TOPEX is a new approach to operating cost estimation of oil and gas installations. It does not rely on knowledge of the capital cost of the installation and uses a computerized expert system (or knowledge base).

Estimates are generated from specific details of the equipment and systems and general databases of prices and manhours.

A novel methodology has been developed for quantifying the operational complexity of an installation which is then correlated with operations manpower.

The use of a computerized application allows rapid calculation of estimates so that what-if and sensitivity studies can be readily done. The knowledge base provides a powerful tool to handle the large amounts of data involved and acts as a repository for the expertise used in its development.


Several methods are used at present for estimation of opex. The simplest is based on the use of multiplication factors which are applied to estimates of capital cost, installation weight or total number of personnel. Generally this method produces overall opex figures, possibly broadly divided between manpower costs and IRM costs. Two more detailed methods are: the use of a predefined manual spreadsheet which prompts the user to fill in calculated or estimated values for individual costs; and ad hoc comparisons with previously estimated or known costs for other similar plants. These methods are generally time consuming and require the collection of considerable amounts of data and the close involvement of an expert to ensure good results.

Problems exist with these approaches which to date has limited either the accuracy or the utility of opex estimates for use in early stages of project planning. These problems are discussed in detail in the next section.

This paper describes a new approach to opex estimation whose aim is to overcome these problems. This approach involves three main parts: a standardized definition of opex and the elements of that cost; direct estimation of opex from the physical parameters of an installation using a new method of correlating operating personnel numbers to the "complexity" of an installation; and the use of an expert system (also known as a knowledge base) for manipulation of the data and expertise involved.

These are combined into a computerized application which allows the user to prepare rapid and accurate opex estimates without the direct involvement of an expert. For convenience this application has been given the name TOPEX. The final section describes areas of application of TOPEX.


For the simple methods of opex estimation using multiplication factors the fundamental problem is one of accuracy. This is for several reasons. The figures, put into these calculations are themselves estimates which puts an inherent limitation on the accuracy of the result.

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