Introduction
- WindSOIL - Design optimization of offshore foundations using improved soil-structure interaction models based on in-situ measurements and medium-scale experiments
- 2 partners
- Coordinator: VUB
- Project type: Fundamental research
- Start date: 1/3/2020
- Duration: 4 years
- Total Budget: €1.975.279
- Funded by the Energy Transition Fund (ETF)
Partners
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Project Context
The offshore wind industry is rapidly pushing forward new designs in a continuous effort to reduce the cost of offshore energy. This cost-reduction is primarily achieved by increasing the average wind turbine capacity. In the current market a 7+ MW offshore wind turbine is soon to become the standard and 10MW to 15MW wind turbines are expected in the very-near future. This increase in turbine size has a big influence on the size of the support structure. In a few years’ time the size of monopiles, with 81.7 % the most dominant foundation design, has doubled in size.
Combined with cyclic wind loads, the controlled thrust loads and periodic wave loading means Offshore wind energy has rapidly ventured outside the scope of geotechnical design codes for offshore Oil and Gas (O&G) that primarily focused on ultimate soil capacity rather than small strain behaviour or behaviour after strong cyclic loading, i.e. soil degradation.
Several authors have discussed the inadequacy of soil-structure interaction models adopted from O&G guidelines for large diameter piles employed in the offshore wind industry. A clear example of this is the large study on 400 offshore wind turbines presented by Kallehave, showing that a 10% underestimation of the first resonance frequency was common. This observation was also confirmed by measurements performed by OWI-lab (VUB) within the various Belgium offshore wind farms. And most recent measurements confirm that there still exists a mismatch with as expected structural dynamics and the as-built values for wind turbines in the Belgian Offshore zone.
This inability to accurately predict real world dynamics from soil-investigations lead to an erroneous estimation of consumed fatigue life and, consequently, a sub-optimal design of the monopile foundation . Both industry and academia agree that if ever an optimal structural design is targeted, a fundamental understanding of the geotechnics for offshore wind is needed.
An opportunity exists as the current zone for offshore wind energy is representative for both the future zone for offshore wind energy in Belgium as well as nearby sites in the UK, Netherlands and France. Focusing on the expected site for the post-2020 offshore wind zone a maximum water depth of 42m (LAT) is assumed and including parts of several sand banks ( e.g. Fairy bank, West Hinder and North Hinder) with significant smaller water depths. Soil conditions are similar to those in the current offshore wind zone given the general morphology of the seabed and the small distance. Given the current market position for the monopile foundation, it is highly likely that the future concessions in Belgium again will opt for the monopile foundation. Examples abroad such as the Veja Mate Offshore wind farm share similar water depths, 39 to 41m, already demonstrated the applicability of monopile foundations. By 2020 the current concessions will be completed with approximately 350 monopiles of different sizes installed, with Northwester 2 expected to use the currently biggest 9.5MW wind turbine.
Figure 1: Current (Blue) and expected (Orange) sites for offshore wind energy in Belgium. Some sand banks are highlighted in yellow
The novelty of this project is to profit from using dynamic data from these existing wind turbines to retrieve key geotechnical insights, relevant for both current and future wind developments in the Belgian offshore zone. To do so novel monitoring strategies need to developed applicable within the specific boundaries of the offshore wind industry able to translate dynamic measurements into geotechnical relevant results.
Project Description
The project aims to initiate pre-normative development of methodologies that may later be included in national and international codes or professional standards. The research proposed here will generate new driven pile design guidance for XL monopiles through a comprehensive programme of high quality field data, involving multiple loading scenarios, on specially instrumented driven monopiles. This will form a benchmark set of results that will be complemented by comprehensive advanced drilling, sampling, in-situ testing and laboratory experiments, supported by rigorous analysis and close analysis of other case history data. The key aim is to develop design procedures that overcome the current shortfalls in knowledge regarding pile driving, ageing, static and cyclic response under axial and lateral loading. The main deliverable will be new guidelines for practical design that will be suitable for offshore applications, supported by measurements in-situ.
Project Objective
The project aims to bring down the Levelized Cost Of Electricity (LCoE) of offshore wind by improving the design methodologies, optimizing O&M and extending the lifetime of offshore monopile foundations through the development of new Soil-Structure Interaction (SSI) models founded by experiments and the outcomes of novel monitoring strategies.
The projects targets the development of following innovative models and techniques:
• A combined hydrodynamic and soil constitutive model (taking into account the nonlinear and cyclic behaviour of the soil) is developed able to investigate soil ageing, soil stiffness/strength increase over time in granular soils and soft rocks after soil compaction or pile installation, and soil degradation under cyclic loading
• From the model and sensitivity analysis a reduced model is defined that can serve early stage design and can be updated through measurements
• An (cost-)optimized site-investigation strategy is defined
• Defined methodology for improved geotechnical design, which among other is able to predict the first order resonance frequency with an error less than 2%
• Controlled experiments in a representative environment confirm the phenomena considered
• A Fleet wide model updating strategy is developed that translates the results from insitu measurements to relevant parameters for geo-technical research and design
• An improved modal parameter estimation algorithm is developed better able to determine and track higher order dynamics, a vital input for reliable model updating
• A model in the loop monitoring strategy is developed that allows to detect and quantify any variations in the soil conditions and support O&M decisions
Figure 2: Example of 3D monopile model and simulation results (normalized bending moments) versus measurements.
Figure 3 Layout of the monitoring set-up to gain insight of the real OWT behaviour.
More info and related articles can be found on Researchgate
Subsidy
The WindSOIL project is financially supported by FPS Economy, as a fundamental research project within the Energy Transition Fund (project call August 2018- duration of project 01.03.2020 – 30.11.2024)
