ABSTRACT

This paper describes the formulation and application of a finiteelement based approach for geometric and material nonlinear analysis of framed structures, based on the use of refined meshes of a nonlinear version of the classic beam element, called herein Standard Linear- Cubic Beam (SLCB) element. The main application is the collapse analysis of fixed offshore platforms, that may be required both in the design and in the operation phases, to determine the behaviour of the damaged or undamaged structure, under operational, extreme or accidental loads. Collapse analyses are also performed to generate data for the study of the reliability of the platform. The finite element formulation implemented assumes large displacements and small strain. Geometric nonlinearity is taken into account by means of an updated Lagrangian formulation. Material nonlinearity is taken into account by means of an elastic perfectly plastic model derived from plastic hinge theory. A brief description of the program and its main features is presented, and numerical examples are used to demonstrate the attained accuracy.

INTRODUCTION

One of the most outstanding characteristics of the Finite Element Method (FEM) is the convergence to the exact solution as the element mesh is refined. Until the late 1980s, due to computational limitations it was not possible to explore this characteristic in collapse analyses of fixed offshore platforms. Up to that time, it was computationally expensive to perform full nonlinear analyses of offshore platforms modeled with refined meshes. In this method, the platform is divided into the largest possible structural units, whose geometric and material nonlinear behavior are idealized. The basic idea of ISUM is to use one element, called Idealized Tubular Structural Unit (ITSU), for each physical component (tubular member).

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