A class society's view on rationally based ship structural design

摘要

The paper describes a class society's view on rationally based ship structural design. Where ships were designed earlier solely on the basis of prescriptive rules from classification societies, the introduction of the finite-element method advanced rational analysis and design methods, giving the designer greater scope, capability and efficiency. However, a thorough knowledge is required of structural design and analysis together with practical experience in structural design. A 13,000 TEU containership is taken as a typical example for the present day rational ship structural design and analysis as conducted by a ship classification society. The core of this analysis is the three-dimensional overall finite-element model of the whole ship. Software packages efficiently generate long-term hydrodynamic loads. Usually, these tools are based on the design wave approach, finding the most relevant load combinations to dimension the ship's structure, whereby hull girder sectional forces and moments match design values specified in class rules. Action by class societies also addresses other design issues, such as the prediction of design accelerations that affect container stowage, the accurate assessment of slamming loads and the estimation of sloshing-induced pressure loads caused by fluid motions in partially filled tanks. Although class rules include formulas to estimate these loads, advanced numerical techniques that directly solve the Reynolds-averaged Navier-Stokes (RANS) equations are better able to describe the physics associated with these phenomena. Larger ships with more flexible hulls are subject to increased risk caused by hydroelastic high-frequency hull girder loads. Although at present class rules rely on constant overall safety factors to account for the dynamic amplification of stresses in the ship structure, first principal numerical methods are being developed, which couple the non-linearly solved rigid-body ship motions and a simple finite-element Timoshenko beam model with the RANS equations.