Non-linear Dynamic Site Response Analysis for Subsea Pipeline and Structures Seismic Design Considering Variable Site Conditions

Abstract

Site response analysis is essential in assessing the offshore pipeline integrity as well as the stability of subsea structures during an earthquake event. Depending on the functionality, the length of offshore pipeline can be tens to hundreds of kilometers running through different underlying soil conditions as well as traversing submarine slopes. Therefore, it is important to identify the effect of variable site conditions on the earthquake response to accurately predict the stability of offshore pipeline as well as subsea structures. A study was undertaken to determine the ground responses along an offshore pipeline route across escarpments and connected to subsea structures at the termination points. The pipeline route was divided into multiple segments depending on the soil variability. Due to high site seismicity, the soft soil is susceptible to high shear strain during strong earthquake shaking; thus requiring non-linear dynamic site-response analysis to accurately capture soil dynamic response. A finite element based non-linear site response analysis was selected for the entire pipeline route. The dynamic site response analyses were carried out using PLAXIS 2D and the soil behavior was modeled considering non-linear variation of stiffness and damping with shear strain. The input ground motions to the dynamic site response analysis were site specific stiff soil earthquake motions scaled to Abnormal Level Earthquake (ALE) and Extreme Level Earthquake (ELE). The segment-specific response spectra and time histories at mudline were developed as input to the seismic design of pipelines and subsea structures. Despite a stable escarpment during a seismic event, the site response analysis showed localized high deformation which can significantly affect the pipeline seismic response. Considerable variations in mudline response spectra was observed depending on strength, thickness and depth of soft soils. This paper provides an overview of dynamic site response analysis performed for an offshore site, and focuses on the effects of variable ground models, as well as escarpment on the seabed level seismic motions for subsea pipeline and structures seismic design. The importance of accounting each of these effects in the development of a robust and safe seismic design has been highlighted.