Abstract

Earthquakes, and their effects on offshore facilities, are an important consideration in the Mediterranean Sea. This paper provides a high level overview of the problem; our goal is to give managers a basic understanding of the issues and general guidance on when and how to address seismic hazard in their projects. As risk strongly depends on the level of expected seismicity, we quickly review earthquake hazard in the Mediterranean region and offer guidance as to when site specific studies are needed. Fortunately, from a project point of view, earthquake design methodologies are well specified in the existing codes. However, experience is required to assess the possible benefits of more detailed analyses (e.g. time histories analyses, non-linear site response) compared to simplified approaches typically used in practice. We review some of the more common codes, and give guidance on application to various situations. The importance of a clear, well thought out, seismic design basis cannot be overemphasized; again we offer lessons learned to help keep projects on track. Finally, we share some of our corporate experience in offshore earthquake design, highlighting applications for offshore platforms, subsea structures and pipelines.

Introduction

Offshore developments in the Mediterranean Sea require the consideration of earthquake effects on facilities. An overview of the earthquake hazard in the Mediterranean Sea is provided to highlight the highest risk areas from a seismic point of view. A review of main seismic codes for offshore structures, typical design approaches and requirements for the seismic design basis are then discussed. Finally, some key aspects and case histories from our experience are reported to draw attention to some specific issues related to seismic design of offshore facilities.

EARTHQUAKE HAZARD IN THE MEDITERRANEAN SEA

Seismicity in the Mediterranean Sea is due to the interaction between the Eurasian, African and Arabian plates (Herman et al., 2015; Vannucci et al., 2004). The region is undergoing rather rapid deformation. Present day Africa-Eurasia motion ranges from about 4 mm/yr in a northwest-southeast direction in the western Mediterranean to about 10 mm/yr (north-south) in the eastern Mediterranean (Herman et al., 2015). The Africa-Eurasia plate boundary is complex, and includes extensional and translational zones in addition to the dominant convergent regimes characterized by subduction and continental collision (Herman et al., 2015). Subduction occurs adjacent to back-arc extensional basins in the Calabrian-Tyrrhenian and Hellenic-Aegean systems. Transcurrent motion along the Northern and Eastern Anatolian faults mark the escape of the Anatolian block towards the west (Vannucci et al., 2004). These areas present the highest earthquake hazard, but significant current seismicity and large historical earthquakes have occurred throughout the Mediterranean region (Herman et al., 2015).

Figure 1 shows epicenters of all events with magnitude greater than 4.0 and focal depth shallower than 50 km which have occurred since 1904 and for which instrumental records are reported in the ISC catalog (Vannucci et al., 2004).

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