The growth in offshore wind energy has resulted in an increase of the size of the wind turbines and consequently, the monopiles supporting them. Due to limited space on decks of the installation vessels, an alternative method of transportation of the monopiles to the installation site is desirable. One possible approach is towing the monopiles directly on the sea by using plugs to seal both the ends. The towed monopile may experience lateral instability during transport. This thesis presents a theoretical linearised rigid body model to investigate the stability of the system.

The analytical equation of motion for a towed flexible cylinder in still water is derived using linear assumptions, which is then further simplified using rigid body assumption. Using the frequency domain analysis, the analytical equations are solved for complex frequency roots to draw conclusions regarding the stability of the system. Additionally, stochastic lateral distributed drag is also incorporated in the model. However, the analytical model does not include the effect of added mass and damping due to the fluid. The necessary numerical hydrodynamic added mass, damping and forces are obtained through ANSYS Aqwa, using linear potential theory with one way fluid structure interaction.

The hydrodynamic numerical results are however, only for real and positive frequencies and are not solvable analytically. The D-decomposition method is employed in order to circumvent this limitation of the numerical model with added mass and damping, that maps the critical axis of frequency roots on a complex plane of a chosen parameter. This allows the formation of the zones of stability through which critical values can be obtained for a parameter.

A parametric analysis, excluding the geometric parameters of the monopile, shows that the axial drag resistance terms do not affect the stability of the system, while the stochastic drag is an unreliable means of stabilising the system due to wave angle dependence. On the other hand, the added mass and damping tend to destabilise the system as tow velocity increases and their effect is significant. Furthermore, the mathematical boundary damper shows that the stabilising appendage at the tail, in the form of a skeg, can stabilise the system. Though, a detailed study on the stabilising appendage needs to be performed.

In conclusion, this study provides a basis for a linearised dynamic stability analysis of a floating monopile under tow with added mass and damping effects, upon which further studies, experimental as well as theoretical, can be performed for validation and improvements. ... Read more

Offshore wind turbines are usually transported upright on board a jack-up vessel, supported by a sea fastening system. The structural assessment of the towers during transportation and installation is currently done using a local hull and grillage model, assuming that the tower dynamics are decoupled from the jack-up. As the offshore wind turbine towers continue to increase in size, their natural frequencies approach those of the jack-up. This may result in dynamic interaction between the towers and the jack-up. This introduces uncertainty in the assessment of flow-induced vibrations (such as vortex-induced vibrations and interference galloping) of the towers. The question therefore is whether such a local model remains valid for larger towers.

This thesis investigates how the dynamic response of the tower group on board the jack-up vessel differs between a global jack-up model and local hull and grillage model. A computationally efficient finite element model of the jack-up-tower system was developed. This was achieved by combining a Craig-Bampton reduced hull model with beam representations of the legs and towers. The resulting model was shown to accurately capture both global and local dynamics. A local model was created that consisted of a foundation stiffness matrix, representing the local hull and grillage, and beam representations of the towers. These models were used to compare the dynamic behaviour of the towers between the two modelling approaches. In addition a reduced-order CFD method was developed to estimate the vibration amplitudes of the towers subjected to flow-induced vibrations. The method utilizes forced vibration CFD and is based on the principle of energy balance.

Structural analyses showed that the jack-up dynamics can significantly influence the dynamic behaviour of the tower group. This happens when the natural frequencies of the jack-up and the towers are sufficiently close, which is a relatively wide range. This interaction changes the shapes and frequencies of the tower-bending modes. Furthermore, it reduces the tower response. In addition, a global model introduces a significantly lower dynamic stiffness at the tower bases resulting in a different base response compared to a local model. A global model also show increased coupling between the towers and a larger sensitivity to the forcing direction. These effects are not captured by a local hull and grillage model.

The proposed CFD method was successfully implemented and demonstrated for a single vibration mode. Although the predictions are uncertain due to limitations in the simplified CFD model, it is shown that the energy-balance-based approach can, in principle, be applied to a group of offshore wind turbine towers.

It is concluded that a local tower model is only suitable when the global jack-up modes do not interfere with the tower-bending modes. When this interaction occurs, a global model is needed to accurately capture the dynamics of the tower group. ... Read more

Significant wave height (𝐻𝑠) is one of the sea-state parameters on which offshore workability decisions depend. Personnel transfers are commonly restricted around 𝐻𝑠 =1.5 m, motion-compensated lifting around 2.5 m, and many operations cease above 3 m. Near these limits, even a small error in the estimated sea state can change the operational decision. For 𝐻𝑠 estimates to be useful in this setting, they therefore need to be accurate to roughly 0.25 m RMSE across the workability range where such decisions are still being made.

Several methods are already used to estimate or describe offshore wave conditions, but each has shortcomings when considered against the needs of real-time vessel operations. Numerical wave models provide useful regional context, although their resolution is too coarse to capture the local conditions
around a single vessel. Satellite altimetry can support large-scale sea-state observation, but revisit times are too long for workability decisions made from hour to hour. Buoys provide direct measurements, but only at fixed locations, while onboard physics-based radar methods depend on processing assumptions
that may break down in the same conditions where reliable estimates are most needed. None of these sources fully provides a local, real-time, and sufficiently accurate estimate at vessel scale.

This thesis examines whether deep learning can narrow that gap by estimating 𝐻𝑠 directly from operational vessel data. The dataset combines X-band radar imagery, six-degree-of-freedom vessel motion measurements, and reference 𝐻𝑠 values from open-source wave buoys and ERA5 reanalysis, drawn from three operational vessels over several years. A Vision Transformer backbone is applied to preprocessed radar images, with optional vessel-motion fusion. In the sequence variant, the model is given a short series of consecutive radar images rather than a single image, allowing it to use temporal information in the sea surface pattern. Eight model variants are trained across a four-axis ablation covering preprocessing route, backbone initialisation, motion inclusion, and single-image versus sequence-based input.

On the development vessel, the best model comfortably reaches an RMSE lower than the set target under normal wind across the operationally relevant 𝐻𝑠 ≤3 m range, with little systematic bias. The strongest within-vessel configuration is the radar-only sequence model. Adding vessel motion does not consistently improve the estimate and, in the tested configuration, tends to introduce a positive offset at low sea states. For use on the same vessel, the radar-only sequence variant is therefore the preferred model.

Cross-vessel transfer remains the main unresolved part of the problem. Blindly applying successful models from one vessel to another does not meet the operational target, although the reasons are partly identifiable. In one direction, performance is mainly limited by the available sea-state coverage, while in the other it is dominated by a vessel-specific calibration offset. Low wind is the most consistent within-vessel failure mode, which is consistent with the weaker radar wave signature expected under reduced Bragg scattering. Overall, the thesis shows that deep learning can provide vessel-scale 𝐻𝑠 estimates from X-band radar imagery with accuracy well below the defined operational target, while also showing that reliable use across vessels requires either vessel-specific adaptation or broader multi-vessel training data. ... Read more

Mid-20th-century slender orthotropic steel highway bridges exhibit a significant vulnerability to high-cycle fatigue accumulation induced by modern heavy goods vehicles. As complete bridge replacements impose severe economic and environmental burdens, alongside significant traffic disruptions, the civil engineering sector urgently requires sustainable, low-intervention solutions. As an alternative to traditional stiffening, which adds mass statically, this research evaluates the efficacy of implementing passive vibration-control devices, specifically tuned mass dampers, as a dynamic retrofitting strategy to extend the fatigue life of critical welded details within these structures.

A multi-scale numerical investigation was conducted utilising a finite element model based on the steel arch Bridge Roosteren as a reference structure. The transient structural dynamic response was simulated for a single passage of a moving load, corresponding to the Eurocode 1 lorry silhouette A, along the most critical path. From this passage, the fatigue damage was systematically quantified at three primary connections (Rib-to-Deck, Rib-to-Crossbeam, and Deck-to-Crossbeam) using the hot spot stress method, rainflow-counting algorithms, Miner's linear cumulative damage rule, and Eurocode 3 S-N curves.

The integration of parametrically tested passive tuned mass dampers did not result in an overall extension of the structure's global fatigue life. Whilst secondary connections, such as the Deck-to-Crossbeam detail, demonstrated a localised fatigue life extension, the governing Rib-to-Deck connection experienced a fatigue damage increase across all evaluated configurations. A unique geometric alignment was induced by the relationship between the reference structure and the considered vehicle. Specifically, the relation of the 4.5 m longitudinal vehicle axle spacing to the 2.424 m transverse crossbeam spacing induced substantial secondary bending moments. This alignment, coupled with the inherent stiffness of the finite element model, limited the dynamic mitigation potential of the tuned mass dampers. Furthermore, the spatial arrangement of the tuned mass dampers, which primarily targeted global dynamic behaviour, was deemed ineffective because the governing fatigue-critical details are mostly driven by high-frequency excitations rather than low-frequency global bending. Consequently, the tuned mass dampers did not induce a significant reduction in the stress ranges at all details, thereby failing to yield a notable improvement in the overall fatigue life.
Although the study did not yield an overall extension of the fatigue life, the findings indicate the potential viability of dynamic retrofitting strategies. To further define this potential, future research should explore the application of semi-active vibration-control devices to capture a broader bandwidth of traffic-induced vibrations. Alternatively, investigations should target localised high-frequency responses by attaching smaller tuned mass dampers directly to the stiffening ribs rather than the crossbeams. These studies should include influential dynamic factors such as stochastic traffic modelling and vehicle-bridge interaction, explicitly accounting for vehicle inertia, suspension mechanics, and road surface roughness. Until effective dynamic retrofitting strategies are fully validated, static stiffening should continue to be prioritised.
... Read more

Hyperloop is a novel high-speed transportation system that combines electromagnetic suspension with a low-pressure environment within a tubular shell to enable travel velocities well above those of existing high-speed rail. While this concept promises high efficiency and reduced resistance, it also introduces dynamic effects and potential instabilities that become increasingly significant at such speeds. This thesis investigates these phenomena with particular focus on the influence of periodic supports and shell behaviour on the structural response, aiming to provide a more realistic guideway model than those previously adopted in the literature.

The guideway is modelled as a thin-walled cylindrical shell with discrete supports, while the vehicle is represented as a moving mass suspended through a non-contact electromagnetic force governed by a proportional–derivative control system. This setup enables more physically representative modelling by incorporating discrete support spacing and allowing for the inclusion of circumferential pre-stress from the vacuum environment, a feature intrinsic to Hyperloop systems.

Two themes are central to the study. The first concerns the steady-state response of the shell under a constant moving load, which isolates the structural behaviour of the guideway. Using a semi-analytical approach, the governing equations are projected onto circumferential modes, transformed into the frequency–wavenumber domain, and solved with a periodicity condition to reconstruct the steady-state response. This analysis shows that periodic supports strongly modify wave propagation, leading to multiple resonance peaks, including in ranges where operational velocities may lie. As a result, simplified continuous models risk overestimating safe operating speeds and overlooking significant amplifications. The results also demonstrate how geometric and damping parameters affect the critical velocity, offering practical strategies for vibration reduction. Furthermore, the inclusion of circumferential pre-stress is shown to be essential, since vacuum-induced compression reduces the effective stiffness of the shell and shifts the system closer to resonance conditions.

The second theme addresses the stability of the coupled vehicle–structure system, where two distinct instability mechanisms are considered: wave-induced instability from anomalous Doppler waves, and electromagnetic instability from the suspension control system. The periodicity of the support structure enables the potential manifestation of wave-induced instability in the form of parametric instability, which is absent in continuously supported models. The analysis is carried out using a semi-analytical approach that combines Floquet theory, Fourier expansion, and harmonic balance to reformulate the problem as an eigenvalue analysis, from which stability boundaries are identified. The findings highlight the need for careful controller design at operational speeds, with periodicity shown to play a role in shaping instability zones.

Overall, this research demonstrates the importance of accounting for both shell behaviour and support periodicity when assessing the dynamic performance and stability of Hyperloop systems. In doing so, it advances the understanding of Hyperloop dynamics and provides a foundation for future research and the further development of this emerging mode of transportation. ... Read more

As offshore wind turbines (OWTs) grow in size and are installed in deeper waters, the use of traditional jack-up vessels for installation faces increasing limitations. Semi-submersible crane vessels (SSCVs) offer a promising alternative; however, their floating nature introduces complex dynamic behaviour that can adversely affect installation operability. This thesis investigates the operability of single blade installation using an SSCV.

A numerical model was developed in OrcaFlex to simulate the relative dynamic behaviour between the hub of a bottom-fixed 15 MW OWT and the root of a suspended blade on an SSCV, during the alignment phase of the blade installation. The analysis focuses on the Thialf, an SSCV from the Heerema Marine Contractors fleet.

The relative motions of the OWT hub and blade root were assessed through modal and time domain analyses under environmental conditions representative of a Baltic Sea site. Results show that wave induced motions dominate the system’s dynamic response. Hub motions were strongly amplified when wave peak periods approached the natural frequency of the turbine, while blade root dynamics were governed primarily by vessel motion and the coupling between vessel response and the pendulum behaviour of the suspended blade.

Sensitivity analyses demonstrated that operational parameters, including vessel draught, suspension length, and tugger configuration, significantly affect the system’s dynamic behaviour.

Operability was evaluated using limiting criteria consisting of a maximum displacement threshold and a maximum outcrossing frequency. The analysis revealed that operability is restricted under conditions of hub resonance, vessel resonance, or coupling between the vessel and the pendulum behaviour of the suspended blade, and that the choice of limiting criteria strongly influences the overall operability.

SSCVs can perform comparably to jack-up vessels in moderate sea states, where vessel motions remain limited. In more demanding conditions, however, amplified dynamics and resonant responses may induce excessive motions that render the installation inoperable. The applicability of SSCVs for OWT installation must therefore be evaluated with respect to both site specific environmental conditions and the operational configuration. With careful tuning of the installation setup, SSCVs can provide a viable alternative under favourable conditions. ... Read more

In the past, development of offshore wind energy was mainly focused on ice-free regions, but increasing demand has introduced the need for development in sub-Arctic regions, such as the Southern Baltic Sea. The expanding offshore wind development in this area not only presents new opportunities, but also new engineering challenges. In these regions, sea ice may drift against the foundation of offshore wind turbines, introducing dynamic ice loads which can significantly affect the design and safety of offshore wind turbines. Extensive research has been performed on the ice-structure interaction and the resulting ice-induced vibrations. However, this thesis studies the ice-structure interaction on a macroscale, considering how ice fields interact with offshore wind farms as a whole. The main motivation for this focus is that the presence of multiple structures instead of a single structure will affect the ice drift- and growth within an offshore wind farm. Currently, the designers of offshore structures often assume that the ice drift speed may be correlated to the surface wind speed at 10 m height by a constant factor (Hendrikse, 2024). However, the presence of both a single- and multiple turbines may slow down the ice. Previous research indicates that for low ice velocities, severe loading can occur in the form of intermittent crushing and multi-modal ice structure interaction (Hammer et al., 2023). The presence of multiple turbines may also promote ice growth during winter - leading to stationary ice - as they can function as anchor points for the ice to grow onto. If the wind picks up, the (re)-initiation of the stationary ice sheet may lead to simultaneous loading of all turbines at low relative velocities, resulting in severe ice-induced vibrations (Hammer et al., 2023). This thesis therefore aims to explore the influence of an offshore wind farm on the ice drift speed, leading to the consideration of two scenarios: (1) the drift of an ice floe field that originate from outside the offshore windfarm and (2) the drift of an ice floe that is frozen in around the turbines. This is formulated in the main research question: How does the presence of an offshore wind farm affect the ice drift speed for both an ice floe field that originates from outside the offshore wind farm and a grown-in condition in the Southern Baltic Sea? The first objective of this thesis was to establish if the ice drift speed can indeed be correlated to the surface wind speed by a constant factor in the presence of multiple structures. The second objective was to acquire the combinations of ice thickness and wind speeds for which the motion of a frozen in ice sheet could be (re)-initiated. This thesis applies image processing to the satellite images that were captured from the Baltic Sea during the Copernicus programme. The resulting ice floe size distribution is then coupled to a synthetic floe field generator that randomizes ice floe shapes, orientation and placement within predefined boundaries. This enables the generation of ice floe fields having different sea ice concentrations and maximum floe sizes. These ice floe fields are then converted to SIBIS to enable performing simulations. This thesis only considers ice drift in 2D in the horizontal plane of motion, which implies that bending, rafting and buckling of ice are not modelled, as this concerns ice that is forced up or down due to compressive stress. Rather, the ice-structure interaction is dominated by crushing and splitting. The ice drift is simulated on a timescale up to a few hours and a scaled-down version of a typical offshore wind farm was used due to limitations of computational resources. Whereas typical offshore wind farm layouts consist of 50 turbines, this thesis adopts layouts using an evenly spaced 3 x 3 and 4 x 4 grid, having an inner distance of 1 km. When the floes are initially positioned outside the offshore wind farm, the results shown that the correlation between the ice drift speed and the wind speed is lost during the ice-structure interaction. The lowest ice drift speeds are found to occur for the floes that are directly in contact with the turbines, experiencing an average velocity reduction of 41.6% as compared to the case without turbines present. These floe speeds are in the range where they can promote intermittent crushing and multi-modal icestructure interaction. Furthermore, the results show that ice floe velocities at the turbines are affected by the interaction among ice floes that are not in direct contact with the turbines. Due to the rotation of ice floes around structures, the velocities at a turbine may also be influenced by the presence of neighbouring turbines. These effects lead to a misalignment of the ice- and wind loading, highlighting the importance of macroscale modelling. Results for the frozen-in scenario indicate that (re)-initiation of the stationary ice sheet may occur for several combinations of wind speed and ice thickness that are typical for the Southern Baltic Sea. The critical wind speed across multiple offshore wind farm layouts - and corresponding ice sheet area - was found to depend on the critical wind drag force. This is the wind drag force for which the resistance of the turbines to the motion of ice is exceeded. For the case of an offshore wind farm modelled after the existing Baltic Eagle wind farm layout, it was found that all wind speeds above 26 m/s can lead to simultaneous loading of all turbines. For both scenarios, it was not validated how using a low number of turbines - as compared to a reallife offshore wind farm such as the Baltic Eagle wind farm - affects the results that are presented in this thesis. For the simulations in which the floes originate from outside the offshore wind farm, this thesis identifies several effects - such as the spatial extend of the speed reduction across the floe field - that can be attributed to the presence of very large floes within the ice floe field. Furthermore, it is expected that the shielding of secondary- and tertiary rows of turbines may be related to the obstruction of these very large floes. To further explore these phenomena, a high number of simulations should be performed - that include a high number of turbines - to address the stochastic behaviour of an ice floe field interacting with an offshore wind farm. For the grown in scenario, the sensitivity of the identified critical wind speeds to changes in the offshore wind farm layout and the geometry of the ice sheet has not been validated. Furthermore, this thesis assumes a water current equal to zero, as both the magnitude and the direction of the current in the Baltic Sea were found to be highly varying. However, an offset between the direction of water currents and wind is commonly observed under natural conditions. Hammer et al. (2023) showed that misalignment of wind- and ice loading direction can result in ice-induced vibrations developing for higher ice drift speeds as compared to an aligned scenario. This effect has thus not been incorporated in this thesis. Finally, the crack paths are only drawn in straight lines during splitting and crack propagation is not modelled. It is speculated that this can affect the macroscale ice drift. ... Read more

The study explores different system identification techniques that utilize machine learning, including the Restoring Force Surface (RFS) method and Sparse Identification of Nonlinear Dynamics (PySINDy).
These methods have potential to help uncover physical models for soil-pile interaction.
To test these methods , the research first applies them to well-known benchmark systems—simple mechanical models with known nonlinear behaviours. This ensures that the identification techniques work correctly before applying them to real pile-driving experiments.
The experimental data comes from lab-scale vibratory pile-driving tests using strain gauges and accelerometers. The study analyses how forces acting on the pile change over time, focusing on both the tip and shaft resistance. Various mathematical models are tested to see which best captures the nonlinear behaviour. ... Read more

Due to urbanization, improved living standards and electrification, approximately five times more raw minerals are necessary in 2050 compared to 2018. In deep oceans, the seafloor contains these minerals in the form of polymetallic nodules. Nodules are about the size of golf balls that grow throughout the ocean at depths between 3500 m and 6000 m. They contain a wide variety of metals, such as manganese, copper, nickel, cobalt. Nowadays, for large-scale applications, hydraulic lifting is almost exclusively considered for vertical transportation through the water column. However, there is little research available about using other techniques instead. To tackle this knowledge gap, this thesis studies the feasibility of transporting the nodules using a concept of mechanical lifting. The concept used in this thesis consists of two alternating containers that are lowered and hoisted by lifting and guidance wires. Due to the conditions, such as the large depth, the environmental characteristics and the positioning and heading of the vehicles, there are technical uncertainties regarding mechanical lifting. Risks include the yaw rotation of the container, which might result in rope entanglement and wearing of the ropes. This thesis presents a study into the yawing stability of the concept of mechanical lifting for the vertical transportation of polymetallic nodules, which is a crucial factor to operate reliably.

The research question is answered by performing an experimental test and a CFD analysis. The experimental tests include the dynamics of the system while testing various configurations and is validated by an analytical integration in time and a CFD simulation at model scale. The CFD analysis takes away the uncertainties and unknowns: the drag force, the yawing moment and the fluctuation magnitudes and frequencies. The CFD analysis is performed using the open-source software OpenFOAM and simulates multiple configurations. The results of the simulations are compared to the restoring moment by the guidance wires, by transforming the excitation moments into static and dynamic responses of the system. The CFD model is validated by testing the model with a 2D cylinder and 3D sphere, and by performing a mesh convergence study. The CFD simulations are validated by literature. With the obtained drag forces, the energy consumption is calculated.

From the results, it can be concluded that the system can stably be transported at 2 m/s, as the static and dynamic responses are well within the safety limits. The largest response occurs in the middle of the water column, as the rotational stiffness is the smallest at that location. The dynamic response is smaller compared to the static response, as the high frequent fluctuations (f > 0.075 Hz) are damped. Rope entanglement will not occur during normal operation at 2 m/s. However, critical situations due to incidental events can arise, including a winch failure, friction or a sudden high current. This has not been evaluated in this research and therefore stability cannot be guaranteed. As lowering at 3 m/s with an inclined system and including the current results in a static maximum yawing rotation larger than the safety limit, the stability cannot be guaranteed for operating at 3 m/s. ... Read more

The rapid development of the wind industry over the past few years has pushed turbine manufacturers to meet the growing energy demands by designing and producing large scale wind turbines.This also means development of larger monopile foundations for the foundation designers in the case of offshore wind turbines.\ Generally, the turbine tower and monopile are modeled together and the loads from the rotor-nacelle assembly are provided by turbine manufacturers.\ The offshore industry is now showing more interest in extracting the loads from the top mass by developing their own tools in order to reduce the dependency on the manufacturers. In order to aid in this process, the present master thesis aims to develop a linear model based on the concept of Dynamic Substructuring which employs a set of equations to compute the interface forces using the kinematics.\ Furthermore, the developed prediction model is used to analyze the loads occurring at the interface between the rotor-nacelle assembly and the tower for different wind speeds and wind conditions.Consequentially, the model was found to produce acceptable loads at higher wind speeds for selected degrees of freedom at the interface while failing to do the same for other degrees of freedom.The results in time domain were converted to the frequency domain to analyse the resonance.The influence of resonance on the interface degrees of freedom was found to be higher at wind speed below the rated condtion.\ These findings can be used as a basis to conduct further investigations into the application of numerical integration concepts to aeroelastic structures. ... Read more

With the Paris climate accords signed in 2016, most countries have committed themselves to ambitious climate targets during the next decades. One of these targets is a dramatic increase in the overall energy portfolio's market share of renewable energies. This increase in renewable market share will, for a large part, consist of newly built offshore wind farms. In turn, this rise in offshore wind energy projects is expected to be especially dramatic in northern regions, where high and constant wind speeds prevail. However, as offshore wind farm projects move further north, additional challenges need to be faced. One of these is the technical challenge to design offshore wind farms for possible encounters with drifting sea ice. To tackle this challenge, a proper understanding of the mechanics associated with encounters of drifting sea ice with offshore wind turbines is essential.
Such encounters are currently primarily understood phenomenologically, and the associated models simulating these encounters – or ice-structure interactions – therefore are phenomenological as well. Moreover, most ice-structure interaction models are fundamentally one-dimensional, whereas ice-structure interactions are generally not one-dimensional. This mismatch holds especially for ice-structure interactions with offshore wind turbines, where wind loads are generally misaligned with ice loads causing highly two-dimensional ice-structure interaction problems. Therefore, the first half of this work sets out to extend one of the industry-leading phenomenological one-dimensional models – the Hendrikse (2017) model – to a two-dimensional environment. The ultimately developed Zero-friction contact Area variation Model By Omnidirectional Numerical Ice (ZAMBONI) attempts to do so by introducing practical extensions rather than introducing new assumptions. Nevertheless, one extension does entail a shift from current one-dimensional ice-structure interaction models. Namely, the assumption that ice experiences neither friction at the ice-structure interface nor internal shear forces. Consequently, much of the correctness of this model hinges on this extension. To assert the correctness of ZAMBONI. A comprehensive verification campaign is performed as well as a simple order-of-magnitude validation campaign. Although both confirm the extensions' correctness, further validation is required, especially concerning the zero-friction principle. Upon developing and discussing this two-dimensional ice-structure interaction model, the second half of this work couples ZAMBONI to an offshore wind turbine model to gain further insight into ice-structure interactions. These dynamically coupled two-dimensional simulations serve two purposes. Firstly, to compare one- and two-dimensionally simulated load cases of aligned ice and wind. Secondly, to perform newly simulable load cases of misaligned ice and wind. Four primary findings are discussed. Firstly, as hypothesized, introducing a disturbing wind load lowers the ice-structure contact area, causing smaller loads and displacements due to ice loads. This effect is especially well observable for misaligned wind loads and low far-field ice velocities. Secondly, a new ice-structure interaction regime is observed where ice and structure synchronize in the structure's first bending mode. This synchronization occurs most dominantly for two-dimensional ice. Thirdly, frequency lock-in occurs solely in the second bending mode and is terminated at lower ice indentation speeds for two-dimensional than for one-dimensional ice. Finally, small ice-wind misalignments, which are most common, appear highly similar to load cases of fully aligned ice and wind. ... Read more

Deep sea minerals can offer an additional resource to meet the increasing mineral demands, instigated by population growth and technological advancements. Deep sea minerals exist in different forms at the bottom of the ocean. In this research, polymetallic or manganese nodules are the kind that are of interest. The nodules are 1 to 12 cm large and contain a variety of minerals like copper, nickel and cobalt, but owe their name to its main component manganese. The region with the highest approximated resource of polymetallic nodules is the Clarion-Clipperton Zone (CCZ), situated in the Pacific Ocean between Hawaii and Mexico. The CCZ has water depth reaching 6000 meters, which is a significantly larger working depth than state-of-the-art deep sea projects within the offshore industry. These depths are accompanied by challenging environmental conditions exerted on the deep sea mining system. A deep sea mining system typically consists out of three components: 1) Production Support Vessel (PSV), 2) Vertical Transport System (VTS) and 3) Seafloor Production Tool (SPT). The SPT harvests the nodules from the seabed, the VTS transports the mined nodules through the water column to the surface where they are transferred to the PSV. The focus in this thesis will be on the Vertical Transport System. Where most deep-sea mining developments are considering hydraulic vertical transport with a riser, Boskalis introduces a concept that utilizes mechanical lifting for the vertical transport. This allows for energy efficient transport, relative simplicity of concept and a maximization of the amount of power units above water. The objective of this thesis is captured in the following research question: What is the behaviour of the combined mining system (ropes, skip and SPT) during vertical transportation by means of mechanical lifting? To answer this question, a wide overview is given of the deep-sea minerals that exist on the seafloor and the existing technologies to harvest them. Whilst the system is in principle relatively simple (just two containers (skips) that are alternatingly filled, hoisted to the surface, emptied and lowered again) many challenges arise. To identify these challenges, the system and the production cycle are discussed in detail. Literature research has been done to ensure realistic modelling of characteristics like structural damping of the rope, drag forces and added mass. The safe working load of steel wires is mostly consumed by its self-weight at a length of 4000 meters, making them unsuitable for deep-sea mining. Instead, the less common but naturally buoyant synthetic fibre rope is envisioned. Many of the challenges in this deep-sea mining system originate from the environment as the system is subject to wave action and currents. Therefore, the current profile and wave spectrum typical for the Clarion-Clipperton Zone are obtained to serve as input for further investigation in the hydrodynamic analysis software Orcaflex. The system will be deployed over the entire 6000 m water column, causing the current but also the forward velocity of the system to possibly lead to high drag forces. A reduction of the forward velocity by introducing a new harvesting method is implemented, resulting in a large reduction of the drag forces and offset. The system consists of at least eight ropes, with two moving skips. Combined with the current and vessel motion, rope entanglement is a risk. A solution to prevent the rope entanglement is presented in this thesis. Possible occurrence of vortex-induced-vibrations (VIV) is identified and future research is recommended. The offset analysis shows that a large offset (500m) between the PSV and SPT results in relatively low horizontal forces on the harvester. The system is connected to the PSV, which is subjected to the Pierson-Moskowitz wave spectrum environment it is situated in, resulting in vessel motions. These motions will govern the dynamic behaviour of the system. The skips with attached fibre ropes have different eigenfrequencies on different water depths, as a longer rope will make for a softer system. This causes both skips, full and empty, to resonate in some regions. Consequently, the dynamic tension in the ropes is higher than the static tension, although it does not come forward as problematic. However, undesired slack rope conditions can occur when lowering the empty skip. To conclude, an analysis of the deep-sea mining system has been done in which the eventual design has been modelled to the best extent currently possible. This research underlines the technical feasibility of this deep-sea mining concept. This research also evaluates the questions that have not been answered yet and recommends a variety of interesting topics for future research. ... Read more

The BAM Tidal Bridge is a proposed bifunctional concept of a bridge connection between two Indonesian islands, and the world’s largest tidal power plant. The wave forcing on the floating structure leads to an undesired dynamic response and a decreased operating reliability. The thesis objective is about designing an additional structure or a design modification to the Tidal Bridge that reduces the downtime to a maximum of five days per year. A model has been developed to analyse the dynamic response of the original Tidal Bridge design, and to test possible design optimisations. Three successive design loops lead to the resulting design of an innovative sway plate structure which fulfils the design objective well. ... Read more

Oceans account for 71 percent of the earth’s surface, marine resources and energy are abundant. Therefore, making full use of marine energy is a good choice for humans to solve the energy crisis. One way to capture ocean energy is converting wave energy to electrical energy, by means of devices called wave energy converters (WECs). This project introduces a new type of wave energy converter named “Gyroscopic-Pendulum Wave Energy Converter (GP WEC)”. Compared to the classical vertical axis pendulum WEC, a flywheel is added in the system. In combination with the floater motions it creates a gyroscopic effect on the pendulum causing it to rotate, a power take-off device is connected directly to the rotating pendulum shaft in order to harvest the wave energy and generate electrical energy. To investigate whether this new type of WEC will generate more energy than the classical one, this thesis proposes a dry test setup for the gyroscopic pendulum allowing for systematically investigating the gyroscopic effect on its power output. This thesis starts from the design of the GP WEC dry experiment and then provides the clear definition of all the components of equipment, along with the applicable scaling laws of all the components and parameters. Also, the requirements as to limits of the equipment are studied with a parameter study. Using the results of the parameter study a numerical model of the GP WEC is used to simulate the dry-tests. Based on these simulations the range and number of parameters that will be tested in the future experiment are confirmed, and test matrices defined. Some interesting observations from the numerical simulations are further studied looking into the time domain response. This thesis concludes with the definition of a test setup for dry experiments to be executed at TU Delft in a follow-up study, test matrices for investigating the gyroscopic effect on the power output are defined, and simulation results are presented which can be used for later validation of the physical model. ... Read more

The oceans, which cover nearly 70% of the earth’s surface, can be considered as an inexhaustible energy source for renewable electricity due to its size and predictability. One way to capture ocean energy is by harnessing the energy produced by waves at sea, by means of devices called wave energy converters (WECs).

Delft University of Technology is developing a new floating WEC concept called the ”gyroscopic-pendulum”. This concept is a modification of the so called ”classical vertical axis pendulum”, which is capable of producing mechanical power harvested from the rotations of the pendulum around the vertical axis.

The new concept is proposed by adding a flywheel with the aim to enhance the rotations of the pendulum about the vertical axis. The enhancement comes from gyroscopic precession which is created due to a change in the angular moment of the spinning flywheel caused by the torque originating from the weight of the pendulum.

This thesis starts with a general introduction about wave power followed by the mathematical and numerical model of the gyroscopic-pendulum. Numerical simulations are performed in which the gyroscopic-pendulum and the classical pendulum are both imposed with the same harmonic roll motion, while the gyroscopic-pendulum system also receives some power input to rotate the disk. The main objective is to find out in which ranges of amplitude and frequency of imposed motions, the gyroscopic-pendulum results in an improvement of the power efficiency compared to the classical vertical axis pendulum.

The results obtained from tests performed in the simulated conditions, shows us that the gyroscopic-pendulum has a significantly higher efficiency compared to the classical vertical axis pendulum when the frequency of the imposed roll motion is in the range of 1.4 to 1.75 rad/s and the amplitude is in the range of 0.6 to 0.95 푚. ... Read more

For the offshore market an increase in renewable projects can be observed. The technological innovations are continuously decreasing the costs of renewable projects. Especially offshore wind is getting more cost effective, and is receiving a lot of attention. However, with the growth of the sector, new challenges arise. Water depths are increasing, soil parameters worsening and greater distances from shore need to be overcome. Whereas prices of installation are under pressure. This drives the market towards larger and more innovative installation vessels. A trend can be observed towards monohull craning vessels which combine a large crane with a large storage space on deck. During lifting the monohull vessel experiences large motions of the lifting configuration at even small wave loading. The large motions are mainly the result of resonance within the system, which results in large crane forces. It is these forces which decreases the vessels work-ability. Applying damping to the system is a way to counter the resonance. The tugger winches can be used to apply damping to the lifting configuration. The aim of the thesis is to investigate the potential damping effect of a tugger damping system for the Bokalift 1 during a jacket lifting operation. With special emphasis on the effect of different control systems of the tugger winches. Two different models are used in the thesis to research the effects of tugger damping on the dynamic behavior of the lifting configuration. Namely, a 2D matlab model, and a more extensive 3D Orcaflex model. The Orcaflex model is build for researching the effects of tugger damping and different control systems. Comparing the responses of the model to airy waves loading. To have control in both longitudinal and transverse direction boom winches on the crane are used, in combination with deck winches. Which are located on both ends of the deck. The hydro static properties of the model are calculated with the program GHs. The hydro dynamic properties are calculated with the help of AQWA. The matlab model is used to enable quick research on the effects of tugger damping and different control systems. The model is based on a 5 degree of freedom (DOF) mass-spring-damper-system. The equations of motion (EOM) are derived using the lagrange formalism with help of the program Maplesoft. The model is validated with help of proven software Orcaflex. Seven different control systems are made and tested in the matlab model. The control systems are examined in two different simulations. Firstly the roll motion is studied when the model is subjected to wave loading. Secondly the damping capabilities for the lifting configuration with initial displacement are tested. The first analysis shows the PID controller has the highest roll motion reduction of the jacket. Linear control system scores higher than quadratic. The results from the second analysis shows that stepwise controller takes the shortest to fully damp the lifting configuration. Following on the results of the 2D model analysis the 3D model will further inspect the effect of tugger winches for the linear, quadratic, and PID controller. A model analysis shows there are 4 important modes within the wave excitation range. The combination of these modes results in the highest crane forces at a wave period of 5.5s. The linear and quadratic control system are once again compared, only now in the 3d model. The assessment shows the linear model as more efficient in reducing the crane tip forces. Lastly the linear and PID control systems are compared towards each other and a model without tugger damping. Based on the results, the linear control system increases the total forces on the crane. Where as the PID reduces all forces. It is shown that tugger damping does not necessarily decrease forces acting on the crane tip for head on waves. From the tested control system the PID controller reduces the forces most effectively and efficiently. However the effect of the PID controller depends on the loading wave frequency and for which it is tuned. ... Read more

Deep-sea mining is becoming more popular. The Atlantis II Deep is a deep sea mining field of metalliferous muds in the Red Sea. This field could be exploited by transporting the sediments through a pipeline laid from shore onto the sea bed to the field. From determined concentrations with their shear stress-shear rate relation, parameters for the power law and Bingham plastic model were determined, relayed to test data, so that in the end the Bingham Plastic model was used to calculate pwoer consumption for a variety of cases. ... Read more

The offshore wind industry has been extended over the last years in areas of active seismicity, such as East Asia, where the design of offshore wind turbines becomes significantly challenging, because albeit aerodynamic and hydrodynamic loads mainly act on the offshore structures, earthquake could emerge as a potentially enormous threat. The demand for reliable and economical design of offshore wind turbine foundations has driven the research for analysis of the structural behaviour under the combined action of loads and the study of the parameters that could influence it. The present master thesis deals with the dynamic analysis of the response of an offshore wind turbine monopile, one of the most common types of foundations, subjected to the application of hydrodynamic and earthquake loads. This study focuses on the understanding of the dynamic properties contributing to the dissipation of energy experienced by the structure. More specifically, the sources of damping leading to reduction of the structural vibration in time are investigated, of which the numerical determination is considerably uncertain, while emphasizing on the hydrodynamic and the soil damping. A numerical approach for the estimation of the hydrodynamic viscous damping is presented based on the calculation of the drag coefficient CD and its dependency on the Reynolds number (Re), the Keulegan-Carpenter number (KC) and the surface roughness (k/D). The drag coefficient, and accordingly the hydrodynamic viscous damping, are derived over the length of the monopile where the waves act, highlighting also the consequences of the changes in diameter and depth. Furthermore, the soil radiation damping due to the seismic waves is studied by including the interaction of the soil with the structure. Particularly, the supporting soil is modelled around the monopile with frequency-dependent springs and dampers to represent the soil stiffness and damping, respectively. The estimation of the soil coefficients is accomplished by integrating in the model of the structure, an advanced soil model developed by Dr. J. De Oliveira Barbosa, which gives the dynamic impedance function for the desired band of frequencies. The analysis of the structural response is executed by examining three load cases for the hydrodynamic and earthquake loads. The overall outcome reveals that a noticeable amount of energy is dissipated because of the presence of the soil radiation damping, drawing also the conclusion that the soil-structure interaction should be considered as frequency-dependent during earthquake. Despite the fact that the approach for the estimation of the hydrodynamic viscous damping constitutes a more precise method, its participation in the specific tested cases is limited to the total amount of damping. ... Read more

In order for the offshore wind industry to be competitive, the cost has to be driven down. One of the major factors that contribute to the overall cost is the over-dimensioning of the substructure. The response of the structure under various loading conditions can provide valuable insight in the design phase so as to provide a structural design that is cost effective yet reliable, and can withstand the loads that are considered to act on the structure.

The objective of the present thesis is the development of a 1D finite element model, that allows for a dynamic analysis of an offshore wind turbine under the combined actions of wind and wave or wind and ice. For this purpose, different models have been combined and improved or extended. Through this model the importance of accounting for non-linear and breaking waves, the effect of the kinematic stretching on the response and the manner in which the misalignment of the load affects the response can be investigated.

A detailed design of the NREL-5MW offshore wind turbine supported by a monopile is subjected to wind, wave and ice action. The aerodynamic action is evaluated through a model valid for the above rated regime when pitch control is active, using a turbulent wind signal resulting from the Kaimal spectrum. The hydrodynamic action is calculated either with the Morison equation or the MacCamy and Fuchs equation with the use of either linear or nonlinear water particle kinematics. An approach towards the calculation of the load from a breaking wave is considered accounting for the wave skewness and asymmetry during such an event. The ice action is calculated through a model that evaluates the force while in the crushing regime. The soil is represented with linear soil springs.

The structure’s response is investigated for all the loads separately at first. The next step is the combined analysis. Aligned and misaligned cases are considered. Results show that wind load is dominating the response in the aligned and misaligned wind and wave case regardless of the method used to calculate the hydrodynamic load in the case of a small wave height. In the case of a larger wave height, using Stokes theory and the Morison equation, the hydrodynamic load is contributing to the resulting response. Concerning the ice loading, the intermittent crushing and the continuous brittle crushing regimes occur for the turbine. The response to the combined wind and ice action appears to be affected by both loads in all examined cases.
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