On the 15th of May 2020, Ritesh Gupta will defend his PhD entitled "Behaviour of monopile under combined cyclic load"
This PhD was supervised by Christophe Dano (3SR).
On account of the COVID-19 epidemic, Ritesh will present his PhD defense virtually, via a zoom meeting.
Monopile is the most common foundation system for offshore wind turbine structures, statistically about 80%, as per Wind Europe, 2018 report. A pile can be defined as exible or rigid
depending on the embedded length to diameter ratio (Le=D) and the relative stiffness of pile and soil. The existing codes for pile design are mainly developed for flexible piles,
whereas monopile for new offshore wind turbines typically falls in the rigid category. The dominant complex cyclic wind and waves loads on offshore wind turbine structure and consequently
on the monopile foundation, act in the lateral direction. The API design procedure, representing the lateral and vertical soil response through uncoupled non-linear springs, is developed
for flexible piles and is recognised as conservative for rigid piles. The behaviour shall represent a coupled vertical and lateral soil-structure interaction because a rigid pile
presents rotation deformation mode instead of detection. The deformation mode further demands a different formulation mechanism, including the distributed moments along the pile shaft and
shear & moment behaviours at the pile base as an essential part of the soil response investigations.
This work presents the numerical models aimed to address the limited understanding of coupling consideration of monopile installed in sand. The cone penetration tests performed in the calibration chamber, data treatment with ICP method and available Fontainebleau sand NE34 properties database in the literature provide the constituent parameters for model definition.
First, a PLAXIS 3D finite element model presents the model pile in the calibration chamber conguration with representative boundary conditions and the constitutive behaviour of the sand.
The model pile geometry and load magnitudes are the outcomes of a similitude relationship with a representative prototype. A constant mass placed on the pile head represents the vertical
load (the dead weight of the wind turbine structure). A simplified lateral point load represents the complex environmental loads, acting at a distance above foundation level, represents
the lateral and moment load at mudline. Thus, vertical (V), lateral (H) and moment (M) collectively represent the combined load, investigated in both monotonic and cyclic loading cases.
Different combined loading case in the limits of horizontal and vertical load capacities represent the overall behaviour of the model pile. The observation of normal and shear stress
changes close to the pile-soil interface at different depths quantify the pile-soil interaction. The response investigation at some strategic stress points in the FE model soil volume
provides a basis for soil-stress transducers (SSTs) layout plan in the experimental soil volume. A methodology to formulate the lateral and shear stresses evolution close to the pile
surface as representative of the coupled interaction is presented.
Second, a local-macro element (LME) model, an assembly of non-linear springs formulated using a Matlab toolbox ATL4S, presents the soil-pile interaction with inherent coupling considerations at different embedment depths. The PLAXIS model outcomes define the basis for a corresponding model scale LME model. The obtained results from both numerical investigations demonstrate the significance of vertical-lateral coupled interaction as a set of hypothesised equations representing the coupled interaction for rigid monopile foundation. A similitude work provides a relation between a prototype scale and the lab-scale monopile model. It further aids in comparison and validation with the available experimental and numerical results in the literature.