The likelihood of leaks from process equipment is a key input to any Quantitative Risk Assessment. This study is aimed at developing a methodology supporting the tailorization of leak frequency values. Specific modification factors for the facilities of the Oil & Gas (O&G) upstream sector are used for this purpose. The method (TEC2O-frequency modification methodology based on TEChnical Operational and Organizational factors) is based on an aggregated set of indicators, whose contribution to the expected leak frequency is systematically evaluated through a specific procedure. Periodic revision and update represents an added value to the method, because it may be adopted to drive the identification of critical safety issues in a facility, integrating technical and managerial aspects and supporting continuous monitoring. The method is compared with similar literature methods and applied to a representative case study in order to demonstrate its potential and advantages. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.