Pushover analysis of a fixed steel offshore platform - Part I

Abstract

Part I: The computer program INTRA models the collapse behavior of trusswork offshore structures with the aid of two types of elements: 'Beam-column' elements, designed to simulate bending dominated failure of structural members, and 'strut' elements, that can represent normal force dominated failure. The behavior of both types of INTRA elements has been investigated by comparison with the results of analytical solutions and of the MARC finite element program. A satisfying performance has been found for the beam-column element BEMC and the strut element ISTR. A Fortran routine has been established to generate the required input parameters for INTRA strut elements, resulting in more exact input values than can be derived with the aid of the default property generation. A parameter study has been carried out to the behavior of strut elements under conditions that may occur in the modelling of a large North Sea platform. It appears that lateral loads and imposed end rotations each may result in a maximum reduction of buckling strength of 15%, but that, however, the average reduction is much lower. Also the post-buckling strength is only modestly influenced. A plane frame pushover analysis has been performed with both the INTRA and the MARC programs. The influence of schematizations in INTRA strut elements on the overall structural beahavior appears to be acceptably small. However, serious numerical problems occurred during the INTRA analysis, resulting in a too low prediction of the ultimate load of the platform. Part II: As a preparation to pushover analysis of a complete fixed steel offshore structure, failure of the foundation has been analyzed in a qualitative way. Structure-foundation interaction is considered and some methods are treated to describe load-deformation behavior of piles loaded up to failure. Main attention is paid to methods in conjunction with the computer program INTRA. Computational rules to achieve load settlement curves for pile groups under extreme loads have been derived with the aid of modified t-z curves. Alternatives are presented to model the foundation in both extensive and more simplified ways.