Collision risk for fixed offshore structures close to high-density shipping lanes

Abstract

Fixed offshore structures in the vicinity of intense ship traffic pose a potential collision hazard such as risk of loss of life, economic loss, environmental damage and other possible unwanted events. Therefore, one of the many performance goals in the design phase of such structures is to ensure that the risk of such major accidents and service disruptions is low enough to be acceptable to users, the public and those responsible for public safety.The present paper uses a risk analysis performed for a large suspension bridge with approach spans as an example of the key elements required in a rational risk analysis procedure for fixed offshore structures in shipping lanes with high traffic densities. It deals with the following items: the basic information and the navigational studies needed for a rational collision risk assessment, exemplification of target risk acceptance criteria for the bridge structure, an analysis procedure for ship collision frequencies as a function of the structural layout, elements in a calculation of the probabilistic distribution of collision forces on the fixed structure, given a collision has taken place, and finally it is shown how the consequences due to accidental ship collisions can be assessed, i.e. the risks that have to be compared to the target risk acceptance criteria.The main objective of the procedure is to provide the designers with the maximum flexibility to develop new cost effective offshore structures based on performance standards for the structural requirements and the requirements to the navigational arrangements, instead of basing the design on the more traditional prescriptive rules or codes. For the example bridge structure it is shown that fulfilment of the established risk criteria substantially influences the geometric requirements for the navigation clearance and the needed resistance to ship, impacts. In addition it was found necessary to establish a vessel traffic system (VTS).