Seismic assessments for offshore shallow foundations in carbonate sediments

This paper presents a methodology for the cost-effective seismic geotechnical design of shallow foundations for typical subsea structures in carbonate sediments. This methodology is focused on the particular design issues experienced in these soils and so the methodology includes the calculation of: (i) seismic shear stress and excess pore pressures induced in the soil column during a seismic event; (ii) post-seismic foundation settlement, which can be a governing design criterion in carbonate sediments due to their compressibility; (iii) bearing capacity during the seismic event allowing for appropriate strength degradation under the cyclic seismic load. Finally, the paper discusses conditions where the underlying 1-D shaking assumption of the simplified approach is not appropriate, with an example of a high aspect ratio subsea structure where 2-D and 3-D dynamic analyses were performed.