A large number of facilities and parts of the infrastructure of offshore oil and gas reservoirs worldwide are approaching or have exceeded their original design life. The petroleum fields are still producing substantial levels of hydrocarbons which are recoverable and profitable if the field’s lifetime is extended. Thus, parts of this infrastructure are being considered for use beyond their planned design life. However, focusing on safety considerations, the condition of systems, structures and components (SSC) may not be acceptable for extended operation. The purpose of this paper is to discuss guidelines for assessing life extension in order to ensure the technical and operational integrity of these ageing facilities. The objective is to highlight those mechanisms by which the installation is degraded physically and functionally including the human factors and organizational issues. This documentation on ageing mechanisms will provide the foundation for a service lifetime extension process.