The expanding offshore wind industry is facing challenges with excessive movement of the power cables that connect offshore turbines to the grid. The casings that surround the exposed parts of cables near structures, known as cable protection systems (CPS), show damage before the end of their expected service life, requiring costly repair and maintenance. A potential solution is to stabilise the CPS by placing one or more rock-filled mesh bag (rock bag) on top of it. There are multiple rock bag manufacturers and distributors. The bags are made of polyester netting that is relatively flexible. The sizes considered for this study are 4 and 8 ton (4000 and 8000 kg), but smaller and larger sizes are available. Currently, little is known about the behaviour and stability of these rock bags in offshore environments. The goal of this study is to expand the knowledge through empirical model testing.
480 experiments were conducted in a flume in the hydraulic engineering laboratory of Delft University of Technology. Two set-ups were used, one with a flat rough bed, to test two or three rock bags side by side and grouped formations of bags. The other is a realistic halved model of a monopile, scour protection and CPS. Regular wave, irregular wave and combined wave-current conditions have been tested.
Analysis of before and after top images of the tests is used to establish a failure criterion. In this way, a stability limit applicable for irregular wave fields is found. The effects of grouping and near monopile flow amplification are quantified.

The transition to more sustainable energy has led to a growing demand for offshore wind energy, necessitating larger structures and heavier foundation piles. During pile installation this increases the risks of uncontrolled pile run, which can have fatal outcomes and project delays. Offshore superstructures predominantly rely on largediameter open-ended driven piles. Intermediate soils pose significant challenges in determining soil resistance during driving and pile run, with calculations typically performed using Static Soil Resistance to Driving (SRD) methods. The Alm & Hamre method is preferred for deep foundation piles in mixed soils due to its inclusion of friction fatigue.
The goal of this research with corresponding research objective is to improve pile run velocity and trajectory predictions for offshore open-ended pile installation in intermediate soils. This objective is reached through research on drainage state, identifying soils susceptible to a shift in this drainage state, and analyzing velocitydependent resistance for CPT and pile velocities. The findings are then incorporated into a modified SRD model. The results are compared to a case study using CPT and borehole data as input, and installation video’s and driving data as validation material.
Key findings indicate that with increased pile velocity the drainage state of several soils can shift towards the more undrained spectrum and therefore the soil will have a smaller soil resistance. These soils with a lower soil resistance during pile installation velocities then predicted include intermediate soils such as silt, sandy silt, and silty clay. Thin alternating layers of sand, clay, and silt are also likely to experience a shift in drainage state. Later silty sand is identified as a soil with a high possibility of being prone to such drainage state shifts.
The SRD method, incorporating velocity-dependent resistance, predicts pile run 31% more accurate than models without this consideration. By including velocity-dependent resistance drops, the model accounts for the changes in soil resistance that occur during pile run, leading to more accurate predictions compared to the standard SRD model. The model used in this research uses a single SRD update for velocity dependent resistance. However, in scenarios with large pile runs trajectories and high pile velocities, or when a substantial portion of the soil is prone to a drainage shift, performing a single update for velocity-dependent resistance will not result in a converged solution. As such for a correct solution, multiple iterations are necessary.
When the model predicts a deeper Self weight penetration depth than observed, the predictions for pile run initiation are not reliable. Given that pile run initiation can be very delicate, further research is needed for locations with CPT and borehole data directly beneath the pile. Additionally, incorporating hammering parameters, such as the added weight due to hammer momentum, should be explored to improve these predictions.

This thesis explores the occurrence of salt dry-out and hydrate formation when injecting CO2 into porous media. In large-scale CCS projects, injecting CO2 can potentially lead to salt precipitation or hydrate formation. These processes diminish injectivity and negatively alter reservoir rock properties. To gain deeper insight, experiments were conducted utilizing microfluidic setups, which allow for visual observation of salt-crystal or hydrate formation. Using microfluidic chips, ten salt dry-out experiments were conducted with varying pore sizes and six hydrate experiments were conducted with varying pulse trigger times. For the dry-out experiments, it was shown that salt crystals form mostly at the outlet side and that heterogeneity has a large impact on the precipitation process. A heterogeneous pattern results in a shift in salt distribution to the small pores, with results showing salt saturation at 7% in the small pore section of the medium-small pore chip, exceeding the 3% in the larger pore section. This shows the significant role of capillary action on salt precipitation. Results highlight that higher CO2 flow rates accelerate water evaporation and salt formation, yet final salt precipitation levels remain similar across varied flow rates. For instance, in the small pore size chip, final salt saturation was observed at 7% with a flow rate of 4.38 mm/s, decreasing to 4% at 0.78 mm/s. Additionally, the importance of high water saturation for salt dry-out and the impact of water backflow is shown. For hydrate formation, the importance of temperature and pressure was noted in these experiments. Three different pressure pulses were employed: manual control, a 0.5-second electronic pulse, and a 0.2-second electronic pulse, which all showing great effect on hydrate formation, yet no correlation could be determined between pulse length and hydrate saturation. Specifically, manual control yielded a 15% hydrate saturation with a 9.4% conversion factor, while the 0.5-second pulse achieved a 7% saturation and 9.1% conversion factor. The 0.2-second pulse resulted in 8% saturation and a 5.9% conversion factor. For the dissociation, the large effect of temperature was observed. All experiments showed a stable hydrate concentration, and a dissociation temperature between 5 and 9°C where temperature differences as small as 0.1°C were shown to be the difference between no dissociation and complete disappearance of all hydrates. Next to this kinetics, an interesting observation regarding the hydrate morphology was made. In addition to the five hydrate morphologies found in literature, a sixth, ‘sheet’-like type was observed.

In recent years, there has been growing interest in exploring the deployment of wave energy converters (WECs) in remote and harsh environments. However, research in this area remains limited, particularly concerning offshore environments with sea ice. This study focuses on investigating the energy production and economic feasibility of a point absorber in the Baltic Sea, specifically off the coast of Åland, which experiences seasonal ice cover. Four winter seasons with varying ice conditions are examined, ranging from ice-free to severe ice conditions. Additionally, the study aims to assess the survivability of the WEC under extreme level ice action and extreme wave conditions.

Based on literature review, a hexagonal slope-shaped buoy has shown promise in withstanding ice conditions up to 15 cm thickness in the Baltic Sea and is selected as the WEC design in this study. Metocean and sea ice data spanning from 2006 to 2021 are analysed from the NORA3 database. Through extreme value analysis, key parameters such as wave height, period, and ice thickness are determined. Survivability analysis is conducted to understand the forces exerted on the WEC during extreme ice load cases and extreme sea states. To evaluate energy production, hydrodynamic coefficients are computed using the Boundary Element Method solver Capytaine in the frequency domain. Subsequently, simulations are conducted using WEC-Sim to derive the power output of the WEC under varying sea states. Optimisation of the Power Take-Off (PTO) damping is performed to enhance performance for the specific site conditions. A comparison of power output is made among different WEC configurations with varying translator sizes.

The survivability analysis reveals important design considerations, especially regarding extreme ice conditions. When subjected to an extreme level ice thickness of 60 cm this results in calculated horizontal and vertical forces of 615 kN and 315 kN, respectively. In extreme sea states, simulations in WEC-Sim shows a maximum heave response of 4.16 m and a maximum heave force of 133 kN. Additionally, the investigation reveals significant fluctuations in wave energy converter (WEC) power production across the analysed winter seasons, characterised by varying ice conditions. During severe ice conditions (2009-2010), energy output decreased by nearly 50% compared to ice-free periods (2019-2020), potentially leading to a 95% increase in the levelised cost of energy (LCOE) if solely derived from that single season. These findings give valuable insights into the optimal WEC configuration, the maximising of power output and offer important considerations to WEC survivability for deployment in ice-covered regions.

This report, titled Expanding Puerto Rawson: Enhancing Fishery Capacity and Project Cargo Logistics, presents a conceptual masterplan for the eco-friendly expansion of the Port of Rawson. The port’s proximity to a region abundant in marine resources makes it an attractive location for expansion. However, the port is facing strategic expansion problems and it could fail to accommodate the rise in demand from the fishery industry. This report carefully maps out the gap between the expected rise in demand and the currently existing infrastructure. After that, two models are created to transform the rise in demand that is expected by 2030 & 2040, to nautical demands for the port, like number of berths and required waterway width. The results from these model in combination with a strategic overview of the landscape, were used to come up with three conceptual designs that eventually converged into one final conceptual design by conducting an MCA. This conceptual design, together with the port waste management plan, could enable an eco-friendly and future-ready expansion of the Port of Rawson.

Sea turtle nesting beaches are under increasing pressure from climate change, rising sea levels, and human activity, making the protection of critical habitats like Ostional Beach, Costa Rica, an urgent priority. This study integrates the analysis of wave runup dynamics, groundwater behavior, stakeholder collaboration, and coastal squeeze mitigation to address the unique challenges at this vital olive ridley turtle nesting site.

Wave runup was examined using timestack imagery analysis. Two automated extraction models— entropy-only and entropy-saturation—were compared against manually digitized data. Results show that the entropy-only model is more reliable in capturing peak wave run-up values, a critical measure for understanding inundation risks. Challenges in accuracy, particularly for the entropy-saturation model, were linked to the site’s unique environmental conditions, such as dark volcanic sand. The findings fill a gap in understanding how specific extraction methods perform under unique site conditions, with implications for improving future modeling efforts.

Groundwater dynamics were studied using pressure sensors installed in custom-built wells, revealing significant interactions with tidal forces. These measurements highlighted the role of tidal cycles in influencing groundwater levels, providing crucial insights into the potential for nest inundation. These findings extend existing knowledge by combining tidal and hydrodynamic factors specific to turtle nesting sites.

Collaboration within the TURTLE project was analyzed through semi-structured interviews and structural evaluation of partner interactions. A tailored framework was developed to enhance communication and coordination among stakeholders, addressing identified gaps and leveraging existing strengths. This framework contributes to more effective project management and the application of scientific insights in conservation strategies.

To mitigate coastal squeeze—a phenomenon where natural habitats are compressed by rising sea levels and human development—the study evaluated strategies such as foreland restoration and managed retreat. Findings suggest that integrating habitat restoration with community involvement is critical to preserving the ecological and social balance at Ostional Beach.

This study takes an interdisciplinary approach to explore how wave runup, groundwater behavior, and collaboration strategies can be effectively combined to support conservation efforts. The results stress the need for tailored, site-specific solutions that blend engineering, ecological, and social perspectives to protect endangered species and their habitats. By bridging knowledge gaps and offering practical recommendations, the research bolsters both local and global initiatives aimed at preserving vulnerable coastal ecosystems. 

The erosion problem along Playá Union presents significant challenges for future port expansion at Puerto Rawson. This research seeks to address the question of how to achieve a sustainable and durable port expansion, while minimizing environmental impacts, particularly concerning sediment imbalance along the coastline? Using 30 years of wave data, both normal and extreme wave conditions are simulated with SWAN, a numerical based wave model. Conceptual port expansion designs are developed, resulting in a final design with an integrated fully dimensioned breakwater. Based on visual inspections, data, and research, a new cement mixture is proposed for the breakwater armour units. A Life Cycle Assessment evaluates the environmental impact, while the effect on alongshore sediment transport is assessed using the SWAN model outcomes and the CERC formula. Visual inspection of the current breakwaters lead to a 6.3% reduction in material use in the new breakwater through reuse of armour units. Furthermore, the proposed cement mixture integrates porphyry quarry waste as the coarse aggregate, a choice also supported by prior research in sustainability. With the new End-of-Life approaches added, the total shadow costs are reduced by 22%. The hydrodynamical analysis and model result in extreme wave heights up to 3.98 m at the toe of the breakwater. By applying a neural network and a k-means algorithm on the wave data, five regular wave conditions are run in the SWAN model. The alongshore sediment transport is impacted by new breakwater concepts. Relying solely on the breakwater layout to counteract erosion north of the port, however, does not prove to be a viable approach. Based on design criteria and aspirations, the final conceptual design proposes the removal of the existing southern breakwater, while retaining sufficient space for future port expansion. The breakwater, integrated in the final design, is dimensioned based on standard design principles. To conclude, the data and model provide valuable insights into the coastal dynamics around Puerto Rawson. The proposed concrete mixture, along with the End-of-Life solutions, minimize environmental impact and enhance durability of the armour units. The sustainable breakwater design, effectively integrated into Puerto Rawson, accommodates for future port expansions

Offshore wind turbines, given their structural properties, are expected to experience severe ice-induced vibrations. However, full-scale events neither have been observed nor published yet and thus predictions of existing numerical models could not yet been validated. Model-scale experiments, aiming to investigate ice-induced vibrations of offshore wind turbines, have been inconclusive as structures with low natural frequencies would exceed the capacity of test facilities (i.e., size and weight limits), while geometrical scaling introduced scaling effects (e.g., buckling) compromising the validity of conducted experiments.

In the absence of full-scale testing capabilities, the main goal of this work was thus to demonstrate how offshore wind turbines behave under dynamic ice loads in smallscale experiments. In total four research questions (RQ) have been formulated and are addressed in this thesis, collectively serving to achieve the primary objective of this thesis.

RQ1: How can ice-induced vibrations of vertically sided offshore structures be scaled?

RQ2: How can offshore structures with low and multiple eigenfrequencies be tested in
ice tank experiments?

RQ3: What types of ice-induced vibrations can an offshore wind turbine experience?

RQ4: What is the effect of wind-ice misalignment on the development of ice-induced
vibrations of offshore wind turbines?@en

Arribadas, a phenomenon of mass nesting behavior of sea turtles, attract millions of olive ridley turtles (Lepidochelys olivacea) to Playa del Ostional, a nesting beach in Costa Rica. The timing and size of these arribadas are influenced by various environmental factors, including temperature, tides, and moon phase [1]. The sea turtles are threatened by a variety of factors, like amongst others climate change.
Rising temperatures and sea levels, changes in ocean currents, and more frequent and intense storms are all likely to have negative impacts on sea turtles [2]. Without intervention, climate change could lead to the disappearance or flooding of sea turtle nesting beaches, resulting in a loss of critical habitat for these creatures. To prevent such an outcome, it is imperative to gain a deeper understanding of the morphological and hydrodynamic characteristics of nesting beaches, as well as identify the factors that influence sea turtle nesting behavior.
This study aimed to identify and map the critical factors that must be considered to ensure persistence of the olive ridley sea turtle and arribadas at Playa del Ostional, Costa Rica. The objectives of this research were, therefore, (1) to map the seasonal morphological and hydrodynamical differences of the arribada nesting beach, (2) to identify the environmental parameters that have the greatest influence on the occurrence of an arribada, and (3) to map out the stakeholders involved. The study site is the beach that ranges from the northernmost part of Playa del Ostional down to the southernmost part of Playa Nosara, which is located on the northern peninsula at the west coast of Costa Rica. The part at Playa del Ostional where most turtles nest is called ‘Main Nesting Beach’ (MNB). A field investigation was carried out to determine the seasonal morphological and hydrodynamical differences of the nesting beach. This field study comprised of two distinct components: (1) a characterization of the morpho- and hydrodynamics of Playa del Ostional in the dry season, and (2) a comparative analysis of these conditions during the wet and dry season. The morpho- and hydrodynamic beach characteristics consisted of the beach profile, sediment composition, hydrodynamic properties and other general environmental characteristics, such as vegetation and nearby rivers. The beach profile was measured by walking transects perpendicular to the shoreline using RTK-GPS equipment. Moreover, a drone was flown that made an orthophoto and collected 30 million data points. The difference in sediment composition was analyzed by obtaining sediment samples in the dry season, sieving these and comparing the obtained particle size distributions and D50 values of the dry and wet season. The hydrodynamical properties and the other general environmental characteristics are analyzed by means of literature review, observations and photography. In order to identify the environmental parameters that have the greatest influence on the occurrence on an arribada, an autoregressive logistic regression model was used. The model that was made the previous research of 2022, was updated and automated. Also, design choices of the model were made and new data was added.
To map out the stakeholders, interviews have been conducted and a stakeholder map was created. Through the use of GPS transects the beach profiles taken in dry season (February 2023) were compared to wet season (October 2022). To tackle normal spacial variance the comparison is done through the calculation of averages on three beach stretches with equal characteristics. Main findings were that beach width is equal in both seasons, slopes are more gradual in dry season, beach plateaus are on average 3.0m wider in wet season. Crossing rivers do not influence the beach profile below waterline in the dry season. For more river characteristic more offshore research is needed. The sediment composition of the beach turned out to show significant differences between the dry and wet season. A significant difference is present in D50 values between the dry and wet season for almost all sediment samples. Moreover, during the wet season, the sediment tends to be coarser compared to the dry season. Additionally, during the dry season, coarser sediment tends to accumulate at the top of the slope, whereas during the wet season, coarser sediment accumulates near the waterline. These observations suggest that coarse sediment may move from areas close to the waterline to the submerged part of the slope over time. This behavior implies that sediment transportation is affected by the seasonal fluctuations in wave energy. The findings altogether indicate that the sediment composition at Playa del Ostional, particularly at Main Nesting Beach, is notably affected by seasonal changes. The impact is more pronounced from the low tide waterline to the high waterline’s end at the top of the slope, with a particular emphasis on the low tide waterline. The wave climate surrounding Playa del Ostional is expected to be less turbulent, with lower wave energy during the dry season. However, the exact distinctions in both wave climate and tidal surroundings between the two seasons cannot be ascertained due to inadequate data availability.
The different stretches of Playa del Ostional demonstrate notable differences in environmental characteristics during the wet and dry seasons. The majority of rivers that flow out during the wet season are absent during the dry season. In addition, a beach scarp appeared during the dry season and not during the wet season, and an estuary that was observed in the dry season was not reported during the wet season research. On the other hand, the beach is mostly surrounded by vegetation in both seasons, with comparable grass and trees. Moreover, no significant difference in wildlife presence was observed between the dry and wet seasons at Playa del Ostional.
The autoregressive logistic regression model was trained on five year of arribada data and 116 individual environmental parameters. The weights of the parameters were plotted and analysed in multiple groups. This resulted in six parameters with the biggest influence: pdTIDE_P1, pdTIDE_mf, pdVELOCITY_IHC_rho, pdVELOCITY_IHC_rho, Mooncycle_third and Moon_v. The maximum probability of an arribada occurring during a certain day was 80%.
By conducting interviews and conducting a stakeholder analysis, the degree of awareness about climate change is assessed and mapped out, which appears to be quite high. The residents of Ostional are aware of the changes and willing to work in new projects. Moreover, the analysis showed that it is important to engage with two key stakeholders: the Refugio Nacional de Vida Silvestre Ostional and CITES.

The Vietnamese Mekong Delta, a vital region in the country’s economy, faces the dual challenges of coastal erosion and mangrove degradation, which threaten its long-term sustainability and flood protection capabilities. This research focuses on the coastal area of the Bac Lieu province, characterized by severe erosion and degrading mangrove forests. The study investigates the applicability and potential impacts of hydraulic measures to decrease the net rate of coastal erosion, utilizing numerical modeling with Delft3D and a comprehensive socio-economic analysis. The research hypothesizes that the coastal erosion is partly driven by the placement of a sea-dike to protect aquaculture farms, initiating a positive feedback loop. This loop explains the relation between coastal erosion and mangrove degradation. The proposed hydraulic measures to interfere with this feedback loop are a porous detached breakwater, a shoreface nourishment and the removal of the existing sea-dike. The socio-economic analysis involves questionnaires for local residents, field investigations, and insights from experts in Ho Chi Minh City. While the questionnaires provide inconclusive results, the overall socio-economic impact of the nourishment and breakwater is deemed positive and worth further exploration, particularly in light of the critical role of mangroves in future flood protection. On the other hand it is concluded that the measure of removing the sea-dike will have a negative impact on the coastal area of Bac Lieu due to the intensive land-use and the lack of individual protection of the farms and villages. Therefore, this measure is not modelled. Numerical modeling with Delft3D assesses the hydraulic impact of the breakwater and nourishment on the heavily eroded and partially eroded coasts of Bac Lieu. Results indicate that the nourishment method exhibits a positive effect in reducing net erosion, especially in low energy conditions. Conversely, the porous breakwater shows minimal impact on cumulative erosion and sedimentation. Since this is against all expectations, the validity of the schematization of the porous breakwater is questioned. It is observed that the schematization does not grasp the complex behaviour of the breakwater and therefore it is concluded that Deft3D is not a suitable modelling tool for modelling a porous breakwater. The findings suggest that the nourishment method is a promising approach for reducing erosion in Bac Lieu, benefiting both the heavily and partially eroded coasts. To determine the best course of action for Bac Lieu, further research into the long-term effects and configurations of nourishment is recommended. Additionally, informing local inhabitants on the threats of relative sea-level rise and flood protection, and fostering consensus between the government and engineering agencies on the importance of protecting the Mekong Delta and its mangrove ecosystems are essential steps toward a more resilient future.

Playa Unión and Puerto Rawson are facing severe coastal erosion and increasingly the negative effects. Since the first half of the 20th century there have been signs of erosion, but also of sedimentation. However, the coastline in the project area is retreating over the years. The situation has recently been declared an emergency. Furthermore, there are several expansion plans for the port consisting of new quay walls and dredging. It is unsure if these plans will influence the erosion. In this report, the following research question is formed: ”What are valued, preferably nature-based, interventions that can mitigate the coastal erosion in Playa Unión and Puerto Rawson considering the planned expansion of the port?” Due to the limited information and data available, assumptions have been made during the research. A site analysis and a CoastSat analysis are conducted to research the morphology of the coastline and its drivers. The coast has been shaped into a steep upper section of coarse granular material and a gentle lower slope with finer material. The main driver of longshore sediment transport in the area are swell waves, with predominant directions SSE and E. This transport is directed from south to north, resulting in the inflow and outflow of sediments. Furthermore, there is a sediment flow from the Chubut river. Due to the construction of the port’s breakwaters, the longshore sediment transport is interrupted, which causes an imbalance in the sediment flow in the system. More sediment flows out of the system than enters, causing coastal erosion. The development of the port contains public as well as private expansion plans and maintenance dredging. The expansion plans will have little effect on its surroundings. The private plan include a parallel breakwater, which alters the natural balance in the area and dredging works, for which the stability of the existing breakwaters has to be figured out. A stakeholder analysis was done to get insights in the opinions and visions of the stakeholders. This was done by interviewing stakeholders and by doing a questionnaire, resulting in a power-interest grid and overview of interests and attitudes. The boundary conditions for the interventions and criteria for the multi-criteria analysis are partly formulated as a result of the stakeholder analysis. The following interventions were considered in this report: • Permeable pile groynes and low crested groynes
• Opening the northern breakwater: opening and reshaping with a curve, constructing tunnels underneath the breakwater and a sediment bypass
• Port expansion and a sediment bypass with power supply southern of the port
• Dredging and moving sediment
• Sediment trap
• Plant vegetation with beach nourishment
• Gravel engine
• Temporary longitudinal flood barrier as a short term intervention.
A conceptual multi-criteria analysis in combination with a nature-based assessment has been conducted to distinguish the most promising interventions in the conceptual design phase. The criteria formulated were effectiveness, easiness of implementation, maintenance, environmental impact and the benefits for recreation. From this, it can be concluded that the gravel engine and the plant vegetation with beach nourishment score the best.

The increasing popularity of offshore wind as a result of the demand for renewable energy, forces the industry to consider the development of wind farms in sub-arctic areas. One of these areas is the Baltic Sea, where the potential for wind energy is very large, but which is also prone to the occurrence of ice. For the design of offshore foundations it is paramount that ice-structure interaction is appropriately considered as ice-induced vibrations are known to significantly increase the loads. As a result, ice-mitigating measures may need to be included in the design. However, such measures will substantially increase the cost estimates required
for competitive tenders. Hence it is valuable for developers to have early screening tools to assess this. The aim of the research was to provide a ‘feasibility map’, which predicts the necessity for ice-mitigating measures in the entire Baltic region. Feasibility of monopiles was considered both technically and economically. The former by imposing design, installation & fabrication constraints and the latter was measured in terms of weight increase of monopiles.

To accommodate the growing demand for renewable energy, installation of new offshore wind energy capacity is increasing in many countries. New projects will be proposed for waters in which floating (sea) ice is a common occurrence in winter. Currently, projects remain limited to regions with mild ice conditions, such as the Southern Baltic Sea, but projects further north are expected. In preparatory work for this thesis, performed at Siemens Gamesa Renewable
Energies, it was shown that (edgewise) blade loads increase significantly for Northern Baltic sites when compared to Southern Baltic sites due to ice induced vibrations.

The phenomena of ice extrusion and rubble loads are expected to provide added damping to an ice-structure interaction system. This added damping may lead to less severe structural vibrations, and thus blade loads for an offshore wind turbine, than for a system with only intact ice crushing. VANILLA is the leading ice crushing model in the industry to evaluate the effects of ice induced vibrations for vertical offshore structures such as wind turbines, but the model
currently does not (explicitly) consider ice extrusion and rubble loads. Therefore, the aim of this work is to investigate the phenomena of rubble and ice extrusion in crushing and propose a modelling approach.

Following a literature study, a rubble model and an extrusion model are proposed as extensions of the VANILLA ice model. The rubble model shows to be consistent and compute plausible rubble loads for model and full scale, while the extrusion model only predicts reasonable extrusion loads at full scale. Forcing and dynamics found from using the Standard and Adjusted VANILLA model with simplified global bending mode shapes of the SG 14-222 DD offshore wind turbine were compared.

Rubble loads were found to be small in magnitude and of negligible influence on the dynamics. The loads stemming from extrusion introduce additional damping to the system when the relative velocity between the structure and the ice increases, such that (i) the immediate load drop to zero after a failure event observed in Standard VANILLA is omitted, (ii) initiation ice drift velocities for IIV regimes become lower, (iii) dynamic amplitudes reduce, and (iv) a positive force-velocity gradient arises in the CBR regime.

The Arctic is warming more rapidly than other latitudes, which can result in the release of additional greenhouse gasses, global sea level rise and increase in extreme weather events. Additionally, this causes the rapid decline of sea ice and an ice free Arctic might occur during the summer in the 2040s. The decreasing sea ice cover accelerates the warming of the Arctic, which is known as the albedo feedback system. Solar radiation management (SRM) can be a solution to diminish or possibly stop sea ice decline. Within SRM a proposed technology, known as Arctic Ice Management (AIM), is distributing water on top of existing sea ice to increase the ice thickness enough to survive the summer melt. This raises the question: What water volume should AIM distribute on top of existing sea ice to counteract the annual Arctic sea ice volume loss? Based on data obtained during the period 1979-2020, the September trends for ice extent, ice area and ice volume are -83 400 km2yr-1, -49 200 km2yr-1 and -322 km3yr-1 respectively. The ice volume is considered as target parameter, as it accounts for both absolute areal ice loss and overall decreasing ice thickness. There are two main ice drift patterns in the Arctic: The Beaufort Gyre in the Beaufort Sea and the Transpolar Drift, of which the latter exports ice through Fram Strait into the Greenland Sea. Literature shows the ice remains within the Arctic for about five years when located in the Beaufort Sea and one to two years when located in the Transpolar Drift. For both locations, the ice decay is determined using an analytical approach first. This approach shows resemblance for ice located in the Beaufort Sea, but generally overestimates the ice decay in the Transpolar Drift. For this reason, an empirical approach is developed to determine the survival ice thickness. This results in accurate trends for ice decay of -2.1 to -2.7 cm day-1 in the Beaufort Sea and -0.8 to -1.4 cm day-1 in the Transpolar Drift. Considering 91 melting days results in an average survival thickness of 2.18 and 1 m respectively. AIM can be used to increase the ice thickness beyond this survival thickness and an AIM model is developed to show ice growth including AIM. The model concludes the AIM thickness, initial ice thickness prior to flooding and freezing duration after AIM define the effective ice thickness increase. The model is validated with small scale experiments, which indicate a delay between the flooding phase and continued natural ice growth. This delay can be the effect of the duration required to restore the temperature profile in the ice after flooding as shown by COMSOL Multiphysics simulations. Considering the AIM model, it is discouraged to implement AIM on ice thicknesses below 0.6 m and suggested for ice thicknesses approaching 1 m or higher to optimize the effective increase. The required water volume to compensate the annual sea ice volume loss highly depends on the location, initial ice thickness and target ice thickness and varies between 707 to 1095 km3 in the Beaufort Sea and between 386 to 464 km3 in the Transpolar Drift for the methods discussed in this research. To pump up this water volume, the expected power requirements are 4.5 to 7.0 GW and 2.5 to 3.0 GW respectively.

Thermal ice pressure is one of the eight ice loads to consider for the support structure design of bottom-fixed offshore wind turbines defined by international standard IEC 61400031. Temperature changes in ice drive its deformation and cause structures restricting expansion to experience loading. The effective coefficient of thermal expansion (ECTE) can be used to quantify the difference in sea versus fresh ice behavior under thermal action. Since the thermal expansion of fresh ice is generally well understood, fresh ice thermal expansion experiments can be compared to sea ice, which has a more complex structure consisting of a solid ice matrix containing liquid and gas. Fiber Bragg Grating (FBG) strain and temperature sensors enabled measurements of thermal expansion in fresh and saline ice samples under different boundary conditions in the cold laboratory. Experiments were compared amongst fresh and saline ice samples for three distinct applied thermal actions: air temperature change, floating in water, and flooding the ice surface. Fieldwork on fresh and sea ice under the same meteorological conditions was performed to compare thermal stresses. The thermal expansion air experiments yielded no significant differences in ECTE values for fresh, granular, and columnar saline ice. However, it was observed during the unconfined ice floating experiments that granular saline ice resulted in a larger ECTE than fresh ice. Negative values for ECTE were obtained for columnar sea ice air temperature change experiments and during granular saline ice floating experiments, both occurring during heating cycles. Flooding experiments described the thermal response of latent heat release during water freezing on the ice surface, and it was observed that fresh ice showed stronger hysteresis in results than saline ice. Thermal stresses monitored for approximately one month during winter in the Vallunden lake sea ice attached to Van Mijenfjorden and in a nearby freshwater lake demonstrated higher values for fresh ice than sea ice and increased over the depth for both ice sheets which experienced the same meteorological conditions.

With a growing global demand of clean renewable energy, offshore wind activities will extend to more harsh environments, including sub-arctic areas like the Baltic Sea. Sea ice can occur here which needs to be taken into consideration in the design of substructures, i.e. jacket substructures, of offshore wind turbines. Several ice mitigating measures exist for jacket substructures, of which one is disregarding braces crossing the waterline. In this way, ice cannot induce loads to the relatively slender braces in the jacket. However, disregarding these braces has disadvantages in terms of structural integrity in comparison to a jacket including these braces.

In Part I of this thesis, two types of jackets, one without (Type 1) and one with (Type 2) waterline crossing braces, are implemented in a numerical model and subjected to ice loading in order to conclude whether the use of a Type 1 jacket design can be justified or whether a Type 2 jacket design is also suitable in sub-arctic areas.

Since over the past years the modelling techniques regarding ice-structure interaction have been updated and improved significantly, assessing the loads at the jacket braces can be done more accurately, allowing to give a more thorough conclusion on whether to use braces crossing the waterline on jackets in sub-arctic areas. As a result, a design including these braces could become feasible, whereas in the past it would be disregarded.

Two types of ice failure are considered in this research, being ice bending failure and ice crushing failure. First, a model is developed to quantify the ice actions occurring at a jacket substructure as function of the approach angle of the ice direction relative to the structure and as function of an introduced 'threshold angle'. It was found that ice failing in bending is equally significantly present as ice failing in crushing. Subsequently, numerical models are introduced that describe the failure behaviour of bending and crushing ice and the force that both ice failure types will induce to the structure. The structure is represented by a 5 [MW] offshore reference turbine supported by a jacket substructure. Using these models, dynamic simulations are performed in order to investigate the local susceptibility to ice induced vibrations (IIV), as well as assessing the Ultimate Limit State (ULS) load at the braces. As a result, local brace IIV regimes could be recognised, however sustained frequency lock-in could not be observed. Furthermore, it was found that a Type 2 jacket could be suitable to use in sub-arctic areas considering the ULS load effects due to ice loading, but care should be taken for the ULS brace load, as that is of great importance for the design.

In Part II of this thesis, it is assessed whether coupled dynamic ice-structure interaction can be modelled using a dynamic substructuring modelling technique, also known as Craig-Bampton method, since it is industry practice to use such a method when jackets are involved. This method is based on a model order reduction of the full finite element model representing the structure, resulting in improved computational time and allowing assembly of multiple (reduced) substructures. The method is successfully implemented allowing to solve coupled ice-structure interaction problems and a framework to do so is provided. It is shown that excluding the loaded degrees of freedom (DoFs) in the jacket from the reduction leads to more accurate results and is considered convenient since few DoFs are loaded by ice, maintaining the effectiveness of the reduction method. As a result, IIV can be investigated using this reduction method, however challenges related to the accumulation of numerical accuracy errors arise with truncation of more higher frequency modes and require further development to be resolved.

The world is transforming its energy production towards more sustainable sources of energy. In Europe, there is currently 25 GW of installed offshore wind power capacity. This is expected to grow with 29 GW over the next five years. Offshore wind farms can be expensive and challenging to build, design and maintain. Understanding the offshore environment will ensure that the to be produced offshore wind turbines are of sufficient quality while reducing costs. Monopiles are currently the most common sub-structure, but jacket sub-structures are becoming more relevant due to increasing water depth or changing soil conditions. Structures in icy waters, such as the Baltic Sea, may be subjected to ice induced vibrations while they encounter sea ice. These vibrations have to be considered in vertically-sided offshore structures' design and are the most critical load case when ice is concerned.

Multi-legged sub-structures, such as jackets, can have a problem that does not exist for monopiles, namely ice jamming, where ice fills the space between the legs of a multi-legged sub-structure. The legs and the jammed ice may then act as a single structural unit. Which leads to the main research question: how does an ice jam influence ice-induced vibrations of a multi-legged sub-structure?

First, a literature study of ice jams and multi-legged sub-structures was performed. This study concluded that different ice jamming situations are possible and have occurred with multi-legged structures, which not all have survived. The ratio between leg spacing and diameter plays a vital role in the ice action on multi-legged structures. Furthermore, the combination of ice-induced vibrations and ice jamming had not been studied yet.

Secondly, a model is made based on a phenomenological ice crushing model using COMSOL Multiphysics and MATLAB to simulate the structural response. The sub-structure is based on the jacket design for the NREL 5-MW reference turbine. Different situations from the literature study are used to make several design scenarios for which the structural response is calculated. In total, there are five situations: a base case, an angled base case, an internal jam, a frontal jam and an angled frontal ice jam. The base case does not have an ice jam, and the angled frontal jam has an increased thickness of the jam to twice the incoming ice. For the other jams, the thickness is equal to the incoming ice.

The different scenarios are simulated for a range of ice drift velocities to capture the different ice-induced vibration regimes and see how the structural response changes due to the presence of an ice jam. First, a baseline was established of the jacket's structural response for the base case. Afterwards, the three different ice jams were simulated. Results show that the base case is excited in all three ice-induced vibration regimes. At lower ice drift speeds, intermittent crushing is observed. Then at around 0.05 ms^-1, it transitions into the frequency lock-in regime. Here the structure is excited at its second natural frequency. For higher ice drift velocities (>0.2 ms^-1), continuous brittle crushing is seen. For the angled base case, the transition between intermittent crushing and frequency lock-in happens at around 0.1 ms^-1, and it stays longer in the frequency lock-in regime. The internal stresses around the contact area between ice and leg for the internal and frontal jam did significantly exceed the ice strength. Thus these jams would have failed on crushing at the contact area. The stresses inside the angled frontal jam exceed the ice strength but by a small margin. With all the assumptions made taken into account, the jam might hold. The structural response shows an increase in period for intermittent crushing and a lower amplitude in structural displacement than the base case.

The main conclusion is that an ice jam that would significantly impact the ice-induced vibrations cannot be sustained. The internal stresses exceed the ice strength which would cause the jam to fail. The ice jam that can be sustained acts as additional stiffness for the system and decreases the structure's displacement amplitude for the intermittent crushing regime. In the frequency lock-in regime the structure's displacement frequency increases a bit. But the amplitude is similar in all scenarios because the maximum velocity of the structure will roughly be the same as the incoming ice floe because that is what excites the structure, and this doesn't change.

Arctic and sub-arctic regions are more and more used to build offshore wind farms. There is a lack of model-scale tests where dynamic sturcutral properties were systematically varied to derive the influence of these properties on boundaries of the three crushing regimes. Those regimes are intermittent crushing, where both the ice load and displacement signal have a sawtooth pattern, frequency lock-in, with its characteristic sinusoidal displacement signal and continuous brittle crushing with smaller random ice loads and structural displacements compared to the two previously mentioned regimes. The first goal of this thesis is to test if an error in the dynamic structural properties (mass, stiffness and damping) has an effect on the transition velocities of the crushing regimes. Secondly, a parameter study has been done to derive trends in the transition velocities when changing dynamic structural properties. Ultimately, in literature proposed dimensionless numbers, which are particularly only taking linear elastic ice parameters into account, have been checked for rightness.
To be able to achieve the above mentioned goals of the thesis, a model-scale test campaign has been performed at the Aalto Ice Tank as part of the SHIVER test campagin. 36 unique single degree of freedom configurations have been tested to derive trends in ice-structure interaction when changing dynamic structural parameters. The most notable element of this test campaign was the usage of a real-time hybrid test setup, which allowed the testing of multiple sets of structural parameters while only one physical structure was used.
Those test signify that there is a relation between mass, stiffness, natural frequency and damping and transition velocities inbetween the three crushing regimes. When mass, stiffness and natural frequency increase, the boundaries of those regimes shift to lower velocities. An increase in damping caused the velocity of the boundary between frequency lock-in and continuous brittle crushing to go up, while the boundary between intermittent crushing and frequency lock-in was not influenced by the change in damping. Furthermore, from the tests it could be conlcuded that dimensionless numbers based on only linear elastic parameters are not correct and more parameters should be taken into account as well.
The thesis is a steping stone into more research into the dependency of ice-induced vibrations on dynamic structural properties with different initial conditions or structural properties. Furthermore in future test campaigns other dimensionless groups could be proposed and validated.

Ocean Thermal Energy Conversion (OTEC) has been heavily researched and developed over the last decade. However, no commercial scale for OTEC has been reached yet. OTEC is a clean and renewable source of energy which utilizes the seawater temperature difference between the upper and deep ocean layers to generate electricity. The heat exchangers and the cold water pipe design are considered to be the most challenging part. The cold water pipe is often stated to be the largest risk. The installation of the cold water pipe, the high investment costs of the cold water pipe and the large forces acting upon the cold water pipe are crucial factors for the future success of OTEC. In this thesis, a new kind of cold water pipe concept is introduced. The FLEX-hose concept consists of a thin sheet of nylon, supported by concrete rings and polyester cables. The total length of the foldable concept design is 1000 m. The nylon sheet is attached to the concrete rings and the cables are attached to the concrete rings. The cables run through the concrete rings and can be used during installation to lower the pipe. Deployment of the foldable pipe is expected to be relatively easy thereby lowering the installation costs. The Multi Criteria Analysis performed in this thesis show the FLEX-hose foldable concept to be attractive compared to a foldable spiral concept, a flexible composite concept, a rigid composite pipe and a HDPE pipe. However, the inclination angle occurring while pumping may not become significant, meaning above 7.5 degrees, since this will lead to lower energy efficiency. In addition, no current design is available on the market, meaning heavy research and development is needed for the foldable concept to succeed. Possible challenges for the FLEX-hose concept that have been established through this research include instability occurring due to buckling and a pressure drop becoming larger than 0.6 bar, thereby significantly reducing the net power of the OTEC plant. The maximum allowed angle of inclination of the sheet to stay below 0.6 bar is calculated to be 7.5 degrees at a flow speed of 2 m/s. During pumping a discharge of 20.000 kg/s is pumped through a thin walled pipe with a diameter of 3.5 m, which leads to a radial displacement of the nylon sheet. Collapse and too significant deformations of the thin walled sheet are prevented by applying 10 MN of pre-tension in the thin walled sheet. However, in order for the nylon sheet to stay in the linear elastic regime, the calculated wall thickness is 0.04 m, thereby making foldability questionable. A total of 200 concrete rings is present in the design, one every 5 m over the total length of 1000 m of the pipe. In addition, a large clump weight of 12 MN is needed at the bottom of the pipe. The total system weight of the foldable pipe with concrete rings is calculated to be 1840 mT, which could be installed by an offshore installation vessel. Finally, creep has also been established to be a challenge due to the life-time of 30 years for the cold water pipe. A synthetic plastic material such as nylon experiences relaxation of the material when stressed for a longer period of time, leading to large deformations.

For the development of Arctic offshore structures, design ice loads are required. These design loads represent the ice loads a structure may be exposed to for specified requirements. The design ice loads depend on magnitude of the ice loads and on the probability of exposure to the ice loads. Both are impacted by local sea ice conditions.

Climate change affects the sea ice conditions causing change in the ice loads Arctic offshore structures are expected to experience. To obtain accurate design loads, the effect of climate change should be considered in defining those.

Conventional methods to determine design ice loads are based on historical data and assume this data can be used to represent the loads during the lifetime of the structure. However, historical data cannot represent future ice conditions if climate change is considered as sea ice conditions will change. This means that the effect of climate change is not incorporated in the design ice loads when these are based on conventional methods. To include the effect of climate change a new method needs to be developed.

In this thesis, such a new method is proposed that enables to include the effect of climate change into the design ice loads. Instead of historical data, the new method considers the future ice conditions. The method allows to base the design ice loads on sea ice conditions that change over time.
To determine design ice loads, extremal distributions are used. Extremal distributions describe the probability of seasonal maximum ice loads. Commonly, the extremal distribution is based on data covering multiple seasons and cannot properly include inter-seasonal change in sea ice conditions. The new method allows to determine the design ice loads based upon changing extremal distributions. The extremal distributions are determined for seasons separately based on sea ice conditions of one season only.

For the proposed method, a concept referred to as an 'ice state' is introduced. Ice states describe a period of time in which the sea ice conditions are assumed to be constant. When sea ice conditions are constant, the corresponding short-term ice load distributions and the ice load frequency can be determined. Both are required to be able to determine the extremal distribution for one season.

According to the new method, the increase of drift speeds causes increase of design ice loads whereas the decrease of ice concentration, thickness and compressive strength causes decrease of the design ice loads. Based upon expected future climate change scenarios, the new method indicates that design ice loads are lower when compared to the design ice loads according to the conventional method.