A Perspective on the Development of a Basis of Design for Evaluation of Seismically-Induced Offshore Geohazards

摘要

Although numerous studies have been conducted, a recommended practice for incorporation of earthquakernloading into the geotechnical design considerations for subsea structures such as wells, manifolds, andrnPLETs, and pipelines is not consistent. Industry guidance provided in ISO 19901-2 and the more recentrnAPI adoption, RP2EQ, provide performance-based recommendations for selection of a two-level seismicrndesign. At the first level the structure is designed to retain its full capacity (and likely its operability) atrnground motions with a return period typically under 200 years that depends on the exposure level of thernstructure. At the second level ground motions that are less likely to occur over the lifetime of the structurernare used to ensure global failure is avoided. The guidance, however, was written specifically for fixed steelrnstructures and fixed concrete structures for which exposure incorporates first and foremost life safety, withrnsecondary consideration for environmental exposures caused by system failures, and economic losses tornthe owners and operator. Subsea field architectures that only include subsea gathering and distributionrnsystems and structures present no exposure to human life, therefore the decision to incorporate earthquakernloads should be done with consideration given to mitigation and prevention of environmental releases andrntolerable levels of damage. The latter include consideration of: (ⅰ) costs associated with production downrntime and schedule; and (ⅱ) costs for replacement and rehabilitation of damaged facilities.rnIn this paper we provide: (1) a discussion on existing industry guidance for reliability-based design; (2)rnreliability-based process for incorporating triggering of seismically-induced soil liquefaction and itsrnconsequences into foundation design, and design criteria in a moderately seismic environment with arnseabed architecture that only includes manifolds, PLETs, pipelines, umbilicals, and associated structures;rnand (3) a reliability-based process for evaluation of seismically-induced stability of slopes and itsrnincorporation in the evaluation of pipeline routes.