Unexploded Ordnance

The presence of an unknown amount of unexploded ordnance devices (UXO) on the sea floor is a risk for life and equipment of fishery and other offshore underwater works touching ground. Especially for the installation of Offshore Windfarms and their grid connection -subsea cable connections- during the change to renewable energy supply, the areas and routes have to be well prepared i.e. they have to be cleaned from dangerous objects to establish a safe working environment. UXO campaigns carry the risk to be time and cost intensive since the number of targets to be identified and cleared cannot be estimated reliably. This uncertainty can have a knock-on effect on time schedule and cost of the subsequent works. Due to the weather situation in the North Sea, UXO campaigns commonly start in early spring and last until late summer or autumn. As this is also the main season for construction, the following installation campaign typically starts in spring of the following year. The main disadvantage of this procedure is, that the clearance of UXO is often limited by time due to the assumption that UXO can migrate on the sea floor. Therefore, UXO-measures might have to be revisited before installation or a repair. Difficulties obviously increase in cases, when Windfarms and cables are already installed and an UXO free sign off is expired since it is generally given for a limited time period. Therefore a better understanding of the current- and wave-induced mobilisation of objects on the sea floor is needed and can be given by this report. The risk of mobilisation and migration due to fishing activities can explicitly not be covered by this study.