Experimental Research for Offshore Wind Turbines Andreas Rogge 1
Pile Foundations Types of pile foundations 2
Pile Foundations Monopile vs. Multi-pile Cyclic moment loads from wind, waves, blade rotation etc. Cyclic loads on pile foundation: mainly LATERAL mainly AXIAL 3
Pile Foundations Examples Source: http://www.alpha-ventus.de 4
Pile Foundations Particularities of the Offshore Pile Foundations Huge experience from oil and gas but... Water depths: 20 35 m, max. 50 m Irregular cyclic loading: up to 10 9 cycles High load ratio H/V Large diameters: D 3 to 5 m; up to 8 m! Natural frequency of founding structure close to turbine s operative frequencies (resonance danger) Cyclic fatigue & deformation play a relevant role in design Serial production: pressure on cost effectiveness 5
Pile Foundations Pile s interaction with the Pore Water Pressure Hydromechanical coupling Possibility of PWP accumulation Special relevance of soil s constitution: Highly nonlinear behaviour! 6
Pile Foundations Pile s interaction with the Pore Water Pressure Potential consequences of PWP build-up: Bearing capacity: Deformations: Dynamic behaviour: Fatigue: Reduced effective stresses in the soil Lower pile capacity Softer soil behaviour Larger soil plastic strains Larger permanent pile displacements Stiffness reduction, change of eigenfrequencies Dynamic resonance? Varying dynamic behaviour Change in fatigue loads of structural components 7
Pile Foundations Research strategies Field observations and tests Offshore prototypes Field tests onshore Experimental testing in the lab Physical model tests in reduced scales Elementary soil tests (triaxial, direct shear,...) Computational models Coupled FE models: Water-Soil-Structure interaction Simplified models for design (rheological,...) 8
Pile Foundations Structural Health Monitoring Testfeld Alpha Ventus Aims: BARD Offshore 1 - Development of strategies for foundation monitoring and assessment - Optimisation of the measuring equipment for offshore applications - Development of standards for monitoring methods 9
Pile Foundations Validation of methods for experimental proof of pile bearing capacity Loading device at BAM test site: - 10 rammed piles (steel tubes) - length18 m, diameter 71 cm - homogeneous sand - tensile/compressive load 3,6 / -1,8 MN 10 Piles, L = 20 m, D = 0.7 m 10
Pile Foundations Validation of methods for experimental proof of pile bearing capacity Load implementation 11
Pile Foundations Validation of methods for experimental proof of pile bearing capacity right: left: cyclic loading pile bearing capacity using pile driving analyser (PDA) 12
Pile Foundations Lab testing facility (Monopiles, M 1:30) 13
Pile Foundations Numerical Model Seabed 2-Phase- Idealisation Coupled material model SP: solid phase LP: liquid phase GP: gaseous phase material model acc. to Pastor-Zienkewicz for fluid-saturated, porous media no elastic region; plastic deformations on all load levels reduction of pore volume under cyclic loading with increasing pore water pressure free values: deformation vector and pore water pressure calibration in triaxial tests 14
Pile Foundations Densification and convection processes Lab findings: - Development of settlement mould around the pile - Consolidation of deformation increase after 10.000 to 100.000 loading cycles (without reaching a final value) - Additional deformation increase due to convective grain movements 15
Pile Foundations Densification and convection processes Modelling: 16
Pile Foundations Calculated pore water pressure under different soil permeabilities Pore water pressure and deformations validated for lab tests Transfer of numerical procedures to real applications 17
Pile Foundations Effects of realistic loading Moderate storm cyclic loading (1-year Return Period) Excess pore pressure accumulates progressively but eventually stabilises Global stiffness decreases in the same fashion Significant impact of PWP-coupling, so far neglected in the praxis Stiffness reduction ~ 17 % 18
Pile Foundations Current practice in the offshore industry Uncoupled (dry) analysis of the foundation simple and straightforward, but neglects possibility of soil softening due to PWP buildup Overprediction of foundation s stiffness Underprediction of residual displacements 19
Pile Foundations Something to keep in mind Importance of constitutive model for the soil Key role of progressive DENSIFICATION of the soil under CYCLIC loading Classical elastoplasticity cannot reproduce it! Advanced models required (P-Z, hypoplasticity, etc ) 20
Fatigue of Concrete under Compression Heavy weight foundations Shift from pile foundations to heavy weight foundations for economical and environmental reasons Construction on-shore leading to shorter installation times Open technical questions especially concerning fatigue strength and life cycle of concrete under compression Bilfinger 21
Fatigue of Concrete under Compression Technical questions Small gravity loads, high horizontal loads Complicate load history (wind, waves, blade rotation, shutdown) Non-uniform loading cycles N > 10 9 Compressive fatigue due to high prestressing level Economical questions High investment costs (structure, installation offshore) High maintenance costs Assessment of remaining life-time and life cycle costs Real-time updating by structural health monitoring FIRST STEP: Identification and characterisation of damage process and relevant damage parameters 22
Fatigue of Concrete under Compression Aim: Insight into compressive fatigue behavior of concrete Monitoring of damage process by continuous NDT methods Characterisation of damaging process (crack development) Continuously updated life-time prediction Evaluation of monitoring results in view of actual damage state Identification of suitable damage parameters (deformation, elasticity modulus, ultrasonic velocity, acoustic emission ) Correlation of damage parameters with test observations (number of crack events, cumulated crack lengths per unit, ) Modelling of damage process on a meso-mechanical level Accordance of calculated damage pattern with experimental findings 23
Fatigue of Concrete under Compression Deformational behaviour under cyclic loading 10 20% 80 90% initial phase steady phase final phase 24
Fatigue of Concrete under Compression Deformational behaviour under cyclic loading Experiments Continuous crack growth during cyclic loading S-shaped degradation behavior Curvature typical for all deterioration parameters Increasing scatter of results at final phase due to localisation effects 25
Fatigue of Concrete under Compression Experimental Setup 26
Fatigue of Concrete under Compression Experimental Setup 80 cyclic tests on cylinders h/d = 30cm/10cm Loading frequency 5 Hz Concrete strength f cm = 57 MN/m 2 Sensor arrangement 27
Fatigue of Concrete under Compression Loading regime Elastic Modulus (different levels) Entire hysteretic loading loop Ultrasonic velocity Interval i Additional Interval i+1 Measurements 28
Fatigue of Concrete under Compression Cyclic load levels S max S min S m DS Interval Cycles Low-cycle 0.825 0.425 0.625 0.4 250 10 4-10 5 High-cycle 0.75 0.35 0.55 0.4 10,000 10 6-10 7 29
Fatigue of Concrete under Compression Life-time prediction Standards: Failure criterion no damage indicator Linear damage accumulation Load-history effects not covered Huge scatter of results Reduction of scattering: Number of loading cycles exponentially dependent on uniaxial concrete compressive strength N f = f(s min, S max ) ; S i = s i / f c Deviation of 5% in f c results in factor 5 to 6 for N f Strength of actual specimen unknown (standard deviation uniaxial: 4 %) Other damage parameters needed to predict lifetime 30
Modulus of Elasticity Fatigue of Concrete under Compression Relation of maximum loading cycles to modulus of elasticity Max. load cycles [10 6] No correlation! Lower bound of factor 5 to 10 in scatter of bearable loading cycles 31
Fatigue of Concrete under Compression Alternative damage parameters: longitudinal strain Identical creep portion not subtracted Similar strain development No indication of bearable cycles 32
Fatigue of Concrete under Compression Alternative damage parameters: hysteretic stress-strain-behavior Change of curvature at higher cycles No significant development in hysteretic areas 33
Fatigue of Concrete under Compression Alternative damage parameters: ultrasonic velocity Significant and reproducable decrease during damage process Relatively small scatter of results (compared to other possible indicators) 34
Fatigue of Concrete under Compression Alternative damage parameters: acoustic emission events US velocity 35
Fatigue of Concrete under Compression Alternative damage parameters: acoustic emission events Good correlation between three damage phases and measured crack events Spatial distribution of events using 6 sensors Quantification of damage and prediction of life-time not possible 36
Fatigue of Concrete under Compression Suitability of NDT methods for life-cylce monitoring Acoustic emission analysis: Good transition indicator of the three damage phases (beginning of localisation) Spatial distribution of crack events using 6 sensors Not suitable for quantification of damage and continuous prediction of remaining life-time Longitudinal strain No reliable information about remaining life-time Absolute value and rate of increase strongly dependent on concrete mixture and environmental conditions Elastic modulus Good correlation with damage process Not practicable for application on-site 37
Fatigue of Concrete under Compression Suitability of NDT methods for life-cylce monitoring Ultrasonic amplitude Signal change very sensitive to damage process Measurement technically vulnerable and not robust (contact between sensor and specimen) Ultrasonic velocity Good correlation with damage process (comparable to elastic modulus) Significant relative change of signal compared to spread of results Practicable execution on-site 38
Fatigue of Concrete under Compression Suitability of NDT methods for life-cylce monitoring Optimum method: ultrasonic velocity But: Prediction of remaining life-time only in relation to initial value Continuous structural health monitoring required Next Steps: Quantification of signal decrease in relation to experimentally observed state of damage Experimental verification in bigger scale (columns 40 cm / 120 cm) Meso-scale modelling of damage process 39