Buoyancy coupling with structural deformation analysis of ship based on finite element method

Abstract

The hydrostatic balance of a ship floating on the water is the result of buoyancy coupling with structural deformation. As to the ship with a relatively flexible hull girder, this coupling phenomenon is very important for the accuracy of general design and structural analysis. However, the existing ship design methods could not solve this coupling problem accurately. Aiming at this problem, a new numerical method based on FEM, called Buoyancy Coupling with structural Deformation Analysis (BCDA), is presented in this paper to improve both efficiency and accuracy of ship design. In BCDA, the concept of buoyancy element is proposed to simulate element on hull wet surface. As to a buoyancy element, the buoyant force is regarded as properties of element rather than external load. The buoyant matrices for both quadrilateral and triangular elements are derived, and the structural stiffness matrix of buoyant element is corrected by buoyant matrix. Being different from the traditional FEM, constraints on translation freedom along vertical direction should not be added in BCDA. The generalized displacements for each node, which consist of displacements due to buoyancy change and structure elastic deformations, are obtained by solving the global equilibrium equations. The results of BCDA satisfy both structural mechanics equation and gravity-buoyancy balance equation. A 15,000t launching barge is taken as example to verify the validity and accuracy of BCDA. With this example, it also proved that the coupling analysis of buoyancy and hull structural deformation is necessary for the general design and structural analysis of floating structure with relatively flexible hull.