Impact of local site conditions on seismic performance of free-spanning submarine pipelines: Underwater shaking table tests and numerical simulations

Local site conditions may pose a significant influence on the seismic responses of submarine pipelines by altering both the offshore motion propagation and soil-structure interaction (SSI). This paper aims to provide an in-depth understanding of the influence regularity of local site conditions on the seismic performance of free-spanning submarine pipelines (FSSPs). For this purpose, a suite of underwater shaking table tests were performed to investigate the seismic responses of FSSP subjected to the offshore spatial motions at three site categories. Response comparison factor (${\chi }_{R.ij}$) is defined to quantify the structural response discrepancies caused by the seismic inputs at different sites. The test results indicate that responses of the studied model FSSP gradually increase as spatial offshore motions at softer soil sites are employed as inputs; and the values of ${\chi }_{R.ij}$ vary with a maximum magnitude of up to 40%–60% for different response indices when the site soil changes from fine sand to clay. Subsequently, the corresponding numerical simulations are carried out to reproduce the seismic responses of the test model. The experimental and numerical results meet a good agreement, indicating that the developed numerical modeling method can accurately predict the seismic responses of FSSPs. Following this verified modeling method and using the p-y approach to address the SSI effect, fragility surfaces of the studied FSSP are derived in terms of PGA and site parameter $V_{S30}$ (shear-wave velocity in the top 30 m of the soil profile) via probabilistic seismic demand analyses. The impact of local site conditions on the seismic performance of the FSSP is quantitatively examined by comparing the fragility curves corresponding to various $V_{S30}$. Furthermore, a fast seismic damage assessment method is proposed for efficiently evaluating the performance of FSSPs buried in various offshore soil conditions. This approach proves beneficial for designers and decision-makers, enabling accurate estimation of seismic damage and facilitating the implementation of post-earthquake maintenance measures for FSSPs.