Stability Requirements of Multi-Purpose Ships for Heavy Lifts

New minimum intact stability criteria are presented to ensure safety against capsizing invoked by sudden loss of crane load during heavy lifts at sea, followed by typical sample stability assessments for a lifting operation on four multipurpose ships. For added stability, two of these ships had a pontoon attached at their sides opposite the lift. Two numerical time-domain methods assessed the transient dynamic heel after a sudden loss of crane load. With the ship at equilibrium, both analyses started by releasing the crane load, simulating a sudden failure of the lifting gear. The first method solved the roll motion equation as a one-degree-of-freedom system; the second method used a Reynolds-averaged Navier-Stokes equations solver. The first method relied on appropriately chosen linearized roll damping coefficients, and the nonlinearity of the righting moment function had to be accounted for. The second method required creating extensive numerical grids to idealize the ship’s hull, including the counter balancing stability pontoon, rudder and bilge keels, as well as all parts of the ship’s superstructure that effect the righting moment at large heeling angles.