The Norwegian Petroleum Safety Authority (PSA) requires offshore petro-leum operators on the Norwegian Continental Shelf (NCS) to perform risk assessmentsof impacts (allisions) between passing ships and offshore installations. These riskassessments provide a basis for defining the allision accidental load that the installationshall be designed for. Even though the risk of allision is small, the potential conse-quences can be catastrophic. In a worst-case scenario, an allision may result in the totalloss of an installation. The ageing industry standard allision risk model, COLLIDE,calculates the risk of impacts between passing (non-field-related) ships and installationsbased on Automatic Identification System (AIS) data. Both the COLLIDE risk modeland a new Bayesian allision risk model currently under development are highlysensitive to variations in vessels’passing distances, especially close proximity passings.Allision risk assessments are typically performed during the design and developmentphase of an installation, which means that historical AIS data are usedBas is^,disregarding future changes to the traffic pattern when the new installation is placedon a location. This article presents an empirical study of one of the most importantvariables used to calculate the risk of allision from passing vessels, namely passingdistance. The study shows that merchant vessels alter course to achieve a safe passingdistance to new surface offshore petroleum installations. This indicates that the resultsof current allision risk assessments are overly conserv

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• Allision risk analysis of offshore petroleum installationson the Norwegian Continental Shelf—an empiricalstudy of vessel traffic patterns