A quantitative analysis of economic risk associated with large investments in offshore oil and gas field development and production is presented. The analysis is intended as a supporting tool in decision-making faced with uncertainty and risk, to study the effect of alternative decisions in an easy manner. The descriptors for the project assessment, such as the Internal Rate of Return (IRR) and Net Present Value (NPV) are applied. The study demonstrates first the impacts of early pilot production (EPP) prior to a main oil field development on the field economy of an oil field development and production installation. Furthermore, the result of cases which reflect relevant situations connected with cost overruns are presented, as well as derivation of rational decision criteria for termination/continuation of a project subjected to cost overruns. Finally, an oil field development project scheduling is demonstrated.