I agree that welder qualification an expensive proposition, but it is less costly than making on the site repairs.
Several years ago there was an article in the Welding Journal about the cost of a weld based on the total cost of engineering, design, implementation, and inspection. Granted this was for a nuc., but the total cost was much higher than one would think. In comparison, the cost of qualifying a welder is "peanuts".
I used to think as you do when I was burning rod for a living, and yes, at times it seemed like a wasted effort to retest several times a year. However, in hind sight and with my experience as an inspector, the time and effort expended to test a welder on site is a worthwhile investment. I have listed several reasons why I believe it is worthwhile:
a) it gives the welder an opportunity to become familiar with the project requirements and the WPSs that will be used on the project.
b) most companies do not use the same welding procedures or techniques, thus the welder may have different requirements to meet.
c) each welding code has its own acceptance criteria which the welder may or may not be familiar with. I test the welder using the acceptance criteria that will be imposed on that project, i.e., B31.3 normal service is different from the requirements for high pressure or B31.1, or Section VIII.
d) unfortunately the welder's skills do change over time. Usually it is related to age and declining visual acuity with people over 40.
e) you never know if the requirements of the code were met on the "last test" because some inspectors take exceptions to the codes.
f) while some projects do have stringent inspections, i.e., either UT or RT, many project have little inspection or nothing more stringent than visual examinations.
So, after two and a half decades of welding inspection experience and more than twenty years of experience under the welding hood burning wire, I feel fairly confident I am speaking with a good amount of knowledge about the subject of welder qualification.
By the way, the purpose of qualifying the welder is to demonstrate the welder has the minimum level of skill required to deposit a sound weld. If he or she can't make a good weld in the booth, do you really expect to see an improvement in production?
Gotta go, I have a few welders and an inspector to qualify this morning.
Best regards - Al