Seismic Analysis and Design of Offshore High-Voltage Cable in Young Bay Mud for M7.8 Earthquake on the San Andreas Fault

Seismic analysis performed in support of design of the offshore segment of a high voltage (230 kV) transmission line that connects two substations in San Francisco, California, is the focus of this study. The offshore portion of the transmission line is located approximately 2,000 feet from the western shoreline of the San Francisco Bay, has a total length of approximately 2.5 miles, and consists of three parallel double armored cross-linked polyethylene (XLPE) cables. The transitions from the offshore portion to onshore portions occur in high density polyethylene (HDPE) conduits, approximately 1,500 ft each, installed from the land using horizontal directional drilling (HDD) and are included in the seismic analysis of the offshore segment. Seismic design criteria for the line are based on 84th percentile ground motions from an Mw7.8 earthquake on the San Andreas Fault, which is located approximately 9 miles to the west. Geologic conditions along the offshore portion of the line were evaluated by a review of bathymetric and geotechnical data and consist of very soft to soft fine-grained sediment of Young Bay Mud underlain by older alluvium deposits over bedrock. Nonlinear geotechnical analysis was performed to calculate the expected permanent ground deformations along the cable alignment. Seismic assessment of the offshore marine cable was performed using non-linear soil–structure interaction analysis to compute maximum axial force under the earthquake-induced permanent ground deformations. The analyses included consideration of material and geometric nonlinearity of the cable, nonlinear response of the surrounding soil, as well as the effects of the operating cable temperature and the friction between the cable and the conduit within the HDPE pipes. Cable properties were obtained from mechanical and electrical cable tests on a prototype cable performed by others. Several parameter sensitivity studies on cable–soil interaction, compression stiffness, and anchorage capacities were performed. Multiple alignments with S-shape slack were evaluated for optimal combined seismic and thermal loading. Overall seismic reliability of the cable was evaluated through analysis that considered uncertainty in earthquake ground motions, geotechnical response, and the corresponding response of the cable. Monte Carlo simulations were performed to estimate the scenario and annual probabilities of failure. The target audience for this study includes engineers as well as public utility company stakeholders, who can benefit from the methods and lessons learned as presented here.