It sure would have helped if the OP would have told us the kind of EPC project is being considered.
Pipeline?
Process Plant?
Power Plant?
Pharmaceutical?
On Shore?
Off Shore?
Grass Roots (Green Field)?
Revamp?
Stick Build?
Module Build?
Until we know the answers to these questions it is not possible to give mush help to the OP.
The OP did however specifically ask about the "Piping Design Basis". The Client is normally required to provide the "Piping Design Basis" as a part of 'His' Basis of Design Development (BEDD) Document. This will include the "Code" (ASME B31.1, 3, 4, 8 etc) to be used. The Client is also required to state the "Plant Life" (20, 25, 30 years, etc.) to be used for calculating pipe corrosion allowance and wall thicknesses
Sometimes its possible to do all the right things and still get bad results