The demand for hydroelastic studies in floating structures applications has increased in recent years. Their applicability extends the potential for floating structure development in offshore environments. For instance, it is applicable to offshore photovoltaic (OPV), wave energy converter (WEC), floating airport, and future floating city development. However, this branch of fluid-structure interaction (FSI) problem requires a solid foundation in understanding the two-way interaction between the floating structure and the fluid.

Computational methods are the most suitable option for studying hydroelasticity due to the ease of problem setup and are comparatively cheaper than experiments. Despite this advantage, this method also faces problems in the coupling strategy that lead to high computational cost. According to a previous study, monolithic coupling shows better computational performance than partitioned coupling. But since it solves a high-order matrix system, it may still require a long computation time. This research aims to investigate the applicability of a reduced-order system in solving the hydroelastic problem by using the monolithic finite element framework. Modal superposition methods will be employed within the framework to compare the computational time between the full-order and reduced-order models.

A reduced-order-modelling (ROM) framework through modal superposition is developed in this research, including a new structural formulation using Mindlin theory to cover a wide range of floating structure problems in a unified formulation. The analysis is conducted using the Gridap FEM library in the Julia programming language and comprises: 1) dry and wet modal analysis, 2) frequency-dependent added mass, added damping, and force calculation, 3) modal superposition analysis, and 4) full-order monolithic FEM analysis.

This research has developed two modal superposition frameworks, namely the matrix and integration methods. These methods have been proven to accurately estimate the full-order solutions and are in good agreement with the existing literature. The matrix method shows better computational performance compared to the full-order solution, up to hundreds of times faster, and up to thousands of times faster than the integration method. The integration method requires longer running time than the full-order solution due to the expanded weak formulation. Furthermore, this research also compared the dry and wet modal solutions and found that the dry modal solution performs better than the wet solution. The wet modal analysis is solved by using a nonlinear eigenvalue problem (NLEP). However, the method used in this research still needs to be validated with a more robust NLEP method for subsequent research.

Overall, this research shows good applicability of the reduced-order model using the monolithic FEM framework. Both methods give a good estimation of the full-order solutions. Despite the heavy computational time, the integration method provides a good implementation of modal superposition directly to the weak formulation within the Gridap ecosystem. The newly developed unified structural formulation also shows good applicability in covering a wide range of VLFS hydroelastic cases. ... Read more

Reliable prediction of mooring-line motion and tension underpins the station-keeping safety of every floating offshore platform, whether built for wind energy, wave energy, or oil and gas production. High-fidelity finite-element solvers resolve the nonlinear dynamics that govern this safety, including snap loads and peak tensions, but their computational cost makes repeated evaluation across design iterations or environmental conditions impractical. Deep-learning approaches promise to overcome this cost. Yet existing mooring-line surrogates either frame the problem as failure classification or reduce the line to a lumped fairlead reaction force, so none reconstructs the full spatial field of motion and tension along lines of different discretisations with a single trained model, nor biases its predictions toward the conservative peak estimates that structural design requires.

This thesis develops a surrogate that couples a graph attention network (GAT) spatial encoder to a long short-term memory (LSTM) temporal encoder, denoted GAT+LSTM, and that addresses this gap. A mooring line is represented as a chain graph whose nodes and edges carry static geometric and mechanical features alongside dynamic motion, tension, and contact features. A two-layer GATv2 encoder captures the coupling between neighbouring nodes, a per-node LSTM encoder captures each node's temporal evolution, and a compact prediction head returns the future motion and tension of the whole line in one non-autoregressive pass. The network is trained under a mixed-topology scheme on finite-element simulations spanning ten offshore locations and node counts from N = 4 to N = 21, with two locations withheld to test generalisation, under two compared training objectives, a fixed-weight loss combining data fidelity with soft physical penalty terms in physical units, and a Kendall multi-task uncertainty-weighting scheme intended to learn those weights automatically.

One set of trained weights serves every discretisation from N = 4 to N = 21 nodes and both withheld locations, with test accuracy meeting or exceeding validation accuracy at every node count. The surrogate reconstructs the spatial field with a global tension R^2 of 0.98. A variance decomposition shows this accuracy certifies the response's spatial profile and per-sea-state level, together at least 98.5% of its variance, not the wave-frequency fluctuation, which the model returns as a near-flat mean. Under the fixed-weight objective this yields a positive mean peak-tension bias of +197.5 N and an underprediction rate of 17.6%. The uncertainty-weighted alternative matches this safety margin only once its one-sided penalty is reformulated as a two-sided quantile loss, and even then it does not surpass it.

The thesis therefore contributes a validated, topology-general load-field surrogate, a safety-aware training methodology with the diagnosis of its failure mode, and a rigorous characterisation of what the surrogate's accuracy does and does not certify. The surrogate is deliberately not a wave-by-wave forecaster of mooring tension. Its value lies in reproducing the spatial load field, across topologies and locations, at a fraction of the finite-element solver's computational cost. ... Read more

Inland floating photovoltaic (FPV) systems are exposed to wave-induced motion and forces, which drive fatigue loading on the structure and mooring and reduce energy yield through suboptimal module orientation. Floating wave barriers can mitigate these effects, but conventional barriers provide no ecological or aesthetic value. Floating ecosystems, pontoons supporting vegetation above and below the waterline, are designed to improve water quality and provide habitat, but whether they can also serve as wave barriers has not previously been quantified.

This thesis addresses how the integration of floating ecosystems influences wave–structure interaction, structural response, and energy yield of FPV systems in inland water bodies. Three wave-barrier representations are modelled using a monolithic finite-element fluid-structure interaction framework: a freshly placed floating ecosystem as a thin elastic beam, a mature ecosystem with an added porous root zone, and a rigid, hollow HDPE pipe as a reference barrier. All cases are applied to wave conditions derived for the Willemspolder, a lake adjacent to the River Waal in the Netherlands.

Results show that both the unmoored elastic beam and the unmoored pipe transmit nearly all incident wave energy at the peak frequency, with spectrally weighted load reductions below 13%. A pipe moored with stiffness precisely tuned to the peak frequency achieves near-complete wave reflection through resonant scattering, extending FPV fatigue life by a factor of 10 or more. The mature root zone introduces broadband porous dissipation as an additional attenuation mechanism, independent of mooring. The impact of wave-induced energy yield losses is small across all configurations; structural fatigue is the dominant design driver rather than energy yield. Floating ecosystems offer ecological and aesthetic value that conventional pipe barriers cannot, making a hybrid configuration a promising direction for future development.
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Floating offshore wind deployment requires dynamic power cables, as part of the system that transmits the generated electricity from the floating platform to the onshore grid. These cables are loaded continuously by waves and current and follow the platform motions, so they experience cyclic loading throughout their design life. Vortex-induced vibration adds a further dynamic contribution, which makes fatigue one of the governing limit states in their design. The industry prediction tools, however, are based mostly on rigid or straight cylinders, and field data, design standards and operational experience remain scarce, so their design relies on large safety margins.

To address this gap, a coupled cross-flow and in-line wake oscillator model was implemented to assess whether it predicts the response of a lazy wave power cable more accurately than the single-degree-of-freedom models common in industry. The model of Qu and Metrikine (2021) was adopted, in which a single wake oscillator drives both directions, forced by the cross-flow acceleration and by the in-line motion through a parametric term, so that the lift and the fluctuating drag originate from the same wake. In-line amplitudes are typically smaller than cross-flow ones, but for long flexible cylinders neglecting the in-line motion under-predicts the fatigue damage, which motivates the coupled formulation.

The model was implemented in a nonlinear finite element solver in Python and as an external function in OrcaFlex, the latter placing it within an industrial framework. The two implementations were verified against each other and validated against a tensioned vertical riser, where they reproduce the measured cross-flow and in-line response and recover the in-line motion that cross-flow-only models cannot capture. The model was then applied to an experimental lazy wave power cable in OrcaFlex and compared with the industry models in the same framework.

The coupled model reproduces the measured frequency content and the main responding modes, and recovers the shared in-line and cross-flow frequency that arises from the curved geometry. It over-predicts the in-line content at twice the cross-flow frequency, which appears only weakly in the experiment. Its Strouhal number can be set for each case, whereas the industry models are fixed at a single value above the measured one. The cross-flow amplitude is over-predicted by about a factor of two, mainly as a result of the selected tuning parameters, an over-prediction shared by the industry models. Overall, the coupled model predicts the response more accurately than the industry models, while the amplitude remains conservative. ... Read more

This thesis examines the occurrence and interpretation of compressive wave phenomena in subsea power cables during normal lay operations, with a focus on addressing a central uncertainty in the field. In current industry practice, the compressive behaviour predicted by numerical cable-lay simulations is frequently regarded as a limiting factor for installation, however, a significant research gap remains regarding whether these compressive waves correspond to actual physical cable responses or are merely artefacts of modelling assumptions. Accordingly, the primary aim of this research is to clarify this gap by evaluating whether the compressive waves observed in OrcaFlex simulations can be substantiated as phenomena that occur during normal lay operations.

To address this question, a comparative modelling, verification, and field-data assessment approach is adopted. First, the theoretical possibility of axial compression is demonstrated using a simplified Kirchhoff rod formulation, showing that time-dependent transverse excitation can induce compressive forces near the touchdown region. Subsequently, a Python-based finite element model is developed to capture both static and dynamic cable responses and is compared with OrcaFlex simulations. The validation strategy proceeds stepwise: starting with analytical catenary theory, followed by static verification across multiple cable types, and finally dynamic comparison using measured operational input data.

This thesis investigates the occurrence and interpretation of compressive wave phenomena in subsea power cables during installation operations. In industry practice, the compressive behaviour predicted by numerical cable-lay simulations is often treated as a limiting factor, although it remains unclear whether these compression waves reflect a physical cable response or are partly due to modelling assumptions. The objective of this research is therefore to assess whether the compressive waves observed in OrcaFlex simulations correspond to phenomena that can occur during normal lay operations.

To address this question, a comparative modelling, verification, and field-data assessment approach is adopted. First, the theoretical possibility of axial compression is demonstrated using a simplified Kirchhoff rod formulation, showing that time-dependent transverse excitation can induce compressive forces near the touchdown region. Subsequently, a Python-based finite element model is developed for both static and dynamic cable response and is compared with OrcaFlex simulations. The verification strategy proceeds stepwise: starting with analytical catenary theory, followed by static validation across multiple cable types, and finally dynamic comparison using measured operational input data.

The simulations are driven by manufacturer cable data and operational data obtained from DEME's cable-lay vessel Living Stone, including vessel motion data from Grafana and onboard sources, as well as ROV observations of the touchdown region. Within the simplified Python model, vessel motions are translated into chute boundary motions, allowing measured operational data to be applied to the cable model in a controlled manner. And the touchdown point is added as an extra boundary condition to verify the catenary shape.

However, an important limitation of this approach is that no sufficiently complete dynamic field dataset was available for the specific severe conditions in which compressive waves are observed in OrcaFlex. As a result, the Python model could only be verified for relatively mild or near-static conditions, and not for a measured case in which the suspected compressive-wave behaviour occurred. Consequently, the combined Python model and vessel data approach is not sufficient on its own to fully answer the main research question.

For that reason, an additional tension analysis is carried out using recorded cable-lay data. Time periods corresponding to normal lay operations are selected and screened based on the availability of reliable tensioner measurements. Particular attention is given to tension peaks, irregularities, and intervals in which negative tension values are observed, as these may indicate conditions relevant to compressive behaviour. These selected cases are subsequently analysed in OrcaFlex using the appropriate vessel motions and cable properties, while the corresponding ROV footage is reviewed in parallel to investigate whether any visually observable compressive behaviour can be identified.


The tensioner-selected cases did not show clear travelling compressive waves in the corresponding OrcaFlex simulations. Although several cases contained irregular or negative tensioner readings, these did not develop into sustained regions of axial compression near the touchdown point. This indicates that negative tensioner values should not be interpreted directly as evidence of compressive-wave behaviour in the suspended cable span, since the tensioner measurement represents a differential force across the onboard tensioner system rather than the true top tension of the free-hanging cable.

Because the project-data-based cases did not reproduce compressive waves, an additional parameter-variation study was conducted. The bending stiffness, touchdown point location, and imposed chute motion were varied to investigate which factors can trigger the phenomenon. The results show that changing the bending stiffness or touchdown point location mainly influences the local cable response but does not, in itself, generate travelling compressive waves. The imposed chute motion, and especially the resulting chute velocity, was found to be the dominant trigger. Compressive waves were only reproduced when the imposed boundary motion became sufficiently dynamic.


The thesis, therefore, combines theoretical analysis, numerical model development, validation, tensioner-based case selection, and parameter variation to investigate compression in subsea power cables from multiple perspectives. The results show that compressive waves were not observed in the analysed normal lay operations. However, the phenomenon cannot be entirely ruled out as physically possible under more severe dynamic excitation. The main conclusion is that the available project data were not dynamic enough to reproduce compressive waves. At the same time, the numerical parameter study indicates that sufficiently high chute velocity can trigger the phenomenon. Further validation requires complete operational datasets from more dynamically severe installation conditions.

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From the existing literature, no scaled models have been reported that combine slamming on cylindrical bodies with global structural compliance. The absence of validated construction methods hinders experimental validation of slamming predictions, which constitute a design-driving load case for the presented structure. To enable controlled investigation of slamming-induced structural response, this study presents the design, construction, and experimental validation of a scaled flexible model representing a three-floater platform using a beam-based structural idealisation. A coupled FEM–BEM numerical framework was developed to design the model based on scaled wet natural frequencies and global mode shapes. The beam system was fabricated using SLA printing with Tough2000 resin, while the floaters were constructed as rigid elements to concentrate flexibility within the connecting beams. Static and dynamic modal testing was performed in both dry and wet configurations. Following material calibration, the numerically predicted natural frequencies and dominant global mode shapes showed good agreement with experimentally identified modes. Natural frequency deviations remained within 7%, and strain-based measurements confirmed accurate reproduction of the primary global deformation patterns. The results demonstrate that the proposed idealised beam and additive manufacturing approach provides a viable methodology for representing global hydroelastic behaviour of Merganser-like platforms ... Read more

There is a growing need for new floating structures for renewable energy and offshore infrastructure, driven by abundant offshore resources and limited land space. Current design processes rely heavily on physical experiments and conventional grid-based numerical tools, which require mesh generation for each design iteration. This meshing step is identified as a major bottleneck, as seen in aerospace and automotive industries, due to its cost and complexity. The same challenge is presumed to apply to offshore floating-structure design. To circumvent this, one can use unfitted, embedded, or immersed boundary methods that reduce human intervention in the meshing step. These unfitted methods have seen limited application in modeling offshore structures. In this thesis specifically, we investigate unfitted finite element methods as an alternative simulation approach for floating offshore structures.

We introduce and justify the fundamental assumptions of the physical modeling for both the fluid and the structure. Specifically, we derive the linearized potential flow formulation in reduced form for the fluid and model the structure as a rigid body. We demonstrate how we couple the fluid and the structure in a single system of equations in the time domain. In this work, the structure is only incorporated as a boundary condition, due to the rigid body assumption. As we work with linearized potential flow, we also derive the system of equations in the frequency domain representation. We transform the systems of equations to their corresponding weak formulations such that we can deal with a challenging integral on the dynamic boundary condition for the structure more easily by assuming a constant wetted surface of the structure.

The traditional finite element method is introduced, and the time and frequency domain weak formulations are transformed to their corresponding discrete weak formulations suitable for the finite element method. A concise historical overview of unfitted, embedded and immersed boundary methods is presented, which highlights the motivation behind their development and challenges encountered. We then compare the newest unfitted methods and select the cut finite element method (CutFEM), aggregated unfitted finite element method (AgFEM), shifted boundary method (SBM), and weighted shifted boundary method (WSBM), because of their suitable properties for moving domains and complex geometries. After which, each of the four aforementioned methods is explained in detail. We describe the discrete spaces and discrete weak forms for each method. And we derive the shifted boundary condition for the kinematic and dynamic boundary conditions at the structural boundary.

To assess and compare the four unfitted methods, we perform a benchmarking study in 2D and 3D, using both implicit and explicit geometry representations. Convergence rates, condition numbers, computational performance, and implementation complexity are compared. CutFEM and AgFEM exhibit the expected convergence rates, whereas SBM and WSBM converge one order lower than CutFEM and AgFEM because no gradient recovery is used. Note that this is only because the problem considered here is Neumann type boundaries. For Dirichlet type boundaries, all four methods should have optimal convergence rates. Regarding numerical conditioning, SBM has the smallest condition numbers overall; for \(p_e = 1\), AgFEM is comparable to SBM, and for \(p_e = 2\), AgFEM’s condition numbers are similar to CutFEM’s. WSBM has the largest condition numbers. Performance tests in 2D show consistent trends in runtime and memory allocations: SBM is fastest and most memory-efficient, followed by CutFEM, then WSBM, and finally AgFEM. AgFEM’s main bottleneck is the costly initialization of the finite element space due to its aggregation algorithm. In terms of implementation complexity, AgFEM is the most complex. CutFEM and WSBM both pose challenges, with CutFEM further complicated by its reliance on a tessellation algorithm for integration. SBM is easiest to implement, although its discrete weak formulation is relatively more involved.

We demonstrate the time domain capabilities of the numerical framework for both 2D and 3D problems, including multi-body simulations, using simple and realistic floating structures. All four unfitted finite element methods are shown to agree with a body-fitted method and to reproduce experimental heave free-decay results, provided sufficient mesh resolution and polynomial order. An adaptive mesh refinement strategy is proposed, because Cartesian grids cannot efficiently represent fine geometric details in 3D. This strategy is completely implemented for CutFEM and AgFEM, but for SBM and WSBM, the Hessian operator on the reference element for adaptively refined grids is still missing, so a term in the shifting operator is currently neglected. With appropriate mesh refinement and polynomial degree, all unfitted methods give consistent results; CutFEM and AgFEM are less sensitive to coarse meshes, while SBM and WSBM may require gradient recovery to reach high accuracy with fewer elements. WSBM in particular, can show spurious oscillations or divergence on non-adaptive meshes, highlighting the critical role of mesh adaptivity and the need to complete the Hessian operator definition for a full comparison of SBM and WSBM.

The numerical framework’s performance for frequency domain problems is demonstrated by applying CutFEM, AgFEM, and SBM to 2D cases to compute added mass and added damping coefficients, and extending AgFEM to 3D realistic floating structures using unstructured refined meshes from GMSH. The methods successfully reproduce these coefficients for simple geometries and, with AgFEM, for realistic ones. The accuracy of added mass solutions appears more sensitive to mesh refinement near the structure than that of added damping. A remaining challenge is optimally balancing domain size and local mesh refinement for each wavelength, rather than relying on a single background mesh.


For future research, the dissertation’s completeness can be improved by implementing gradient recovery for SBM and WSBM and by defining the Hessian on the reference element. For frequency domain problems, an optimal, wavelength-dependent mesh strategy should be developed using the adaptive approach from the time domain studies, balancing domain size, refinement level, and automated background grid generation. An extension of the solvers to parallel computing is underway to enable large-scale multi-structure simulations, the challenge of which is parallal adaptive AgFEM. The numerical framework should have validated wave generation and damping modules, enabling longer simulations within a limited domain. For large-scale simulations, the first priority is validating the multi-body frequency domain formulation. Comparable large-scale validation studies must also be identified and performed for time domain simulations. For the time domain studies, mooring systems should be incorporated, either via linear stiffness matrices or more advanced models. The current framework can be expanded upon by incorporating higher fidelity models. This also introduces the challenges of moving free surfaces and moving structures that require remeshing. One step avoiding free surfaces with higher fidelity models, is modeling of fluid structure interaction of submerged risers or mooring lines which have marine growth. Lastly, we have proposed the generalized SBM (GSBM) to address challenges encountered during this project. This method should be subjected to the benchmark initiative and it can be a promising tool for the time domain simulations. ... Read more

Floating Offshore Wind Turbine (FOWT) unlock far-offshore wind resources in deep waters that can’t be harvested under economic aspects using bottom-fixed wind turbines. Numerical modelling tools are employed to assess different FOWT designs under various environmental conditions. In order to be competitive and yet guarantee working designs, the numerical tools need to be reliable and computationally efficient. This justifies the need for assumptions simplifying the models.

One such simplification is the rigid floater assumption, where the FOWTs’ substructure is assumed rigid. This reduces the computational effort but at the same time alters the results, like the tower’s first natural frequency and corresponding mode shape.

Multiple approaches are used in literature to match the tower’s first natural frequency of flexible and rigid floaters. This involves adjusting the tower properties, such as length or Young’s modulus, or alternatively, implementing a flexible element between a rigid floater and a flexible tower. Siemens Gamesa currently makes use of the latter method by tuning the flexible elements’ properties to achieve a match in the models’ tower first bending natural frequency.

So far no studies have been conducted on the impact of the flexible element parameters on the tower first mode shape when tuning towards a matching tower first bending frequency. Additionally, the effect of differently correct mode shape variants on the tower dynamics is investigated. This leads to the goal of improving the tower dynamics for a FOWT with a rigid substructure.

The analysis was based on two versions of the U-Maine FOWT model. One fully flexible floater design served as a reference, whilst a fully rigid floater design was used to incorporate the different flexible element designs. Various flexible elements with distinct properties were evaluated to understand the sensitivity of the mode shape to these parameters. Subsequently, selected designs exhibiting varying degrees of accuracy of the mode shape were compared to the flexible floater design in time domain simulation. Furthermore, three separate methods of identifying the tower’s first bending mode are proposed.

In the course of modelling the flexible floater design, a modelling error was made that resulted in double counting of the heave motions. Despite this error, it was concluded that for constant and turbulent wind, all flexible element designs outperform the rigid floater design. Furthermore, a close mode shape match likewise results in an increased match of bending moment and tower top rotation for high and low wind speeds. In the range of rated wind speeds, the shortest flexible element design with the worst mode shape match performs best. Comparing the tower top acceleration also indicates an overall improvement of the results, but less significant. This is expected to result from tuning towards the first tower mode rather than higher-order modes.

Generally, using any flexible element design already results in an improved mode shape match with minor differences. The impact of these discrepancies on the tower dynamics is small. Therefore, it is concluded that any flexible element, even when only tuned to match the tower’s first natural frequency, is an improvement over the rigid floater design. ... Read more

This thesis investigates the use of Multifidelity and Stochastic Polynomial Chaos Expansion for the assessment of fatigue damage in Floating Offshore Wind Turbine (FOWT) mooring systems. Specifically, the work aims to improve efficiency in providing uncertainty quantification, a probabilistic approach to computer experiments, by balancing prediction accuracy with computational cost. This is crucial for the design and analysis of complex FOWT configurations, where high-fidelity simulations are expensive. The research adopts two approaches: multifidelity (MF) modeling and Generalized Lambda Models (GLaM). While assuming an underlying polynomial structure in the data already provides a speed-up over conventional Monte Carlo estimates. The MF framework builds further on this Polynomial Chaos Expansion (PCE) by employing multiple models of varying resolution, from computationally inexpensive approximations to more accurate versions of the same solvers. Certain optimizations are applied within this method, sparse basis selection, Gaussian process regression, and basis adaptivity, which enable further speed-up and improved accuracy. In addition, when the seed is not fixed (i.e., when the stochastic nature of the problem is preserved), GLaMs are used to approximate the entire output probability distributions. Once trained, these PCE models also allow straightforward sensitivity analysis of the input-output relationships. The primary quantity of interest is mooring line fatigue damage, which results from the cyclic environmental loading experienced by the FOWT. Input uncertainty arises from environmental parameters such as wind speed, wave height, and peak period. Since wind and waves are correlated, their joint distributions are modeled using copulas. Validation is performed using the irregular Ackley function as a benchmark, alongside two realistic mooring case studies that compare quasi-static (MoorPy) and dynamic (MoorDyn) models, and investigate the impact of simulation duration on the fidelity hierarchy. Results for the Ackley benchmark confirm that the MF algorithm captures uncertainty effectively and matches the accuracy of single high-fidelity surrogates, while reducing computational cost, provided the polynomial order and truncation norm are tuned to balance bias and variance. Initial mooring studies show that quasi-static models replicate mean tensions but systematically underpredict fatigue damage. While the MF framework shows promising results for these mooring models, further investigation is needed to address and improve its limitations. The MF approach provides comparable prediction accuracy and slightly reduces the number of required dynamic samples for simpler configurations such as fairlead motion, but its performance degrades in more complex coupled two-dimensional FOWT simulations, where low-fidelity surrogates fail to capture essential coupled dynamics, limiting the effectiveness of MF corrections. Similarly, fidelity hierarchies based solely on varying simulation durations show limited benefit and require further refinement. Overall, with the added cost of the low-fidelity model, there is no clear cost benefit. Results from the GLaM framework also remain subject to further investigation due to limited time. ... Read more

Mangroves play a vital role in coastal ecosystems by dissipating wave energy, protecting against erosion and supporting biodiversity. Despite their importance, quantification of their attenuation capabilities remains challenging. This study explores the potential of a two-dimensional (2DV) OpenFOAM model to simulate wave attenuation through mangrove forests using a layered approach to capture the vertical variability in mangrove frontal area.

The model is first validated against flume measurements under a range of different hydrodynamic conditions to assess its capability in a controlled, well-measured environment. The goal is that by applying the Reynolds Averaged Navier Stokes equations and a layered approach, a more realistic bulk drag coefficient can be calibrated for different hydrodynamic conditions. One of the objectives is to explore whether this approach can yield a stronger correlation between the bulk drag coefficient (Cd) and the Keulegan-Carpenter (KC) number.

Then, the model is applied to a real-world case study for the Mandai mangrove forest on the north-west coast of Singapore. his application demonstrates the model’s ability to simulate complex coastal environments, including variable bathymetry, forest extent, and wave conditions. A sensitivity analysis is also conducted to evaluate the effect of different parameters on the modelled wave attenuation

Results confirm the model's capability to accurately represent the vertical variations in vegetation structure and their influence on wave attenuation. It showed great agreement with the measured wave attenuation of the flume after calibration. Despite its expected improvement on the Cd-KC relation, it did not succeed in this, but it does reveal the impact of depth-variable drag forces and their effect on the velocity profile and wave attenuation. Furthermore, the case study showed its applicability to real-world coasts. Despite numerical dissipation standing in the way of any quantitative analysis, the runs in combination with the sensitivity analysis provided valuable insights into the model's behaviour and wave attenuation predictions. ... Read more

This thesis investigates the hydrodynamic behavior of a barge-type Floating Offshore Wind Turbine (FOWT) equipped with a central moonpool, with particular focus on the influence of the damping lid method, a numerical approach used to approximate viscous damping in potential flow simulations. The damping lid factor ε was varied systematically to evaluate its impact on moonpool resonance, platform motions, and mooring system loads under both idealized and realistic sea states.
Hydrodynamic coefficients and motion response amplitude operators RAOs were first analyzed in the frequency domain using DIFFRAC, revealing strong sensitivity of the system to internal resonance modes when no damping is applied. The time domain simulations were then performed in aNySIM to assess the mooring tensions and the motion behavior under regular and irregular wave conditions. A minimum damping lid factor of ε = 0.035 was identified as sufficient to suppress unrealistic resonance amplification.
Finally, a case study for a North Sea scenario was carried out to compare the performance of the system between ε = 0.035 and ε = 0.1 in terms of serviceability and operational availability. Results showed that once the critical damping threshold is reached, the system response becomes relatively insensitive to further increases in ε.
The findings demonstrate that the damping lid is an effective and practical tool for modeling moonpool effects in barge-type FOWTs, and that a carefully selected damping factor for this specific barge type, can eliminate the need for more computationally intensive viscous simulations when evaluating global system behavior.
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In this thesis report, we investigate the difference in using physics-compatible elements and standard elements in the context of fluid flow through porous media. For this, we compare the H(div)-conforming Raviart-Thomas elements to Lagrange elements. Our model involves a channel flow over a porous media, resulting in a fluid and a porous subdomain. We distinguish between three approaches, as based on a research paper. The first is the one-domain approach called PE, which sets one equation on the entire domain. The other two approaches are two-domain approaches, which use different equations in the two subdomains, with interface conditions to couple them. The first of these is NSD, which couples Navier-Stokes with the Darcy equation. The second is NSF, which does the same but also incorporates the Forchheimer term, which models inertial effects.

In general, we were not able to make a completely successful model, as the PE case did not match the paper, and the results found when using Raviart-Thomas elements had some unexplained anomalies. We were also unable to carry out an accurate error analysis. We were able to see that Raviart-Thomas produced an exactly divergence-free solution, as expected from the theory, at the cost of more computing power and time needed. ... Read more

As offshore wind energy expands, the environmental impact of monopile foundations on ocean stratification has become a growing concern. Thermal stratification occurs when layers of seawater with different temperatures form due to solar heating, creating a stable density gradient that limits vertical mixing. However, monopile-induced turbulence can disrupt this layering, a process known as destratification, which may have significant ecological consequences.

This study investigates a novel perforated monopile design as a potential solution to mitigate these effects. Using Computational Fluid Dynamics (CFD) simulations in OpenFOAM, the research compares the hydrodynamic behavior of regular and perforated monopiles, focusing on turbulence intensity, wake formation, and destratification rates.

The results indicate that perforated monopiles reduce turbulence intensity and modify wake dynamics compared to regular monopiles, by allowing partial water flow through the structure and reducing recirculation zones. However, their impact on destratification rates remains minor. While perforations alter hydrodynamic behavior, they do not significantly mitigate stratification breakdown.

These findings suggest that while perforated monopiles influence flow dynamics, their effectiveness in reducing destratification is not significant enough to definitely recommend these structural modifications. Future research should explore different perforation patterns and other alternatives develop more sustainable offshore wind turbine foundations. ... Read more

To meet the ambitious targets outlined in the Paris Climate Agreement of limiting global temperature rise to 1.5 degrees Celsius, global renewable power generation must triple by 2030, with offshore wind energy expected to increase exponentially to 500 GW, a fourteen-fold rise from 2020. Despite a 48% cost reduction from 2010 to 2020, offshore wind energy remains more costly per megawatt-hour than fossil fuels, highlighting the need for cost-effective innovations in offshore wind farm foundations in the North Sea. Offshore wind farms are increasingly moving into deeper waters and utilizing larger turbines, which arise challenges for traditional monopile foundations. The Hybrid Monopile (HMP) is a promising al- ternative. It integrates features from traditional monopiles and jackets, potentially enhancing structural stability, reducing environmental impact, and optimizing costs in deeper waters. This thesis evaluates the technical and economic feasibility of the HMP compared to traditional foundation types such as the traditional monopile and jacket structure, aiming to accelerate development of offshore wind energy and contribute to global climate objectives. This study uses a comprehensive methodology to assess the HMP’s structural integrity, installation feasibility, and cost-effectiveness relative to traditional monopiles and jackets. The structural analysis begins with a preliminary design phase to establish input parameters for the finite elements analysis in Abaqus. This analysis evaluates natural frequencies and stress levels under varied conditions such as water depth, soil types, and turbine sizes, assessing the HMP’s feasibility across different conditions. The installation procedures are investigated, evaluating various strategies for HMP’s and compare it with the installation strategies of traditional monopiles and jackets. The most efficient way of installation is determined for the installation of the Hybrid Monopile while considering various options. This is compared to industry standards for the installation of the traditional monopile and jacket structure. The economic evaluation involves cost modeling. This analysis provides a cost comparison based on the manufacturing and installation of the structures, highlighting the economic advantages of the HMP in deeper waters and with larger turbine. Structural assessments demonstrate that the HMP is capable of deployment in water depths up to 80 meters and supporting offshore wind turbines of up to 22 megawatts in the North Sea. Design simulations indicate the HMP’s resilience against operational stresses caused by various environmental forces. The installation strategies emphasize efficient methodologies for HMP installation. Due to the fact the hybrid monopile requires an increased number of pin piles compared to jackets, the installation cycle of the hybrid monopile is less superior to the jacket, where the traditional monopile outperforms both of them based on installation time. Economic analyses highlight that while traditional monopiles are cost-effective in waters until 40 meter, the HMP emerges as a competitive solution for deeper waters and larger turbine configurations compared to jacket structures. In conclusion, the HMP represents an advancement over traditional jackets assuming a 20% reduction in interface piece manufacturing costs. Further optimizations in the design of the Hybrid Monopile and installation procedures could enhance its competitiveness in relation with a jacket. The traditional monopile is a more cost-effective solution in environments where possible, however the HMP offers a promising solution where noise and environmental limitation arises ... Read more

In the current energy market, where increasing demand for renewable energy is met with larger turbines which require larger installation vessels, understanding operational limits of such vessels becomes invaluable. For the installation of wind turbine generators at great heights, a stable working platform is offered by jack-up (JU) vessels, which use legs to elevate the hull and thus crane of the vessel above the waves. The operation taken into consideration is the preloading process where load is increased on the legs to ensure enough foundation capacity for later operations such as lifting of wind turbine generators. Traditionally, vessels have only been positioned above flat seabeds, but with the increased demand in the wind sector, unfavorable locations with sloped seabeds also need to be considered.
In order to capture global behaviour of a JU on a sloped seabed, two modelling methods are combined to create a framework; soil-structure interaction and structural finite element model. This framework implements a sub structuring approach by cutting and simulating a single leg on a sloped seabed using soil-structure interaction modelling. The results of the first method are foundation stiffness values and reaction loads during preloading of the JU while the second method takes those results as input to calculate internal loads of the JU.
The results show that the proposed framework is able to fulfill the task of combining two models and obtain meaningful results that can inform the JU during preloading phase. Then with this framework, the results show that sloped seabed induces an extra set of moments and lateral loads due to the asymmetric seabed causing an eccentric reaction load. These moments are then redistributed through the JU to other legs and their footings.
These results imply that for a JU on a sloped seabed, a framework combining geotechnical and structural domains can be applied to show the increased load due to the sloped seabed. Additional to the results, a sensitivity study is implemented for a variation of soil type and soil-structure geometry, the latter providing a more realistic seabed slope and larger reaction loads. ... Read more

Rising global CO2 levels have created a growing need for renewable energy resources. Offshore wind energy is a promising solution, with monopile (MP) foundations preferred for their relatively low cost, provided the site conditions are suitable. Recently, wind turbines have significantly increased in size for greater cost efficiency, leading to the development of 3XL MPs with diameters of 12 meters or more. Depending on the site and marine contractor preferences, a jack-up vessel may be used to install a 3XL MP. Marine contractors typically employ dynamic models to determine suitable weather windows for each installation phase. Lowering the 3XL MP through the splash zone and gripper to just above the seabed is a critical phase. To optimize the dynamic models for this phase, understanding the 3XL MP’s behavior during splash zone lowering is essential. This thesis therefore investigates the dynamic behavior of a 3XL MP during lowering phase through the splash zone until just above the seabed using a jack-up vessel.
A 1-DOF model, considering only the rotational DOF of the MP, is established to examine this behavior. Results from this model show that the most probable maximum (MPM) of the MP rotational motion response, evaluated with a dynamic lowering simulation for irregular spread waves, is 75% lower compared to the conventional steady-state simulations at critical MP drafts. Additionally, the dynamic behavior of the MP is highest for increasing peak periods, as these coincide with the natural periods of the 1-DOF model at larger MP drafts where wave-excitation loads are greatest. Several physical phenomena which are not included in the model are examined to assess their impact. Ambient currents traveling in the same direction as irregular unidirectional waves during a dynamic lowering simulation, reduce the MP motion response by 25%. Vortex shedding frequencies do not coincide with natural periods and are thus neglected. The same goes for the 2nd order difference frequency wave drift forces. Both the dynamic amplification and the mean motion displacement due to these second order wave forces are negligibly small. Lastly, the influence of jack-up motion is negligibly small during peak periods of 8 and 9 seconds but reduces the MP rotational motion during a peak period of 7 seconds. ... Read more

The exploration of bottom-founded offshore wind turbine development, focusing on monopiles, indicates that while the floating offshore wind sector is emerging to exploit deeper sites, there is a continued rationale for advancing XXL monopiles in the realm of bottom-founded offshore wind. Cost-effectiveness stands out as a pivotal factor among several compelling arguments.
This study aims to improve the economic viability of XXL monopiles by developing a more accurate methodology for calculating wave loads. The introduction of time-varying hydrodynamic coefficients is central to this effort. Building on an extensive literature review, a novel time-varying coefficient method was proposed. This method accounts for subperiodic fluctuations in wave force coefficients by introducing dependencies on the Keulegan-Carpenter and Reynolds numbers, which are determined using zero-crossing periods. This approach marks a significant methodological shift from previous attempts.
The newly developed method was implemented in a MATLAB environment and is compatible with both first and second-order wave theory, as well as the Morison and Rainey load models. Efforts were undertaken to replicate the results of previous experiments. The model successfully reproduced both regular and irregular waves, with particularly accurate results for the least steep regular waves. However, for steeper waves, the second-order wave theory did not fully capture all characteristics, leading to less accurate reproductions.
A simplified fatigue assessment was carried out using MonoPoly, an in-house software package developed by Sea and Land Project Engineering (SLPE). The North Sea Wind Farm, a reference project by SLPE, was used for site characterization and structural configuration.
The fatigue assessment evaluated various test cases to examine the impact of different analysis approaches in the time-varying coefficient method model. Overall, results showed that the time-varying coefficient method decreased the maximum Damage Equivalent Moment (DEM) values by 0.2 to 0.7 percent, depending on the analysis approach. The mean DEM values decreased by 0.2 to 0.5 percent. These findings apply to a more advanced constant coefficient method used in the industry, compared to the one recommended by ISO. A side study suggested that the decrease in DEM values would be even greater when comparing this new method to the ISO-recommended method.
For thin-walled cylinders like monopiles, the relationship between moment and thickness is essentially linear. Therefore, a 0.5 percent reduction in mean DEM allows for a 0.5 percent reduction in monopile thickness while maintaining stress levels. This reduction in material thickness has the potential to save several million euros for wind farms, demonstrating the significant economic benefits of the newly developed time-varying coefficient method.
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Plastic has many valuable properties and has become an intrinsic part of our everyday life. An approximated 400 million tonnes is produced every year, consequently, vast quantities of plastic waste find their way into the oceans. The question "Why don't we just clean it up?" led to the founding of The Ocean Cleanup by Dutch entrepreneur Boyan Slat in 2013.

As of the writing of this thesis, The Ocean Cleanup has removed 12,989,690 kilograms of trash from oceans and rivers. System 03, consisting of two towing vessels towing a large net, is utilised to extract plastic waste from the oceans. To catch as much plastic at minimum time, effort and money, it is essential to have a comprehensive understanding of the behaviour of this net in water. A 3D Finite Element model is employed to simulate the system. Drawbacks in this model are the high computational demand and inaccuracies. These inaccuracies are caused by missing knowledge regarding drag coefficients at low angles of attack and the extrapolation of net behaviour to large nets.

The main objective of this research is to develop a simple, yet accurate model to simulate the physical behaviour of a high length-to-depth ratio net towed through water. Achieving accuracy in the model requires a comprehensive understanding of the net's behaviour in water, which has been investigated through a series of experiments. The simplicity of the model ensures low computational demand, enabling its use in offshore operations for swift decision-making on board. The developed model is validated using measurements obtained from campaigns conducted by The Ocean Cleanup. The performance of this simplified model is then compared to that of the 3D Finite Element model. Finally, the simplified model is applied to predict the behaviour of potential future systems.

In the experiment, the loads on two different nets were measured as they were towed through a basin at various velocities and angles of attack. The results show a significant decrease in drag at low angles of attack, attributed to the shielding effect. Experiments with two-meter and six-meter nets revealed that the shielding effect becomes insignificant beyond these lengths. Consequently, the drag coefficients calculated at various angles of attack for the six-meter net are utilised in the model.

To predict loads on the system, the model estimates the system's shape. For an accurate simulation of reality, the model makes an initial guess on how the system could look like. After this guess, an iteration loop starts until convergence is achieved. Convergence indicates that the system has reached equilibrium, accounting for external influences, internal forces, deformations, and the shape of the system. The model uses system properties, vessel positions and environmental conditions as input to generate the predicted shape of the system and the predicted loads acting on the system.

The performance of the simplified model is assessed by examining the error between the predicted and measured loads under various conditions. This performance is then compared to that of the 3D model. Both models demonstrate accuracy, with a MAE of just over 20\%. In the wide-span cases, which occur most frequently in the campaigns, the simplified model performs better than the 3D model. In terms of simulation time, the 2D model significantly outperforms the 3D model. While the 3D model requires approximately one week to complete its simulations, the 2D model finishes in 0.14 seconds, making it a flexible and user-friendly option. Altogether the simplified model is a simple, yet accurate model to simulate the physical behaviour of a high length-to-depth ratio net towed through water.

To demonstrate the model's functionality, it is applied to one of the trips, showing the maximum sailing velocities to ensure the system remains within its limits. The same trip is then simulated using potentially new nets and increased sailing velocities, resulting in a higher plastic catch. To improve the model's accuracy, aspects like wave direction, wind speed and wind direction can be implemented. Additionally, conducting a CFD analysis is recommended to gain a deeper understanding of net behaviour, which can help avoid the expense of additional tank tests.
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Thin and large floating structures are increasingly used for a variety of applications, such as floating solar platforms, aquaculture, storage facilities, and even housing. For example, in the Netherlands, where thin plate-shaped floating solar platforms are being deployed in near-shore regions due to their benefits, such as uninterrupted solar irradiance and land conservation. With these deployments, it becomes essential to assess the risk posed to these structures by wave forcing in near-shore environments. Due to the large size of these flexible structures, the potential large impact of long ship-generated waves can be a key scenario of concern.

This thesis explores this scenario using a hybrid model, which couples a weakly nonlinear 2D finite element-based Boussinesq wave-generation model (FEBOUSS) with a 3D linear monolithic fluid-structure interaction (FSI) model. A key contribution of this work is the implementation of a novel, one-way partitioned fluid-fluid coupling algorithm, which efficiently transfers wave-information from the FEBOUSS model to the FSI model. The coupling is achieved by (1) enforcing the normal wave particle velocity from FEBOUSS as a boundary condition at the vertical inlet of the FSI model, and (2) using a damping zone that not only absorbs reflected waves from the membrane but also gradually enforces wave-elevation and normal wave particle velocity at a subsection of the free surface boundary of the FSI model. The accuracy of the coupling algorithm can be adjusted by refining the partition of feeding particles in the overlapping zone while simultaneously controlling the length of the damping zone. This approach ensures both computational efficiency, in terms of total simulation time and data storage requirements, as well as high physical accuracy at the coupling interface. The scenario of ship-induced waves entering a narrow harbour housing a floating structure is demonstrated, focusing on how the velocity of the moving vessel, represented non-dimensionally by the depth-Froude number, influences the hydroelastic response of the membrane. The analysis revealed that for all tested depth-Froude numbers, the membrane initially follows the long ship-induced waves, with subsequent responses that may be either (significantly) amplified or dampened with respect to the incoming waves, depending on the specific case under consideration.

The work presented in this thesis makes a significant contribution to offshore engineering literature by introducing a novel fluid-fluid coupling algorithm that can be extended to even more complex scenarios. Both the FEBOUSS wave-generation model and the FSI model are highly versatile, making them suitable for studying a broad range of fluid-structure interaction problems. Future research could explore the effects of variable bathymetry, irregularly shaped floating structures, and other relevant factors, expanding the scope of this study to include a broader range of physical phenomena.

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