Analysis of environmental risk and extreme roll motions for a ship in waves

Within the last two decades more than one hundred large tankers, passengers, container or tanker ships have been lost in severe weather conditions or by meeting with a dangerous train of short number of steep waves causing large roll angles and, ultimately, capsizing. In addition to the loss of cargo and human lives, such an event also involves severe pollution of the sea and adjacent shores, especially if the ship is an oil tanker. The motion of a ship in long beam seas could be described by a second-order non-linear differential equation, having the roll angle as variable and depending on four parameters. With the direct forcing amplitude as bifurcation parameter, the dynamical system exhibits either periodic or chaotic behaviour, the route to chaos being realised by a period doubling sequence of periodic motions. Some accepted indicators, like bifurcation diagrams, phase planes and Poincare sections have been computed and they confirm the transition from order to chaos, and finally to unbounded solution (capsizing). A parametric analysis emphasises the role played by each of the system parameters on transient ship capsize. A sketch of an oil spill risk assessment process associated to a capsized ship event is suggested, as a first step in a general risk management plan involving an oil tanker failure/capsizing.