As some other members have already posted some very good advice. I'll try to keep my suggestions short.
Your main responsibility as a field piping engineer should be to support construction activities. What that means is help the construction supervisor, general foreman, piping foreman, fitter and welder by supplying them any and all information they require to complete their tasks. You should also be responsible for making sure that all components on the isometrics are installed per the drawings. (i.e... valves, pipe supports, pie shoes, guides, spring supports, insulation, tracing, etc…). You may also be asked to chase down materials to support the construction activities.
If you don't have the experience, do not redesign any piping systems. If a problem is encountered in the field, you should gather any and all relevant information and forward it to engineering contractor who designed the system. It should be their responsibility to resolve these issues.
This is a great opportunity for you to get some valuable experience. Don't be afraid to ask questions.
Good Luck!
Hench