Domestic U.S. flagged liftboats have relatively low length-to-beam ratios (L/B), compared with typical vessels, and operate with minimal freeboard and high vertical centers of gravity. As such, domestic liftboats can have poor righting arm characteristics that may not be revealed by conventional regulatory stability analyses that evaluate in the pure heel direction. This study adapts a novel energy-to-incline method and varying axis stability analysis developed by Andrew Breuer for Mobile Offshore Drilling Units to low L/B liftboats. This method of generating energy-to-incline surfaces was applied to the limiting case of a low L/B=1 liftboat and to the liftboat SEACOR POWER (L/B=2.02) to produce energy contour plots for a range of heel and trim values and to identify capsize points. Righting arm curves were mathematically computed from these surfaces by extracting incremental energy along pathways to capsize. For the low L/B ratio vessels, this resulted in truncated righting arm curves for inclinations in many directions, associated with the computational approach and the physics of orthogonal tipping. Analysis of the results revealed that low L/B liftboats have their lowest-energy pathways to capsize in inclination directions different from the traditionally assumed heel direction. As such, it is proposed that much of the U.S. domestic liftboat fleet may also exhibit such “off axis” weakest capsize directions. Finally, standard hydrostatic stability software was used to compute righting arms for the L/B=1 liftboat when inclined directly from the point of static equilibrium to a known capsize point, while fixing the angle of rotation in the direction orthogonal to the straight-line path. The resulting righting arm approximated well the lowest-energy righting arms from the more computationally intensive energy-to-incline method. Future research would be to assess whether such straight-line, fixed-rotation paths provide a good approximation for the minimum energy righting arms for liftboats over a range of L/B. If so, this method would provide an efficient means to produce liftboat righting arms with two zero-crossing points necessary for comparison with stability regulations.
liftboat; intact stability; potential energy; righting arm; orthogonal tipping
