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ABSTRACT A novel feature of the pseudo-force influence method's formulation (presented in Part I) enables a non-linear dynamic simulation system to be developed around an unmodified, linear finite-element program. Such a simulation system is used to investigate the dynamic collapse resistance of an offshore platform subjected to extreme storm overload. From this study it is concluded that, for structures possessing ductile (post-ultimate-strength) failure modes, the peak dynamic load that can be resisted may exceed the static ultimate capacity by a considerable margin. This conclusion provides a major incentive for designing ductile behaviour into all new offshore platforms and not only for those operating in seismically active regions. For certain existing platforms that are required to meet operational demands outside their original design intent, expensive remedial work may be avoided owing to the increased resistance rating. 1. INTRODUCTION With the recent advent/availability of non-linear analysis techniques (e.g. USFOS, FENRIS, INTRA, and the method developed by Stewart & van de Graaf, 1990), structural integrity assessments may now be based on the static collapse strength of the entire system rather than on the strength of each component. Therefore, the reserve capacity beyond first component failure may be evaluated and utilised, and a more realistic indication of the structure's performance may be obtained. Assessments based on these more advanced methods can compliment traditional linear analysis procedures (e.g. API-RP2A). They have been used, for example, to:verify the fitness for purpose of existing platform sub-structures whose present or envisaged operational demands are outside the original design intent (Stewart et aI., 1988; Bea et aI., 1988). (Thus, unnecessary and expensive in-situ strengthening can be avoided and/or it may be possible to tie in a satellite field without the need for a costly new platform.)
ABSTRACT Starting from the principle of virtual work, a general method for non-linear (dynamic) structural analysis/re-analysis is derived using pseudo-force concepts and a reduction technique based on influence coefficients. This pseudo-force influence method (PFI-Method) is particularly efficient for the class of problem in which non-linear (or modified) material behavior is restricted to a limited number of elements and inertia/damping properties are lumped at only a few discrete points. It is emphasized that the PFI-method is simply a convenient reformulation of the discrete equilibrium equations and as such is directly analogous to more conventional forms of the non-linear finite-element approach. 1.0 INTRODUCTION This paper is presented in two parts. In this first part a fresh look is taken at the formulation and solution of the (dynamic) equilibrium equations for a solid continuum. This new perspective leads to an efficient alternative to the more conventional implementations of the finite-element (f.e.) method for the solution of a certain class of problem in mechanics, namely those for which:the material properties over most of the domain are constant and linear-elastic, but a limited number of elements exhibit material non-linearity (or their properties are to be varied); and if dynamic response is to be considered, time-dependent resistances (inertia/damping) are present at only a few locations. This is discussed in Part II of this paper, as is the application of the method to the dynamic collapse behavior of an offshore structure subjected to extreme storm loading. 1.1 Historical development of pseudo-force and related methods Given a large linear model of a structure, it seems self-evident that if the properties of only a few elements are altered, this should not entail carrying out a complete re-analysis to obtain the new response.