Inelastic Analysis Of Fixed Offshore Platforms For Earthquake Loading
Authors: Kallaby Joseph , Millman David N. 
Published: October 2008

Fixed offshore structures in seismic areas may be subjected to intensive ground shaking causing the platform to undergo deformations well into the inelastic range. This paper presents a procedure that may be used to account for such inelastic behavior using an investigation of a major steel platform for illustration. A post buckling capacity curve for bracing that may undergo column buckling is presented based on tests carried out in 1974.

INTRODUCTION

This paper presents a method which may be used to investigate energy absorption capacity or ductility of fixed offshore structures loaded into the inelastic range by earthquake excitation. This method describes a procedure whereby framing members that have either buckled or yielded are replaced by their post yield or buckling capacity. The procedure is repeated until either the required energy or ductility are met or the structure becomes unstable. For illustration purposes, the Maui A Drilling and Production Platform, a braced steel tower supported on pile foundation and shown in Figure 1 is used. It is under construction in Japan and is scheduled to be installed in 360 feet or water depth offshore taranaki, New Zealand, in early 1976 for Shell BP and Todd Oil Services.

PLATFORMS IN SEISMIC AREAS

Offshore platforms have been installed in various parts of the world where seismic activity was recorded or the area considered active. Such areas include offshore Southern California, Alaska, certain regions of the Gulf of Mexico, Southeast Asia and the Persian Gulf. Recent intensive search for oil has pushed exploration to other areas such as the Sea of Japan, and the China, Tasman, and Black Seas.

PRESENT SEISMIC DESIGN PROVISIONS

Present provisions for offshore platforms as given by the API code, follow essentially recommendations of the Uniform Building Code, with the further stipulation that the K factor be not less than 2. The API, recognizing the inadequacy of such criteria formed an Earthquake Mini Committee in late1974 to develop appropriate seismic design provisions. Meanwhile, major platforms including the 900-foot Exxon in the Santa Barbara channel and the Maui were designed for the stringent earthquake criteria formulated independently of the present codes.

DUAL EATHQUAKE CRITERIA

Design of structures for earthquake resistance should consider operation and safety requirements. This dual criteria is usually provided for by designing a platform to remain within yield or buckling stresses for the maximum level of earthquake which can be expected during the life of the structure, and to maintain its structural integrity during a maximum credible earthquake. Operation criteria insure minimum acceptable levels of structural strength and stiffness. Safety criteria require adequate ductility and energy absorption capacity to prevent collapse. These seemingly opposed requirements are two sides of the same earthquake coin. Furthermore, recognizing the need for both these criteria is the fundamental prerequisite of good seismic design.

SAFELY LEVEL REQUIREMENTS

To insure platform structural integrity during severe earthquake, it must be shown to possess adequate energy absorption capacity, which is most efficiently achieved through ductile action. Figure 2 describes performance of a typical frame under lateral load.