Miter Cut for Water Pipeline and Hoop Stress

Simple. Follow the rules of the design/construction code in the agreed contract.

We can not answer this for you. Read the contract/purchase order to determine which to follow.

Recheck your dimensions and/or calculations. A 28-inch pipe of 0.5mm wall thickness can't handle anything like 60 bar....

It'd be easy to cut, but tough to weld.

it 0.5 in (12.7 mm)

sorry

A 28" pipe, constructed of general use pipe steel, with welded seam, loaded to 60 bar pressure, with an acceptable safety factor, should have wall thickness of appx 1 inch. This is a rough, on the fly approximation. A careful calculation is required.

Do it right; buy enough 90 degree ells to cut segments for what you need. Use transition pups if the walls are too different.

As stated by another poster, the applicable code is the one in your contract or as required by the Authority Having Jurisdiction.

That being said however, please note that AWWA codes apply to water pipelines, while ASME B31.4 applies to Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids. The ASME code is more restrictive due to the nature of the fluids transported, and in this case would likely not apply (unless by contractual agreement).