The Prinos Development in the North Aegean Sea is the first offshore oil and gas resource to be developed in Greece. The field, which is west of the Island of Thasos, lies in an area of potential seismic risk. To account for this risk, the facilities were designed for two levels of earthquake activity. The lower level Design Earthquake (DE) is the earthquake which is reasonably postulated to occur during the intended life of the structures and for which structural design is carried out to assure continued operation of the facility during and subsequent to the earthquake. The Maximum Credible Earthquake (MCE) is the maximum earthquake reasonably postulated to occur at the site. For this earthquake, the partial destruction or complete loss of the facility is allowed, as long as public health or safety is not endangered. Probabilistic and deterministic procedures were followed to define the two design earthquakes.
The DE for the Prinos site was derived using a probabilistic approach where the peak horizontal ground acceleration was developed as a function of return period. The two seismotectonic provinces found to control the seismic risk for the Prinos Field were the Rhodope Massif, in which the site is located, and the North Aegean Compressional Zone, south of the Rhodope Massif. The recurrence rate of earthquake activity was defined for each of the provinces and the DE site acceleration was then derived for different return periods using a magnitude-distance-acceleration relationship. The MCE was evaluated using a deterministic approach. This method considers regional tectonics and geologic features capable of generating seismic events. The proximity of the site to such structures and the general characteristics of the tectonic provinces were input to a magnitude-distance relationship to determine the MCE.
The results of the analyses yielded a O.2g peak ground acceleration for the DE corresponding to a 100-year return period and a O.4g peak ground acceleration for the MCE. These results plus time histories of ground motion consistent with a site dependent design response spectrum comprised the seismic design criteria for the Prinos Facilities.
The Prinos site is located in the North Aegean Sea, between the Island of Thasos and the mainland. The offshore facilities associated with the development of the Prinos field include a series of drilling/ production platforms; a gas well platform; a remote spread mooring; and the associated submarine pipelines interconnecting the various structures (Figure 1).
For structures whose damage or destruction could endanger public health and safety, two earthquakes are normally postulated for seismic design. The lower level DE is the earthquake which is postulated as reasonably occurring during the intended life of the structure and for which the structural design is carried out to assure that failure will not occur. For this earthquake, structures are designed to remain within the elastic range of deformation. The higher level event, the MCE, is the maximum earthquake which can be reasonably postulated to occur at the site.