Due to recent developments in computer technology, computer-aided investigations of structural movements in a maritime environment have become more relevant during the last years. With regard to mechanically coupled multibody systems in fishery and offshore operations, the analysis of such systems is in the focus of research and development. To analyse multibody systems, forces and moments of all included bodies have to be defined within the same reference frame, which requires a transformation algorithm. Showing the correctness of the transformation algorithm, it must be also applicable for six degrees of freedom (6DOF) motions of a free floating single body in seaways. Therefore, the computation of irregular waves is discussed before the traditional motion description of a floating structure by using the Kirchhoff equations. With these basics, an approach to calculate the motion equations of single bodies within the earth-fixed reference frame is presented before the method of the inertia value transformation. To compare the body-fixed and earth-fixed calculation method, a free-floating crew transfer vessel in irregular waves is simulated and the results are discussed. Finally, the inertia value transformation will be proved by the energy conservation principle on the example of a pure rotating rigid body with none digital calculations.Keywords: inertia value transformation, wave-disturbed ship motions, wave-structure interaction, six degrees of freedom (6DOF), hydromechanics, inertial kinematics, Euclidian room
IntroductionModern simulation techniques enable a more profound analysis of various engineering problems in an early design stage. In maritime kinematics, the focus is on the behaviour of offshore structures under natural environment conditions like wind, waves and current. Detailed knowledge of loads and motions is required for proper dimensioning of efficient and safe systems. Furthermore, simulations are increasingly used for the design of controller-based automation and assistance systems. In addition to the accurate calculation of the structural movements, the calculation speed of a simulation is an important quality feature for these applications, e.g. realtime constraints. To determine the kinematics model of a free-floating structure in a seaway, different approaches are commonly used. The equations of motions can either be defined in the inertial reference frame or alternatively in the body-fixed reference frame. The difference between dry mechanics and maritime mechanics is based on additional hydrodynamic effects, namely, the hydrodynamic added mass, e.g. [1][2][3]. The hydrodynamic mass force is an inertia force. The relative acceleration between incompressible fluid and structure induces a pressure field, which results in a hydrodynamic force that is formulated as the product of the relative acceleration and a 'virtual' mass. The size of the hydrodynamic added mass depends primarily on the direction of movement and the geometry of the structure [4].Due to the phenomenon of added or...
