What are the special requirement in designing the electrical part an Oil and Gas installations?

There are not thumb rules, you need years of experience and practice. However there are some references that can provide you a good starting point to understand the oil and gas facilities.

API 540 " Electrical Installations in Petroleum Processing Plants"

or "Handbook of electrical engineering, For practitioners in the oil, gas and petrochemical industry" by Alan L. Sheldrake edited by Wiley

You should check another references.

The book Enrique has quoted is an excellent reference for Oil and Gas. I would also recommend reading the guide to the oil and gas industry that ABB produce (you should be able to find in on the internet). It will give you an idea of the type of equipment to be expected in a typical O&G installation.