Offshore Oil&Gas fluid production installations may be the target of intentional malicious acts (security attacks) carried out by adversaries of different nature and different motivations which may generate major events with severe consequences on workers, the environment and the property. The current study reviews the state of the art concerning the security of Offshore Oil&Gas operations, which is typically addressed according to best practices and qualitative or semi-quantitative methods. However, systematic approaches or guidelines in support of the analysis are still lacking. The current study investigates the possibility of using Adversary Sequence Diagrams (ASDs) as Security Vulnerability/Risk Assessment (SVA/SRA) supporting tools. A case study addressing a fixed Offshore Oil&Gas fluid production platform proved the ability of ASDs to provide credible attack modes within the Physical Protection System (PPS) that the adversaries have to perform in order to accomplish their objectives, as well as the security barriers that can potentially be effective in delaying and detecting the attacks, which are information required by SVA/SRA studies.