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Design optimization of the offshore wind turbine support structure is an expensive task; due to the highly-constrained, non-convex and non-linear nature of the design problem. This report presents an analytical gradient-based method to solve this problem in an efficient and effective way. The design sensitivities of the objective and constraint functions are evaluated analytically while the optimization of the structure is performed, subject to sizing, eigenfrequency, extreme load and fatigue load constraints. A case study is carried out for the OC4 jacket substructure to evaluate the method. Results shows that an optimal jacket design with 52 percent structural mass reduction is attained in 27 iterations, while satisfying all design constraints under the simplified load cases used. Besides, it is shown that the analytical sensitivity analysis is more accurate and efficient than the often used finite difference approximations. It can avoid numerical artifacts that typically occur in the analysis of extreme load constraint sensitivities.

Large structural systems such as bridges and offshore structures are subject to deterioration processes during their service life. Specifically, fatigue and corrosion are of major concern. To ensure that the structures are operated within acceptable safety levels, actions to mitigate the risks associated with deterioration need to be conducted throughout their life cycle. Examples of mitigation actions are to increase the reliability of the deterioration-sensitive components at design (e.g., by increasing their resisting cross-section area), to choose more robust structural configurations to minimise the consequences of deterioration, and to repair structural components based on identified damage. Generally, the optimal mitigation strategy will be a combination of several mitigation measures. The limited societal budgets and the increasing environmental concerns demand an efficient use of the construction materials. To efficiently manage the large portfolios of existing and planned infrastructure systems, the decisions made throughout the life cycle of the structures need to be optimised within a common framework. The aim of this thesis is to improve the design basis of mitigation strategies for structural systems. Structural reliability methods are used to assess the effect of the mitigation measures on structural safety. At the design phase, the effect of future inspections and monitoring on the system reliability can be estimated by integrating over possible outcomes using prior information. Similarly, one can perform the same integration at any stage of the service life based on information updated with obtained data. Reliability- and risk-based design are applied to the assessment of the optimal mitigation strategies at the system level. System-level design is of particular relevance for the present research, because aspects such as spatial dependence of the deterioration processes or structural redundancy have a strong effect on the system reliability and the value of the information that can be obtained. A framework for simultaneously assessing optimal design and inspection strategies is developed. This framework is then applied to the investigation of how the optimal mitigation strategy varies as a function of system redundancy and size. These results aim at generalising decision rules for the standardisation of mitigation strategies. Moreover, a novel method for assessing system reliability bounds of multimember offshore structures subject to extreme environmental loads and high-cycle fatigue is developed. This method is applied to the assessment of the reliability associated with existing system-level design methodologies and the evaluation of system effects, such as fatigue dependence and progressive collapse. In addition to the developed theoretical frameworks, the articles include several examples of applications, specifically on the design of monopile support structures for offshore wind turbines subject to fatigue and extreme environmental loads, and on the design, reassessment and inspection planning of mooring systems affected by combined fatigue and pitting corrosion.

Floating wind turbines (FWTs) are considered a promising solution for wind energy harvesting in deep water, but are currently too expensive to compete with other energy sources. Being a relatively new and immature technology means that there still is a large potential for cost reductions through optimization of the FWT structure. Optimized designs will bring the construction costs of FWTs down and increase profitability, which is currently the major challenge for the industry. Optimized designs can also result in increased reliability, which is an important issue. The main purpose of this work was to improve the design process for FWTs and thus contribute to reducing the cost of energy. This is addressed through two overall research objectives, which consider i) increased computational efficiency of global design analyses for FWTs, and ii) methods for numerical iii iv design optimization which can help identify cost-effective and reliable design solutions. The main focus was on the support structure and controller for 10 MW spar-type turbines, considering fatigue and ultimate loads. A linearized aero-hydro-servo-elastic model was shown to yield good results for the fatigue loads in the support structure. Acceptable agreement was also observed for short-term extreme response, especially for the support structure bending moments, which were quite Gaussian also in harsh environmental conditions. The resonant platform pitch response was overestimated by the linear model, especially in near-rated conditions. A gradient-based optimization approach with analytic derivatives was developed to perform integrated design optimization of the support structure, blade-pitch controller, and mooring system for an elastic 10 MW spar FWT, including the scantling design of the hull, where the goal was to minimize a combination of design costs and rotor speed variation. Different control strategies were compared through integrated design of the controller and support structure, which allowed for identification of optimal control parameters in a lifetime perspective, and fair comparisons between different strategies. The impact of environmental modelling on the long-term fatigue reliability, and associated design costs, of the support structure was also assessed through re-design of the tower and platform. The methodologies for global response analyses and integrated design optimization developed in the present work have been shown to be suitable for preliminary design of spar FWTs, where they can provide a starting design for later and more detailed design phases. Different modelling and design aspects for cost-effective and reliable solutions have been identified and assessed. The methodologies can be further extended to account for different FWT concepts, additional design parameters, and other load cases, and may help identify novel design solutions.

Tretthet er ofte en styrende designfaktor for havvindmøller. Siden utformingen av offshore vindturbiner inkluderer konservatisme, kan den faktiske akkumulerte tretthetsskaden være lavere enn hva turbinen er beregnet på. I dette tilfellet kan operatøren avgjøre levetidsforlengelse av eksisterende vindturbiner. Derfor er det viktig å estimere den faktiske akkumulerte tretthetsskaden for å støtte beslutningsprosesser ved forlengelse av levetiden, og for optimalisering av støttestrukturdesign. Imidlertid er tretthetskritiske steder lokalisert i nærheten av mudline hvor det er umulig å installere strain gauges for å måle den akkumulerte tretthetsskaden. Det første formålet med denne oppgaven er å undersøke om data-drevne tilnærminger (lineær regresjon og feed-forward neurale nettverk) kan brukes til å estimere akkumulert utmattelsesskade både i individuelle turbiner og gårdsbaserte nivåer. Det andre målet er å bestemme minimumsantal sensorer og mengde data som kreves for nøyaktig estimering. Mot dette målet har man brukt reelle måldata for to offshore-turbiner i samme vindmøllepark. Spesielt har de data-drevne tilnærmingene blitt anvendt med ekte måledata fra SCADA-systemet, målinger øverst og nederst i tårnet, og data fra et bølgemålingssystem. Disse dataene ble brukt til å estimere den akkumulerte utmattelsesskade på flere steder (tårnbunn, overgangsstykke og to nivåer på monopilen) i form av skadeekvivalente belastninger. Gjennom hele studien, 10 min. Statistiske egenskaper til måledataene har blitt brukt som input til læringsalgoritmer. En bemerkning er at estimeringen ikke har blitt utført for tretthetskritisk lokalitet i nærheten av mudline selv, men det forventes at estimering med disse tilnærmingene kan utvides til tretthetskritisk plassering dersom nøyaktig responsestimering på flere steder på understøttestrukturen er mulig. Resultatene av denne oppgaven viser at de data-drevne tilnærmingene kan gi nøyaktige estimater skade ekvivalent last på individuelt turbinivå på flere steder på støttekonstruksjonen når øyeblikk eller hellingssignaler ved tårnbunn brukes. For estimering av bedriftsnivåbelastning har det også vist seg at de data-drevne tilnærmingene kan gi ganske nøyaktige estimater for skadeekvivalentbelastningen. Det skal imidlertid bemerkes at turbiner som brukes i denne studien har lignende dynamiske egenskaper. Derfor bør estimatene for farm-wide level belastning med data-drevet tilnærminger undersøkes videre i fremtiden. Fatigue is often a governing design factor for offshore wind turbines. Since the design of offshore wind turbines includes conservatism, the actual accumulated fatigue damage can be lower than what the turbine is designed for. In this case, the operator can make a decision on life time extension of existing wind turbines. Therefore, it is important to estimate the actual accumulated fatigue damage to support decision making on life time extension, and for optimization of support structure design. However, fatigue critical locations are located near mudline where it is unfeasible to install strain gauges to measure the accumulated fatigue damage. The first purpose of this thesis is to investigate if data-driven approaches (linear regression and feed-forward neural network) can be applied to estimate the accumulated fatigue damage both in individual turbines and farm-wide levels. The second purpose is to determine the minimum number of sensors and quantity of data required for accurate estimation. Towards this goal, real measurement data of two offshore turbines in the same wind farm have been used. Specifically, the data-driven approaches have been applied with real measurement data from the SCADA system, measurements at the top and bottom of the tower, and data from a wave measurement system. This data was used to estimate the accumulated fatigue damage at multiple locations (tower bottom, transition piece and two levels on the monopile) in the form of damage equivalent loads. Throughout the study, 10 min. statistical properties of the measurement data have been used as input to the learning algorithms. One remark is that the estimation has not been performed for the fatigue critical location near mudline itself, but it is expected that estimation with these approaches can be expanded to the fatigue critical location if accurate response estimation at multiple locations on the support structure is possible. The results of this thesis show that the data-driven approaches can give accurate estimates damage equivalent loads on individual turbine level at multiple locations on the support structure when moment or inclination signals at tower bottom is used. For farm-wide level load estimation as well, it has been proven that the data-driven approaches can give quite accurate estimates the damage equivalent load. However, it should be noted that the turbines used in this study have similar dynamic properties. Therefore, the farm-wide level load estimation with the data-driven approaches should be further investigated in the future.

Offshore wind turbines are getting larger and the water depths where they are located are getting deeper as well. This causes problems of eigen frequencies for rigid structures where a jacket structure could be too expensive in depths around 50 meters. At this depth, floating structures will not be efficient due to the hydrodynamic behaviour and the mooring system for this depth can be challenging. That is the reason why compliant structures are analysed for depths around 50 meters due to their low eigenfrequencies of the structure while having a load path into the seabed. For the realization of the thesis, two main tasks have been done. First, the implementation of a code to calculate all the steps in the design of an offshore wind turbine. This method has covered from the determination of natural frequencies of the structure and the solution of the dynamical equation of dynamics to obtain displacements depending on time for different environmental conditions in the tower, to the obtention of the force response of the structure and the code checks necessary to satisfy the guidelines and requirements for Ultimate Limit State, Fatigue Limit State and Serviceability Limit State. This methodology will be the base for a preliminary design of the structure. The main theory to develop this program has been the finite element method and the type of beam used for the calculation can be decided by the user between the Bernoulli beam and the Timoshenko beam. The second main task is the design of a compliant tower for a specified site location using the methodology for preliminary design create beforehand. The location of the compliant tower is the Dutch North Sea and an analysis has been performed to try to locate the optimal natural frequencies for the structure based on the frequencies of the environmental loads acting on the tower. Different tests have been done and different options for the design have been studied like the use of guy cables to control the displacements, the incorporation of a mass trap in the structure and changes in geometrical aspects have been done to achieve a lower natural frequency. These changes are the length from the seabed level to the rotor-nacelle assembly and the slenderness of the monopile studied. All the elements introduced in the code can be changed easily and therefore, this program will be useful for many different cases. The rotor-nacelle assembly used has a rating of 5 MW and a rotor radius of 126 m. Possibility of a larger wind turbine has been studied to reduce even more the natural frequencies. Another important factor like the soil-tower interaction has been also considered and the analysis of the behaviour of the soil has been done according to p-y curves. Differences in behaviour for different types of soil has been evaluated. After arriving to a final design of the compliant tower satisfying all the limit states evaluated, the results of the program are shown and it can be seen that the representation of deflections are reliable while the data for bending moment reactions and shear forces is not completely exact due to the higher influence of bending strain energy than for the shear energy for high flexibility members. Although further research is needed about compliant towers, this preliminary analysis is satisfactory to ensure that compliant towers could become much more common in the future.

This master thesis project concerns the structural monitoring applied to a windturbine. The algorithm analysed in this master thesis is an extended Kalman Filter called the joint input-state estimation algorithm. Kalman Filter algorithm is usually fed with a measured signal that is to be filtered and some model parameter. The filtering process is based on a weighted average between the model and the measure at each time step. The weighting process involve calculation of noise covariance matrices. This master thesis is an analysis on Kalman Filtering in case of an operating windturbine, the measured signal correspond to motion data, either displacement, velocity or acceleration taken at a given height. As on a windturbine, the excitation force applied through the rotor can not be exactly known, an extended version of the Kalman Filter called the joint input-state estimation algorithm will be used. This algorithm estimate at the same time the state and the input -excitation force - of the system. A special attention is given to detection and quantification of the modeling error which is the main personal contribution to this field. A process to include modeling error in calculation of covariance matrices is given and an analysis of the impact of each dynamical model parameter error on the overall estimation error is carried out. Besides, the stability of the algorithm is also a concern in this thesis. Finally, an optimisation of the sensor layout is performed. As the Kalman Filter is linear, the algorithm will be analysed for periodic sinusoidal signals as every signal can be seen as the infinite sum of harmonic signals. Then, algorithm will be analysed on a virtual windturbine through FedEm.

Stochastic environmental modelling, and structure response calculation are two major steps in determining fatigue damage for offshore wind turbines. However, there are challenges ahead of each step. First, site-specific or high-quality met-ocean data that contain joint measurements of wind and wave are not always available. To generate joint met-ocean data, copula was used. Copula is a function that can capture autocorrelation and dependence of two random variables. The copula was calculated for a specific location in the North Sea for pairs of random met-ocean variables (wave height, wind speed) and (wave height, wave period). This joint function was transferred to another location where only marginals were known. New data were generated by combining these data sets. Second, even though integrated time-domain analysis of offshore wind turbines can capture the dynamic response of a structure, such simulations are time-consuming. Lumping is a way to reduce a full sea-state to some load cases by weighting environmental parameters by a certain criterion. A simple lumping method was applied to generate lumped sea-state representing pairs of random variables. This lumping method is applied on both real data and copula-generated data. In addition, the difference between fatigue damage caused by real data and copula generated-data were calculated using a formula based on the quasi-static response of a wind turbine. The results show using copula together with simple lumping method can provide a very good estimation of lumped wind and wave height, and fatigue damage. The average difference measured was less than 15%. The copula accuracy reduces in predicting lumped wave height and period, and fatigue damage due to less correlation of wave height and wave period.

To maintain the current momentum of increasing interest and development in offshore wind energy, there is a need for novel training tools for engineers and researchers. Concurrently, additional educational outreach activities are required to inform the general public on the cost of energy from offshore wind and the research to improve it. A serious game may enable a new group of learners to explore the topic of offshore wind. The objective of this study is to develop a serious game for the design and management of offshore wind farms to be used for training and dissemination. The game s effectiveness is measured in terms of its simulations and its educational power. The study includes a literature review of serious game design and offshore wind energy followed by the development of game design and a functioning prototype using Python. This prototype was playtested and evaluated for educational impact and playability. The game design involves the joint tasks of building a game framework and developing a simplified offshore wind farm simulation. This simulation addresses weather prediction, offshore wind farm design, operation and maintenance, energy demand, climate change, finance, and stakeholder influence. The weather prediction model uses a Markov chain matrix to generate different sea states for every iteration of the game. The game is considered effective with respect to its simulation efficiency and its ability to produce realistic values for offshore wind energy. Playtesting demonstrated immersion and informed decision making among participants. Additional surveys revealed that the participants knowledge on offshore wind had increased while playing the game. Key recommendations for future versions of this serious game about offshore wind energy are listed.

The objective of this thesis is the development of a gradient based optimization framework for the local optimization of structural members of an offshore wind turbine with jacket support structure and the implementation of an algorithm using the principle of decomposition. Here, it is assumed that when changing dimensions of a structural member, sectional response forces and moments remain constant. This assumption is made for both the actual changed member and for all other members of the structure. Additionally, this is assumed to be valid for a simultaneous change of all members all over the structure. It is the so-called local optimization approach and allows for an individual (local) optimization of each member of the structure. The development of the gradient based method for the local optimization necessitate the study of changes in global performance when modifying structural members locally and a method to receive these sensitivities in structural performance elementwise. Thereafter, the algorithms were used with the local optimization approach. The implementation was carried out using customized Matlab scripts for the automated load analysis with the finite element solver Fedem WindPower. The UpWind reference jacket used within the OC4-project and the generic NREL 5MW turbine atop were used as the case study for this work. In addition, as the objective function for the optimization process, a cost model that considers both manufacturing and material costs was developed. Finally the evaluation of the speed and the accuracy of the methods proposed is presented and the efficiency of the method is proved.

Optimization of structures in a domain with large uncertainties is rather difficult. This also applies for the offshore wind energy sector. For current offshore wind energy development locations with monopile-based support structures the fatigue limit state is the driving design criteria. These analyses are connected with long time domain evaluations to cover non-linearities. Model, statistical and data uncertainties lead to a combined fatigue damage prediction uncertainty. The former are either covered by a design fatigue factor or a material factor, which are stated in certification standards, e.g. DNVGL-ST-0126. The influence of mass changes regarding different lifetimes and the impact of this design fatigue factor has not been published yet. Based on this, within this graduation project, the monopile support structure is optimized for different lifetimes in order to identify mass changes and influences of the design fatigue factor. Literature shows that automatized optimization using genetic algorithms in offshore wind energy is possible but limited, due to the algorithm methodology including a large number of design evaluations. This graduation project shows the applicability of Importance Sampling for load case reduction in a genetic algorithm optimization for offshore wind. Compared to previous approaches Importance Sampling assists to use a full certification procedure for fatigue limit state computations in a feasible amount of time with high fatigue life estimation accuracies. Subsequently, the fatigue limit state load case table is reduced by 93%. By optimizing the monopile with this reduced amount of load cases the algorithm is computationally feasible for the industry. Rambøll simulation software for offshore wind turbine support structure design is used in combination with the genetic algorithm function in Matlab®. The combination of the software leads to the optimization of monopile based offshore wind support structures for different lifetimes. The algorithm runs with a reduced amount of load cases. Resulting critical fatigue damage values of converged designs are showing deviations from actual fatigue damage values using full fatigue limit state load case tables at maximum 6.6% and minimum 1.7%. This high accuracy leads to an optimization of monopile structures for desired lifetimes and consequently to the mass versus lifetime curve. A mass increase of approximately 22% is observed from 25 to 100 years lifetime. After reaching 75 years lifetime the curve shows a flattening behavior. Besides, parameter evolutions of optimized monopile designs are discussed in terms of different fatigue life. The design variables are embedment depth, cone angle, and corresponding wall thicknesses of monopile sections. Summarized, this thesis proved the implementation of a full state of the art fatigue limit state computation in the genetic algorithm by Importance Sampling with reduced load cases and also visualized the impact of mass changes for different projected lifetimes. As a conclusive remark, the application of Importance Sampling for load case reduction in the design process opens new possibilities of optimization in the offshore wind energy sector.

Very recently, offshore wind energy has begun expanding into subarctic regions with seasonally ice-infested waters like the Baltic Sea. For offshore wind turbines with conventional support structures including monopiles and jackets that are typically flexible and vertically sided, the phenomenon known as ice-induced vibrations can develop. In response to the growing interest in offshore wind in the Baltic Sea and in further validating a state-of-the-art numerical model for ice-structure interaction, the Ice-induced Vibrations of Offshore Structures (IVOS) project has been coordinated with the Hamburg Ship Model Basin (HSVA) and many academic and industry partners including TU Delft and NTNU in order to enhance understanding of the topic via a comprehensive laboratory testing campaign. In this thesis, specific data are selected from the extensive IVOS Phase 2 tests such that a comparative analysis is performed between two different model-scale structures, one with circular cross-section and another with rectangular cross-section, that were subjected to ice-induced vibrations in the frequency lock-in regime. Preceding the comparative analysis, the data from the IVOS Phase 2 tests are organized into a database and post-processing tools are developed to facilitate further research by supplying analysis-ready data sets. Included in the post-processing tools is an attempt to determine ice-induced global loads from uncalibrated tactile sensor data, which offers insight into the relationship between the relative pressure distributions along the ice-structure interface and the ice-induced global forces on the model structure. The comparative analysis garnered qualitative information about the frequency lock-in regime and buckling failure of ice. It is observed that frequency lock-in vibrations usually persisted regardless of buckling events. For the frequency lock-in regime, the ice-induced global loads on the circular cross-section structure are generally lower than those for the rectangular cross-section structure. It is unclear whether the difference in global loads between the structures is caused by the difference in cross-sectional shape, other structural properties, ice properties, or combinations thereof. The analysis of the energy of the system is intriguing but does not offer lucid conclusions. However, the quasi-figure-eight pattern from the x-direction and derived y-direction structural displacements is an enlightening discovery that may explain inconsistencies in the energy of the system. Based on the general configuration of the test apparatus from the IVOS Phase 2 tests and the results from the comparative analysis, it can be concluded that ice-induced vibrations of the model-scale structures in the frequency lock-in regime should be regarded as a two-dimensional problem.

Jacket support structures are a preferred solution for offshore wind turbines in deeper waters. Extensive knowledge exists in relation to its construction technique as well as its crucial components, but due to considerable cost pressure, continued optimization is essential for the future competitiveness of the concept in the offshore wind business. A significant share of the overall production costs of jacket structures is related to the joints connecting the tubular members. Hence, these joints along with their complex welds are of special interest in terms of cost reduction. The design of tubular joints is generally driven by fatigue resistance. Due to the size, complexity and cost of these joints, this is assessed by using detailed FE models. When defining a numerical model, assumptions have to be made. These decisions influence the obtained output. Several aspects that have an impact on the results of the models are found to require further research and are investigated within this project: (1) influence of using solid versus shell elements in the modelling of the joint members; (2) degree of accuracy of the Efthymiou equations; (3) influence of the carry-over effect in multiplanar KK-joints; (4) differences in the fatigue life predictions between the hot-spot and the effective notch stress methods. Guidelines recommend the use of both solid and shell theories for the definition of the FE models used in the hot-spot fatigue assessment. Both options are compared in terms of accuracy of the results and computational time. Generally speaking, significant differences are found between both models. The background of the differences is studied. The employment of the Efthymiou formulae is common in practice. These equations provide the SCF at the locations around the weld where they are found to be maximum. The use of this approach carries two sorts of uncertainty. One is directly related to the values that the equations predict. The second one stems from the lack of information on the results at the other positions around the weld. The output of this approach is compared with the results of numerical models. The validity of its use is quantified. Offshore wind jacket joints are mainly multiplanar KK-joints. Loading in the braces of one face of the jacket may yield significant carry-over effects on the out-of-plane braces connected to the same joint. A parametric comparison is carried out to study the accuracy of modelling the joint as a planar K-joint. In general, it is found that the carry-over effect cannot be neglected and the assumption is not accurate. The fatigue assessment of tubular joints by means of the hot-spot method is subjected to several assumptions that limit the optimization of the members. The notch concept is a more realistic method that is presented as an alternative. However, this method is not widely used in engineering practice due to the difficulties in building the numerical model and the high computational requisites. An algorithm to carry out the effective notch stress assessment, based on the sub-modelling technique, is proposed. A comparison of the fatigue life prediction, between the hot-spot and the effective notch methods, is presented. The latter method is found to predict a higher fatigue life for many of the situations tested. Furthermore, since this method allows for a more detailed modelling, the weld profile can also be designed in order to optimize the fatigue resistance. The weld slope is found to have a significant impact on the results.

A fatigue analysis gives an answer to the question of whether a structure will resist fatigue throughout its lifetime or not. The hot spot stress approach is one of the methods used to find the solution, by looking at points around the circumference of a weld, and finding the stress values at these points. This master thesis investigates fatigue analysis using the hot spot stress approach, through a sensitivity analysis By changing what parameters and methods the fatigue analysis uses, several fatigue damage values are calculated for each joint. The design chosen for the study is a single support structure for an offshore wind turbine (OWT) the UpWind reference jacket from Phase I of the Offshore Code Comparison Collaboration Continuation (OC4) project. [5] The jacket structure supports the NREL 5-MW reference wind turbine. [6] It consist of K-joints, X-joints and Y-joints. With fatigue damage being the design driving factor, the method for estimating fatigue damage may necessitate different design in the structure. [12] That means that different fatigue damage estimations may result in different costs and a more reliable structure. As anything can be safe and reliable if you just overspend and over-design it, an ideal fatigue analysis can give reliable results, without being too conservative. During the master work, an application was developed in the .NET Framework to work in conjuncture with FEDEM WindPower, a simulation program used for wind turbine systems. The developed application performs all extractions and calculations needed for estimating the fatigue damage using the hot spot stress method. The HSSs calculated are linearly extrapolated structural hot spot stress, with the nominal stress in the chord included for the HSSs on the chord side of the weld. The application performs the fatigue damage estimations using several procedures of the hot spot stress approach. This leads to several fatigue damage values for each joint, giving the basis for the sensitivity analysis concerning the hot spot stress approach. Common for all variations in the options, are the load simulations and the usage of hot spot stress approach, rainflow counting, S-N curve and Palmgren-Miner rule. This is all explained in chapter 2. They also have in common general fixity conditions, chord-end fixity parameter equal to 0.7, and for the joints with two braces, loads occur in both. The variations consist of the L parameter used in the SCF equations, the order in which maximum fatigue damage concerning one hot spot was kept while the others were excluded, the total number of hot spots around the circumference of the weld, and the choice regarding which equations to use for the SCFs at the crown for axial load. For the choice of equations to use for the SCFs, one can choose between solely geometrical SCFs, and SCFs including nominal stress. Here, both options are used. Only Y-joints are affected by changes in the L parameter after a certain limit, but until this limit is reached it affects all joints. This is because Y-joints are the only ones, regarding the structure analysed and the choices made in this thesis, that have SCFs containing L. K-joints are those joints that are most affected by which order one chooses to keep the maximum value amongst the hot spots. This indicates that the spot most prone to fatigue damage varies more within the K-joints than in the other two joint types. X-joints and K-joints are equally affected by the number of hot spot one uses. Increasing the number of hot spots from 8 to 16 can for several joints give an increase ratio equal to 16 %, the ratio being the fatigue damage estimated using 16 hot spots divided by the fatigue damage estimated using 8 hot spots. Increasing the number of hot spots either gives the same value or a higher value, because increasing the number of hot spots indicates adding hot spots in between the existing ones. The optional SCFs at the crown for axial load are only evaluated for Y-joints. The equations containing nominal stress shows, for most of the joints, much lower fatigue damage estimations than the solely geometrical ones, for L values at 7 and higher. The limited selection of parameters, due to limited amount of time, results in a sensitivity analysis that is not sufficient to come up with a final, clear answer regarding the ultimate fatigue analysis. This thesis is therefore intended to be used, with the developed application, as the basis for a larger sensitivity analysis that could provide further knowledge on the preferable procedure concerning fatigue analysis using the hot spot stress approach. This master thesis has been performed in cooperation with Fedem Technology AS in Trondheim. The app development was assisted by Fedem Technology AS representatives Ole-Ivar Holte, Runar Heggelien Refsnæs and Daniel Zwick. Inge Lotsberg at DNV GL AS has contributed concerning the fatigue analysis, as has Sebastian Schafhirt, who has also helped in relation to the research method. Michael Muskulus has been the main supervisor and contributed regarding the research question.

Floating offshore wind turbines are taking on more and more prominence, as the industry moves towards larger turbines, farther offshore, in deeper water. The increase in turbine size can reduce the costs of offshore wind energy, but requires larger support structures. Rather than redesigning the structure completely, a rational methodology for upscaling an existing floating substructure can improve the efficiency of the design process. This work presents a guideline for the optimization and upscaling of a semi-submersible floating platform, addressing also special challenges related to changes in turbine technology, as well as design criteria for floating platforms. The OC4 semi-submersible platform is used as starting point of this study. Based on analysis results of an initial elementary upscaling procedure, the main criteria, when dealing with a semi-submersible floating platform, are specified. Optimization is then carried out, focusing on stability and eigenfrequencies. Reducing the upper column diameter and changing the ballast position within the columns, yields longer natural periods in heave and pitch, a lighter and cheaper platform, and a less over-conservative, but still stable and safe system. Based on this optimized design, a guideline for an upscaling procedure for any other turbine is given. The main scaling factor is determined from the mass ratio of the top structures, rather than the turbine rating. The main column is scaled, so that it fits the new tower base diameter. The scaling factor for the upper columns is computed, based on the ratio of the overturning moments, and considers the contribution of the different columns to the stiffness component in pitch. The mooring line length is scaled, such that it can yield a predefined stiffness. Finally, the controller gains are recalculated, based on the expected natural frequency in pitch. The optimization and upscaling process is carried out for Fraunhofer's offshore wind turbine IWT-7.5-164 and the DTU 10 MW reference wind turbine. The floating wind turbines are analyzed, modelled and simulated by means of simplified spreadsheet methods, linear frequency-domain calculations (in DNV's software HydroD), and detailed equation-based models (in Fraunhofer's software Modelica). The systems are evaluated regarding their eigenfrequencies, nominal pitch, stability and global performance in selected sea states, taking variable buoyancy and center of buoyancy into account, and adjusting the blade-pitch controller gains. The results, obtained from both computer programs and the initial hand calculations, are comparable and satisfying. The structural integrity is proved by a simplified approach, using tank pressure and sea pressure for computing the equivalent stress. More detailed strength checks for fatigue and ultimate limit states, as well as optimization of the mooring system, are left for future work.

Offshore wind structures located in ice infested waters are subjected to actions from moving ice and forces are generated when a drifting ice crushes against the structure. The main purpose of this Master s thesis is to implement numerical ice loading models on an offshore wind turbine monopile structure, analyze ice induced vibrations and assess the importance of ice loads on offshore wind turbines. A model of offshore wind turbine monopile structure for time domain simulation will be established on FEDEM and ice loading spectral model will be implemented first. Ice force spectrum will be calculated from empirical formulas and ice loading time series will be generated and applied on structure model directly. In order to assess the performance of the spectral model, another ice-structure interaction model, Määttänen-Blenkarn model, is also applied on the offshore wind turbine monopile structure. The time domain simulation results from two models are analyzed and compared. The feasibility of both models for different ice speeds are investigated. The complex ice properties define the difficulty in ice load calculation. The relative speed between structure and ice will influence ice crushing strength. The existing spectral model is only feasible for continuous brittle crushing regime where structure response have little influence. But Määttänen-Blenkarn model considers structure response through stress rate dependent ice crushing strength and is also applicable for low to intermediate ice speeds. Simulation results also illustrate that different ice crushing modes of different ice speeds can be obtained from Määttänen-Blenkarn model and the results of spectral model for different ice speeds have similar characteristics. To better evaluate the impact of ice loads on offshore wind turbine structure, the coupling model of wind and ice loads will be applied in time domain simulation to investigate the joint effect of wind and ice loading. Fatigue damage assessment is also performed to see the structure damage induced by ice loads. The results illustrate that based on current model and data, ice loads are larger than wind loads and the fatigue damage caused by ice loads should be considered.

Offshore wind energy faces three important trends: (1) wind farms grow in size, (2) monopiles are installed in deeper water, and (3) cost reduction remains the most important challenge. With wind farm size, the importance of variations in environmental site conditions across the wind farm increases. These site variations, e.g. water depth and soil conditions, can lead to significant differences of loads on support structures. For monopiles in deeper water, design is dominated by wave-induced fatigue loads. Since full fatigue load calculations are computationally demanding, they can typically not be performed for each turbine within large wind farms. Therefore, turbines must be grouped into clusters in early project phases, making time-efficient approaches essential. Optimization of design clustering is necessary to reduce design conservatism and the cost of offshore wind energy. Hence, the goal of this thesis is to investigate load site variations and clustering. Therefore, a probabilistic fatigue load estimation method is developed and verified with aero-elastic simulations in the time domain. Subsequently, the developed method is applied for an exemplary wind farm of 150 turbines in 30-40m water depth to perform (1) sensitivity studies of loads to changes in MSL, soil stiffness, and wave parameters, (2) probabilistic assessments of data, statistical and model uncertainties, and (3) deterministic and probabilistic design clustering. The estimation method is based on frequency domain analysis to calculate wave-induced fatigue loads, a scaling approach for wind loads, combination of wind-wave loads with quadratic superposition, and Monte-Carlo simulations to assess uncertainties. Verification confirms an accuracy of 95% for lifetime equivalent fatigue loads compared to time domain simulations. The computational speed is in the order of 100 times faster than typical time domain tools. Sensitivity studies show a significant influence of water depth and wave period on EFLs. The influence of soil on EFLs is minor for high soil stiffness but can increase significant for soils with low stiffness. Normal distributed input parameters in a probabilistic assessment yield a positively skewed probability distribution of EFLs. Design clustering is optimized based on site-specific fatigue loads using brute-force and discrete optimization algorithms. Results for the exemplary wind farm show a design load reduction of up to 13% compared to standardized design. Probabilistic clustering proved to be only relevant at cluster borders leading to a difference in allocation for 12 out of 150 turbines. Project results show that it is essential to account for load differences in large wind farms due to varying site conditions. This study improves clustering and provides a basis for design optimization and uncertainty analysis in large wind farms. Further work is needed to extend tool verification and formulate design clustering for cost optimization.

The purpose of this master thesis is to develop and evaluate a novel approach for performing structural reliability analysis and robust design of offshore wind turbine support structures. Offshore wind turbines are structures especially prone to uncertainties due to its exposure to environmental conditions. Therefore, the models that take into account probabilistic descriptions of variability are more natural approaches. In this sense, a structural reliability analysis and a robust design are two complementary approaches that allow for incorporating uncertainty and randomness in the design process. To achieve the objective of this study, it has been developed a simplified model of a monopile structure that allows for doing the necessary calculations efficiently. The simplified model is implemented as a time-domain simulation in MATLAB and allows to solve the transient dynamics of a beam. In parallel, the rotor loads are obtained from rotor simulations in FEDEM Windpower software. Uncertainty is studied in the following system parameters: aerodynamic damping, soil stiffness and cross section of the support structure. Simple probability distributions are used in order to describe the uncertainty on these parameters. The basis of the research is a probability analysis based on several approaches. First, it is raised an individual analysis for each system parameter and wind speed. Secondly, the way used to add the aerodynamic damping in the simplified model has given rise to two different analyses. The effect of the soil stiffness and the structure cross section is investigated through a correlated and uncorrelated sampling of both variables. Finally, a combined probability analysis is performed in order to observe the effect of analysing the three system parameters at the same time. Additionally, the reference offshore wind turbine is also analysed in order to enable evaluate the effects of the other analyses. The probabilistic analysis is performed with a variable number of samples and the displacements at the tower top and the bending moments at the mud-line of the structure are obtained as a result. These outputs are used to evaluate the equivalent fatigue damage of the support structure. The wind turbine considered is the 5-MW NREL, with the reference support structure defined in the OC3 project. The results show that the response of the proposed simplified model is significantly different depending on the system parameter. The soil stiffness and the aerodynamic damping have shown a weak effect on the response of the structure. The reason could be an underestimation of the uncertainties associated to these parameters. Conversely, the structure cross section has shown a greater impact, probably due to dynamic amplifications linked to resonance problems. The uncorrelated analysis of the soil stiffness and the structure cross section has shown the greater effects on the response of the structure, given that the sampling approach of this case leads to a greater scattering of the system parameters and, consequently, of the results.

This master thesis considers the strength analysis of the offshore wind turbine s jacket support structure with one structural member damaged or weakened, not capable of transferring the loads. A wind turbine and its substructure are usually exposed to environmental loadings (i.e. wind, waves) and described by computer simulations. FEDEM Windpower environment is used to analyse the model of the offshore wind turbine. Firstly, the thesis is to indicate the areas of the largest fatigue damage accumulation in the remaining structure members and to estimate their ultimate strength. The fatigue evaluation of the steel structure elements is based on the Palmgren-Miner linear damage hypothesis. The steel brace member damage has been modelled by reducing the elastic modulus (Young's modulus) or inserting a free-joint mechanism in the middle of the element. Then the results of structure strength calculations before and after damage are collected and compared. Secondly, the analysis assumes a reduction in the fatigue loads applied to the support structure by changing the wind turbine control system. These changes are aimed at decreasing mainly the rotor revolution frequency and generator torque at the same time. The adjustment is achieved by varying a turbine blade pitch. The controller has also been modified to avoid continuous turbine operation at frequencies close to resonance.