Wave loads and soil support for extra large monopiles

In many offshore locations, including the North Sea, offshore wind turbines are an important and growing part of the renewable energy mix. Larger turbines can help reduce the cost of energy production, but they introduce new technical challenges. Apart from construction, installation and maintenance issues, towers with larger diameters interact with waves and soil differently than smaller structures do. In some cases, the existing models and theoretical solutions need to be extended and validated. The WAS-XL project aims at better understanding the forces from severe waves on large-diameter piles and evaluating the impact of long-term variation of wind- and wave-induced loads on the behaviour of the soil and its stability. The project addresses these challenges by combining advanced computational tools for analysis of turbine subjected to wind and hydrodynamic loads. In order to verify mathematical models and computational analyses, model tests are performed. These tests involve both basin tests of monopiles subjected to hydrodynamic loads, as well as geotechnical small-scale tests of representative sections of soil subjected to stress from the monopile. The key output from the project is to provide knowledge about the response of extra-large monopiles, which may be applied to drive down costs in a design situation. As part of this, the project also applies the results from tests and analyses to develop new simplified methods which can be used in design practice. The project was completed in May 2021. Since November 2020, the project has completed the work with numerical wave tank and methodology for calculating wave loads, completed and processed results from hydrodynamic model tests, validated and reported methodology for analysis of structural-earth response, and completed and given recommendations for improved analysis methods for fatigue calculations in the design of large wind turbine foundations. The project has now been formally closed, but parts of the work on completing scientific articles will continue throughout the year. This work will be performed by NTNU, PhD student and post doc. Project partners are SINTEF Ocean, NTNU, NGI, NRC, Equinor, EDF, Innogy, Vattenfall and Multiconsult.

KPN WAS-XL har bidratt til å øke kunnskapen rundt analysemetodikk innen temaene: respons av sjøtilstander hydrodynamisk respons, globalrespons og geoteknisk respons for offshore wind turbiner. Videre har det blitt utviklet analyseverktøy og ny metodikk for modelltesting. Følgende virkninger er registrert: - Bidrag til sammenstillelse av FME North Wind - Applikasjon av beregningsmetodikk og modelltestmetodikk for Equinors Empire Wind prosjekt I tillegg til forbedrede analysemetoder etablert i WSA-XL, forventer vi også at genererte data fra modelltester vil anvendes i fremtidig forskning, e.g. som valideringsbasis.

Economies of scale are pushing for larger (6-12MW) offshore wind turbines (OWT), which require larger support structures. For shallow and intermediate water depths, large-diameter monopile foundations are considered most promising with respect to the levelized cost of energy. However, the eigenfrequencies decrease - getting closer to the primary wave frequencies - as structure size increases, making the structure more susceptible to nonlinear wave loading, for which more accurate and validated analysis methods are needed. Also, the wave load history on the monopile has an impact on the soil response, which current design practices do not address properly. The research proposal addresses uncertainties related to nonlinear wave loads and responses of large-diameter wind turbines with a focus on integrated engineering analysis including soil responses. The proposed work combines numerical and experimental methods. The project consists of the following WPs: WP1: Modelling of nonlinear wave kinematics and loads. WP2: Interaction between hydrodynamic loads and nonlinear soil response. WP3: Long-term structural response analysis methodologies. WP4: Hydrodynamic experiments validation. The combination of expertise in hydrodynamics and geotechnical engineering permits detailed study of non-linear phenomena and application of new models to long-term structural response modelling. The project aims to produce and disseminate results of academic and practical importance: -New model scale data for validation of nonlinear hydrodynamic wave kinematics and loads on monopiles in steep waves. -Advances in numerical and empirical engineering methods for including nonlinear wave loads in the design of offshore wind turbines. -Improved and verified design tools for integrated analyses. -Recommended practice on the use of monopile soil models accounting for irregularity in cyclic load histories and partial drainage in the soil. -Recommendations to design practice and standard.