Collapse Analysis Of Framed Offshore Structures

ABSTRACT The paper presents new approaches for nonlinear system analysis of trusswork platforms. The main idea behind the technique is. to minimize the cost of the nonlinear analysis by reducing the number of parameters and the size of element model. A brief description is given on the theoretical basis of the computer program including the formulation of incremental stiffness with modifications for plastic hinges. Special emphasis is given to the choice of adequate interpolation functions and to the evaluation of nonlinear element characteristics. The solution strategy follows the conventional incrementation procedure with special algorithms for handling stability problems and load reversal. The numerical procedure is demonstrated on buckling analysis of simple columns and frames. Comparison with experiments and alternative numerical computations proves the accuracy and efficiency of the procedure. Practical design examples are given on progressive collapse analysis of a jacket structure, a deep water tripod typer of platform and a module support frame. INTRODUCTION The implementation of accidental loads as a design limit state in offshore rules (DnV,1981) has strengthened the need for rational tools for such calculations. It is of interest to know the amount of damage as well as the residual strength in damaged condition. Much effort has been given to the development of refined nonlinear finite element programs during the last decade and several program system are available for special purpose analysis. However, for the designer the step from conventional ultimate load design to the use of advanced nonlinear finite element or finite difference programs is still difficult. Special problems arises during the choice of numerical model, e.g. in the subdivision of a bracing element into an adequate number of finite elements so as to pick up possible local element buckling. The aim of the present work is to create a more design oriented numerical tool in the sense that only one numerical element is used per physical element. Further, the plastic behaviour is described by interaction formulas for cross sectional capacity which also are well defined characteristics in conventional design. The basic idea behind this so called Idealized Structural Unit Method was described (Ueda and Rashed, 1974) for ultimate strength analysis of transverse frames in ship structures. The procedure was extended for ultimate strength analysis of tubular frame structures (Rashed, 1980). Further development and adoption to progressive collapse analysis of mobile offshore platforms was presented (Aanhold, 1983) including the modelling of damaged structural members. The dented region was replaced by an eccentric circular cylinder with equivalent cross section properties. The extension of the technique (Søreide, 1986: includes refinement of the formulation for large deflections as well as the solution strategy. Due to the coarse element mesh nonlinear geometric behaviour on element level is modeled allowing moderate deflections between nodal points. The basic continuum theory behind the method is also revised so that the stiffness is derived from the incremental form of the virtual work principle. Interesting developments are now going on to incorporate concrete elements of different cross sections.