| dc.description.abstract | Abstract
The aim of this study is to link the impact of ice accretion on ship motion with its impact on static stability. If such links are demonstrated, ship motion may be applied in more basic and affordable stability monitoring systems. Moreover, as more advanced stability monitoring systems base ship response prediction on the state of predefined load cases; these may not perform well during undefinable icing incidents. A motion-based system may offer improvements to their load condition monitoring during ice accretion, improving ship response predictions.
This study applies a simulation-based methodology, in which a single degree of freedom model is derived from the linear equation or roll motion. This equation is expanded to include a trochoidal waveform excitation term, and an inclination dependent ice moment. Ice impact on roll response is further modelled by including the ice influence on system mass distribution, and the curve of static stability. Finally, the model incorporates a non-linear righting moment expressed by a fifth degree polynomial.
Time and roll decay simulations are performed on four different load conditions of a sample ship, in order to investigate steady state and transient stage response, in addition to natural roll periods. One load condition is an ice-free reference at design waterline, and the three others applies different DNV regulation ice mass distribution requirements. The roll response characteristics of interest are roll amplitude, phase and period. Applicability for stability assessment considers if separation between load conditions are consistent and proportional, or if reference to natural frequency or period is possible.
The simulation results suggest only the periods of transient stage secondary roll motions to be applicable for stability monitoring. At higher excitation frequencies, these correspond to the natural periods demonstrated in the roll decay simulations. However, due to non-linear effects, these periods deviate from analytically derived natural periods, and linking the periods to metacentric height is not straightforward. This is however achieved in conjunction with information an integrated ship monitoring system may provide. By comparing consumed weight loggings to draught-based displacement estimates, the difference will correspond to accreted ice mass. Inserting the formula for metacentric height into an adjusted roll period-ratio equation, solved for ice radius, the height of the ice mass center is achieved by adding the height of the axis of rotation. This suffice for adding ice weight correction to the current system metacentric height estimates. Although information requirements supersedes the potential for basic stand-alone applicability, this approach may offer improved accuracy in metacentric height estimations in larger system integrations. | en |
