Accurate ship motion prediction is crucial for safe and efficient operations at sea. Previously studied Single Input Single Output (SISO) system-based method often falls short in providing robust and precise predictions. This thesis explores the potential of using a Multiple Inputs Multiple Outputs (MIMO) system identification approach to provide more accurate and ro- bust ship motion prediction in offshore operations.
We investigate two MIMO systems, the force-to-motion system and wave-to-motion system, employing Spectral Analysis method and Subspace method, respectively. Both synthetic and real-field data are utilized to study the methods. While Spectral Analysis method demon- strates better accuracy, it relies on the assumption of perfect accuracy of the pre-computed force Response Amplitude Operators (RAOs), which is a limitation of this approach. Sub- space method is less precise, it also requires real-time re-evaluation of the system order due to its sensitivity to wave conditions, making this method less versatile.
Comparing these methods with SISO system method and conventional RAO-based method, Spectral Analysis method emerges as the most accurate, followed by Subspace method. These findings strongly support the potential of MIMO system identification to significantly improve ship motion prediction accuracy.

Modelling of Floating Offshore Wind Turbines (FOWTs) is challenging due to the strong coupling between the aerodynamics of the turbine and the hydrodynamics of the floating structure, which makes the dynamic response of the whole system highly complex. A variety of numerical approaches can be used to predict this response.

The frequency-domain method (FD) is chosen for further development. These methods are considered as efficient tools for analyzing first design iterations. Given the ongoing development of new concepts for FOWTs, the industry could benefit from such a tool. Currently, only one such open-sourced method is developed (RAFT), but the method still has room for improvements.

After enhancing RAFT's control modeling, the frequency-dependent aerodynamic added mass and damping coefficients are determined following RAFT's methodology. These coefficients are then implemented in a wave diffraction program to predict the surge, pitch, and mooring line tension response of FOWTs for four different design load cases (DLCs). These results are compared against time-domain (TD) simulations.

The method proved capable of predicting the surge and pitch motions well, but the mooring line tension prediction still needs improvements.

Operational optimization of vessels is valuable for the planning and execution of maritime operations. Accurate and efficient models to predict vessel motions are needed to make reliable operational decisions. The wave-induced vessel response can be modelled in terms of a Response Amplitude Operator (RAO) and a wave spectrum. Uncertainties related to the parameters that govern the RAO can significantly influence the reliability of the vessel motion prediction. To decrease these uncertainties, the maritime sector has realized the potential of using vessel motion measurements. As a result, it is envisioned that a vessel response model might include an identification module that searches for model parameters using measurements of responses to make a reliable prediction.

This study presents an identification procedure to handle the inherent uncertainties of vessel model parameters, aiming to improve vessel motion prediction. The identification procedure identifies the vessel's RAO by the measured response spectrum and nowcast wave spectrum, with the goal of finding the heave and roll natural frequencies. The natural frequencies provide information on the vessel’s parameters. This is used to identify the parameters related to the mass distribution and damping of the vessel. These were found by minimizing a cost function, that quantified the difference between the measured and predicted response spectrum, using an optimization method. Identifiability analyses of the parameters were performed on two case studies.

For the first case study, a synthetic data set is created with the vessel response model to simulate the vessel motions. Tests were conducted with five different wave spectra and several vessel headings, constituting diversified scenarios. The RAO was identified by the measured response, the wave spectrum, and a sinusoidal function to describe the directional dependency of the RAO. Using the synthetic data set, the identification algorithm successfully identified the parameters with good agreement to their actual values. The second case study involved the examination of parameter identification on real onboard vessel motion measurements. In most of the cases, the RAO could be identified from the measurements and the natural heave and roll frequency was found. The identified parameters resulted from the identification procedure and improved the vessel motion prediction compared to the initial prediction, but still, deviations remained. The identified parameters are verified against a different measured data set. The results show that the identified response spectra approach the measured responses, indicating that the identified parameters are reusable.

In summary, it was found that the parameters have a great influence on the output of the vessel response model. Therefore, it is essential to have a thorough understanding of the correct operational parameters for accurate motion prediction. The established identification procedure shows to be a good addition to existing vessel motion models to identify input parameters at relatively low computational cost.

Deterministic predictions of wave induced ship motion are increasingly often used to enlarge the oper- ational envelope of ships that perform complex operations at open sea. The large amount of complex operations performed nowadays, motivates the relevance of motion predictions. The company Next Ocean provides these forecasts, based on linear seakeeping theory. While highly linear degrees of freedom are predicted accurately, roll motion predictions prove to be more demanding and are less accurate. This research aims to improve roll prediction accuracy by adding (non)linear damping to the equations of motion. The prediction optimization is performed on data of the platform supply vessel Acta Auriga.

To enable the use of nonlinear forces, the equations of motions are first implemented in the time domain using Cummins’ equations. The linear coefficients in this equation are determined from the frequency dependent coefficients in Acta Auriga’s hydrodynamic database. To ensure the correctness and investigate the limitations of the implemented Cummins equation, the results from the time and frequency domain are extensively compared. Both monochromatic excitations, as well as spectrum (JONSWAP) excitations are considered. In contrast to linear force coefficients, nonlinear force coefficients are generally not known for a given vessel. Implementing these forces into the equations of motion imposes the need for an approximation of these coefficients. Estimates for the linear, quadratic and cubic viscous damping forces are made, using both the measured motions of a vessel operating at sea and the predictions of the corresponding excitation force, made by Next Ocean. A multivariate regression algorithm is used for the identification.

The Cummins equation was successfully implemented. However, the translation from frequency to time domain was more error prone than anticipated; the frequency dependent coefficients need to meet requirements that are typically not met by databases meant for frequency domain calculations only. Furthermore, degrees of freedom without a restoring force showed running away behaviour that could not be negated without adding extra damping. The roll motion was susceptible to instabilities. The exact origin of this instability was looked for, but could not be found. Adding damping resolved the instability. Furthermore, the interpolation in the roll response amplitude operator introduced errors in the initialisation of time domain calculations, which led to inaccurate results when spectrum excitations corresponding to more severe sea states were used. Only under specific conditions, these high energy spectra led to accurate roll predictions. Adding damping made for a close match between frequency and time domain calculation for all degrees of freedom. The complexities mentioned made that an easily scalable algorithm could not be obtained using time domain calculations. Easy scalability be- ing important to Next Ocean means that running time domain simulations in Next Ocean’s product is deemed unrealistic. This also means that no nonlinear forces can be used in real time applications.

In an attempt to improve linear predictions, coefficients in the linear seakeeping model, including the linear damping coefficient, are identified and vessel motions are re-predicted with the added damping and updated linear force coefficients, using the Cummins equation. None of the identified parameters led to better predictions than were obtained by Next Ocean. Identifying and updating parameters was therefore concluded to be not beneficial to the quality of the motion predictions. Adding a fixed amount of linear roll damping, which was not identified from the field data, did lead to improved prediction quality; correlations between predictions and measurements increased by 9.6%.

While the motion prediction could not be improved using the Cummins equation, valuable information on the time domain simulations was obtained. The finding that better predictions were consistently obtained by adding a fixed amount of damping, sparks an opportunity for further research into the ideal amount of damping. This, and more elaborate identification schemes could provide meaningful insight to improve motion predictions.

\chapter*{Abstract}
Larger wind turbines are being developed to fill the growing demand for green energy. Wind turbines that need to be installed in the next decade will reach a capacity of 15 MW, with an approximate rotor diameter of 240 meters. The currently used wind turbine installation vessels cannot install these larger wind turbines; hence, investments need to be made. An opportunity arises from the oil and gas industry: investments in this industry are decreasing. Thus, the floating monohull fleet working in this industry will need repurposing in the upcoming decade.

In this thesis, the operational limit of a single blade installation with a floating monohull crane vessel is sought. An installation method is proposed where taglines are used to compensate vessel motions and wind loads on the blade. Motion compensation with taglines is a technology that is already being used in the industry, which makes it attractive to contractors who are hesitant to use new technologies. The proposed installation concept uses three different compensation systems, allowing the blade motions to be decoupled and controlled separately. One of the compensation systems is selected and investigated in more detail in this research. The selected compensation system compensates for the blade motion in y- and yaw-direction with two dynamically controlled taglines horizontally attached from the blade to the crane boom.

The operational limit is determined for the final installation stage, the mating phase between the blade and the hub. The alignment of the blade and the hub is modeled in frequency- and time-domain. From these models, it can be concluded that the control system can limit the y-motions at the blade root below typically accepted motion limits, when the wave direction is limited to an interval of 150-210\deg, for a sea state up to a wave height of $H_s$= 5.6 m and wave peak period $T_p$ of 10 s. However, this is only true when the predisplacement of the blade towards the crane boom is larger than the maximum crane tip displacement.

Based on these preliminary findings, it seems that floating installation of turbine blades could be feasible with proper tagline control. Further investigation, involving dynamic control in all degrees of freedom, is therefore recommended.

In offshore operations a trend is forming where vessels are more often required to do multiple short operations within a small-time frame. Traditional mooring systems require execution time far beyond the operation time. Dynamic positioning systems offer great advantages for short time span operations such as crew transfer or lift operations.

Currently operations are planned based on DP capability plots and experience of captain and DPO. DP capability plots have little operational value as this is a static calculation and only provide information for average station keeping capability. During operations, the displacements made by the vessel around the DP set-point, also referred to as DP offset, are of great importance to determine the operability of an operation. Currently, the only way of calculating the DP offset is by conducting extensive time domain simulations, which are hard to integrate in the operational workflow of a DP vessel involved in walk-to-work operations. Therefore, a new approach is developed which predicts the vessel’s DP offset in the frequency domain, which enables a quick and robust calculation of the DP offset which is suited to merge into the on-board workflow. A frequency domain model is per definition a linear model. This leads to the main challenge of this research. A vessel operating on DP is non-linear. Currently there is no insight in what the effect is of non-linear components present in a DP system, on the linear approximation of a frequency domain model.

To investigate the effect of non-linear components onto the DP frequency domain model, a time domain model is developed that is capable of systematically enabling/disabling different non-linear components. The time domain model will serve as the ’truth’ in this research as no actual vessel data is available. Furthermore, this helps identify the effects more easily, as the input for both models are identical. From the time domain model transfer functions can be derived that serve as the basis for the frequency domain model. The transfer function is a linear relation between two variables. In this case, between second order wave drift forces and displacement of the vessel in surge, sway and yaw direction. The following non-linear components are investigated in this research: Thruster ramp up, thruster turning rate, forbidden zones, saturation and thruster allocation. Thruster allocation is present in each model that will be tested, as this is an essential part of a DP system.

Using two methods of determining transfer functions the model and the effects of all non-linear components are tested. The model is subjected to a variety sea-state, with different wave directions. Both methods offer similar results even though different approaches to determine the transfer functions are used. The selected method is capable of accurately predicting vessel offsets, although some extreme offsets are not captured.

It is concluded that the presence of non-linear components have little to no effect on the DP offset as calculated by the time domain model. Because natural frequencies characteristic to these non-linear components are expected to exist at much higher frequencies that naturally present in second order wave drift forces. Thus, making a linear frequency domain model suitable for DP offset forecasting. It is advised to investigate the effect of including 2D input spectra as this is expected to improve the current model.

Within this thesis an attempt is made to improve the seakeeping of a Service Operation Vessel (SOV) by means of an active anti-roll tank (ART). It is researched if the addition of a wave prediction system can improve the ART’s performance. As the seakeeping of the SOV is most important for the installed motion compensated gangway, the workability is assessed for different control methods based on the design criteria of the gangway. The computations have been made in frequency domain. A one directional model is used with two degrees of freedom: the ship roll angle and the tank’s fluid angle. Due to the presence of a wave prediction system, a feed forward loop can be introduced in the control system. It is found that if the pump moment leads the wave excitation moment, the tank’s fluid will oscillate in a less power demanding way. This is caused by the better interaction of the action and reaction forces that are caused by the pump. The action force on the ship’s structure will always oppose the wave moment, but the force acting on the tank’s fluid will only stabilise the vessel at certain frequencies. To obtain a robust model, the feed forward loop is combined with a feedback controlled model. The stability of the models’ feedback loop is assessed using Nyquist plots and sensitivity plots. It is found that for the damping factor belonging to the ART of the SOV, the feedback loop is unconditionally stable. On topic of the powers related to the active ART the dissipated and actual power are analysed. Based on the dissipated powers it is found that for the same amount of dissipated power by the pump, the feed forward controlled model causes a lower root mean square value of the ship’s roll in comparison with the feedback controlled model. This is explained by the better interaction of the forces generated by the pump. Based on the obtained RAO for the roll motion, the workability of the SOV is assessed for a significant wave height of 3m. It is found that even though the seakeeping of the vessel increases because the roll response decreases, the gangway motions aren’t significantly affected by the active control of the ART. The reduction in roll response is not proportional to the reduction of the gangway motions since other vessel motions such as heave and sway also effect the gangway’s motions. All in all, it is concluded that adding a feed forward loop to a feedback controlled model improves the seakeeping of the SOV. However, to improve the workability of the SOV it is recommended to include a passive ART in the design, instead of an active ART. This is because the active control has little impact on the gangway’s motions compared to the passive ART.

The demand for renewable energy is growing, due to a growing awareness among people about the effect of burning fossil fuels on the environment. Governments have signed climate agreements and are obliged to decrease their level of exhaust gasses. This change in attitude regarding the environment led to an upcoming market for renewables. A significant part of the demand for renewable energy is covered by the wind industry, both on- and offshore. Especially the offshore wind market holds huge potential, due to stable wind conditions and growth opportunities. A side effect of the evolving industry is that turbines are increasing in size and are being installed in deeper water depths. It is expected that the methods of installation, currently used in the offshore wind market, will no longer be suitable for the installation of new generation turbines. In this research, installation of offshore wind turbine towers using a heavy lift vessel (HLV) is opted as an alternative to the current installation methods (jack-up installation). The major benefits of floating installation, when compared to jack-up installation, are the independency on both soil conditions and jacking operations. A challenge of floating installation, however, is the introduction of vessel motions resulting in excitation of the tower. The main objective of this research is to find the most efficient way to control the motions of the tower during installation using a HLV. To fulfill this objective, first, a conceptual design study has been conducted, which focused on developing concepts for the safe installation of offshore wind turbine towers using a HLV. Safe installation was assumed if the displacements of the tower bottom maintain within limits, as specified in a list of design requirements and criteria. Before examining any concept in more detail, the response of the tower and the effect of applying a form of motion compensation has been examined in a general way. To test the response of the tower in the crane in a general way, with and without motion compensation, the tower in the crane has been represented by a double pendulum system with a moving support point. Then, using the numeric model, the response of the tower in the crane has been computed for certain sea-states in both frequency- and time domain and for both the free hanging- and the motion compensated case. The motion compensation system has been represented by a virtual spring damper system, of which, by changing the simulation parameters, the optimal configuration (position, stiffness and damping) has been determined that minimizes the tower response. From the results, it can be concluded that a significant reduction of the tower bottom displacements can be achieved under the application of an active motion compensation system, the requirements for safe installation, however, are not yet fulfilled.

New floating wind turbine designs are needed to reduce production costs and to increase mass production feasibility. The TetraSpar floating wind turbine achieves these goals by being constructed using components highly suitable for standardization and industrialization. The design makes use of a suspended submerged counter weight to obtain a low center of gravity for the floating system, while also allowing a low draft during transport and installation. An analytic analysis of the counter weight suspension system is presented that investigates the behavior of the system. The main concern of the novel system is its motion relative to the upper floating platform which it is connected to. It is demonstrated that the suspended system's motion is described by the axial elongation of the suspension lines when positive tension is maintained in all suspension lines. An analytic solution for the equivalent stiffness of the suspension system is given. This suspension system stiffness introduces additional natural frequencies into the system, and the effects of these frequencies are demonstrated.

This novel concept requires a multibody modeling approach to perform a dynamic loads and response analysis, as the stiffness between the floating platform and the counter weight is provided by chains. Additional design criteria are required for the counter weight system dependent on a combination of chain capacity and maintaining positive tension in all of the suspension lines. To satisfy these design criteria a global hydrodynamic load and response analysis of the floating system is performed. In this concept, the counter weight depth contributes significantly to the dynamic properties of the system and therefore a parametric study is conducted. The global response parameters of the rigid-body motion, natural frequencies, nacelle accelerations, counter weight chain tensions, and maximum platform-pitch angles are compared. Following the parametric study, an ultimate limit state analysis is conducted on the original and alternative designs. Design recommendations are made for the counter weight depth and configuration of the suspension system layout.

The rudders of moored FPSO’s are fixed in the center to avoid unwanted rudder motions. This is often done by mechanical locking of the rudder machine. Due to waves and current, the loads on the rudder are higher than expected, causing failure of the locking system. An empirical study is executed to predict the combined wave and current loads on the rudder in moored conditions.

In the transition to renewable energy, harvesting the power from oceanic waves is often mentioned to be next step after the deployment of wind energy technology. Although many different solutions for wave energy conversion (WEC) are being developed and financial support schemes exist, commercially extracting this energy seems however to be rather challenging. Major troubles are over-priced first projects and incapability to endure year-round oceanic conditions. While the costs are expected to reduce once a successful WEC-device enters large scale production, it is unclear what device type would be the most viable. This study presents a combination of available methods and tools that together allow to assess viability of WEC-devices in early stage of development. Prior to including economic factors, the most important aspect that defines viability is power performance. Comparing simulated power performance of different concepts for WEC is complicated by their different level of optimization of control strategies. Renowned studies that compared the power performance of various device types have been extended by calculating the viability for one particular promising device type: the quasi-rigid, submerged pressure differential device. This device appears promising being protected by its submerged position, while performing as if it was placed in the high intensity waves at the surface. The diffraction software tools commonly used for hydrodynamic calculations, could not be applied to calculate quasi-rigid (volume-changing) objects. For this problem a method is suggested which adds the functionality of panel selection to exclude surface areas of a body design for hydrodynamic calculation. In this thesis both the average power performance in time-domain simulations and the maximum power limits were calculated in order to compare the submerged pressure differential device to other point absorbing WEC-devices. The suggested methods contribute to defining a generic way of comparing future concepts for WEC to converge towards a single approach and select WEC-devices for investment.

Real-time ship motion predictions contributes to safer operations at sea and increases workability. Nowadays, a handful of people and companies are actively working on this subject. Most approaches use the ship’s navigational radar to first predict the wavefield surrounding the vessel and then calculate the motion response. One of the methods used to calculate the ship’s motions is a linear ship motion model based on a frequency-domain approach. The downside of this approach is that the accuracy of the motion predictions is affected by (for example) uncertainties in the Response Amplitude Operators. In this thesis, it is shown that estimating transfer-functions from measured motions and a so-called "now-cast" prediction of the forces will counteract for such uncertainties. For the estimation of these transfer-functions, different methods and smoothing techniques are evaluated. Based on sea-trial data, it is shown that the accuracy of the motion predictions increases with ~ 1-10 % when estimated transfer-functions are used, compared to the solutions obtained by pre-calculated transfer-functions.

Experience-based decision making by crew plays a key role in the safe and effective execution of maritime operations. For yachts and naval vessels, common operations can be identified as helicopter operations (take-off and landing), launch and recovery of small craft and (dis)embarkment to and from these small craft. All of these operations are limited by wave-induced ship motions.

For these operations, experience-based decision making does however not always guarantee that the operations are carried out in the most safe and effective manner possible. Also, the level of comfort as experienced by a yacht-owner or guests can be directly affected by this type of decision making. With this in mind, Feadship aims to develop a Feadship Comfort System (FCS), which can contribute to the decision making process. One of the elements of this system is ought to be a waveradar. The aim of this thesis is to describe how the waveradar can be included effectively in the FCS, and how it can be used for the above mentioned operations.

The waveradar is in basis a common navigation radar that can perform measurements of the surface elevation in the direct surrounding of a vessel. These measurements can be used to predict the ship's motions up to approximately two minutes ahead in time. With this, windows of opportunity can be distinguished when operations can be carried out best. These windows depend on the corresponding limiting criteria that are set on the ship's motions. A major issue is however that sea trials have shown that the roll prediction is inaccurate.

Due to this problem, two methods are evaluated in this thesis to improve the accuracy of the roll prediction. These methods are set up such that they are practical in use and can be applied directly to the deterministic wave predictions from the waveradar. The first method is scaling the response based on the roll motion history, whereas the second method is based on scaling the roll damping. The latter is effectively linearising the viscous contribution to the roll damping.

As no data from sea-trials is present to compare these methods, a benchmark calculation is carried out based on Cummins approach. In these time-domain calculations, a linear and non-linear roll damping coefficient are taken into account, which are based on a decay test that is available at De Voogt Naval Architects (DVNA). This follows from model tests performed by MARIN.

Comparing both methods to this benchmark calculation showed that the method of scaled damping resulted in the best approximation, with a Pearson correlation coefficient of (only) 33 %. Also, in terms of the behaviour of the motion envelope, this method showed the closest approximation of the two methods. It is found that the way in which both methods influence the RAO of the roll motion has a great influence on the results found. More suitable methods that should provide a better approximation are suggested from this, such as a frequency dependent scaling of the roll damping. This is however not discussed further.

With the method of scaled damping, the practical application of the waveradar within the FCS is evaluated. For the common operations of yachts and naval vessels, limiting criteria on ship motions are obtained from literature or stated by the author. The majority of these criteria are RMS values, for which a method is suggested to use them real-time. In this method, the most probable maxima are obtained from the RMS criteria, which are then applied as real-time criteria. This showed impractically large results, which showed that this method does not suffice.

From this conclusion it follows that the analysis of the practical application of the waveradar is only carried out for the (dis)embarkment of small craft. From this, design considerations for the FCS are obtained, from which the main result is that heading suggestion is one of the most important tasks that the system needs to be capable of.

Finally, a performance monitor has been evaluated for yachts at anchor. This performance monitor displays the predicted Comfort Rating (CR) real time, which can be used by the yachts crew to evaluate the current anchor location. This is a specific desire of Feadship and is developed next to the other operations. For this application, heading suggestion is also one of the most important tasks that the FCS needs to be capable of.