Finite element analysis software developer Plaxis has a new tool for the design of laterally-loaded monopile foundations.
According to Plaxis, Mo De To takes the research led by Oxford University under the Pisa project and takes it into daily engineering practice to extend the conventional design approach to accurately model the response of large diameter monopile foundations for offshore wind turbines.
Despite 80% of existing offshore wind farm foundations being monopiles, the convention p-y method design approach is largely disputed and can lead to over conservative designs. The p-y method was developed for long and slender foundation piles and only takes into account horizontal soil reactions along the shaft.
Plaxis said that the main alternative, 3D FE analysis, was computationally too demanding for the thousands of load cases contemplated in offshore engineering, however, the Pisa research created a new design approach that is incorporated into Mo De To.
The Pisa method addresses these shortcomings by introducing a pile model that does not depend on slenderness assumptions and also considers shaft shear and base reactions. These are most relevant as the diameter grows larger with respect to the pile length, as it is the case for monopile foundations, leading to a significant optimisation in structural dimensioning.
Plaxis worked in collaboration with the Pisa academic team on the project to validate the design method using 3D finite element models.
Mo De To allows users to undertake the conventional rule-based approach but it also implements an automated workflow for a numerical-based methodology.
Through Mo De To’s numerical approach, site-specific soil reaction curves are calibrated using the actual soil characteristics from a small set of Plaxis 3D FE models that define the project design envelope. This allows specific, precise, soil reaction curves that accurately model the site soil conditions and geometry space of the project. It also provides a means of advancement towards a global database of project-specific soil curves. Further improvements in numerical methods or constitutive models will enrich this database and enhance existing soil reaction data sets.
Plaxis has said that Mo De To uses a robust and highly-efficient 1D finite element solver to model the behaviour of the embedded monopile under lateral loading. It makes use of Timoshenko beam theory to account for shear deformations, while the soil response is captured through a set of soil reaction curves (under the Pisa method: distributed load, distributed moment, base shear, and base moment), either user-defined (rule-based design) or calibrated from a set of Plaxis 3D models (numerical-based design).
The result is design iterations for multiple geometries and load cases that can be run in seconds rather than in hours. An excellent match is obtained between numerically-calibrated 1D and full 3D results.