B31.3 uses a mixture of a percent of SMYS or a percent of UTS and choses the lowest as the allowable stress. So it does the calculation for you.
B31.8 just uses SMYS and then applies different design factors depending on your location. So out in the middle of no where it could be up to 0.8.
Middle of a heavily populated town or similar it is 0.4.
31.3 is not that bothered about optimising pipe thickness as the cost of the pipe is a small amount of the material cost compared to pressure vessels, flanges, valves etc. 31.8 though really wants to optimise wall thickness and cost of pipe as it is a very significant percent of the cost of a pipeline.
Does that make sense?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
