Abstract
The Norwegian Petroleum Safety Authority (PSA) requires offshore petroleum operators on the Norwegian Continental Shelf (NCS) to perform risk assessments of impacts (allisions) between passing ships and offshore installations. These risk assessments provide a basis for defining the allision accidental load that the installation shall be designed for. Even though the risk of allision is small, the potential consequences can be catastrophic. In a worst-case scenario, an allision may result in the total loss of an installation. The ageing industry standard allision risk model, COLLIDE, calculates the risk of impacts between passing (non-field-related) ships and installations based on Automatic Identification System (AIS) data. Both the COLLIDE risk model and a new Bayesian allision risk model currently under development are highly sensitive to variations in vessels’ passing distances, especially close proximity passings. Allision risk assessments are typically performed during the design and development phase of an installation, which means that historical AIS data are used “as is”, disregarding future changes to the traffic pattern when the new installation is placed on a location. This article presents an empirical study of one of the most important variables used to calculate the risk of allision from passing vessels, namely passing distance. The study shows that merchant vessels alter course to achieve a safe passing distance to new surface offshore petroleum installations. This indicates that the results of current allision risk assessments are overly conservative.
Highlights
Operators of offshore petroleum installations on the Norwegian Continental Shelf (NCS) are required to assess the risk of ship impacts from both field-related ship activity and passing vessels
This study has demonstrated that shipping traffic adjust their sailing track when new offshore petroleum installations are commissioned, by generally altering course to achieve a passing distance of at least 1 nm
This article compares the vessel traffic patterns for seven installations on the NCS, comparing the vessel installation passing distance found in Automatic Identification System (AIS) data before and after the installations were placed in their locations
Summary
Operators of offshore petroleum installations on the Norwegian Continental Shelf (NCS) are required to assess the risk of ship impacts from both field-related ship activity and (unrelated) passing vessels. Allision risk assessments of passing vessels typically start with a mapping of the traffic pattern around a proposed location, using Automatic Identification System (AIS) data for a (recent) 12-month period. Since these assessments must be completed before commissioning, the AIS data available at the time of such studies represent the traffic pattern before the physical installation is placed in the field. The lack of procedures based on empirical evidence may result in too much subjectivity in risk assessments, making replication of results difficult, even when based on the same input data (Goerlandt and Kujala 2014)
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