Severe Storm Wave Loads on Offshore Wind Turbine Foundations (SEA-SWALLOWS)

Lead Research Organisation: University of Bath
Department Name: Architecture and Civil Engineering

Abstract

Offshore structures, including offshore wind turbine foundations, marine renewable energy device support structures, bridge piers, and floating vessels, are routinely exposed to harsh environmental loads. These frequently drive the design. The physics and statistics of wave-structure interaction are complex and still not fully understood for strongly non-linear loads as experienced in the most severe conditions.

The particular focus of this project is fixed offshore wind turbines. These are one of the most promising sources of clean energy; and central to the UK's ambitions to become carbon neutral. The price of offshore wind has fallen significantly over the past ten years. Part of this reduction has been due to improvements in technical understanding leading to less conservative designs. Recently, there has been a trend to move to more exposed and deeper water locations with 'better' wind resources. However, such locations are susceptible to more extreme wave heights and subsequently more severe loading. These changes have increased the importance of wave loading models able to give accurate predictions of base shear and moment time-series. It is important that such models predict not only the magnitude of the load but also the correct frequency content of the loading. For instance, a large slamming load may be of sufficiently short duration that the load is not simply transmitted to the foundation. Further, structures are typically designed so as to avoid the natural frequency of the storm waves. However, if loading was to occur at higher harmonics of the fundamental wave frequencies these may coincide with the structure's natural frequencies, thus greatly increasing their importance for design. For structural fatigue assessment very long time series are required. Therefore, experimental and high-fidelity numerical models are too resource-intensive to be directly useful for practical engineering calculations. A highly efficient yet still sufficiently accurate alternative is required.

The physics of wave loading is typically split into non-breaking and breaking loads. These have different magnitudes and timescales as they are dominated by different physical phenomena. For non-breaking waves, traditionally the Morison equation has been widely accepted as the starting point for calculating wave loading on offshore structures by most modern design standards. For slender cylinders in the inertia regime such as the monopiles used for offshore wind, extensions have been made to the Morison model, taking wave kinematics as inputs. Predicting wave kinematics is itself a difficult task, particularly for severe yet random sea-states where both standard regular wave stream function theory and 2nd order random wave theory are imperfect models.

Breaking waves are notoriously difficult to model numerically and to measure experimentally due to the violence of the hydrodynamics and scaling issues. Various models have been proposed to simulate the time history of the loading. However, when calculating extreme responses and foundation reactions for dynamically sensitive structures, it is generally sufficient to know the total applied impulse (and where it acts) for impact loads rather than the exact time-history. Estimating the impulse is far more robust, quicker and the physics can more easily be modelled.

We aim to revolutionize load calculations on offshore structures using novel fluid mechanics to develop fast reduced-order engineering models. While the focus of this work will be examining the impact of extreme wave loading on offshore wind turbine foundations, the ideas and tools generated will be more broadly applicable. We will develop a computationally fast method and an open source tool to be used by practicing engineers in industry to model long-term cyclic loading, leading to more efficient designs of offshore structures, reducing construction cost whilst preserving function and reliability.

Publications

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Zhang Z (2024) Spatial estimation of unidirectional wave evolution based on ensemble data assimilation in European Journal of Mechanics - B/Fluids

 
Description We have made significant advances and revealed new findings in this project. The summaries of each contribution are given below.

1. We expanded our novel method on nonlinear wave loading which was published in the Journal of Fluid Mechanics, 2018, and established a new Gaussian Process-based model for nonlinear wave loading on offshore wind turbine monopile foundations with a Stokes-type harmonic expansion model. Based on this Gaussian Process-based model, we developed an engineering prediction model for the nonlinear loading on such structures using machine learning data assimilation.

2. We designed and performed three phases of new unique laboratory experiments for further investigation of non-breaking nonlinear wave, breaking wave, and directionally spread wave impact on offshore wind turbine monopile foundations under a range of carefully designed realistic sea states. These new experimental studies aim to provide new physics of severe storm wave loading on offshore wind turbine foundations, and further expand the parameter space of the database for our new Gaussian Process-based load model.

3. Advanced numerical simulations were performed to provide assistance and support to the three rounds of laboratory experiments in this project. The in-house developed Particle-In-Cell (PIC) model and widely applied OpenFOAM CFD tool were used to provide cross-checking to ensure the accuracy of the numerical results. When modeling severe wave loading on offshore wind turbine monopile foundations, the numerical results revealed the new phenomenon of high-order wave loading and how it related to the secondary load cycle, a feature associated with severe wave loading, and there are still a lot of unknowns about what may cause the secondary load cycle. Due to the accuracy of these two numerical methods, numerical modeling will provide additional data that is required but difficult to achieve in the laboratory, such as breaking wave cases.

4. A new transformation of the FNV theory for calculating wave loads on a cylinder was proposed as part of this project. This T-FNV expresses force with only nonlinear wave kinematics at the free surface. We further proposed an approximated T-FNV that uses only nonlinear surface elevation.This work was not included in our original research proposal and research objectives. We are delighted the new T-FNV formulations demonstrate good accuracy for wave forces for both deep and shallow-water cases against the original FNV model, and they show an increased role of higher harmonics in force than surface elevation.
Exploitation Route Our ultimate goal is to develop a new method for accurately calculating severe wave loading on offshore wind turbine foundations and an open-source engineering tool that can be directly used by design engineers to improve their determination of environmental loading, leading to reductions in construction costs. We would like to make this engineering tool open access to ensure all design engineers working in offshore wind can benefit from the new technology and advance in our research.

The research methodology and the application of machine learning in offshore hydrodynamic loading will benefit broad areas and foster new innovations in general coastal and offshore engineering. The methods developed could expand to the wave impact on floating offshore wind systems, offshore wave energy devices and other offshore structures.
Sectors Aerospace

Defence and Marine

Construction

Energy

Environment

URL https://www.sea-swallows.org/
 
Description High End Computing Consortium for Wave Structure Interaction HEC WSI
Amount £355,960 (GBP)
Funding ID EP/X035751/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2023 
End 01/2027
 
Title A new Gaussian Process based model for non-linear wave loading on vertical cylinders 
Description We expanded our novel method on nonlinear wave loading which was published in the Journal of Fluid Mechanics, 2018, and established a new Gaussian Process-based model for nonlinear wave loading on offshore wind turbine monopile foundations with a Stokes-type harmonic expansion model. Based on this Gaussian Process-based model, we developed an engineering prediction model for the nonlinear loading on such structures using machine learning data assimilation. 
Type Of Material Model of mechanisms or symptoms - in vitro 
Year Produced 2023 
Provided To Others? Yes  
Impact The paper was only published 3 months ago; we will monitor the impact in the near future. 
URL https://www.sciencedirect.com/science/article/pii/S0378383923001515?via%3Dihub
 
Title Transformed-FNV: Wave forces on a vertical cylinder - A free-surface formulation 
Description A new transformation of the FNV theory for calculating wave loads on a cylinder was proposed as part of this project. This T-FNV expresses force with only nonlinear wave kinematics at the free surface. We further proposed an approximated T-FNV that uses only nonlinear surface elevation. This work was not included in our original research proposal and research objectives. We are delighted the new T-FNV formulations demonstrate good accuracy for wave forces for both deep and shallow-water cases against the original FNV model, and they show an increased role of higher harmonics in force than surface elevation. 
Type Of Material Model of mechanisms or symptoms - in vitro 
Year Produced 2024 
Provided To Others? Yes  
Impact This paper was only published online very recently, we shall monitor the impact resulting from the development of this method in near future. 
URL https://www.scopus.com/record/display.uri?eid=2-s2.0-85183468345&origin=resultslist
 
Description PRIMaRE Partnership 
Organisation Cardiff University
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description PRIMaRE Partnership 
Organisation Plymouth Marine Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description PRIMaRE Partnership 
Organisation University of Bristol
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description PRIMaRE Partnership 
Organisation University of Exeter
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description PRIMaRE Partnership 
Organisation University of Plymouth
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description PRIMaRE Partnership 
Organisation University of Southampton
Country United Kingdom 
Sector Academic/University 
PI Contribution I became Chair of PRIMaRE Partnership on 1 August 2022. The Partnership for Research in Marine Renewable Energy (PRIMaRE) is a network of world-class research institutions based in the west, south, and south west of the UK who undertake research and development to address challenges facing the marine renewable energy industry at the regional, national and international level.
Collaborator Contribution Within this PRIMaRE Partnership, we actively exchange and discuss emerging issues in the research and development of marine renewable energy, including offshore wind energy development. These discussions have contributed to the research we are currently working on this project.
Impact The HPC-WSI grant has been recently funded by EPSRC, which will support our computations using the national HPC facility.
Start Year 2014
 
Description A Prestigious Keynote Lecture on the outcome of this research 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Prof. Jun Zang was invited to give a prestigious keynote lecture at the RWWWFB held on 1 November 2023. In her presentation, Prof. Zang provided a summary of the project objectives, the methodology used in the project, and the latest research progress. Around 80 people attended this online event, including leading international experts, academia, engineers, and postgraduate students from over ~20 countries.
Year(s) Of Engagement Activity 2023
URL http://www.iwwwfb.org/RWWWFB.htm
 
Description Advisory Board Meeting 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact The intended purpose of this Advisory Board meeting was to inform the international industry experts and renowned professors of the project aims and objectives, research methodologies, and latest research progress., and to have their comments on the project and help steer the research direction of the project. This meeting generated great interest from these experts in the project and strengthened the relationship and collaboration with these experts and their institutions.
Year(s) Of Engagement Activity 2022
 
Description Keynote talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Prof. Jun Zang was invited to give a keynote presentation at the 6th Symposium on Computational Marine Hydrodynamics, held on 14 January 2023. In her presentation, Prof. Zang provided a summary of the project objectives and the methodology used in the project, and the latest research progress. Over 7000 people attended this online event, which sparked questions and discussions afterwards. and further queries from the audience were followed.
Year(s) Of Engagement Activity 2023
 
Description Talk to ABL Group London by Prof. Thomas Adcock, University of Oxford 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact The intended purpose of this talk was to inform the design engineers at ABL Group London of the project research progress. This meeting generated great interest from the audience in the project and helped future collaboration with the company.
Year(s) Of Engagement Activity 2023