Finite Element Analysis of Large Diameter Grouted Connections

When considering offshore monopile foundations designed for wind turbine support structures, a grouted connection between the monopile and an overlapping transition piece has become the de facto standard. These connections rely on axial loads being carried primarily by the bond between the steel and grout as shear. Given the critical nature of the grouted connection in a system with zero redundancy, the current design verification requirement is that a finite element analysis is performed to ascertain the viability of the connection with respect to combined axial and bending capacity whilst pure axial capacity is handled as a decoupled phenomenon using simple analytical formulas. The present paper addresses the practical modeling aspects of such a finite element model, covering subjects such as constitutive formulations for the grout, mesh density, and steel/grout interaction. The aim of the paper is to discuss different modeling approaches and, to the extent possible, provide basic guidelines for the minimum requirements valid for this type of analysis. This discussion is based on the accumulated experience gained though the independent verification of more than 10 currently operational offshore wind farms that have been certified by DNV, as well as the significant joint research and development with industry captured in the DNV Offshore Standard for Design of Offshore Wind Turbine Structures DNV-OS-J101. Moreover, general observations relating to the basic subjects such as overall geometric extent of the model, inclusion of secondary structures, detail simplification, boundary conditions, load application etc. are presented based on the authors more than 3 year involvement on the subject at DNV.