Ivan Ćatipović

Numerical model of towing line in sea transport

Towing as a specific type of sea transport is often used for installing objects for drilling and exploitation of underwater gas and oil wells. Also, towing proved to be a cost-effective solution for the installation of the offshore wind turbine electric generators at sea locations. Because of the mass of these objects the need for towing increases progressively. Time domain numerical model for the wave-induced motions of a towed ship and the towline tension in regular head seas is presented in this paper. For the sake of simplicity, one end of the towing line is attached to ship’s bow and another end has prescribed straight line motion. All considerations are done in the vertical plane so the ship is modeled as a rigid body with three degrees of freedom. Hydrodynamic loadings due to waves are taken into account along with added mass and damping. Dynamics of the towing line is described by finite elements. Due to the nonlinear properties of the problem calculations are done in time domain. Comparison of the obtained numerical results is made with previously published results.Copyright

Calculation of Floating Crane Natural Frequencies Based on Linearized Multibody Dynamics

Natural frequencies and modes of a floating crane with a suspended load are studied. Both the floating crane and the suspended load are modeled as rigid bodies with six degrees of freedom. The suspended load is considered to be partially immersed in the sea water. A wire rope for load suspension is assumed to be non-extensible and without bending stiffness. Linearized governing equations for a coupled dynamic response are derived based on the multibody system dynamics with small displacements assumption. An added mass for the floating crane as well as the partially immersed load is determined using traditional potential theory and tree-dimensional boundary element method (in a frequency domain). Mapping from the frequency to the time domain is necessary to determine the correct added mass value for calculating above described natural frequencies and modes. A damping of the overall system can be calculated as some small portion of a critical damping. A numerical example is based on the floating crane during positioning of precast concrete piles. These piles are main part of foundations for over sea bridges.Copyright