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A PERSPECTIVE ON THE DEVELOPMENT OF A BASIS OF DESIGN FOR EVALUATION OF SEISMICALLY-INDUCED OFFSHORE GEOHAZARDS (2016)
Although numerous studies have been conducted, a recommended practice for incorporation of earthquake loading into the geotechnical design considerations for subsea structures such as wells, manifolds, and PLETs, and pipelines is not consistent. Industry guidance provided in ISO 19901-2 and the more recent API adoption, RP2EQ, provide performance-based recommendations for selection of a two-level seismic design. At the first level the structure is designed to retain its full capacity (and likely its operability) at ground motions with a return period typically under 200 years that depends on the exposure level of the structure. At the second level ground motions that are less likely to occur over the lifetime of the structure are used to ensure global failure is avoided. The guidance, however, was written specifically for fixed steel structures and fixed concrete structures for which exposure incorporates first and foremost life safety, with secondary consideration for environmental exposures caused by system failures, and economic losses to the owners and operator. Subsea field architectures that only include subsea gathering and distribution systems and structures present no exposure to human life, therefore the decision to incorporate earthquake loads should be done with consideration given to mitigation and prevention of environmental releases and tolerable levels of damage. The latter include consideration of: (i) costs associated with production down time and schedule; and (ii) costs for replacement and rehabilitation of damaged facilities. In this paper we provide: (1) a discussion on existing industry guidance for reliability-based design; (2) reliability-based process for incorporating triggering of seismically-induced soil liquefaction and its consequences into foundation design, and design criteria in a moderately seismic environment with a seabed architecture that only includes manifolds, PLETs, pipelines, umbilicals, and associated structures; and (3) a reliability based process for evaluation of seismically-induced stability of slopes and its incorporation in the evaluation of pipeline routes.
This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 2–5 May 2016