ABSTRACT

The paper describes how corrosion engineers can use the decision tree analysis in order to evaluate and select the best materials for the completion of a new oil field, characterised by high C02 and H2S content. The method has been based on the decision tree analysis and the Montecarlo simulation to obtain the probability distribution of some events to cccur (for instance, number of workovers, corrosion inhibitor efficiency, coating damage rate). The corrosion study leads to four different technical solutions, showing different risk and reliability: carbon steel with corrosion allowance and inhibitor injection, coated tubing, and two corrosion resistant alloys, a superduplex stainless steel and a superaustenitic stainless steel. The cost comparison has been carried out by using the Expected Monetary Value criterion applied to the Life Cycle Cost evaluation.

The paper presents and discusses the decision tree and the results of simulations.

INTRODUCTION

From a general point of view, a decision can be thought as a choice made on a set of possible actions, or strategies. Very often at the time the decision has to be made, it is impossible to know with certainty which of these possible strategies will turn out to be the best choice. This typically happens when the degree of success of any particular strategy will depend on external factors which cannot be perfectly predicted in other words, the decision must be made in an environment in which there is uncertainty about the future behaviour of those factors that will determine the consequences following from the possible course of actions1.

A useful criterion to make a decision in an uncertain environment is the Expected Monetary Value (EMV) criterion that dictates the choice of that action for which expected monetary value is optimised. The method2 is well known in petroleum risk assessment and widely used in making decisions where risk of unsuccessful performance is not negligible. The EMV method is then adopted to compare different technical solutions, each of them being weighted, taking into account the associated corrosion risk.

This paper describes the application of this method used to evaluate and select the best tubing material for an offshore oil field in the Mediterranean Sea. Different scenarios have been considered for which Montecarlo simulation has been used.

The use of structured decision analysis to evaluate and select the best tubing and downhole materials has been considered important for a variety of reason:

- the experience achieved in the last years from other oil fields in Mediterranean Sea may lead to less expensive solution;

-because of many factors affecting the technical solution, associated to the high corrosivity, due to the presence of high CO2 and H2S content, different risk scenarios are foreseeable, thus the most suitable approach which properly can take into account the risk relating assessment is the use of decision tree;

-Montecarlo simulations has been used in order to obtain the whole range of possible scenarios with each alternative and the most-likely values for the variables in the decision tree and the sensitivity analysis.

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