Soiltwin

Introduction

• Soiltwin: Data driven design optimization and smart monitoring of monopile foundations using updated soil-structure interaction models

• 2 Partners

• Coordinator: Vrije Universiteit Brussel 

• Start Date 01/01/2020

• Duration: 4 Years

• Total Budget: € 1.203.947

• Funded by The Blue Cluster 

• Soiltwin Consortium Partners: VUB, UGent   

• Industrial advisory board: Parkwind, Otary, Jan De Nul, DEME, 24SEA, COM&SENS, Cathie Associates

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Project Context

Today we see an industry-wide mismatch between design expectations and the as built dynamics of offshore wind turbines installed on monopile foundations. This mismatch results in a sub-optimal (fatigue) design and ultimately a higher cost for offshore energy. It is the general consensus of both academia and industry that this is due to errors in the interaction between the monopile and the surrounding soil.  Current soil-structure interaction models are not “tuned” to correctly assess the soil stiffness at small displacements for short and large diameter piles, i.e. monopiles. 

This project therefore aims to calibrate those models by updating them based on Finite element analysis and lab-experiments at UGent and the Coastal and Ocean Basin (COB) and measurements performed on all Belgian offshore wind turbines.  Moreover, the monitoring data will provide insight in the evolution of the soil behaviour during the lifetime of the structure. A unique dataset of monitoring data is already available to the project which includes both geotechnical and structural measurements. This information contained in this dataset needs to be converted to insights in geotechnical and structural behaviour, otherwise the potential of this dataset will not be unlocked.

Project Description

As a first step, the soil behaviour next to monopile foundations will be better characterised through laboratory tests. Soil behaviour under cyclic loading can lead to both softening and stiffening of the response depending on the loading intensity and number of cycles. The aim of the laboratory testing is to better characterise the thresholds at which cyclic degradation will start to occur and which loading regimes could lead to stiffening of the soil response. All in-situ monitoring data indicate stiffening of the overall foundation response over time (increased eigenfrequencies). Based on the laboratory testing, a constitutive model will be calibrated which can explain the soil behaviour for selected episodes of cyclic loading.

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The second goal of the project is to calibrate the constitutive model and use it in 3D finite element analyses such that the measured bending moment profiles can be matched. The in-situ measured bending moments at Nobelwind along the pile provide a detailed insight in the soil behaviour next to the pile during high and low intensity loading.  The calibrated soil parameters will provide insight in the behaviour of the soil next to the pile and the evolution of soil properties under cyclic loading.

In addition to the in-situ data at Nobelwind, coupled hydrodynamic-geotechnical analyses in the controlled environment of the hydraulic laboratory will also be carried out to further improve the calibration of the models. These analyses support a secondary project goal, which is to improve the readiness of the project partners and the Coastal and Offshore Basin (COB) to perform coupled hydrodynamic-geotechnical tests with appropriate scaling of geotechnical and hydrodynamic properties.

While a sophisticated constitutive model provides detailed insight in soil behaviour, 3D finite element analyses are not well-suited for routine calculations because of their computational expense. Therefore, the project proposes to develop a 1D beam-column model which captures the key trends in the soil response but allows faster calculation of the bending moments and pile deflections. The 1D model will also include base resistance and distributed moment-rotation resistance along the pile which have been shown to be important in recent research.

Continuing from the calibrated 1D beam-column model of the monopile-soil interaction, an integrated ‘digital twin’ model will be created of the entire offshore wind turbine. This model will enable the calculation of the eigenfrequencies of the structure and is a necessary tool for comparison between the measured eigenfrequencies and the modelled values. Any remaining discrepancy between design and measurement for a particular soil type, should be mitigated by further improving the modelling of said soil’s response model.   An extensive measurement campaign of the resonance frequencies, using mobile measurement units, in the Belgian offshore zone is planned to validate the results over an as large as possible set of different offshore (monopile) structures.

Finally, a novel fleet-wide model updating strategy will be developed. Each location within a farm will be paired with an updatable digital twin model. Novel to the updating process is to consider fleet wide similarities, e.g. in turbine masses, within the updating. Ultimately, each Digital Twins will be continuously updated to best represent the local conditions, including soil condition, whenever a new measurement of the resonance frequency becomes available. Through the digital twin operators can monitor elemental concerns such as scour and soil degradation and improve their residual fatigue assessments for optimized service life. 

Results

The Soiltwin project focused on improving the modelling of the interaction between the soil and the structure of monopile foundations for offshore wind turbines. The main results of the project include:

• Refinement of soil material models: The project developed 3D numerical models for the interaction between monopiles and the surrounding soil. These models were optimized using extensive cyclic and quasi-static tests on North Sea soils. The use of improved soil material models, such as the PISA models, led to a significant improvement in predicting the natural frequencies of turbines.

• Development of digital twins: The project created digital twins of offshore wind turbine foundations, enabling the simulation of monopile responses under different loading conditions. This was linked to a cloud-based metadata database system for managing geotechnical and structural data.

• Improved analysis methods: The project results improved the accuracy of predictions of the dynamic response of wind turbines, particularly with respect to natural frequencies and the impact of phenomena such as scouring and protective layers.

• Applications for industrial valorisation: The developed models have strong potential for use in the design and monitoring phases of offshore wind turbines. Specific valorisation cases include the use of these models to support the front-end engineering design (FEED) phase and lifetime assessments of monopile-supported turbines. The project also laid the groundwork for further commercial applications, such as a licensing model allowing geotechnical consultants to use the developed software.