The need for reliable and reasonably accurate methods for predicting the ultimate earthquake resistance of tubular steel fixed offshore platforms has become increasingly evident. A realistic nonlinear dynamic time domain response analysis of an example structure is presented and compared with three simplified methods for determining ultimate earthquake resistance. These simplified methods are:
Ductility modified response spectrum,
Ductility evaluation under equivalent static loads, and
Reserve energy technique.
Over the last five years we have witnessed an order-of-magnitude escalation in the severity of earthquakes which must be explicitly considered in the design of fixed offshore platforms. Consider Figure 1, in which earlier design criteria are compared with more recent proposals. The total range of variation, from the building code requirements to the Maximum Credible earthquake, is a factor of 100. This apparent escalation in design earthquake severity is due to many factors such as: the damage suffered by buildings designed under current codes during recent earthquakes; concern for the economic and environmental impact of structural collapse in addition to the safety aspect; the relatively limited energy absorbing capacity of braced tubular structures in contrast to moment resisting systems used in conventional buildings; the empirical nature of seismic force levels in conventional building codes, and the lack of major earthquake experience with offshore platforms to provide a comparable empirical basis for seismic design criteria. Thus a rational approach to the reserve capacity of offshore structure is needed.
The 1971 San Fernando earthquake provided indications that ground surface motions in the epicentral region of an earthquake can have peak accelerations in excess of 0.5g which had previously been assumed by the earthquake engineering profession to represent the upper bound of soil transmitted ground motion. (8) This earthquake also proved that conventional buildings designed to modern building codes (base shear coefficients of 0.04 to 0.08g) could generally survive the severe ground motions without collapse. However, the energy demands of the earthquake far exceeded their elastic strain energy capacities. Infact, the repair costs of structural and nonstructural damage from nonlinear deformations exceeded the building replacement cost in a few cases. From an owner's standpoint such structures would be considered total failures even though they did not collapse.
In the last five years, major strides have been made in developing realistic methods for assessing earthquake risk and appropriate expressions for describing ground motions. New building codes such as the current work of ATC-3(13) and platform design recommendations, such as the 7th and 8th Editions of API-RP2a, have reformulated seismic design criteria in terms of dynamic force analyses. The resulting force levels from such analyses are normally significantly higher than those computed using code based equivalent static lateral force formulas used in the past, as illustrated by Figure 1.