Reliability of (wind turbine) foundations using nonlinear FEA

Transcription

1 Reliability of (wind turbine) foundations using Lars Andersen & John Dalsgaard Sørensen Department of Civil Engineering, Aalborg University, Aalborg Denmark Outline of the presentation Initiating gproblem Overall strategy Work package 1: Probabilistic subsoil model Work package 2: Computer modelling and new design basis Related research projects 1

2 Initiating problem Foundations for offshore wind turbines are invisible but the cost is typically 25 35% of the total budget Soil properties are hard to determine accurately In situ test t are expensive offshore since special equipment is necessary Soil properties must be determined early in the construction phase Test programmes are hard to design since the subsoil conditions are not known in advance Present methods typically provide a determination of the wrong parameters Hence: What is the optimal test programme? What is the optimal foundation? 2

3 Initiating problem Status for the design of foundations for offshore wind farms Material properties and stratification of the soil are determined by Cone-Penetration Tests (CPT), possibly augmented by geotechnical borings and laboratory tests. Based on measurements, characteristic ti values of the material properties are determined (5% quantile values or cautious estimates ). Afterwards, design properties are determined based on partial safety factors. The soil is assumed to consist of a number of well defined layers. Within each layer the soil is regarded as a homogeneous material with the design values of the strength and stiffness properties. A deterministic design is carried out. What are the design criteria for the size and shape of the foundations? Deformations (settlements) during operation are critical for large modern wind turbines. Damping in the soil can be of high importance regarding cyclic loading and fatigue Ultimate limit state must still be analysed. 3

4 Initiating problem In heterogeneous deposits, failure occurs through the weaker parts of the soil. This leads to failure mechanisms that may be significantly different from the mechanisms occurring in homogeneous soil deposits. Just to provide an example, a vertical load applied to a foundation on homogeneous soil undergoes pure vertical translation with no rotation. By contrast, a foundation on heterogeneous soil may rotate, which may be critical in the design of modern wind turbines. 4

5 Overall strategy Computational models are established for the analysis of settlements and failure in heterogeneous soil The serviceability limit state (SLS) is analysed by a viscoelastic model The ultimate t limit it state t (ULS) is analysed by plastic limit-state it tt analysis The simple models are compared with a fully nonlinear analysis The statistical basis for the development of probabilistic models is established Primarily based on data from existing wind farms Additional data from test field at Frederikshavn (NearshoreLAB) Statistical distribution of final results are found by simulation (MCM) Settlements of wind turbine foundations and deformations during operation Bearing capacity of wind turbine foundations Revised strategy for the design of wind turbine foundations 5

6 Overall strategy Ongoing coupling between tests and computations updated models Prior statistical model based on seismic tests and knowledge from other projects Update of model based on CPTs and geotechnical borings + Bayesian statistics 6 Reli iability of (wind turbine) foundations using nonlinear FEA Lars Andersen Department of Civil Engineering Aalborg University

7 Work package 1: Probabilistic model of the soil The position (depth) of the layer interfaces are regarded as stochastic variables The inclination of the layer interfaces are regarded as stochastic variables The strength and stiffness of each layer are modelled as a stochastic fields 7

8 Work package 1: Probabilistic model of the soil Preparation of data base Existing wind farms: Seismic tests, CPT, geotechnical borings New offshore field at Frederikshavn NearshoreLAB: Seismic tests have already been performed Traditional CPT and geotechnical borings combined with laboratory tests CPT with cross-hole measurements of wave propagation velocities 8

9 Work package 1: Probabilistic model of the soil Known correlations: The material properties in each individual layer are strongly correlated in the horizontal direction but only weakly correlated over depth (secondary stratification). The strength th and stiffness of a material are correlated, but the dependency d is not the same for different materials, e.g. sand and clay. The position of subsequent layers is correlated, i.e. if the top layer lies at a greater depth at one place compared to another, this will also be the case for the next layer. Similarly, the inclination of the interface between layers 1 and 2 must be correlated with the inclination of the interface between layers 2 and 3, etc. Goal 1: To establish a probabilistic model and method that enables a reliable foundation design for offshore wind farms, which requires fewer tests than present methods 9

10 Work package 2: Computer modelling and revised design method Method of analysis Monte Carlo Method + Finite Elements Interface positions and inclinations modelled as stochastic variables Strength th and stiffness parameters modelled d as stochastic ti fields A full three-dimensional model is necessary The soil properties vary in three dimensions Settlements (rocking rotation of the turbine) are typically critical for the design Which properties have a significant impact on the final result? Model uncertainty should be reduced in the most efficient way The computational model should only include what is necessary Is a fully nonlinear model necessary? High computation cost compared to elastic or plastic models Hence, the necessity of a fully nonlinear model is examined Goal 2: A computer based model is developed that can by employed for fast evaluation of deformations (and failure?) of wind turbine foundations in heterogeneous soil 10

11 Related research projects Dynamic stiffness of wind turbine footings: Calibration of lumped-parameter models 11 Reli iability of (wind turbine) foundations using nonlinear FEA Lars Andersen Department of Civil Engineering Aalborg University

12 Related research projects Dynamic stiffness of wind turbine footings: Surface footings on stratified soil 12 Reli iability of (wind turbine) foundations using nonlinear FEA Lars Andersen Department of Civil Engineering Aalborg University

13 Related research projects Dynamic stiffness of wind turbine footings: Bucket foundations in homogeneous soil 13 Reli iability of (wind turbine) foundations using nonlinear FEA Lars Andersen Department of Civil Engineering Aalborg University

14 Related research projects Large monopiles for offshore wind turbines updated design methods (p-y curves) Universal foundation concept which kind of foundation is optimal The composite shell bucket Steel-concrete t composite Flexible design Cost efficient? 14 Reli iability of (wind turbine) foundations using nonlinear FEA Lars Andersen Department of Civil Engineering Aalborg University

15 Thank you for your attention Lars Andersen: 15 Do LPMs Pr ovide the Correct Geometric cal Damp ing? Lars Andersenn Aalborg University Aalborg Denmark