A simplified regulatory-oriented method for relative assessment of susceptibility to parametric roll inception at the early design stage

Parametrically excited rolling motion can occur in relatively mild sea conditions if the ship is prone to suffer large variations of restoring with respect to the calm water restoring and if appropriate conditions are met. Due to its capability of jeopardizing the ship safety, parametric roll has been recognised as a dangerous phenomenon in the recently issued IMO "International Code on Intact Stability, 2008" and appropriate methodologies for dealing with this phenomenon are required in the framework of development of so-called "new generation intact stability criteria". Along the same line, sufficiently simple and robust methodologies need to be developed to support designers in identifying, at the early design stage, designs requiring more attention for what concerns parametric roll. Similarly, methods are needed for relative comparisons between design alternatives. Responding to this call, the present paper presents a simplified methodology for the vulnerability assessment to parametrically excited rolling motion. The method is modular, based on regular waves, and can be implemented exploiting standard hydrostatic software tools. The method, in addition, explicitly considers environmental conditions by means of standard wave scatter diagrams. Scope of the methodology is the determination of a vulnerability index, intended as a measure of vulnerability of the considered ship/loading condition to the inception of parametrically excited rolling motion. Examples of application are reported and an alternative application based on irregular waves is outlined.