Hi

I have a question regarding pipe stress analysis of buried pipes.

I don't know which piping standard do I have to use in order to calculate stresses in light slope piping system in refinery, which is in most of it's part buried underground. Piping material is SA 106 Gr. B and pipe is coated with polyethylene (PE) for corrosion and mechanical protection. Pipe is operating at 260°C.

Can I use B31.3 or some other ASME piping standard, for example B31.8...

You should always use the codes specified in project specification.
You cannot change codes as an option.
Definitely, B31.8 cannot be used for temperature higher than 232°C. Refer to para. 802.1 of B31.8.

if any possible to provide bellow for buried pipe line