Seismic behaviors of free-spanning submarine pipelines subjected to multi-support earthquake motions within offshore sites

Due to the shortage of offshore seismic records and inhere difficulty in simulating artificial offshore ground motions, numerous existing works have been performed to investigate the seismic responses of free-spanning submarine pipelines, not only using onshore seismic records as inputs but also adopting uniform seismic excitations. These analytical schemes may result in severe misestimates of the seismic responses. In this content, this paper numerically investigates the seismic behaviors of a free-spanning submarine pipeline by using multi-support earthquake motions within offshore sites (MEMOSs) as inputs. The soil-pipe and water-pipe interactions are modeled using the nonlinear soil spring and added mass methods, respectively. The effects of spatially varying and overlying seawater are explicitly taken into account in the simulation of MEMOSs. The seismic behavior law of the free-spanning submarine pipeline under the synthesized MEMOSs is analyzed and summarized. Moreover, the impacts of the seismic excitation type, offshore seismic motion, coherence loss, local site condition and free span length on the Mises stress, vertical displacement and failure PGA of the submarine pipeline are examined and discussed comprehensively. Numerical results show that the middle and ends of free span are the dangerous regions; and the MEMOSs, coherence loss, local site condition and free span length have significant influences on the seismic responses and operational capacity of the pipeline. Based on the obtained results, some meaningful advices are recommended to the pipeline designers that (1) more attentions should be paid to the middle and ends of free span in the pipeline design; (2) the MEMOSs should be adopted as seismic inputs to accurately predict the seismic behaviors of pipelines; (3) the construction of submarine pipelines in soft clay and formation of large-dimensional free span are adverse for the safe operation of pipelines under earthquake actions, which should be avoided in engineering practice. •Seismic analysis of a free-spanning pipeline under MEMOSs is performed.•FE model of the pipeline is developed by the soil spring and added mass methods.•MEMOSs considering the effect of spatially varying and seawater are generated.•Effects of spatial variability parameters and free span length are discussed.