Ground response analysis and liquefaction assessment of subsea soils at an offshore platform site

Abstract

The development of underwater infrastructure is increasingly important due to the rising demand for offshore resource utilization and marine spatial expansion. This study evaluates the seismic behavior of subsea soil at a proposed offshore platform site, focusing on liquefaction potential and cyclic softening. Site-specific geotechnical investigations, including Standard penetration tests (SPT), cone penetration tests (CPT), and shear wave velocity (VS) measurements, were conducted to characterize the soil profile. Nonlinear site response analysis revealed significant amplification of seismic waves in the 0.2-2.0 s period range, due to soft clay layers. Liquefaction potential was evaluated using cyclic resistance ratio (CRR) and cyclic stress ratio (CSR) methods, following KDS 17 10 00 and effective stress-based dynamic analysis. Results indicated that the upper soil layers consist of high-plasticity clay, have a low likelihood of liquefaction. Cyclic softening analysis showed that softening may occur at shallow depths under 50-year return period earthquakes, and more broadly under 500-year events. The factor of safety varied depending on the undrained shear strength (Su) estimation method. While the site satisfies international seismic design standards for liquefaction, potential cyclic softening suggests that further design considerations are needed. This study offers a practical framework for evaluating seismic stability of submarine soils.