This paper presents the methodology and analysis procedure required for a nonlinear dynamic soil-pile-structure interaction analysis of the offshore structures subjected to ductility level earthquakes with emphasis on the design of a well-balanced optimized structural system. The analysis uses a 3- dimensional MicroSAS II structural model with full structural representation of the piles to which a series of nonlinear soil springs and dashpots are attached. The nonlinear inelastic soil elements, cyclic strain rate, gapping and hysteretic energy dissipation were properly considered. The nonlinear inelastic behaviors of the structural members were modeled. A new technique has been applied to identify the time-varying power spectral density of the earthquake accelerograms. The advanced nonlinear inelastic finite element analysis was performed to evaluate the structural stability of the legskirtpile connections.
The current design criteria specified in the API RP 2A-WSD address the strength level earthauke design and ductility level earthquake design. The typical design approach considering the strength level earthquake is based on the natural periods of the linearlized structural model, the modal analysis, and response spectra specified in the codes. In the seismically active areas, platform response to the rare, intense earthquake motions or ductility level earthquake may involve inelastic behavior and structural damage may occur but structure collapse shall be avoided. It may be unrealistic to use a linear structural model and response spectrum methods to design an offshore structure subjected to the very intense ductility level earthquake without huge cost penalty. For these reasons, the time history method is recommended in API RP 2A to perform the nonlinear, inelastic, dynamic soil-pile-structure interaction analysis to demonstrate the sufficient structural system redundancy such that the load redistribution and inelastic deformation will occur to prevent offshore structural collapse or the abrupt changes in stiffness in the vertical configuration of the offshore structure. Some examples had been investigated for the dynamic overload analysis of the offshore structure [1, 2, and 3].
In this paper, a four-leg and eight-skirt-pile platform (shown in Figure 1) designed for north offshore Trinidad in 497 ft water was used in the analysis. As per the seismic hazard study, three sets of seafloor time history accelerograms (two for shallow crustal earthquake with Mw 7.3 and Mw 7.4, and one for interplate subduction-zone earthquake with Mw 7.7), which were representative of 2,000-year return period earthquake at the site, were considered. It was demonstrated that the structure-optimized platform can withstand the 2000- year earthquake at the north offshore Trinidad without structural system collapse. It is estimated that over 15% of the structural steel have been saved after the reconfiguration of the structural system.
Figure 1 Outline of the platform topsides and jacket model (Available in full paper)
Three sets of accelerograms (3 components/set) were selected as being representative of the ductility level earthquake seafloor motions for the structural seismic design and analysis. The accelerograms are listed as follows:
Yermo accelerogram recorded during the 1992 Landers, California earthquake of magnitude Mw of 7.3