An Example in the Use of Performance-Based Engineering for Offshore Shallow Foundations Overlying Liquefiable Sand

Abstract

Abstract Geohazards affecting offshore facilities are typically mitigated through avoidance. However, as stated by Cauquil (2014), exploration and production activities are moving to more complex and deeper geo-environmental contexts, and recent projects have shown that it may not be possible to avoid the geohazard and its impact area. Therefore, facilities in these fields must be developed using a design that can overcome the effects of the hazard. If present, seismically induced geohazards (e.g., seabed settling due to liquefaction) can be mitigated by placing facilities on deep foundations that penetrate through liquefiable layers. Alternatively, facilities can be placed on shallow foundations that are designed to tolerate permanent ground displacements. These ground displacements depend on various factors such as the density and thickness of the liquefiable layer, the depth to the liquefiable layer, and characteristics of the ground motion (e.g., intensity, magnitude, and duration). The magnitude of settlement that needs to be incorporated in design must be based on either a deterministically developed design earthquake scenario or a probabilistically developed annual probability of exceedance (or return period). This paper describes how hazard curves can be calculated to estimate liquefaction-induced settlement under shallow foundations overlying liquefiable soils. These hazard curves were developed using the PEER performance-based earthquake engineering (PBEE) framework (Deierlein et al., 2003). The PBEE framework incorporates the probability of seismically induced ground motions developed from a probabilistic seismic hazard analysis (PSHA), the probability of liquefaction conditioned to the occurrence of a ground motion, and the probability of settlement conditioned to the occurrence of liquefaction. The results of the analyses are discussed, and recommendations are made for incorporation in evaluation of geohazard and designs considering their occurrence.