding defect, re-welding and then reinstating the original geometry of the component.
Quantifiable results and statistics of such repairs are often sought for costing exercises, as a reference for presentations, or to rate potential vendors. The range of industry sectors to which this information appeals is wide and disparate and includes oil and gas pipelines, pressure vessels, shipbuilding, power generation and automotive part manufacture.
Some data is available but its range is limited. The information available until now is based on individual experiences or on industry rules of thumb. A more structured approach and an up-to-date collection of data has been identified and driven by two straight forward objectives.
The first aim was to carry out a literature review on the rate of repair and rejection in different industry sectors. The second was to conduct a survey on the rate of repair and the usage of consumables and welding processes in various industry sectors.
Two routes were identified to collect such data: a literature review, to determine the type and quality of information available in the public domain, and a survey of the relevant industry sectors, aimed at obtaining data from individual companies and analysing them to identify common trends or patterns.
A preliminary literature search showed that published data was mainly concerned with repair rates and the factors affecting them. In particular, information such as the extent of individual welding process usage and quantities of consumables are project-specific and proved difficult to retrieve, so it was decided to confine the literature review to weld repair rates.
With regard to the industry survey, a web-based questionnaire was prepared and distributed to companies within TWI's Industrial Membership base, as well as Professional Members of The Welding Institute, members of standardisation committees, readers of the Welding and Cutting journal, and other contacts in industry.
The industry survey concluded:
- Average repair rates for the oil and gas and power sectors range from 1-3% (Figure 1, see note below for details on source data). Peak rates up to 25% in specific locations were recorded and exceptionally values up to 55% were logged.
- Peak repair rates are typically obtained in specific locations within welded products, such as root runs, fillet welds and areas with limited access.
- Major factors perceived to affect repair rates are; the welder's skills, location or accessibility of welds, and poor fit-up prior to welding (Figure 2).
Note to Figure 1: The repair rates provide by the participants were calculated with different methods, as follows:
- Offshore structures and other welded structures: fraction of the length of repaired weld versus the total weld length.
- Piping systems, onshore and offshore pipelines: number of repaired welds versus the total number of welds.
- Pressure vessels: number of repaired welds versus the total number of welds (two entries were based on the ‘length of welds’ method, however, the % values were consistent with the rest).
Further details are available from a summary paper published in Welding & Cutting 11 (2012). No 1.
The complete survey report is available to TWI Industrial Members. For further information, please contact us.