Dynamic Characterization and Site Response Studies for an Offshore Site Based on Detailed Geotechnical Tests

Introduction

Earthquake poses significant threat to offshore and marine structures. As in most of the cases, the offshore structures are constructed for petroleum extraction; any damage to these structures will result in a substantial economic loss. Majority of offshore site consist of very soft to soft marine sediments to a considerable depth (even up to 50 to 60m). Presences of such soft sediments alter the ground motion characteristics of earthquake shaking. Earthquake damages to marine structures have been well documented by many researchers (Wyllie et al.1986; Iai and Kameoka, 1993; Hall 1995; Boulanger et al. 2000 etc.). In the case of offshore structures, the effects due to earthquakes can be classified as soil structure interaction effects and hydrodynamic effects. Both of these effects are need to be considered for the earthquake resistant design of offshore structures. For the design of any offshore foundation system, the dynamic characterization of marine soils in depth is essential. This paper presents a detailed seismic site characterization and site response studies for an offshore site on the Western side of Yemen (Middle East).

The study area is located on the Western side of Yemen within the area of Red Sea. Republic of Yemen is an Arab country in south East Asia located in southwestern to southern end of the Arabian Peninsula. The coastline stretches for about 2,000 km and it is bordered by Saudi Arabia to the north, the Red Sea to the west, the Gulf of Aden and Arabian Sea to the south, and Oman to the east. Figure 1 shows the geographic location of the study area along with location of boreholes in an area of 4km × 8km within the red sea. Detailed site characterization was carried out based on the average shear wave velocity for top 30m depth (V_s30), obtained using seismic cone penetration test (SCPT) data by a geotechnical company. A correlation between shear wave velocity and cone tip resistance (q_c) value for the study area was also determined by the authors. The synthetic ground motion was generated as per the methodology suggested by Boore (2003) using regional seismo-tectonic characteristics. Dynamic soil properties such as shear modulus and damping ratio at different strain levels, using the undisturbed soil samples, were obtained from resonant column tests and cyclic triaxial tests. Based on the resonant column test data and cyclic triaxial test data, appropriate modulus reduction and damping curves were selected for the ground response analysis. Equivalent linear ground response analysis was done using SHAKE 2000 (Ordonez 2000), to evaluate local site response. Design response spectra at ground surface were also evaluated for the study area using Eurocode 8. Peak ground acceleration (PGA) at ground surface, predominant frequency of surface ground motion, and amplification factor for major borehole locations in the study area are presented in this paper.

Figure 1.

Location of the study area