Environmental risk assessment of shipwrecks – Model development and application

Abstract: Potentially polluting shipwrecks containing oil or other hazardous substances pose a threat to the marine environment. This is a global problem and many shipwrecks stem from the Second World War having been deteriorating on the sea floor since then. Only in Swedish waters more than 300 wrecks are estimated to pose an environmental threat. Together, these wrecks are estimated to contain between 1,000 and 15,000 tonnes of bunker oil. Every shipwreck poses a unique case depending on, for example, the type of vessel, cause of sinking, and environmental preconditions. This implies that the problem is complex and also that there are many uncertainties involved. 

It is not feasible to remediate all shipwrecks due to large costs, but a proactive approach would reduce the need for and high costs of reactive response in case of a discharge.  Until now, there were no comprehensive probabilistic method for assessing the environmental risk posed by shipwrecks in order to provide necessary support to decision-makers.

In order to prioritise and effectively use resources, proper decision support is needed. Risk assessments and the overall process of risk management are important means to provide such decision support. The purpose of this thesis has therefore been to develop, apply and evaluate a model for comprehensive risk assessment of potentially polluting shipwrecks. A comparison of current methods for risk assessment of shipwrecks was performed in order to identify development needs. Based on the comparison, a generic framework for risk management of shipwrecks was suggested. Furthermore, a method for estimating the probability of discharge of hazardous substances was developed, using a probabilistic fault tree method. The method makes it possible to consider possible activities that may damage the wreck as well as physical and environmental conditions affecting the wreck. An approach for consequence assessment of discharges from shipwrecks, consisting of an aggregation of methods was also developed within this thesis work.

The generic framework for risk management of shipwrecks clearly shows the important steps required and how they are linked. It also emphasizes the need of proper assessments to facilitate prioritisation of shipwrecks and an efficient resource allocation for these environmental threats. The result is a probabilistic and comprehensive model for risk assessment of shipwrecks including possibilities to cope with the vast uncertainties involved in shipwreck risk assessment.