A computer program for inelastic dynamic analysis of. tubular offshore structures has been developed. This includes a strut element which models the inelastic stretch and post-buckling behavior of braces. and a failure algorithm based on limiting ductility at plastic hinges. Application to a pile-supported example platform, subjected to extreme shaking well beyond normal elastic design limits, demonstrates survival in spite of damage.
Current design criteria for fixed offshore structures specially a design "Strength Level" earthquake for which the probability of occurrence is comparable to that of the design wave. This level of shaking is also consistent with that used to analyze important onshore structures.
In addition, the 1977 edition of RP 2Al will call for specific investigation of platform overload response for a "Ductility Level" earthquake whose-intensity is twice the design Strength Level. Alternatively, designers may further wish to investigate platform survivability for higher estimates of the "Maximum Credible" earthquake having peak ground velocity on the order of 4 to 5 ft/sec.
The cost penalty for having to design for these larger quakes on a strength basis can run to tens of millions of dollars per -structure. However, if offshore structures can be properly detailed to maintain integrity during inelastic deformation, earthquake motions beyond the design elastic capacity may result in permanent distortion and localized damage--but not collapse. See Figure 1. Thus the order-of-magnitude increases in design criteria cited above do not necessarily require a corresponding increase in platform strength and cost, in this inelastic ductility reserve can be taken into account.
Various approaches to inelastic analysis have been reviewed in a comparison paper. 2 A common feature of these analytical efforts is that they rely heavily on empirical descriptions of tubular member behavior in the inelastic range.
Beginning with an API-sponsored research project in 1972, Sherman et al have been steadily expanding our knowledge of the inelastic behavior of tubular members of the types used in offshore structures.
The initial series of tests dealt with the 3 flexural capacity of fixed-end and cantilever beams3. Tubes with a D/t ratio of 36 or less (1300/Fy) were found to qualify as compact sections, in the sense that they possessed sufficient plastic hinge rotation capacity (prior to local buckling) to develop the theoretical limit load for fixed-end beams. Tubes with D/t of up to 48 (1700/Fy) also demonstrated considerable ductility (3 to 10 times yield deformation) and developed the theoretical limit load prior to failure. although local buckling initiated earlier. Tubes with D/t of up to 92 (3300/Fy) easily exceeded yield deformation, but lacked sufficient rotation capacity to develop the theoretical limit load; such behavior might be termed semi-compact.