Prediction of the initial movement of objects on the sea floor

The presence of a huge amount unexploded ordnance devices (UXO) on the sea floor is one of the major problems during installation and maintenance of offshore wind farms and other offshore structures. To identify all targets, time and cost consuming campaigns are necessary. The clearance of UXO is often limited by time due to the idea that UXO can migrate on the sea floor. Therefore the UXO-measure has to be revisited in some cases before installation or repair. Thus it might be possible, that additional UXO-measures are necessary before starting any operations touching the seabed. Insofar a better understanding of the migration of objects on the sea floor may help to improve currently used methods of time-dependent clearance by more knowledge-based decisions. The aim of this study is to investigate the requirements of initial movement of objects on the sea floor. This has been done by the analysis of literature, previous investigation to scour and burial of such objects, wind-tunnel experiments as well as experiments in a water channel. Additionally, numerical simulations allowed comparisons, combinations and a generalization of experimental results. A new mathematical model developed and validated allows a prediction of the incident fluid velocity that is necessary for an inertial motion of defined cylindrical and spherical objects. This model allows a reliable prediction of the initial migration of objects on a sandy sea floor. It is based on physical parameters, which depend on predictable and measurable events like currents, tides and indirectly on the weather conditions.