Page 329 - Marine Structural Design

Part I1

Ultimate Strength

Chapter 15 Offshore Structures Under Earthquake Loads

15.1 General
Bottom supported offshore structures in seismic areas may be subjected to intensive ground
shaking causing the structures to undergo large deformations well into the plastic range.
Previous research in this area has mainly resulted in procedures where the solutions have been
sought in the frequency plane (Penzien, 1976). The present chapter is devoted to time domain
solutions such that the development of plastic deformations can be examined in detail.
The basic dynamics of earthquake action on structures has been discussed in Clough and
Penzien (1975) and Chopra (1995). There have been extensive investigations on earthquake
response of building structures in the time domain (Powell, 1973). Unfortunately, most of
works have been limited to plane frames. Furthermore, for offshore structures hydrodynamic
loads have to be taken into account and the geometrical nonliearities become more important
than in building structures. Therefore, there is a need for a procedure to predict earthquake
response of offshore structures including both geometrical and material nonlinearities.
Methods for analysis of frame structures including geometrical nonlinearities have been based
on either the finite element approach (Nedergaard and Pedersen, 1986) or on the beam-column
approach (Yao et al, 1986). Nedergaard and Pedersen, (1986) derived a deformation stiffness
matrix for beam-column elements. this matrix is a function of element deformations and
incorporates coupling between axial and lateral deformations. It is used together with the
linear and geometrical stiffhess matrices.
Material nonlinearity can be taken into account in an efficient and accurate way by use of the
plastic node method (Ueda and Yao, 1982). Using ordinary finite elements, the plastic
deformation of the elements is concentrated to the nodes in a mechanism similar to plastic
hinges. Applying the plastic flow theory, the elastic-plastic stiffness matrices are derived
without numerical integration.
In this Chapter, a procedure based on the finite element and the plastic node method is
proposed for earthquake response analysis of three-dimensional frames with geometrical and
material nonlinearities. Using the proposed procedure, earthquake response of a jacket
platform is investigated. Part of this Chapter appeared in Bai and Pedersen (1991). The new
extension is to outline earthquake design of fixed platforms based on API RF'2A.

15.2 Earthquake Design as per API FW2A

API RP2A (1991) applies in general to all fixed platform types. Most of the recommendations
are, however, typical for pile steel jacket platforms. The principles and procedures given in