The prediction of vessel motions in a seaway using panel methods based on linearized potential flow theory is a routine practice in seakeeping analysis. Many three-dimensional radiation-diffraction panel programs capable of performing such computations are commercially available. They typically provide forces and motions corresponding to the six rigid-body modes. In the special case of multiple interconnected bodies, motions must be determined subject to the mechanical constraints imposed by the connectors. The loads in the connecting structure are of particular interest in such cases. This type of problem can be analyzed by initially solving the radiation-diffraction problem considering each separate body to have six independent degrees of freedom and then imposing the constraints in a special post-processor. However, this procedure is computationally inefficient since the presence of geometric symmetry planes can not be exploited and the constraints must be imposed in an ad-hoc manner. An efficient alternative is to define generalized modes as shown by Newman (1993, 1994, 1998). Care should be taken to define the generalized modes as either symmetric or anti-symmetric about geometric planes of symmetry of the ensemble. The computational burden can be reduced by doing this whenever the geometry permits. The exciting forces and hydrodynamic coefficients for the set of generalized modes are computed in the usual manner. The desired motions and connector loads can then be determined. This paper presents the above approach. The method is illustrated with two example applications. One is the determination of cross structure loads for a SWATH vessel. The other is the determination of motions and hinge loads for a triple modular assembly. Emphasis is given to obtaining connecting structure loads in body-fixed coordinates.
The prediction of vessel motions in a seaway using panel methods based on linearized potential flow theory is a routine practice in seakeeping analysis.
