4.2.1. Non-Supported Structures

This section provides a brief introduction to pipelines. Submarine pipelines are an important part of the marine oil and gas extraction system, and are currently the most convenient and economical tool for transporting oil and gas. Seismic liquefaction is one of the main causes of damage to submarine pipelines, mostly through the following two failure modes: (1) due to the difference in soil gravity between the pipe and the liquefied seabed, the pipe will rise or sink; (2) seabed sliding causes lateral movement of the pipe. In conventional mitigation measures, the pipelines are buried deeper; however, it is difficult to do so in a marine environment. Ren et al. proposed a new measure for liquefaction damage prevention by reinforcing pipelines with wing plates, and verified the feasibility through shaking-table tests [62]. Yang proposed a simple portal frame to limit the displacement of pipelines. Compared with general anchoring reinforcements, the portal frame allows a certain upward displacement of the pipeline, which greatly reduces the stress of the pipeline and improves the safety when liquefaction occurs [63].
