Stern Slamming of an LNG Carrier

Abstract

Rational assessment of stern slamming of a large twin screw LNG carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behavior of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ship’s stern. Reynolds-averaged Navier-Stokes (RANS) computations based on the volume of fluid (VOF) method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were coupled to a nonlinear motion analysis of the ship in waves. At every time step of the transient computation, the finite volume grid was translated and rotated, simulating the actual position of the ship. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Slamming-induced pressure peaks, typically lasting for about 0.5 s, were characterized by a steep increase and decrease before and after the peak values. Shape and duration agreed favorably with full-scale measurements and model tests carried out on other ships, indicating that computed results captured all essential physical phenomena. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. Short-duration impact-related slamming loads excited the ship structure to vibrations in a wide range of frequencies. Excitation of the lowest fundamental eigenmode contributed most to additional stresses caused by hull girder whipping. Although longitudinal and shear stresses caused by quasi-steady wave bending were uncritical, we obtained a significant amplification (up to 25 percent) due to the dynamic structural response.