New geotechnical design methodologies are now being utilised to develop site-specific monotonic and cyclic soil reaction curves for the structural design analysis of large diameter monopile foundations. Although there has been a number of recent publications outlining new geotechnical design approaches for monopiles (e.g. Peralta et al., 2017; Byrne et al., 2017), there are few examples in the literature of real case studies outlining the practical application of such approaches in design.
This case study presents detailed structural design using a combination of time domain and spectral dynamic analysis techniques considering recommendations from the Pile Soil Analysis (PISA) joint industry project (Byrne et al., 2017) to develop site-specific soil reaction curves. The application of such new geotechnical methodologies presents a number of challenges when applied in design projects with respect to the site characterisation for calibration of the numerical model, suitable rigorous approaches to consider cyclic loading and a suitable workflow for application of site-specific reaction curves (e.g. PISA type approach) within detailed structural analysis. Therefore, this work presents a detailed description of the geotechnical and structural design analysis performed for the Seastar® and Mermaid offshore substation platform, Belgium Sector North Sea. Three-dimensional (3D) finite element analysis is performed using a multi-surface plasticity constitutive model to develop site-specific soil reaction curves, according to the PISA approach, for use in detailed structural analysis. A developed dynamic structural analysis tool capable of capturing ringing effects, due to high order wave loading, and modelling diffraction effects including directionality for appurtenances as well as p-δ effects and linear buckling is used for the structural design analysis and is described in the paper. Using such bespoke models is shown to reduce costs and schedules when compared with more standard codified approaches, significantly reducing the foundation-related project risks.
Presented at the 7th International Conference Offshore Foundations, Bremen, Germany, 9-11 July 2019