Loading And Capacity Effects On Platform Performance In Extreme Condition Storm Waves And Earthquakes
Authors: Bea R.G. , Young Carlton 
Published: October 2008

Dynamic - transient loading effects from extreme storm waves and earthquakes can have important influences on the nonlinear ultimate limit state performance of fixed offshore platforms. Recorded and synthetic storm wave and earthquake time histories have been used to develop loading time histories acting on template-type platforms having natural periods in the range of 1 to 5 sec The interactions of these loading histories with the dynamic, nonlinear, hysteretic performance characteristics of idealized systems have been analyzed. A static pushover capacity modification factor has been developed to recognize transient loading structure performance characteristics. The results from the idealized systems have been correlated with results from time domain nonlinear analyses of platform structure systems subjected to intense wave and earthquake loadings. For the global behavior of the platforms studied, the results based on the simplified systems are in good agreement with those from the complex analyses.

INTRODUCTION

Due to the transient and dynamic aspects of most environmental loadings imposed on and induced in offshore platforms, it can be important to recognize the differences between loadings and loading effects.1,2,3 The term "loadings" is taken to represent the forces that are imposed on an offshore structure that are fundamentally independent of how the structure responds to the imposed forces. Such forces frequently are referred to as being static even though they vary with time.

The term "loading effects" is taken to represent the internal forces that are generated within an offshore structure that are dependent on how the structure responds to the imposed forces.

Loading effects induced in an offshore structure are determined by:the characteristics of the loadings, andthe performance characteristics of the structure, Table 1 summarizes the primary loading and structure performance characteristics (Fig. 1).

SD is the maximum static lateral loading (Fig. 1) determined by the design environmental condition parameters and force determination procedures in design guidelines. Rus is the ultimate limit state maximum lateral loading capacity determined by static push-over analyses. The static Reserve Strength Ratio (RSRs) is based on SDand Rus. Fv is the ratio of the resultant capacity factor (?) to the resulting loading factor (?) (Table 1):(Mathematical Equation) (available in full paper)

The "capacity modifier", Fv, combines the loading and ultimate limit state performance characteristics of the platform structure - foundation system. This paper addresses Fv for extreme condition wave and earthquake lateral forces developed on conventional steel, template-type platforms having natural periods (Tn) in the range of Tn = 1 to 5 sec.

Ductility (?) is used as a primary reflector of the damage producing potential of the transient loadings. The ductility capacity of a platform (Fig. 1) is determined by the lateral displacement at which the structure can no longer support its gravity loadings (? p) divided by the lateral displacement at which the overall system demonstrates its first significant nonlinear behavior ((?e):