The current demands of the Offshore and Maritime industry have lead to the design and
production of larger vessels of complex geometries. These vessels are subjected to unique
loading conditions that have not been investigated extensively. Such a case is presented
in wet deck slamming of large catamaran vessels. These vessels have been designed and
produced to conduct offshore platform installation operations and are expected to operate
in severe conditions where wet deck submergence becomes a threat.
Wet deck slamming is a water entry impact phenomenon that can induce extreme loads
when a part of the floating structure pierces the water surface during severe waves. This
event can trigger a transient dynamic response at both local and global levels. The event has
multiple influencing factors that can contribute to the severity and probability of occurrence.
Although the examination of slamming started over a century ago, the criteria concerning
the incident definition, identification and prediction are applied from different perspectives.
Modern methods identify slamming events based on the structural responses, while the sta-
tistical analysis considers correlation between the events, as opposed to older approaches
treating the incidents as independent.
Progress in understanding slamming is achieved through full-scale measurements, exper-
imental tests, and numerical simulations. Each method presents unique advantages and
drawbacks,and all three are used for knowledge development but also verification and vali-
dation. As numerical capabilities increase, it is also important for the experimental method
to evolve and explore new ways of representation of the complex events.
This project aims to investigate wet deck slamming using a rarely used form of model
that could closely mimic real life responses. This research can enhance the experimental
method capabilities and serve as stepping stone for further evolution of similar research.
The experimental test runs for the needs of the project were conducted in the Ship and
Hydromechanics laboratory facilities of TU Delft.

The offshore wind energy market is expanding and the number of offshore wind turbines being installed in the near future is rising. Offshore wind turbines are being installed further offshore and in deeper waters. Besides, to lower the cost of wind energy offshore wind turbines are increasing in size and power output. Both wind turbines and their support structures are expected to keep increasing in size and weight in the coming years. After fabrication, wind turbine components and support structures have to be transported to onshore storage depots or to their offshore location. To enable safe transports, wind turbine components and support structures are constraint to heavy transport vessels or transport barges by sea fastening structures. As a result of wind turbine components and support structures increasing in size and weight, the reaction loads for which sea fastening structures need to be designed are increasing as well. Since increasing reaction loads have various negative consequences which are expected to become more critical for future transports, there is a need for an optimized reaction load calculation.

The current method of calculating the reaction loads which is widely used in the industry is often referred to as being a conservative method. The aim of this thesis was to enhance the existing calculation method of the reaction loads by shifting from a conservative approach towards a method of calculating reaction loads based on an acceptable probability of occurrence during transports. By calculating the reaction loads for an acceptable probability of occurrence it is avoided that sea fastening structures are designed for overly conservative reaction loads while the structural reliability of these structures will still be ensured.

In this thesis an existing sea fastening design project from the industry was used to perform a case study. Data and information from this project were used as input to perform motion analyses of a vessel which is transporting a jacket support structure. The obtained linear wave-induced accelerations of the jacket CoG were used as the main input for calculating the reaction loads. It was first investigated how these 6-DoF accelerations of the CoG are used in the current method of calculating the reaction loads. This was followed by introducing statistical extreme value theory with the purpose of using the accelerations of the jacket CoG as input for a probabilistic method of calculating the reaction loads.

The findings of this research show that optimized reaction loads can be obtained by replacing the current calculation method by a long-term probabilistic method. It was found that this long-term probabilistic method could be derived by combining 3-hour extreme value density functions of reaction loads with the probabilities of encountering the various sea states at the location on the route for which the most severe environmental conditions are expected. The long-term probabilistic method was used to perform a probabilistic investigation of the reaction loads calculated with the current calculation method. It was found that the return periods of the reaction loads calculated with the current method were significantly different for the individual jacket legs. Moreover, it was found that the sea fastening design for at least one of the jacket legs was expected to be over-conservative. By presenting the long-term probabilistic calculation method, a methodology was introduced which determines reaction loads based on acceptable return periods while avoiding over-conservative sea fastening designs.

This research has provided a new insight into the method of designing sea fastening structures. The long-term probabilistic calculation method can be applied in practice to determine optimized reaction loads incorporated in sea fastening designs. This research therefore makes a valuable contribution to preparing the reaction load calculation method for future transports which are expected to become more critical due to wind turbine components and their support structures growing in size and weight.

Traditionally, the control point (CP) of a dynamically positioned vessel is located around its center of gravity (CoG). However, during offshore operations, other locations, such as the crane tip or gripper position, become more critical due to the load being located there. This thesis proposes shifting the control point from the CoG to an alternative location, specifically the gripper position, to minimize the DP footprint at this location. Minimizing the DP footprint at the gripper location could lead to smaller motion deviations at this critical point. Consequently, this could potentially improve the workability of the vessel, which in turn could lead to higher operational yields. Additionally, the risk of potential damage and unsafe situations offshore is mitigated.

The conventional DP system contains a Kalman Filter, to filter out the first-order motions, a P(I)D controller that calculates the demanded forces to keep the vessel in place, and a thruster allocation algorithm that distributes the demanded forces to all the thrusters in an optimized way.

A new design for a DP system with the control point at the gripper position for the Deepwater Construction Vessel (DCV) Aegir is presented and evaluated. Time domain simulations were performed with the Aegir containing its conventional DP system and with the Aegir containing the newly designed DP system. These time domain simulations were performed using Orcaflex, which contains a model of the Aegir. This model of the vessel is connected to an external Python code, that contains the DP system, including a thruster allocation.


As the new DP system at the gripper location has to cope with coupled equations of motion, it is equipped with a new Multiple-Input-Multiple-Output (MIMO) PD controller, that consists of a decoupling module and separate PD controllers for each DOF. To obtain state estimates for the second-order motions at the gripper location, the state estimates calculated by the Kalman Filter are translated to the gripper location.

The effects on DP performance were assessed by evaluating and comparing the motion responses in the horizontal plane and DP footprint at the gripper location of both models. Also, the thruster behavior and energy consumption of both models are compared. This was done for an incoming wave direction of 135 degrees, a peak period (Tp) of 8 seconds, and significant wave heights (Hs) from 1.0 m to 2.0 m with increments of 0.5 m. The wave spectrum used is a JONSWAP spectrum. As recommended by the classification society, three-hour simulations are performed for the so-called 'base case' sea state with Hs = 1.5m. However, due to the extensive simulation time only one-hour simulations were performed for the cases with Hs = 1.0m and 2.0m.

When comparing the motion responses, one of the most remarkable results is an improved yaw response for the gripper control point model in all sea states that are considered in this study. Further on, the motion responses for sway appeared to be bigger for the gripper control point compared to the center control point in Hs = 1.5m and Hs = 1.0 m. However, the differences in sway are observed to be marginal for Hs = 2.0m.

The results show that the DP footprint has slightly improved in the x-direction for the gripper control point model compared to the center control point model for the base case. The same observation is done for Hs = 2.0m, but the differences found between the models in Hs = 1.0m are marginal. Also, the DP footprint was observed to be slightly larger in the y-direction for Hs = 1.0m and Hs = 1.5m for the gripper control point.

The total thrust outputs as delivered by the DP system during the simulations were converted to power, by using the propeller diagrams (for DP-speed state) for the specific types of thrusters the Aegir is equipped with. From these results, it became clear that the gripper point control model consumes less energy in all tested sea states compared center control point model.

From the results presented in this study, it is concluded that the system itself has potential, but no hard conclusions can be drawn for the system in its current form. Problems were observed with the current thruster allocation algorithm from HES, which need to be explored in more detail and resolved. It is recommended to look into developing a stable working thruster allocation algorithm for both control point models, such that more accurate dynamic simulations can be performed and the system can be assessed under more sea states.

This thesis tests a lumping block method on accuracy in broad and narrow wave spectra. The method aims to construct a robust and efficient alternative for the computationally expensive time domain fatigue calculations of mooring lines in the early design stage of a floating offshore wind turbine (FOWT). A robust way of modelling must be applicable and reliable in all possible sea states, including extreme wavelengths, nonlinear waves, and dynamic amplification caused by the resonance frequencies. The method’s reliability for different wave spectra is validated by comparing the proposed numerical lumped time-domain simulations with the original numerical time-domain simulations.

The lumping block method gives accurate damage results for narrow wave spectra. However, the model’s reliability decreases when the broadening of a wave spectrum causes damage-increasing conditions, like outliers in wave periods and nonlinear waves. The lumped broad wave spectrum simulations underestimate a mooring line’s structural damage, especially for wave periods above resonance. Moreover, the non-Gaussian wave-induced surge responses are the most critical parameter for reliable mooring line fatigue calculations. They must be considered for simplifying fatigue mooring line calculations in general. Furthermore, the non-Gaussian responses cause the underestimation of the proposed lumping block method for the broad wave spectra.

Nevertheless, the proposed method yields accurate but slightly conservative damage results and is robust and efficient in fatigue calculations for sea states corresponding to a narrow spectrum. Hence the lumping block method is safe and usable for significant parts of the North Sea since only middle or narrow spectra occur.

An important trend exhibited by the offshore wind market is the increase in the size of wind turbines, leading to longer and stiffer monopiles with larger diameter-to-thickness ratios. Current transport analysis is focused on loads resulting from hydrodynamic accelerations, without taking loads into account resulting from differences in bending deflection between the vessel and cargo. This thesis covers a study carried out on the structural response of a monopile and sea-fastening system subjected to displacement-based loads. The loadcase follows from a vessel excited using a regular wave leading to bending deflections and rigid body accelerations. The saddles used to support the monopile are modeled as unilateral springs, emphasizing the need to use a non-linear solution method to obtain structural responses. The harmonic nature of hydrodynamic-based loads led to the selection of the harmonic balance method (HBM) used as solution method in the structural model. A novel understanding how cargo properties, seafastening properties and seafastening arrangements influence the structural response of the coupled cargo-seafastening system. Various parametric studies are performed to identify behaviors related to the total structural response. Based on this study, the conclusion can be drawn that a large number of saddles in combination with a low stiffness is desired to minimize the structural response of the cargo and sea-fastening system. Furthermore, the influence of lashing stiffness and pretension is limited with respect to the total response. At last, a parametric analysis on the properties of the cargo showed that certain sea-fastening arrangements can be identified which comply with design criteria.

Reducing the levelised cost of energy is crucial to accelerating the energy transition. To develop offshore wind solutions in greater water depths, a floating solution is required. The time-domain simulations of these Floating Offshore Wind Turbines (FOWTs) under wind-wave misalignment used in research and industry projects are computationally intensive and limits researchers and industry in their developments. To better understand the sensitivities of the fatigue loads of FOWTs to different parameters and environmental conditions, a computationally efficient method is needed. The aim of this research is to develop a frequency-domain method to quantify the effects of misaligned wind and waves on the response of a semi-submersible floating offshore wind turbine. Therefore, the following research question is defined: What is the effect of misaligned wind, windsea waves, and swells on the loads at the tower base of a semi-submersible type floating offshore wind turbine?

Several sensitivity studies are conducted to quantify the contribution of yaw-roll coupling effects and aerodynamic damping to the responses and loads. From these studies, it appears that the yaw-roll coupling can increase the response when excited at wind/wave directions in which the structure is asymmetric.
The magnitude of this effect is related to the wave peak period (and the resulting wavelength), the angle of misalignment with respect to the structure, and the apparent length of the structure. Also, the lack of aerodynamic damping in the direction of the rotor plane (side-side direction) leads to a noticeable
increase in the response, directly or through coupling effects. Finally, the frequency-domain method is compared with the time-domain simulations (BHawC-OrcaFlex) carried out by Siemens Gamesa. Although reasonable agreement is found for the load driving rigid body modes, significant differences in the tower bottom loads are found for the lowest and highest production wind speeds.

These results show that misaligned wind and waves can increase the response for headings where the structure is asymmetric due to coupling effects. Wind-wave misalignment leads to an increased response in the direction of the rotor plane due to the lack of aerodynamic damping. In general, the wind-wave misalignment can also have a mitigating effect on the maximum equivalent moment at the tower base, as the aerodynamic damping also reduces the response in the wave frequency range. Furthermore, the comparison shows the need to extend the frequency-domain method with the first tower bending modes and improvement of aerodynamic/mooring property estimation. Based on the findings and the conclusions, the recommendation is to investigate the floater specific sensitivities at an early stage of the design. Future research should focus on: the implementation of tower flexibility, improvement of the quasi-static estimation of mooring stiffness, frequency dependent aerodynamic properties, and implementation of second-order wave forcing.

The growth in the offshore wind industry sees an increased demand for offshore service vessels (OSVs). These vessels often operate in harsh conditions, and their performance is heavily dependent on their seakeeping characteristics. Conventional ship design processes fail to effectively consider seakeeping early in the design process, leading to sub-optimal vessel design. A design framework has been developed in the software 'NAPA' to efficiently design high-performing OSV concept designs.

The developed framework is able to optimize a hull shape -specifically the main particulars and length of different hull sections- to maximize performance in certain objectives. These objectives are seakeeping, ship resistance, lightship weight, and station keeping power requirements. Regarding seakeeping, the attainable percentage operability is calculated for each iteration, although the Operability Robustness Index (ORI) is optimized. The ORI is a more robust seakeeping key performance indicator (KPI) than percentage operability, which is advantageous when facing concept design uncertainty. The framework maximizes ORI, thereby seakeeping performance, for a particular loading condition, motion sensitive criteria, and operational area. The ORI is evaluated based on the area of operation's scatter diagram and wave spectrum, governing motion limits, and iteration-specific RAOs. Designs are required to satisfy an initial stability constraint, to ensure feasibility.

The framework's output is a Pareto-frontier showing the trade-offs between different KPIs, and the corresponding variable combinations. Thereby, the naval architect can evaluate what design offers the best overall performance.

A 'feeder' OSV, designed to transport wind turbine components to wind installation vessels has been optimized to validate the framework. This OSV is currently being developed as a concept by C-Job naval architects. The ship has been optimized for maximum operability of an Ampelmann motion compensated platform. A single optimization run took three and a half hours to complete 300 iterations, thereby finding the Pareto-frontier. Comparing the Pareto-optimal solutions with the base vessel, the ORI can be increased up to 3.6%, the lightship weight decreased by 21.1% and the ship resistance decreased by 13.0%. The framework showed that smaller vessels can still attain good seakeeping performance, leading to a substantial reduction in lightship weight. The increase in seakeeping performance allows for the use of less expensive motion compensated equipment while maintaining high operability. The framework showed that there is a trade-off to be made with regard to seakeeping, and lightship weight, and ship resistance. The framework presents what variables and ship attributes cause these trade-offs. This information allows naval architects to determine the optimal design direction during concept design. Clients and naval architects can decide what trade-off in performance provides the ideal combination to achieve the ship's mission. Consequently, besides producing high-performance designs, the framework substantially increases early design knowledge. Thereby, the overall design process becomes more efficient.

The framework showed to be a valuable tool for OSV concept design. By the extensive incorporation of seakeeping early in the design process, naval architects can design high-performance OSVs efficiently. The produced designs maximize performance in any of the KPIs, ensuring vessels have a high operability, but do not weigh more, or have higher fuel costs than is needed.

Due to the need for more renewable energy, interest in floating wind turbines has greatly increased over the last years. These turbines have to withstand harsh environmental conditions, such as extreme waves called rogue waves. These rogue waves (or freak waves) are exceptionally large waves compared to the surrounding sea state and are defined by two different size criteria, one relating to the crest height and the other to the wave height. Research has shown that these rogue waves occur more frequently than expected, from which the question arose whether these rogue waves could be dangerous for floating wind turbines.
This thesis aims to answer that question by evaluating the probability of such a wave interacting with a floating offshore wind turbine and the subsequent induced motion response. A location off shore California is evaluated for a spar-type floating wind turbine (SFWT). In this research, different time-extreme (TE) and space-time-extreme (STE) statistical wave crest and crest-to-trough models are used with historical wave data to estimate rogue wave occurrence probabilities. The STE models approximate the probability of a wave occurring within an area in time, as opposed to a point in time (TE), and are considered state-of-the-art. Wave data from 1998-2020 is used, which is a significant amount compared to the life of a typical offshore structure. A frequently chosen model for research into a SFWT is the OC3-Hywind concept, which consists of the National Renewable Energy Laboratory (NREL) 5MW reference turbine and a substructure based on the Hywind spar.

The area size which leads to the STE models estimating a higher rogue wave occurrence probability than the TE models is investigated for the first time. This is done by comparing the TE and STE probability models to each other on the basis of their maximum estimated exceedance probability of the rogue threshold, for different area sizes. Only on rare occasions, with a very small area, did the most conservative TE wave crest model estimate a higher probability than the STE2 (wave crest STE) model. Therefore, the STE2 model appears most conservative when evaluating rogue crests for a SFWT.
The Rayleigh model was the most conservative among TE crest-to-trough models that were considered, and estimated higher probabilities than the STE1QD (wave crest-to-trough STE) model in several sea states up until an area size of 12x12m2.
The influence of the shape of the wave spectrum was investigated by two spectral bandwidth parameters. All four wave crest models showed an increase in rogue crest probability as the wave spectrum became narrower. Regarding the crest-to-trough models, no clear preference was observed.
Long-term rogue wave probabilities were calculated for four sizes of rogue crests and waves, where the squared area of the STE models was based on the waterline diameter of the OC3-Hywind spar (6.5x6.5m2). Probabilities were averaged per sea state bin, and it was observed that for this area the STE1QD model was more conservative than the Rayleigh model. When single maximum exceedance probabilities are compared, the Rayleigh model can be more conservative in several sea states until an area of 12x12m2, but when overall occurrence probabilities are considered, this area appears to be roughly 6.5x6.5m2.
Following the long-term rogue wave probability analysis, four rogue waves were each embedded in random wave series, based on their most likely wave spectrum, using a deterministic extreme wave model. With these wave time histories, simulations were run in OrcaFlex using a publicly available OC3-Hywind model. During the simulation the turbine was assumed to be in parked condition and only the wave loading was considered. Mostly the pitch angles and nacelle accelerations are limiting for safety, due to stability criteria and sensitive components in the nacelle. These limits are imposed by turbine manufacturers but are never shared publicly. Therefore, the results were compared with reference values from the literature. The dynamic response did not exceed these maximum reference values, but they can be considered quite serious as some did exceed lower thresholds.

Based on the long return period of the considered rogue waves, together with their induced dynamic response which does not exceed maximum reference values, it appears that these rogue waves alone are not dangerous for the considered SFWT. However, ultimately this will depend on the sensitivity of the components inside the turbine, as well as the desired risk profile of the wind farm owner.