Nelson complexity index

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The Nelson complexity index (NCI) describes a measure of the secondary conversion capacity of a petroleum refinery relative to the primary distillation capacity. It was developed by Wilbur L. Nelson in a series of articles that appeared in the Oil & Gas Journal from 1960 to 1961 (Mar. 14, p. 189; Sept. 26, p. 216; and June 19, p. 109). In 1976, he elaborated on the concept in another series of articles, again in the Oil & Gas Journal (Sept. 13, p. 81; Sept. 20, p. 202; and Sept. 27, p. 83).

The NCI assigns a complexity factor to each major piece of refinery equipment based on its complexity and cost in comparison to crude distillation, which is assigned a complexity factor of 1.0. The complexity of each piece of refinery equipment is then calculated by multiplying its complexity factor by its throughput ratio as a percentage of crude distillation capacity. Adding up the complexity values assigned to each piece of equipment, including crude distillation, determines a refinery’s complexity on the NCI.

The NCI indicates not only the investment intensity or cost index of the refinery but also its potential value addition. Thus, the higher the index number, the greater the cost of the refinery and the higher the value of its products.

In the second edition of the book Petroleum Refinery Process Economics (2000), author Robert Maples notes that U.S. refineries rank highest in complexity index, averaging 9.5, compared with Europe's at 6.5. The Jamnagar refinery belonging to India-based Reliance Industries Limited is now one of the most complex refineries in the world with a Nelson complexity index of 14.[1] The new refinery began trial production on December 25, 2008. The former BP Texas City, Texas refinery, newly acquired by Marathon Petroleum as their Galveston Bay Refinery has a Nelson complexity index of 15.3 in 2013.

The Oil and Gas Journal annually calculates and publishes a list of refineries with their associated Nelson complexity index scores.


  1. ^ Nelson's Complexity Factor (PDF), Reliance Industries Ltd, retrieved 2009-02-28 

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