More and more frequently Industrial members have contacted TWI in recent
months regarding manufacturing and fabrication welding statistics. Just
what are present rejection and repair rates in production welding?
This was a question TWI put to itself, in the
interests of its Membership, and came up with some interesting findings.
Such 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 by Members 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. So
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).
For more details of this work or to request full report entitled 'Repair
rates in welded construction - An analysis of industry trends' please
contact us.
