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.

Wave energy globally has potential average power slightly less than wind but this has been unexploited to date. We are concerned here with wave energy converters (WECs) offshore, before the energy resource is reduced by shallow-water effects, which would be suitable for grid scale electricity generation. Individual WEC capacity has been considered to be much smaller than for wind turbines and cost of energy (COE) considerably larger. However, with multi-mode, multi-float systems, capacity may be similar to or greater than wind in some locations and COE has been estimated to be similar to offshore wind. Survivability in extreme waves needs to be established, along with reliability of components. The mooring is the most vulnerable structural component of an offshore wave energy converter. Snap loads are a particular problem in extreme waves, and also in intermediate waves affecting fatigue. There is a widespread consensus in the wave energy community that mooring system design and modelling is a major challenge that needs to be overcome. Although literature and design guidelines for conventional ocean engineering applications are abundant, in general they do not account for the requirements of wave energy conversion, where the mooring should not inhibit platform motion causing the energy generation. Design, optimization, and assessment of mooring systems require efficient hydrodynamic and dynamic mooring models, which should be fully coupled to represent all interactions. There are various mooring options: catenary slack moored, elastic taut moored, combinations with single point (buoy) moorings, and nylon/polyester ropes offer an economic option while reducing snap loads. While some progress has been made with nonlinear hydrodynamic WEC loading models for point absorbers, an efficient general nonlinear hydrodynamic loading model for multi-bodies, accounting for wave breaking, is presently not available. Computational fluid dynamics (CFD) simulations require days, even weeks, to run on multiple processors and is unreliable for complex dynamic problems. The intention here is generalise efficient linear hydrodynamic load models by including the fully nonlinear force component due to the pressure field in the waves, known as the Froude-Krylov force. This has improved predictions of response and mooring load, markedly in some cases. This will be advanced through comparison with experimental wave basin tests and formally generalised through system identification, for single and multi-bodies with a range of mooring configurations in representative, generally multi-directional wave fields and currents. The convenient simplification of linear wave input will also be assessed with a revised force formulation determined by system identification. These force formulations will be coupled with the general industry-standard mooring model Orcaflex accounting for dynamic and material properties enabling design optimization using multi-objective genetic algorithms. This will enable survivability, fatigue and reliability analyses.

Although there is a long history of research of wave energy convertors (WECs), there are still many challenges that make it difficult to develop effective, reliable and economically viable WECs. One of the challenges is the lack of robust modelling tools to assess survivability of WECs under extreme marine environments that cause extreme loads and large responses. Survivability of WECs needs to be concerned not only in the design stage but also when operational to maximise the amount of harnessed energy and minimise the risk of damage. To assess and analyse the survivability of WECs, one must identify survival conditions, quantify loadings and responses of WECs and characterise the pressure and velocity field of WECs under survival conditions. Identification of survival conditions for WECs requires not only the consideration of severe storms but also of loads and responses of WECs in shorter steep seas, which is different from that for other offshore structures that may just need to consider severe storms giving the largest wave heights. High precision quantification of loadings and responses of WECs must consider wave breaking and viscosity, which will provide dominate factors for conceptual design and to determine if the device needs to be shut down. Characterisation of the pressure and velocity fields of WECs needs to resolve two-phase flow with vortex structures to sufficient detail, which will provide information for structural and components design. In addition, as the waves in the survival conditions are highly nonlinear, they must be simulated for a long propagating duration in a large domain to allow them to sufficiently evolve. Therefore, the numerical modelling tools for analysing WEC survivability should have the capability of dealing with breaking waves and two-phase flow and accurately estimating the effect of viscosity in turbulent states. In the meantime, the tools must be fast enough so that engineers can simulate the cases within practical time-scales for design. Many numerical models with various levels of accuracy and efficiency exist, but none of them can adequately deal with the extreme conditions found in practice. Some models are phase-averaged, being computationally efficient but not sufficiently accurate. Some models are phased-resolved, based either on the potential theory or the viscous theory. The most advanced potential models are fully nonlinear and much faster than viscous models, but could not deal with wave breaking and turbulence which always occurs for WECs. The viscous models can theoretically deal with the physical phenomena but are generally very computationally expensive, perhaps also suffering from unwanted numerical dissipation. This project will develop a novel numerical modelling suite by combining different models and by proposing new numerical approaches and machine learning techniques, which will be more accurate and require less computational effort. The modelling suite will be able to automatically go up to fully nonlinear simulations and down to linear simulations depending on the level of nonlinearity of waves and their interaction with the WECs. The new modelling suite will be validated by data measured from WEC models in the laboratory and real devices at sea, and will be applied to assess the parameters relevant to the survivability and reliability of WECs. During the project, an advisory board will be set up to give the suggestions on specific research topics, and regular project meetings/workshops will be held to attract the interests of WECs stakeholders and disseminate the research outcomes. Our project partners will be invited to be a member of the advisory board and to attend or contribute to the meetings/workshops. Databases for different types of WECs will be created during this project, which will be accessible by general public.

The aim of this proposal is to create a robustly-validated virtual prediction tool called a "digital twin". This is urgently needed to overcome limitations in current industrial practice that increasingly rely on large computer-based models to make critical design and operational decisions for systems such as wind farms, nuclear power stations and aircraft. The digital twin is much more than just a numerical model: It is a "virtualised" proxy version of the physical system built from a fusion of data with models of differing fidelity, using novel techniques in uncertainty analysis, model reduction, and experimental validation. In this project, we will deliver the transformative new science required to generate digital twin technology for key sectors of UK industry: specifically power generation, automotive and aerospace. The results from the project will empower industry with the ability to create digital twins as predictive tools for real-world problems that (i) radically improve design methodology leading to significant cost savings, and (ii) transform uncertainty management of key industrial assets, enabling a step change reduction in the associated operation and management costs. Ultimately, we envisage that the scientific advancements proposed here will revolutionise the engineering design-to-decommission cycle for a wide range of engineering applications of value to the UK.

Wave energy convertors (WECs) offer opportunities for niche (powering aquaculture and offshore stations) and grid-scale applications. However, disruptive innovation is essential to unlock the potential of wave energy, achieve step change reduction in cost of energy, and prove competitiveness against other renewable energy options. Here we investigate the opportunity to transform the development of WEC systems by utilising intelligent design concepts that exploit novel use of deformable materials. WECs based on deformable materials may offer improved performance, survivability, reliability, and reduced cost compared with steel or concrete alternatives for the following reasons: 1. To achieve a given resonant frequency, a flexible fabric device can be smaller and lighter. 2. Hydrodynamic characteristics of such a device can be modified by controlling its internal fluid pressure, enabling it to be tuned to suit incident wave conditions. These adjustments can be made by an on-board intelligent responsive system. 3. Controlled non-linear changes of geometry would enable a deformable fabric structure to accommodate or shed high loads without reaching critical stress concentrations, improving survivability and reducing installation and lifetime costs. 4. Flexibility opens up the possibility to use a range of PTOs, such as novel distributed embedded energy converters (DEECs) utilising distributed bellows action, electro active polymers, electric double layer capacitors or micro-hydraulic displacement machines. 5. A lightweight flexible structure with largely elastic polymer construction is unlikely to cause collision damage, and so is therefore a low risk option for niche applications, such as co-location with offshore wind devices. The performance of flexible responsive systems in wave energy, their optimisation in operating conditions, and their ability to survive storm waves, will be assessed through a programme of wave basin experiments and numerical modelling of different flexible WEC concepts. Survivability is a critical hurdle for all WEC concepts as by their nature they need to respond in energetic sea states while avoiding critical stresses in extreme seas. For a flexible responsive structure, this means avoiding concentration of stress (naturally avoided by collapse/folding) or of strain (avoided by use of a distributed PTO during operational conditions). Numerical models will be developed that account for complex interactions between wave action, deforming membrane structure, and internal fluid. The models will be informed, calibrated, and validated using results from materials testing and fundamental hydro-elastic experiments. Advantages and disadvantages of rubber-based, polyurethane and other reinforced polymer materials will be assessed in terms of manufacturing cost, join, bonding, and fatigue performance in the marine environment. The research will draw on origami theory and the technology of deployable structures to avoid problems with wrinkling, folding, or aneurysm formation, and an entirely new design may emerge through this innovative approach. We aim to demonstrate a pathway to cost reduction for flexible fabric WECs optimising for performance, structural design and manufacture for both utility scale and niche applications.