The complexity number for a refinery is a measure of the capital intensity of a refinery due the different process units that it contains.
It is frequently used to normalize for differences in refinery configuration when comparing different refineries.
The complexity calculation was originally developed by Wilbur Nelson in the 1960s and published in a series of articles in the OIl and Gas Journal.
The complexity number for a refinery is calculated by multiplying the complexity factor for each process unit by the ratio of the process unit's capacity to the refinery's atmospheric distillation capacity, and then summing across all process units. This results in a number that typically falls between 5 and 15.
All refinery process units have a complexity factor assigned to them. These factors reflect the capital cost of building each process unit (per barrel/day of capacity) relative to the cost for atmospheric distillation. For example, a coker has a complexity factor of 5.5 because it typically costs 5.5 times as much per barrel of capacity to build as an atmospheric crude tower.