Excavations / trenching in Oil & Gas Plant

Hi Everybody,
We have a project related to installation of CP system for a very prestigious Oil & Gas client for its LIVE gas plant. Our company is taking care of the civil part which is mostly related to excavation works, reinstatement after installation of CP anodes, augering for various depths and all sorts of related civil works. As per the strict guidelines of plant management, we can only dig up to 0.9m without needing any approvals. For any thickness higher than 0.9m anywhere in the plant, we need to do a specialized survey including scanning using GPR. In many locations, to route the cables related to CP system installation, we need to do small trenches and then cover them up (back-fill or concrete).
Now my question is; are there any standards / codes / conventions, for digging / excavation works in Oil & Gas plants? If yes then can anybody guide me about the same?
Would really appreciate if somebody having prior experience in oil & gas for digging/excavation works can share his/her experience.
Regards