Response of a Steel-Jacket Platform Subject to Measured Seafloor Seismic Ground Motions
Authors: Smith C.E. 
Published: October 2008

Seismic analyses of offshore platforms may be accomplished using classical finite element techniques with the input forcing function modeled for earthquake loadings. The significant different between an offshore structure and an onshore structure is that the analysis procedure must be modified to consider the fluid-structure and the soil-pile interaction effects. Current practices (1996) use earthquake forcing functions obtained or modified from ground accelerations measured in the vertical and horizontal directions recorded from onshore sites. However, recent research has suggested that the ground motions measured on the sea floor may differ from ground motions measured from onshore sites during an earthquake. This paper will compare the response of a typical mid-water depth offshore platform subject 10 both onshore and offshore recorded ground motions from the same earthquake. Recommendations are put forth for the type of seismic forcing function to be used in the design or re-qualification of offshore structures, as well as relevant research for the future.

Introduction

Offshore platforms have several key structural and environmental different as compared to typical onshore buildings'. The differences can be grouped into the following areas:geology and response of subsea soils (soft-saturated soils and lack of offshore seismic data,a water column that provides a source of loading and dampening to the structure (fluid-structure interactions),special structural characteristics (deep pile foundation, large mass, large number of redundant members, larger overturning moments, and the fact that the mass is concentrated at the top of the platform m opposed to being distributed over its height as in most building structures), andthe unusual engineering properties of offshore soil deposits (often normally consolidated and extremely soft and compressible near the seafloor). Thus, the dynamic behavior of au offshore structure during an earthquake may be significantly different from its onshore counterpart.

The current (1996) methodology for the design and/or reassessment of offshore structures makes use of data obtained or modified from onshore seismic events. However, based on the many differences existing between onshore and offshorestructures cited above, a joint program was started in 1976 between the Department of Energy and the Conservation Division of the U.S. Geological Survey (now :!IC Minerals Management Service) to develop advanced instrumentation systems to assist government and the offshore industry in characterizing environmental and engineering conditions offshore. As part of this program, an instrumentation system was developed and installed offshore southern California to collect strong motion seismic data from remote ocean floor sites. The program was called the Seafloor Earthquake Measurement System Program and was given the name SEMS for brevity.

Sleefe 3 studied data obtained from these units, as well as data obtained from a second series of SEMS instruments installed offshore Long Beach in 1985. Of particular interest was the data obtained from the North Palm Springs Earthquake (6.0 magnitude, 91 mile epicentral distant) of July 8, 1986.