Key Technologies In Preliminary Ship Design Based On NURBS Representation

Preliminary ship design is the key point of the ship design and shipbuilding, in which ship hull representation is the foundation. Presently, There are two main methods of ship hull representation: One is the representation by discrete points, i.e. by the offset table often used. The drawbacks are that the interpolation is necessary to compute the coordinates of the points outside the offset during the different stages of design and shipbuilding, and the different interpolation method leads to different values resulting in the difference of the practical ship hull representation and the errors of the calculation and shipbuilding. The other is the representation by mathematical function including 2D wire frame and 3D surface patches representation methods. Similar to the discrete points method, the errors occur in the former while interpolating the points outside the ones on the wire frame. The boundary conditions of the surface patches in the latter are difficult and fussy to be dealt with to meet with different continuous order due to the complexity of the boundary conditions between the adjacent surface patches. So representing the hull surface with a single surface function is the efficient way to solve the above problems and the basic condition to speed up the 3D design taking place the 2D one.In this dissertation, the ship hull is represented by a single NURBS (Non-Uniform Rational B-Spline) function. Then based on it, the key technologies, including the main algorithms involved in the preliminary ship design, compartmentation, calculation of statics performances and hull structure strength analysis, are studied. The uniformity of ship hull representation, calculation precision and parameterized inputting of data are realized and the intelligent algorithms are applied to this study.The hull surface consists of the fore and aft arc surfaces, free ones at the aft and fore body and the analytical ones in the mid-ship by the different continuous order and it is very difficult to represent it with a single NURBS surface, so the 2D wire frame and surface patches method are still the main methods of ship hull representation. But according to the above mentioned, it is difficult to assure the precision of the later design and calculation and the shipbuilding quality. Here, the ship hull, deck and inner hull are represented with a single NURBS function to ensure the uniformity of representation and make the performances calculation and block building more convenient and precise.Plane-surface intersection algorithm and geometric properties calculation are the core contents in the tanks division and statics performances calculation. These cases, including thetangency, discontinuity and loss of intersection curves, are not solved very well by the existing methods. Here, the half-space property of the plane is applied to solving the intersection between plane and surface appropriate to that between any quadrangular surface that can be discretized into quadrangular elements and plane, and the above cases are handled successfully;The existing geometric properties calculation methods have not perfect universality, with which it is difficult to calculate the properties of arbitrary section area, partial surface area and volume. In this study, the geometric properties combination method is employed to calculate that of the plane, surface and spacial fields. This method is used to the statics performances calculation improving the calculation precision.The intersection algorithm is applied to tank division and the creation of the keypoints in the finite element (FE) modeling of the ship hull structure so as to realize the parameterized method of compartmentation and FE modeling, which enhances the design efficiency. The properties related to water plane, wet surface area, underwater volume and hold capacity are calculated employing this geometric properties calculation method, so that the hydrostatic performances, free floatation, stability, free surface correction and the load applying on the FE model are computed more precisely and conveniently, avoiding the errors resulting from the assumption, interpolation and the fussy inputting data in the 2D design method, and the more reasonable results are obtained.It is difficult to express the relationship of the center of buoyancy and righting lever against the offset of the hull and floatation with determinate function. The influence of the trim should be considered while calculating the floatation and stability precisely due to the asymmetry between the fore and aft body form. So it is necessary to calculate the free floatation and minimum stability using optimization method. But the inappropriate settings of the parameters probably result in the difficulty to obtain the global optimum solution even the divergence in the generic optimization method. Here, the genetic algorithm (GA) and the fussy genetic algorithm (FGA) are applied to the free floatation and minimum stability calculation respectively and the necessary improvements are made to speed up calculation and improve the precision.The studies and calculation cases indicates that the errors caused by the difference of shiphull representation, the assumption and interpolation can be avoided and the precision and theefficency of ship design and the quality of shipbuilding can be enhanced, by the realization ofhull representation with a single NURBS function, the solutions of the main calculation methodinvolved in the ship preliminary design—plane-surface intersection and geometric propertiescalculation, and their application to the parameterized compartmentation, FE modeling of hullstructure, statics performances calculation and the introduction of intelligent algorithms to them.