Since the welder qualified on plate, then he/she could only weld on pipe equal to or greater than 24 inches in diameter with backing, backgouging, or both.
Sub-note c of Table 4.10, AWS D1.1-2006
Minimum thickness would be 1/8" not only because of Table 4.11 but, also because of paragraph 1.2.1, limitations of the code.
Initial plate was 7/8" thick. Thickness range would be 1/8" to 2T(1 3/4"). Table 4.11
What type of repairs are you questioning? Weld repairs or mill induced discontinuities that need repaired?