Inelastic Earthquake Analyses Of An Offshore California Platform
Authors: Craig Michael J.K. , Shekher Vinod J.K. 
Published: October 2008

Inelastic analyses have been conducted on a platform intended for offshore California. The energy absorption capacity of the structure-foundation system, and the collapse mechanism of the structure have been defined. The structure was designed to meet strength and ductility requirements by elastic analyses.

INTRODUCTION

Current recommended practice(l) suggests that major pile-supported offshore platforms intended for installation seismically active areas be designed to fulfill two provisions: A strength requirement in resisting a moderate earthquake with a small chance of being exceeded in the lifetime of the structure, and a ductility requirement to resist a rare, intense earthquake.

The ductility requirement is now formulated in terms of the ability of the structure-foundation system to absorb energy over and above that absorbed in the strength level analysis. That absorption of energy can be entirely through elastic straining of components of the idealized system, or it can include inelastic deformation of soil and structural elements, provided stability of the structure-foundation system is not compromised.

This paper presents results of inelastic analyses of a small platform intended for offshore California, designed to the elastic criteria of the 9th edition of RP2A(2). The objectives of the work are to :Assess the energy absorption capacity of the structure as designed, and compare it to the recommended ductility requirement of the 10th edition of RP2A.Assess the structure's overall ductility and failure mechanism.Present three state-of-the-art analyses required to obtain these results.

These analyses are the first of their kind, in three dimensions, on a full-scale drilling and production platform designed by the response spectrum method.

The initial cost of a platform designed to entirely elastic criteria exceeds that of a design in which inelastic behavior of the structure is accounted for, possibly by as much as tens of millions of do11ars(3). Having assessed the energy absorption capacity of this structure, the next step is to develop a balanced design accounting for inelastic behavior, and quantify a cost saving, in steel weight at least, over the original elastic design. For small structures, the time and effort demanded by an inelastic design may outweigh any such savings. However, a basis can be developed which may aid in the selection of the most rewarding design path for future platforms in seismically active areas. This work is ongoing, and beyond the scope of this paper.

COMPUTER PROGRAM INTRA

A brief description of the analytical tool used in these analyses is in order before proceeding.

The Inelastic Tower Response Analysis computer program is the product of-several years of effort by a joint-industry sponsored group. It has been developed to analyze the three-dimensional response of offshore platforms to earthquake shaking. It is a finite element code, using nonlinear soil, linear and nonlinear structural elements. The code can account for material and geometric soil damping, mass and stiffness proportional structural damping, and hydrodynamic damping. Numerical damping can be used to damp out participation of the higher modes. The user may choose his own mix of iterative techniques, including constant stiffness and Newton Rhapson.