INFLUENCE OF SEAWATER LAYER ON THE SEISMIC RESPONSE OF BURIED OFFSHORE PIPELINES

Previous studies on the seismic responses of buried pipelines mainly focused on the onshore pipelines, investigations on the seismic responses of offshore pipelines are rare. Those studies analysed offshore pipelines adopted the onshore earthquake ground motions as input, which may lead to inaccurate pipeline response predictions. It has been revealed that seawater layer can significantly suppress the seafloor vertical motions near the P-wave resonant frequencies of the water layer. Moreover, it can indirectly influence the seafloor motions by changing the water saturation and pore pressure of subsea soil layers. The direct and indirect influences of seawater layer on the seismic responses of offshore pipelines have never been studied owing to the lack of understanding and availability of seafloor seismic ground motions. This paper carries out a stochastic analysis on the seismic responses of buried offshore pipelines. The direct and indirect influences of seawater layer on the seafloor seismic motions are explicitly considered by using the recently proposed theoretical local site transfer functions by the authors. The seismic responses of the buried pipelines in the axial and transverse directions are stochastically formulated in the frequency domain. The differences between the seismic responses of buried onshore and offshore pipelines are discussed. The influence of seawater layer on the subsea earthquake ground motion prediction and offshore pipeline responses is highlighted.