Study Of Hull Damage Stability Calculation With NURBS Technology

Key techniques involved in ship hull damage stability calculation by NURBS are studied in this paper.The aim is to calculate damaged stability in 3D space based on the hull surface so as to improve reliability and efficiency. With the development of computer technology and free-form surface modeling, body plan description on the computer has been converted into hull surface model simulation in which more information is included so that hydrostatic properties calculation can be carried out with hull information directly. When no damaged compartment, damaged stability calculation is just intact stability calculation and when waterplane is horiental, geometric properties calculation is the base of hydrostatic curves.Because ship hull is a kind of typical free-form surface, it is satisfactory to be modeled with NURBS. The key point to damaged stability calculation is to evaluate displacement and center of buoyancy under various floatations and obtain damaged stability curve eventually that can be generalized to evaluate geometrical properties (i.e. volume, centroid, moments of inertia, etc.) of the complex 3-dimensional closed surface. It can be transformed into 2-dimensional integration oyer boundary surface according to Gauss theorem.Studies having been done in this paper are as follows:(1) The algorithms of interpolation, approximation and boundary conditions of NURBS are studied, in which effects of boundary conditions on interpolating curve are especially discussed. The results indicate that start and end segment of interpolation curve are changed with different boundary conditions. Interpolation and approximation of arbitrary data points and ship hull surface modeling based on NURBS is implemented.(2) Intersectioning algorithm of NURBS surface and general implicit surface is researched in which the parametric formula are substituted into the implicit surface equation and then get the nonlinear two-dimensional equation on parametric domain. To solve the equation instead of iterative algorithm with an indispensable initial value, the theory of extracting isocurve in two-dimension scalar field is used. The results indicate that the methodology is simpler and more practical than other algorithms, where initial value are not used that reduces thein_______________________________________________Abstract____________________________________________of the CAD system and can avoid failure because of incorrect choice of the initial iterative value. It needs neither iterative process nor getting derivative of the intersection point.(3) Computation method of geometric properties of closed surface applied to performance calculation of ship hull surface based on NURBS is presented. The volume integrals can be transformed to surface integrals using Gauss's theorem. Two cases are studied, one is floating upright and another is free floating in 3D space. The former method is parametric surface integral and the latter is to triangulate closed surface based on NURBS.(4) The key technique in probabilistic damaged stability calculation is discussed. Damage stability curves of a series of damaged one-compartment respectively and combinations of damaged multi-compartments are needed in probabilistic method. Systems of equation of floatation and stability may be divergent when damaged multi-compartments are calculated that may lead to programs stop. The reason is studied and that makes the programs can distinguish this case and eventurelly the probabilistic calculation of arbitrary damaged multi-compartments is carried out.(5) Intersecting point distribution between every cross-section of both hull and damaged compartment under arbitrary floatation and waterline with 2D offsets is given particularly. Calculation of underwater geometric properties of both hull and compartment is treated as one process, the same to calculation of ship floatation, intact stability and damaged stability. The results prove that it is much simpler than others before and simplifie...