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Offshore and Dredging Engineering

Role: Daily supervisor

In collaboration with: Allseas

Continuous development in the wind turbine industry leads to increasing size of wind turbines. Allseas investigates the possibility to enter the offshore wind turbine installation industry with Pioneering Spirit. For Allseas’ preliminary wind turbine installation design, wind turbines are assembled offshore, requiring wind turbine components to be brought from shore to Pioneering Spirit by means of a cargo barge. This operation requires a proper mooring procedure of which the mooring system is an essential part. The mooring system secures the barge alongside Pioneering Spirit where it has to stay for multiple days. In this research a pre-defined vessel orientation is analysed where the barge stern is extended 40m in longitudinal direction from Pioneering Spirit stern, to increase the barge area reachable by the unloading crane. This barge position is challenging due to limited shielding from Pioneering Spirit, leading to excessive environmental loads acting on the barge. Due to the barge extension, also properly connecting the mooring system to Pioneering Spirit is a challenge. The above leads to the main objective of this thesis: improve the mooring system to secure a barge alongside Pioneering Spirit in offshore conditions, including an evaluation of the dynamic mooring loads. The main mooring system design requirements are that the system Safe Working Load and motion limits are not exceeded. Evaluation of the mooring systems starts with a multi-body diffraction analysis with hydrodynamic matrices and other hydrodynamic properties as output. This data is imported in a time domain model which enables capturing non-linearities, e.g. the mooring system and wind and current loads. Validation and verification steps are required to assure realistic outcome and understanding limitations of the numerical models. Before new mooring systems are introduced, a base case mooring system is defined and modelled in the time domain. Finding the limiting sea state for which the Safe Working Load limit is reached enables to compute the workability for the reference location defined. Two reference locations are considered: a wind sea area and swell sea area. Wind and current speed is assumed to be constant over time and independent of elevation. Workability is defined as the percentage of time the mooring system is able to operate. Evaluating the base case mooring system shows performance of 67% workability for wind sea areas and 16% for swell areas. Next to the base case, four mooring improvement concepts are evaluated from which the Cavotec Moormaster® system shows most promising results. This system consists of three main components: ‘fixed structure - hydraulic cylinder - vacuum pad’ (Figure 2) that connects the barge to Pioneering Spirit. This system is modelled as a link with constant stiffness and damping properties within its operational limitations. The Moormaster system, shows perspective to improve the workability for both wind and swell seas. Besides workability, the Moormaster system improves the entire mooring procedure by quick connection and safe disconnection upon exceeding operational limits.

Side-to-side mooring; Pioneering Spirit; Diffraction analysis; Time domain modelling; Cavotec MoorMaster

Hydraulic Engineering

Role: Daily supervisor

In the last few years, the climate crisis has been accelerating at a dizzying pace and poses an emergency threat to our planet. Rainfalls have transformed into intense downpours, and flash flooding, combined with the sea level rise, leads to a higher risk of inundation of densely populated coastal cities. Moreover, the melting of the permafrost can lead to significant landslides, triggering the generation of mega-tsunami waves. The latter can have a catastrophic impact, not only on the infrastructure but also on human life. Man-induced climate change is responsible for the increase in frequency and intensity of extreme natural events, such as tsunamis, floods, and storm surges. Recent research indicates that almost one-fourth of the world population lives at high-risk locations to at least 0.15m of inundation depths with a return period of 1 in 100 years. Therefore, the urge of implementing protection measures against unsteady flows is imperative. Undoubtedly, the involvement of engineers can play a pivotal role in order to analyze these flows in the built environment and provide sufficient coastal and building plans to ensure safety and reduce reconstruction costs. The behavior of the unsteady flow around a structure is not a well-understood topic and results in a lack of accuracy and reliability. More insights are required into the fluid-structure interactions to come up with a safe building design. In the present study, unsteady flows are generated using the dam-break technique in line with previous research. The Thesis aims to model, validate, and implement a simplified approach to analyze the complexity of the hydrodynamic behavior of unsteady flows around impervious buildings, with different orientations and blockage ratios. To do so, the research introduces a numerical simulation method of a dam-break wave, using the two-dimensional, and non-rotational shallow water equations. The Galerkin finite-element model is applied for the discretization of the solution on a limited domain. Initially, the flow of the dam-break wave was validated in the absence of the structure, using a dam-break experimental work for comparison. Then, in order to insert a structure in the domain, a second experiment with a structure is used, which generates tsunami-like waves using the vertical release technique. The gate, which represents the dam, is located at x=0 and opens instantaneously at t=0s. Behind the gate, the reservoir maintains an amount of water that flows, after the opening of the gate, into the channel generating the shock wave. At the channel downstream of the gate, initial water levels are considered and the behavior of a bore propagation is simulated. The building is located on the downstream side and different impervious building configurations were studied in terms of orientation and shape to investigate the impact of the unsteady flow. To analyze the complex hydrodynamic processes of flooding, four fixed points around the structure are set to measure the action of the bore on different impervious building configurations. The water elevations and the averaged velocity profiles in time were derived at each point. Moreover, the horizontal forces in the x and y directions are calculated by the model, integrating the stresses over the wet surface of the buildings. The general behavior of the fluid-structure interactions is captured well, especially upstream of the building, and insights are gained regarding the behavior of the fluid around the structure and the parameters of influence. Results showed that orientation changes completely the impact on the building configurations. The separation of the flow and the blockage ratio are the main parameters that are influenced by the angle of rotation and change the behavior of the loading process at the initial impulsive phase, when the wave arrives at the structure, and at the hydrodynamic phase, where the flow has a quasi-steady behavior. Overall, a good agreement is achieved with the experimental data, although the numerical model overestimates the loads acting on the different building configurations. Results proved that the orientation of the building with respect to the flow facilitates the flow around the structure and contributes to lower water levels, and to a better distribution of the horizontal loads on the surface. The best results were achieved for an angle of rotation of 45°, where symmetrical separation of the flow is also playing an important role.

numerical modeling; impervious buildings; bore impact; structure orientation

Offshore and Dredging Engineering

Role: Daily supervisor

Perforated monopiles show promise in providing a better alternative to the commonly used jacket-like substructures used in intermediate water depths in the range of 30 to 120 m. By introducing perforations near the vicinity of the splash zone the wave loads on the monopile can be mitigated and the fatigue damage reduced, which is the main advantage jackets have over monopiles. Furthermore, perforated monopiles would be easier to install and manufacture compared to jacket structures, which has the potential to reduce the Levelized Cost of Electricity (LCoE) considerably. Perforated monopiles also have the benefit of reduced (local) scour, reduced corrosion damage and providing a habitat for marine life. To optimize the design of perforated monopiles fast novel surrogate models are needed to determine the (wave) load reduction by the introduction of perforations on the monopile. As traditional (full) order Computational Fluid Dynamics (CFD) models are slow and expensive to evaluate. Using a surrogate model a wider range of geometries can be used in the early design optimization steps. Promising designs can then be further evaluated using full models. This thesis develops a surrogate model for the characterization of the (hydrodynamic) loads on perforated monopiles using Convolution Neural Networks (CNN). A dataset is created of 15,561 samples, which considers geometries of varying number of perforations, porosity and ‘angle of attacks’. A Finite Element Method (FEM) CFD model is made in Gridap for a ‘creeping flow’ to determine the flow fields. And determines a load Reduction Factor (RF) by the introduction of perforations. Furthermore, using a Signed Distance Function (SDF) a representation of the geometry is made for use in the CNN model. Using PyTorch a CNN model is used to predict the RF for a certain input. The model combines multiple convolutional layers with an optional regression head (with linear layers). Five different models are created. Three for representing the geometry by using the SDF, decomposing the SDF in its x and y distance components and the binary representation. Furthermore, two models combine the SDF or distance components with the flow fields from the CFD model. The results show that a speedup of 386 times is achieved by using the surrogate model, showing the main benefit of using a surrogate model. This is likely to improve considerably in further research when turbulence is taken into account. Furthermore, each of the five models show a good accuracy in predicting the RF. A general trend is observed that the more information is provided in the input to the CNN model the better the accuracy tends to be. The combination of implicit representation of the geometry by using distance components with the CFD flow fields shows the best accuracy with an expected maximum relative error rate of 0.94% within the parametric space of the dataset.

Monopiles; Perforation; Machine learning; Surrogate modeling; Renewable energy; CFD

Offshore and Dredging Engineering

Role: Daily supervisor

In collaboration with: Mocean

In contrast to traditional power cables for bottom-founded offshore wind turbines, power cables for floating wind turbines penetrate the water column and are exposed to cyclic loading. Due to the dynamic nature of the cable loading, the cables are prone to fatigue failure. Since the evaluation of the fatigue life of complex structures, like power cables, has to deal with a certain degree of uncertainty, fatigue safety factors are introduced. These factors ensure a desired probability of failure based on the degree of uncertainty present in the modeling of the fatigue life. Limited experience in the evaluation of the fatigue life of dynamic power cables is found in the literature, and no sources have been found attempting to calibrate the prescribed fatigue safety factor. Currently, a fatigue safety factor of 10 is advised. This implies that there is much room for reduction in the uncertainties, meaning that there is a potential for making the cables cheaper and more reliable, which will in turn make floating wind energy more competitive with other (renewable) energy sources. Due to the lack of available literature, comparisons with umbilicals and flexible risers are made. Different methods for the evaluation of fatigue safety factors for flexible risers are evaluated, and judged on how applicable they are to use on dynamic power cables. It is found that a reliability based approach is most suitable for this purpose. A case study is carried out for a presently relevant scenario to put the proposed method into practice. It is concluded that for a safety class corresponding to a required maximum probability of failure of 10-3 in the last operational year, a fatigue safety factor of 3.5 is needed. This safety factor is required for the wave-load induced fatigue, as opposed to the seabed induced fatigue, to which the dynamic power cable is less vulnerable nearby the touch-down zone. The main parameter driving the uncertainty in the fatigue analysis is the sensitivity of the local stress analysis, which is in line with what has been found for similar studies for the oil & gas industry. In order to bring down the cost of dynamic power cables, and make them more reliable, more accurate local stress analysis models need to be developed, being validated against test data, in order to reduce the standard deviation of the local stress computation.

Floating Wind Turbines; Dynamic power cables; Fatigue; safety factor; Reliability; femap; OrcaFlex

Offshore and Dredging Engineering

Role: External committee member

In collaboration with: Jumbo

Offshore lifting operations must have reduced payload motion to increase safety and reduce operating time. When payload is retrieved from the splash zone to the deck, besides the crane block, no additional control can be applied on the underactuated system. Existing studies either assume more control over the payload or develop a control system based on a new crane. To reduce payload motion on current crane vessels, a conceptual model needs to be developed. In this thesis, various state-of-the-art solutions are considered based on four criteria: Time reduction, motion reduction, initial investment required and power required. Eventually, the quantified criteria and an analytic hierarchy process established that the most promising concept is based on an automated side loader of a garbage truck. The selected concept is developed based on a design process that focuses on optimized material usage. The geometry is determined according to requirements and forms the starting point of the circular design process. A dynamic analysis is conducted to obtain the dynamic response of the payload and eventually the reduced payload motion. The design cycle is complete after a finite element analysis has been conducted to verify the structural integrity of the model. After more than 20 cycles of the design process, the conceptual model is optimized and over 85% of motion is reduced in the X direction. The payload motion in Y- and Z-direction is 20% and 29% respectively. The simulation results in this study show that the conceptual model is able to reduce the payload motion during offshore lifting operations whilst staying within the limits set by offshore standards. The motion reduction of the payload creates a safer and more efficient environment to execute offshore lifting operations.

Offshore lifting operations; Conceptual development; motion reduction; Dynamic analysis; Finite Element Analysis

Structural Engineering

Role: External committee member

The slip joint connection is a relatively new alternative method for connecting offshore wind turbine towers to prior installed monopiles. It consists of two overlapping conical sections that together form a connection. There are several challenges left to be solved before the slip joint can be applied on a commercial scale. One of those challenges lies in the decommissioning process of the slip joint connection. It is proven that build-up settlement can increase the friction force within the joint over its lifetime and complicate its disconnection. One potential method for reducing the friction force in the slip joint is the excitation of one of the structures shell modes localized around the slip joint. Good estimations of the shell modes of the structure are therefore essential. Models for precise estimation of the shell modes of a wind turbine with a slip joint are difficult and expensive to develop due to the complexity of the joint. This thesis is a study into the possibility of using simplified finite element models to estimate the shell modes of a wind turbine connected with a slip joint. This is done by estimating the shell modes of the structure using a reference model and seven simplified models in a modal analysis. The estimated shell modes are compared based on their eigenfrequency and mode shape. Based on this comparison, conclusions about the applicability of simplified models are drawn. The first 300 eigenmodes of the structure are estimated with both the reference model and simplified models. Of these eigenmodes, ten shell modes of interest are selected based on their eigenfrequency and location at which the modes are localized. The modes of interest are than compared for their eigenfrequency and mode shape. The mode shapes are compared based on the Modal Assurance Criterion (MAC), which is calculated at a specific location of interest. In addition to this, the angle in which the modes are orientated is measured and compared, as this can be useful information for the decommissioning process. Results showed that three out of seven simplified models studied resulted in a estimation of the shell modes which was sufficiently accurate. The simplifications used for these models consist of the averaging of wall thickness for the upper and lower slip joint and a simple model for the upper slip joint. From these results it can be concluded that some of the simplifications can be applied when estimating shell modes of a wind turbine tower connected with a slip joint, as they lead to only a small decrease in accuracy of the shell mode estimation. However, using these results to make predictions about other potential simplifications is challenging as each different simplification has a unique influence on the estimation of the shell modes.

Slip Joint; Eigenmode analysis; Wind Turbine; Vibration; Decommissioning

Offshore and Dredging Engineering

Role: External committee member

In collaboration with: Van Oord

Climate change is triggering an ever-growing demand for renewable energy. The U.S. is still far behind Europe when it comes to offshore wind energy. They have made ambitious plans to reach 30 GW in offshore wind energy by 2030 while currently 42 MW is installed. One of the main challenges in the U.S. is the installation method since a legislation called the Jones act prevents the usage of European installation vessels for shuttling (the conventional method). Building a Jones act. compliant installation vessel is a large investment which comes with risks and long lead times. Feedering is an alternative strategy, but barely any research on it is available. Here, a feeder vessel sails back and forth from the storage port to the (non-Jones act. compliant) installation vessel to supply Wind Turbine Generator (WTG) components. These components need to be lifted from the floating feeder in order to be installed. In the literature, this step is deemed to be the riskiest. However, barely any technical research is available with regards to the lift-off. In the first thesis of this double degree program, a lift/installation sequence called the direct installation method is deemed to be highly interesting with respect to the logistics and costs. However, this research misses a technical study in order to understand if it is technically reachable to directly install these components. In this offshore engineering thesis, a barge is used as a feeder vessel and tower segments of a 20 MW WTG are chosen as the to-be lifted components. This research focuses on the pre-tension phase before the lift-off. This contains the steps where the crane of the installation vessel is already attached to the tower, pre-tension is building up and the release of the sea-fastening. Here, pre-tension is a percentage of the load that is taken in the crane before the lift-off. This research aims to increase the understanding of whether a tower can be released safely on the floating barge and what can be done in order to realise the idea of a direct installation method. Frequency, as well as time-domain simulations, are used to investigate the problem. The results show that snap loads occur for pre-tensions up to 10\%. From 30\% and higher, the tower will start toppling. Toppling is initiated due to the inertia of the large tower segment when it is released from its sea-fastening. Toppling the tower is not allowed since this could damage the tower itself, the sea-fastening and/or other components on deck of the feeder. Increasing the limiting wave height is a must in order to make the direct installation method more practicable. This can firstly be done by using more tower segments. Therefore, reducing the size of each segment. Another option is to implement a motion compensation tool that decouples the motions of the feeder and the tower. The third option is to design a seafastening system that reduces the moment after the release, a temporary counteracting toppling system. All in all, can be stated that safely releasing a 20 MW tower segment on a floating barge is highly challenging and more research is required to solve the issues that are found in this research. This is necessary to allow the direct feeder method to be used for future offshore wind installation projects in the U.S.

Random vibrations; Fatigue assessment; Wind forcing; Finite Element modelling

Offshore and Dredging Engineering

Role: Committee member

In collaboration with: Rijkswaterstaat

Nautical radar systems are subjected to random vibrations during their service life, which may lead to the degradation of the materials and a possible fatigue failure. Assessing the impact of these vibrations to the structural integrity of the radar is crucial for drafting appropriate maintenance plans and therefore, the development of a new tool is required, which will be capable of evaluating the fatigue lifetime of the radar system. This thesis examines a 5.7-meter-long radar antenna, supported by a lattice tower with a height of 20 meters, located in Rijkswaterstaat’s test environment in Stellendam. Although the lattice structure and the radar system are coupled, they are treated separately in this study. Firstly, a Finite Element Model of the tower is developed in ANSYS, where the radar is represented by an added mass at the top floor. The aim of the model is to identify the vibrations of the structure under the influence of the incoming wind. The stochastic description of the turbulent wind is taken into account in the evaluation of the wind loads and a random vibration analysis is performed, resulting into the response of the structure in the frequency domain. Furthermore, a second, simplified Finite Element Model of the radar system is built in ANSYS. Time series of the fluctuating wind velocity are generated and the wind load acting on the radar is approximated by the aerodynamic force in the along-wind direction, considering also the rotation of the radar antenna. In addition, the previously obtained response of the lattice structure serves as input for the radar model, representing the vibrations induced to the radar due to the motion of the structure below. The output of the model is the occurring stress response, which is subsequently used to assess the fatigue damage of the radar antenna. The results show that the development of stresses at the bottom surface of the antenna mainly occurs at its central area. Given the assumptions used throughout the thesis, the motion of the lattice tower is found to be the governing loading condition for the resulting stresses and the magnitude of the motion significantly influences the fatigue damage of the antenna. Finally, the fatigue lifetime appears to be very sensitive to the construction materials used for the radar system. These results should be verified by future measurements in order to improve the suggested models.

Random vibrations; Fatigue assessment; Wind forcing; Finite Element modelling

Offshore and Dredging Engineering

Role: Committee member

In collaboration with: Royal HaskoningDHV and Deltares

Weirs are constructed inside rivers to manage the water level. On the one hand they need to be designed sufficiently strong to prevent failure or destruction, while on the other hand they need to be economically feasible. If a structure is designed too strong, unnecessary costs are made and the structure becomes too expensive. One of the potential failure mechanisms for weirs is erosion of the soil downstream of the weir. Bed protections consisting of rock and concrete are placed to prevent this erosion. Numerical modelling can give additional insight in the design of such a bed protection. This thesis aims to improve the design of bed protections downstream of underflow weirs through numerical modelling. This is done through two research objectives, being 1) proposing a numerical modelling strategy to create a design method for riprap bed protections downstream of underflow weirs, and 2) preserving a small computation time compared to full scale 3D models, as this is ideal for a future application in engineering. A new approach is formulated by using the stability parameter of Steenstra (2014), ψRS, as basis. This parameter is based on multiple flow situations, except the underflow gate. For this reason the underflow gate is investigated in the current research. The output of the created OpenFOAM models can be applied to this parameter, leading to ψ-curves that can be compared with measured damage. The first research objective is obtained by the application of three different turbulence mixing length approaches, of which two approaches created interesting results. The first approach is the Bakhmetev approach, leading to a conservative design outcome with a gradual decreasing stability pattern. The second is the Shear Stress Relation (SSR), leading to a promising result with better defined instability regions that compare with measured bed damage. From the desire to work conservatively, the results reveal that in the design phase of a bed protection downstream of a weir, the application of the Bakhmetev approach for the mixing length is recommended over the SSR approach. Extended physical testing containing three aspects is needed to strengthen the promising findings of the SSR. These aspects are 1) a damage stone count including a sieve distribution, 2) a setup with varying gate heights and stone dimensions, and 3) varying water levels and discharges per different gate height and stone size. In addition a preliminary 2D numerical model study is advised, which allows to investigate the areas of interest for PIV flow measurements. The second research objective is achieved by the application of a 2D RANS model and the simplification of the water level through a rigid lid. This simplification still leads to workable results, with computation time that are in the range of two to eight hours, operating on ten cores in one computer. This makes the approach attractive for engineering applications and for future applications in renovation tasks for weirs in the Meuse.

CFD; Weir; Hydraulic jump; Bed protection; OpenFOAM; Underflow gate

Offshore and Dredging Engineering

Role: External committee member

In collaboration with: TNO

The world is currently in a global energy transition. To reach the global energy goals requires massive deployment of clean and efficient energy technologies. The use of solar photovoltaic (PV) therefore has to be scaled up. Offshore deployment of floating PV would be a suitable solution. There are currently different concepts in development for offshore floating PV. One solution is to design the offshore floating solar structures using interconnected rectangular floaters on which the solar panels are mounted. Multiple design choices have to be considered, such as the size of the structure and the design of the connections between the floaters. The main aspect to consider are the forces that arise in the connections due to the motions and loading of the offshore floating solar structure in the waves. One of the main challenges is to quantify the effect of the different design choices regarding size and connection compliance on the forces within the connections between the floaters. The aim of this research is to give insight into the interaction between connection forces and different concept choices regarding the size of the floaters and the compliance of the connections between the floaters. A numerical model of two floaters which are connected with two connections at each end of the structure is developed. The forces and motions of the floating structure are computed using numerical tools in the frequency domain. The hydrodynamic coefficients and wave excitation forces are obtained using the boundary element method software NEMOH. The connections between the floaters are modelled as a set of linear translational and rotational springs in each degree of freedom. The results are validated using earlier studies. The influence of the design choices is investigated for application at the North sea for different wave directions. First, four different dimensions are varied, the length, the width and the draft of the floaters and the size of the gap between the floaters. The results are then analyzed and compared for each dimension. The comparison of the results shows the influence of each dimension on the force in the connections. Secondly, the connection stiffness is varied in four different directions, the axial, shear, bending and torsional stiffness. The comparison of the results show the interaction between connection stiffness and resulting forces in the connections. The developed numerical frequency domain model shows the interaction between forces and moments in the connections and different design choices. The results can be used to give an insight into the occurring forces within the connections between two floater for application in the North sea.

Offshore Floating Solar; numerical modelling; Frequency Domain; Offshore engineering

Offshore and Dredging Engineering

Role: External committee member

In collaboration with: Blue21

With the climate goals of the Paris Agreement and the European Green Deal, countries need to reduce their carbon footprint and increase their renewable energy production. For countries with high population density, land-based photovoltaics (solar) takes up valuable space. Therefore, solar application at sea is considered. One of the proposed designs is by the Joint Industry Project (JIP) Solar@Sea II. The JIP structure is flexible, inflatable, and considerably smaller than Very Large Floating Structures (VLFS). The goal is to mitigate the installation and transportation disadvantages associated with VLFS. Combining multiple of these singular structures in an array allows for the same operational solar footprint as would be the case for a single VLFS. To keep the structure at the intended location, a mooring system needs to be designed. For the mooring line analysis, the motions of the attachment points of the mooring lines to the floating structure must be deter-mined. These motions are dependent on the interaction between the fluid, structure, and mooring line system. The development of a numerical method that is able to determine the flexible motions of the structure is required. This method should be suitable for the initial design stages of the structure and mooring system. A numerical method for the deep-water regime in frequency domain was developed. The structural deformations are determined by means of the Finite Element Method (FEM) in ANSYS. The fluid interactions with the structure are determined by means of a lower order Boundary Element Method (BEM). The combined effect of both structural motions and fluid behavior is captured in an equation of motion. The motions of the flexible structure can then be determined by solving the equation of motion. The method is written in Python and the interaction with ANSYS is achieved by means of an ANSYS APDL interface. The numerical method was successfully verified by comparison of results with analytical solutions. The nu-merical method is validated by comparing the numerical result with experimentally determined responses of a 1:1 scale JIP structure to incident regular deep-water waves as measured by MARIN. Suitable deep-water test cases were determined from the MARIN data. The undisturbed numerical wave was compared with the undisturbed deep wave measured by MARIN. Finally, three test cases were selected for the validation of the interaction of the structure and the wave. These consisted of a wave longer than the structure, a waveslightly shorter than the structure, and a significantly shorter wave than the structure. The numerical method was able to accurately predict the structure motions for waves longer than the structure. For waves shorter than the structure, the error between the numerical method and the experimental data increased as the wavelength decreased. The errors found can likely be attributed to the nonlinear interaction of the ballast bags, which are not considered in the current numerical method. This cannot be confirmed based on the available experimental data. The presented work is part of a larger intended method, which is not yet finished. Satisfactory results were found for the components considered in the verification. The validation provided points for improvement for the current numerical method. The response to waves longer than the structure can be determined accurately. The method is less accurate for waves shorter than the structure. The recommendation is therefore to research the effect of the ballast bags on the structure response in shorter waves. This could result in a better approximation for structural responses in shorter waves.

FEM analysis; BEM analysis; Hydroelasticity; Floating Solar

Offshore and Dredging Engineering

Role: Daily supervisor

In collaboration with: Mocean Offshore

Large-scale cultivation of seaweed presents opportunities for multiple global challenges currently at play. Cultivated seaweed can provide a sustainable source of protein for humans and cattle without competing for land, freshwater supply or the use of fertilisers. Kelp forests are known to be a solid basis for an elaborate biome that supports biodiversity in areas that have been damaged by over-fishing or rising sea temperatures. Additionally, kelp forests can lock-in large amounts of Blue Carbon, expanding the oceans’ buffering capacity to mitigate anthropogenic emissions. Furthermore, with their densely seeded lines, offshore kelp farms are found to attenuate wave amplitude, thus providing coastal protection and benefits like increased workability for offshore operations. Both academic publications and industry reviews underline the potential of this sector and significant growth in cultivation is expected in the near future. Methods currently used for quantification of the damping effects of large-scale offshore kelp farms are diverse and entail varying degrees of accuracy and computational cost. Experimental observations that support the outcomes of these methods are limited to scaled experiments in wave flumes, with various methods used to mimic vegetation. No convergence is found in the most suitable methods for application to large-scale offshore kelp farms. This research presents a novel modelling framework based upon the Finite Element method, implemented using Julia Programming Language. The effects of the vegetation on the wave climate are represented with a Darcy-Forchheimer term borrowed from porous medium flow theory, including a linear and a quadratic resistance term. The framework comprises a numerical wave tank, using the incompressible Navier Stokes equations. The single-phase model captures the free surface using the coupling of dynamic pressure with a virtual elevation variable through a linearized transpiration boundary condition. Wave energy dissipation is shown to increase significantly by moving the farm structure close to the water surface. Similarly, a decrease in relative water depth - compared to the vegetated height - increases damping potential. Wave period is found to be of strong influence on dissipation, where short waves are attenuated more. Scaling vegetation length with wave length, however, diminishes the reduction in damping of longer waves. Conversely, wave amplitude is shown to be of less influence on the transmission of amplitude through a vegetated patch. The framework presents a method that is easily scalable, flexible in application on a wide range of flows and vegetation characteristics, and at reasonable computational cost. Introduction of both the linear and quadratic terms extends applicability compared to traditional methods. The approach is verified using convergence studies, application of the model is validated by comparison to existing experimental data. It is shown that experimental set-ups can be reproduced effectively, and simulation results coincide with experimental findings. Validation of outcomes on scales larger than common wave tanks was found unfeasible due to a lack of measurement data. A theoretical case study was performed to predict wave damping of a full-scale kelp farm, demonstrating promising potential with up to 40% wave energy reduction at the local peak wave period. Further research into the establishment of the Darcy- and Forchheimer-coefficients is recommended. A preliminary range of values has been found, based upon calibration on existing experiments that represent realistic ranges of vegetation characteristics. Furthermore, the main conditions of the flow and vegetation that dictate damping potential are identified. On this basis, research into a physics-based determination of the coefficients is recommended. Additionally, full-scale measurements are advised to validate application on future kelp farm designs. Through this novel approach, the range of application is increased compared to existing methods, while straightforward setup and usage are governed, and limited computational costs allow for simulation without the need for a dedicated computer setup. The framework is shown to be robust by generating consistent simulation results. In summary, the established framework shows to be a good alternative to existing approaches to investigate the wave damping potential of large-scale offshore kelp farms.

Numerical model; FEM; Julia Language; Porous medium; Kelp farming; Seaweed; Wave damping

Hydraulic Engineering

Role: Daily supervisor

In collaboration with: Blue21

For both developed and developing nations, coastal zones form an attractive location for urban settlements. With the expected increase in the earth’s population, coastal areas will experience a further increase of inhabitants. Floating city development could therefore be an interesting alternative for land-based urban expansion on land [41]. Expanding an urban settlement towards the ocean however, will make it more susceptible to extreme forces such as a tsunami waves. By generating more knowledge on how a floating structure interacts with a tsunami wave, it can show the potential value that a floating city can bring. If a floating city limits the effect of (extreme) events such as tsunamis, it can protect the coastal zone which is located near a floating city. To provide an answer to this question, this research will focus on how a floating structure can reduce the transmission of a tsunami. With the construction of an analytical model, representing the floating structure and the tsunami wave as simplistic as possible, the system can be understood more quickly. If the problem is solvable by generic programming language, this would mean that it can solve a larger range in the spectrum of the problem. The conceptual model features two options: one where the platform has no freedom of movement, the other where the platformcan move vertically. They both assume that hydrostatic pressure holds during wave propagation and a linearization of the momentum equation describing the water particle interaction. For each option, the transmitted wave height is determined based on varying the floating structure dimensions. This gives an indication on which parameters are of influence in the transmission of the wave. First, the conceptualmodel is analysed by changing the platformdraft and the length for both the motionless and the vertically moving platform. Both options are influenced most by the length of the structure. The situation with the motionless platformshows this effect earlier, by a higher wave attenuation percentage for the same platform length. Whether the draft also has an influence is strongly dependent on the value representing the length of the platform. The difference between the two platform movement options shows that the effect of changing the allowed movement of the platformis significant. Next to the reduction in transmission, the conceptual model shows signs of resonance. The moving platform option in the model is formed by a second order differential equation. Fitting this equation, resonance is evident and therefore visible for certain combinations of the platformdimensions. In addition damping is present, ensuring that there are some parts where, despite the natural frequency pointing there, no resonance occurs. The amount of damping is strongly linked to the platformlength, with a higher level of damping for a longer platformlength. Finally, the results from the conceptual model are compared to the outcome in SWASH. This numerically based model has the possibility to simulate a tsunami wave in its development towards the coast and also features a buoyancy function for structures. This comparison serves as a validation of the conceptual model. In general, the conceptual model always results in a less reduced wave attenuation percentage and can be said to be more conservative. Due to the assumptions of leaving out the non-hydrostatic pressure and a lower level of detail, a maximum of 5% deviation in both model results occured. This however, matches with the fact that it is a less detailed model and adds to the reasoning that the conceptual model provides what it is meant for. Next to the effect of the structure itself, the positioning of the structure is also of large importance. Wave height and intensity of the wave will vary due to the surrounding local coastal features. Next to that, the local water depth is determinant in compressing the wave, therefore decreasing the wave length when the water depth decreases. The maximum wave attenuation that can be achieved according to both models is 10%, considering platform dimensions and location variations. The conceptual model appears to work for which it is intended: modelling the resulting transmission between a floating structure and a linear tsunami. It is expected that modelling programs can be expanded and/or improved, so that more realistic floating structures can be modelled. However, it will remain difficult to accurately model a tsunami as it is complex in behaviour. Yet, this research brought the field one step closer to evaluating the transmission of tsunami waves when interacting with a floating structure of certain dimensions.

hydroelastic analysis; VLFS; floating city

Hydraulic Engineering

Role: Daily supervisor

In collaboration with: Blue21

This thesis concerns a hydroelastic analysis of a multi-module very large floating structure (VLFS), analysed in the frequency domain. To this end, the fluid-structure interaction is described by a 2D model, where the VLFS is represented by four floating beams interconnected with rotational springs. The fluid is modelled as an ideal fluid, and the floating beams are modelled by the Euler-Bernoulli beam theory. The finite element method is applied to solve the governing equations of the fluid motion and the motion of the beams, where the model is built using the FE-library Gridap, written in the Julia programming language. The aim of the study is to investigate the influence of various module and connection stiffness on the behaviour of the system, with the view to obtain more insight in the complex relation between the hydroelastic response and internal loads, when the system is subject to regular waves.

hydroelastic analysis; VLFS; floating city

Sustainable Energy Technology

Role: Daily supervisor

In collaboration with: SIF Offshore Foundations, Delft Offshore Turbine (DOT)

The offshore wind industry in Europe has experienced significant growth in the past decade, with wind farm development mostly focusing on the shallow area's in the North Sea. Naturally, the market is driven to reduce the Levelised Cost of Electricity (LCoE) to become more competitive with fossil fuels and less dependent on government subsidies. A transition in wind farm development towards deeper waters is expected and already observed in the market, driven by decreasing availability of shallow area's and higher wind resource at far offshore locations. The majority of the northern part of the North Sea is between 60 - 120 meter deep, currently the jacket foundation is deemed as the foundation of choice for this water depth range. Despite several technological advantages of the jacket, the main downsides are the large engineering effort and welds required to produce such a foundation resulting in difficult series production and high costs. This does not align with the industry's ambition to lower the LCoE. As such, the need for a technologically viable and economically attractive foundation concept for waters between 60 - 120 meter deep arises. The goal of this research can be divided in to two parts. The first part is to determine the potential of conventional monopiles in this water range and identify the main limiting factors. To do so, a monopile is dimensioned at a selected reference location for three turbines representing the current, near future and future outlook of the market. The designed monopiles are tested for manufacturabiliy, Ultimate Limit State (ULS) and Fatigue Limit State (FLS) to identify the technical showstoppers. Next, in the second part, a novel monopile design is introduced and analysed to work around the identified limits. To dimension the monopiles for the three reference turbines a parametric dimensioning script is developed. The monopile geometry is dimensioned to have a selected first natural frequency of 0.20 Hz, based on the relevant frequency diagrams. Next, these geometries are tested against mudline ULS failure for the power production and parked condition load cases. Hereafter, an FLS check for the B1, C1 and D S-N curves is conducted based on the obtained scatter tables for site conditions. It was found that D-curve fatigue damage for non grinded butt welds is the main limiting factor for all dimensioned monopiles in deep water. However, industry experts believe that all welds can be grinded in the production process, eradicating the need to assess the D-curve. When assuming this statement to be true the newly obtained limits become ULS failure during parked conditions for the 15 MW reference turbine and manufacturability constraints for the 20 MW reference turbine. The Haliade X showed no limits within the specified water depth range when neglecting the D curve fatigue damage. A perforated monopile concept with reduced available area for wave loading is introduced. A Computational Fluid Dynamics (CFD) model based on the 2003 Menter Shear Stress Transport turbulence model is constructed for a perforated monopile to gain insights into how waves propagate through the structure and the forces associated with this. The CFD model is verified against experimental wave flume data before being used for further analysis showing a root mean square error of 0.0192 between model results and experiments. The CFD model is used to assess three geometries with different perforations and levels of porosity. No increased drag around the first natural frequency caused by the perforations, hinting to favourable dynamics, was found in any of the test cases. As such, the dynamic response of the three perforated monopiles was found to be unchanged when compared to a reference pile without perforations. Despite this, a significant reduction of lifetime fatigue damage was observed caused by the reduced forces acting on the structure resulting from the smaller frontal surface. Next, the mudline stresses are recalculated and a structural finite element model to assess the stress concentrations around the perforations is set up to verify the maximum allowable stress level threshold is not exceeded. A geometry was found which shows a 35.5% reduction of lifetime fatigue damage whilst stresses remain below the maximum threshold, hereby showing the potential of the perforated monopile. Implementing this perforation allows the use of monopiles up to 87 meter deep, limited by D curve fatigue. Reference piles without perforations were found to be infeasible for all assessed water depths, also limited by D curve fatigue. It is shown that the perforation concept can either be implemented to push the monopile foundation to deeper waters, or can be used to realise steel reduction at current water depths.

Monopiles; Offshore Wind; Perforation; Renewable Energy; Wind. Sif; DOT

Offshore and Dredging Engineering

Role: Daily supervisor

In offshore engineering complex simulation models are constructed for design optimization using Monte Carlo methods. These models incur large computational costs. Multi-Level Multi-Fidelity Monte Carlo is proposed as a method to reduce the computational cost of these simulations. In addition, research is conducted on the use of porous media as passive damping systems. Hence, an analysis on the effect of porosity on the vortex shedding frequency is conducted. This thesis is an exploratory investigation on the application of Multi-Level Multi-Fidelity Monte Carlo in fluid dynamics topics and its particular use for analysis of the effect of porosity on the vortex shedding frequency on a porous circular cylinder. Three case studies are conducted. Firstly, applying Multi-Level Multi-Fidelity Monte Carlo on a solid circular cylinder case, which is deemed as a successful application, based on the estimated quantity of interest, variance reduction and computational cost reduction. Furthermore, two parametric studies are conducted: 1) to discover empirical relationships (low-fidelity models) and 2) forward uncertainty propagation with Multi-Level Multi-Fidelity Monte Carlo using a uniform input distribution. Both parametric studies consist of a number of equally distributed points of porosity on a case setup of flow past a porous circular cylinder. The parametric studies use a frequency detection algorithm, which approximates the vortex shedding frequency using the frequency of lift force oscillation. The results of the first parametric study indicate there is a drop in vortex shedding frequency as experienced by the cylinder for increasing porosity. The hypothesis is that for increasing porosity the formation length of vortex shedding increases. Two empirical relationships are derived from the results by curve fitting the Strouhal number (dimensionless form of the vortex shedding frequency) versus porosity. These empirical relationships are incorporated in the Multi-Level Multi-Fidelity Monte Carlo method and applied to a similar parametric study on the effect of porosity on the vortex shedding frequency. The results indicate the presence of systemic errors in the high-fidelity model. The conjecture is that the major influence on these errors is due to the resolution of the frequency detection algorithm being too low. For this reason, no clear conclusion on the validity of the empirical relationships is obtained and further research is required.

Monte Carlo; Porous Media; Multi-Level Multi-Fidelity; Strouhal number; vortex shedding

Offshore and Dredging Engineering

Role: Daily supervisor

In collaboration with: SPT Offshore

In this thesis I will show the realisation of a multiline anchor system using a suction pile anchor (SPA). From the literature research it can be concluded that the SPA is a suitable anchor for a multiline anchor system and that the most probable mooring configurations will be either the original single-line, the 3-line or the 6-line system. Furthermore it could be concluded that when using a catenary mooring solution for the 3-line system, that the anchor perceives a lower horizontal net force due to anchor lines coming from different directions and cancelling parts of each other. This would, in theory, make the SPA for the 3-line multiline system smaller and also there would be 67% less anchors needed in a wind farm array. Furthermore the 6-line anchor does seem to have a bigger horizontal net force which requires the SPAs to be bigger but 83% less anchors are needed in an array of wind turbines. The looked at FOWT-system was the University of Maine VolturnUS-S reference floating offshore wind turbine semi-submersible, which supports the IEA-15-240-RWT turbine with a turbine rating of 15MW. With the original mooring configuration being a catenary one, a comparison has been made with a taut system and its implementation into a multiline anchor system has been researched. Lastly three different soil profiles have been chosen: normally consolidated clay, slightly consolidated clay and loose sand. In Chapter 4 Suction anchor concept design, the forces on the anchor are set up using the data set-up by NREL and the University of Maine and the C. Fontana papers. A SPA has more bearing capacity when the mooring-anchor connection point is below the mudline. Because of this feature the mooring line attaches to the SPA at an angle and in this thesis a range of angles of approach are chosen to be investigated: 25 and 35 degrees for the chain and 45 degrees for the taut line. Because of the angle there is a vertical force introduced to the anchor which need to be added together for each line. Because of this the total net force on the anchor is increased for the 3-line and 6-line anchors compared to the single-line anchor. With the mean and maximum forces the calculations for the holding capacity/pull-out capacity can be set up by looking at the Ultimate Limit State (ULS) or maximum forces in the system. These calculations are taken from the DNV guidelines which are the industry standard. To set-up the first parameters estimation a embedment (h/D-ratio) starting value has to be chosen and from literature and in discussion with SPT offshore the starting values were set at 5 for the clay profile and 1.5 for the sand profile. The weight of the SPA must be defined which was done as the mean vertical force applied to the anchors as such they will not be pulled out over time. The installation and removal calculations are an important step in the design and are also set up. Here the under pressures required to fully install the SPAs are calculated. Furthermore, structural failure due to buckling is checked for and different soil failures are analysed. Lastly the removal pressure of each concept is checked which allows for complete removal of the suction anchor. By lowering the embedment ratios of the different concepts the pressures inside the anchors can be minimised and problems can be averted. In the detailed design the full design of a in use SPA is shown and each part is defined. Furthermore, the one-line-broken criterium is discussed and it can be concluded that in case this happens the 6-line multiline system is very dangerous because a chain reaction can be started which can take out large parts of a wind farm array. Also a weight estimation of each SPA is made from which the extra needed ballast is calculated. Subsequently a cost estimation of each anchor concept can be made by calculating the cost of each system from the structural weight, the ballast and the mooring line lengths. At a depth of 200m, at which this study is situated, the taut multiline system cannot be set-up but the single-line taut system can be compared to the catenary single line system. Lastly a parametric analysis is done where changes in different parameters are compared to each other. What can be concluded from this thesis research is that a multiline system is technically feasible for a 15MW floating offshore wind turbines using SPAs. The 3-line and 6-line systems both have larger anchors than the single-line system although they need less anchors in a system. When including the mooring line costs together with these anchor costs it can be concluded that the 3-line anchor is more economically viable but the 6-line anchor is not. What can also be concluded from these mooring line costs and what is discussed in the parametric analysis is that the system works better if the wind turbines are closer together because the lines will be shorter. This distance is dictated by the wake recovery and an optimisation study is recommended for the 15MW wind turbine but it is also recommended that smaller turbines and deeper depths are looked at.

Anchor; anchor mooring; anchor points; multiline; SPA; suction pile anchor; FOWT; Floating offshore wind turbine; SPT Offshore

Hydraulic Engineering

Role: Daily supervisor

In collaboration with: Mocean Offhsore

At the moment, the world is at the verge of an energy transition. One of the most promising green resources is solar energy, which is a rapidly growing market. However, to fully use its potential of economy of scale, the application of offshore floating solar should be explored. A promising option is the use of a flexible type of Very Large Floating Structures (VLFSs), which are called Very Flexible Floating Structures (VFFSs). They are characterised by their large length to height ratio compared to rigid bodies and depending on their material properties have a hydroelastic response to the incident wave. In the late 1990s, a lot of research has been done on VLFSs by Tsubogo and Okada (1998) who derived an analytical dispersion relation assuming a zerodraught structure. However, only recently, Schreier and Jacobi (2020b) did experimental research on VFFSs in a towing tank at the Delft University of Technology, as little is still known about flexible structures. This report focuses on a numerical alter native that covers both VLFSs as well as VFFSs using a Finite Element Method (FEM) Fluid Structure Interaction (FSI) model which has been built based on potential flow to model the fluid and a dynamic EulerBernoulli beam that represents the floating structure using the Julia package Gridap. One of the main advantages is that the zerodraught assumption is not necessary and, therefore, structures with larger draughts can also be modelled. Next to this, the numerical model is able to cope with irregular shapes, for which no analytical method yet exists. The model is built such that it can handle 2D as well as 3D domains. A 2D analysis has been made to understand the influence of hydroelastic wave deformation of the incident wave, in terms of wavelength dispersion as well as amplitude dispersion on floating structures. To verify the model, the numerical results were compared to the analytical solution and experimental research in a towing tank, which showed accurate results. Test runs were set up that mimicked the towing tank setup and a fullscale solar park. Furthermore, a sensitivity study was executed that shows the limits of the flexible domain and to see in which cases significant (>1%) hydroelastic wave deformation would occur using governing mean and extreme ocean waves, as well as a typical lake wave. Finally, the influence of the draught of the structure was examined. This report provides a good overview of when wave deformation should be accounted for in terms of bending rigidity and density. Confirming existing theory, it was found that the stiffness of the VFFS causes wave stretching and the draught of the structure influences the extent of wave shortening. It was also found that significant wave deformation will not occur for ocean waves as the required stiff ness is beyond existing materials. For extreme ocean waves, there is even no dispersion at all. As the wave frequency increases, the hydroelastic interaction gets stronger. The typical lake wave showed to be well within the flexible regime and also showed significant dispersion with realistic material parame ters. The numerical model is able to cope with large draught scenarios which lead to wave shortening, which in its turn leads to wave focusing. Ultimately, the numerical model showed to be a good alternative to existing methods to investigate the behaviour of VLFSs outside the floating solar domain, where one could think of ice floes, floating islands or floating airports.

Very Large Floating Structures; Fluid Structure Interaction; Very Flexible Floating Structures; FEM; potential flow; Floating Solar

Offshore and Dredging Engineering

Role: External committee member

Using a floating vessel operating on its DP-system and using a motion-compensated pile gripper to install monopiles could be the installation method of the future. Therefore, this thesis focuses on this method. The main objective of this thesis project is to build a model that accurately describes the motions of the Stella Synergy, the monopile, and the motion-compensated gripper, depending on the environmental conditions. This model is built in Anysim, which is a time-domain simulation software program of MARIN based on the RK2 numerical method. The model considers the early pile driving phase because this phase is governing in terms of risk. The monopile acts as an inverted pendulum in this phase, and the motion-compensated pile gripper must guarantee the stability of the monopile. The vessel uses its DP-system for station keeping. The DP-system contains a position reference system, a filter, a control system, and a thruster allocation algorithm. The vessel describes the wind, current and wave forces on the monopile and vessel. The environmental conditions are assumed to be collinear, and wave spreading is added to the model for some simulations. The wave forces on the vessel are determined with diffraction calculations in Ansys AQWA. The diffraction calculation for the vessel is verified with a diffraction calculation of MARIN, and the diffraction calculation for the monopile considers the shielding effect and is verified with a calculation with the Morison equation. A motion-compensated pile gripper with two PD-controllers is built in Python. The gripper considers static and dynamic friction forces and a maximum delta force per numeric timestep to model the pressure build-up time of the hydraulic cylinders. Multiple 3-hour simulations are run to generate results. These simulations, which considers each a different sea condition, are tested by the six limitations of the model. First, the preferable incoming angle of environmental conditions is determined. The workability of the Stella Synergy is calculated operating at the North Sea using this preferable incoming angle of attack. Then, two adaptations to the model are tested to increase the workability. Using fast-rotating thrusters or changing the DP-gains result in the workability of 96.4%. The governing limitation is the pitch motion of the vessel. It is tested if using mooring lines in combination with the DP-system results in a footprint reduction. It is concluded that adding mooring lines could result in a footprint reduction, but it is crucial to gain insight into the optimal axial stiffness of the mooring lines. The monopile's influence on the vessel's motion is also tested. It is concluded that the vessel's surge, sway, roll and yaw motion increases significantly due to the environmental forces on the monopile, which are passed through the gripper to the vessel. Finally, the workability of the vessel during the worst-case single failure is determined. After improving the DP-gains for particular sea conditions, the workability for the worst-case single failure was 96.0%. The failure results thus in a minor difference in the workability.

Workability; DP-system; Motion-compensated gripper; monopile; installation

European Wind Energy

Role: External committee member

Bayesian system identification, including parameter estimation and model selection, is widely used to infer partially known, unobservable parameters of the models of physical systems when measurement data is available. A common assumption in the Bayesian system identification literature is that the discrepancy between model predictions and measurements can be described as independent, identically distributed realizations from a univariate Gaussian distribution. However, the decreasing cost of sensors and monitoring systems leads to more frequent structural measurements in close proximity to each other (e.g. fiber optics and strain gauges). In such cases, dependency in modeling uncertainty could be significant, both in space and time, and the assumption of uncorrelated Gaussian error may lead to inaccurate parameter estimation. The aim of this thesis is to explore how Bayesian system identification can be feasibly performed using large datasets when spatial and/or temporal dependence might be present and to assess the impact of considering this dependence. A pool of models, each assuming a different correlation structure, is defined and Bayesian inference is performed. In particular, stress measurements obtained on a steel road bridge are used to update the parameters of the corresponding FE model and the parameters of the correlation structure. The results are compared to a reference model where only measurements of the response peaks are used under the assumption of independence. Nested sampling is utilized to compute the evidence under each model and Bayesian model selection is applied. The question of efficiently performing system identification for large datasets (N > 102 for temporal dependencies and N > 103 for combined spatial and temporal dependencies) is investigated, and a novel approach for efficiently calculating the exact log-likelihood is derived. An approximation based on the Fisher information matrix is used to efficiently calculate the information content of measurements. It is found that the choice of correlation function can significantly affect the posterior distribution of the model prediction uncertainty. Additionally, it is shown that using large datasets and considering dependence makes it possible to perform system identification for a larger number of parameters compared to the reference model. The results of the case study indicate that using measurements from multiple sensors under combined spatial and temporal dependence and additive model prediction error yields reduced uncertainty in the posterior and up to 29% reduction of the posterior predictive credible interval range compared to the reference case. Furthermore, the efficiency of the proposed likelihood evaluation method is assessed. Using this method, exact calculation of the log-likelihood can be performed for >106 points in under a second in the case of correlation in one dimension. For combined spatial and temporal correlation it is shown to be approximately 900 times faster than naive evaluation for a 64 by 64 grid of observations. The results of the case study indicate that the described approach can be feasibly applied to real-world structures and can potentially improve parameter estimation and reduce prediction uncertainty. These findings suggest that further research into the approach could yield improvements over current methods.

Bayesian Inference; Bayesian statistics; System Identification; Structural Health Monitoring (SHM)

Offhsore and Dredging Engineering

Role: Daily supervisor

In collaboration with: Jumbo Maritime

A QUAD lift is a new lifting method in which dual crane vessels combine their vessel capability to increase their offshore lift performance. The use of the Jumbo J-Class vessels in a QUAD lift creates the opportunity to increase the offshore lift capacity and to install structures with larger dimensions. When floating vessels are close to each other in an offshore environment, their motion will be different than in the freely-floating situation because of hydrodynamic coupling and wave diffraction forces. The main objective of this thesis is to create a model of the QUAD Lift method which predicts the vessel and load motions and evaluate the workability such a lift. Both potential solvers AQWA and OrcaWave are used to assess the hydrodynamic parameters of the interacting vessels. The gap between the vessels is 40 m and the vessel configuration is such that the cranes are parallel to each other. In between the vessels, transversal wave resonance induces peaks in the frequency dependent radiation forces of the vessels. An additional damping lid in between the vessels effectively reduces the resonance behaviour, which is overestimated by potential solvers. The damping lid has an negligible effect on the final workability of the QUAD lift. A 18-DoF linear Matlab model is created which includes the mechanical connection between the vessels and the load. The cranes and the cables are modelled as linear springs. The natural frequencies and eigenvectors show large coupling between the vessel roll and the load sway motion. Tugger lines between the vessel and load are added to shift the natural frequencies of the system and to decrease the large horizontal responses of the load. A parametric study is done on the effect of the load mass, cable lengths and wave directions on the system motion in the most probable wave condition in the Central North Sea. An increase of the mass of the load leads to larger vessel and load motions. The shorter the cable length, the larger the vessel and load motions. The motions are most severe in beam and quarter waves. Depending on the stiffness of the tugger lines the workability can be improved up to 85, 55 and 24 % in respectively head, quarter and beam waves. The limiting factor for the workability is the off-lead angle of the cranes. Broadening of the off-lead angle limit of the crane shows great potential to further increase the workability.

Diffraction/radiation model; Coupled Multibody and Hydrodynamics; Vessel motions; Frequency Domain; Offshore Installation; Offshore Heavy Lifting; Workability