A conventional dynamic positioning system is used to control vessel motions in the horizontal plane, i.e. surge, sway and yaw. However, the safety and operability of several offshore operations can be increased when also the roll motion is actively controlled, especially in beam seas. A control system model is proposed for combined thruster induced roll reduction and DP station keeping when the vessel is operating in beam seas. The proposed control system model is applied to a offshore construction vessel and the performance is demonstrated by time domain simulations. The DP footprint is compared to a conventional dynamic positioning control model. The proposed control model enables active roll reduction while the effect on the station keeping performance is unaffected.

Currently there are three methods of calculating the Dynamic Positioning capability of a vessel namely: static calculations, real-time time domain simulations and fast-time time domain simulations, although the outcome of the last two should be identical. In each of these methods a set of input variables is required to perform the calculations. These inputs are not always exactly known and are therefore sometimes estimated or taken from databases. It is not always clear how big the uncertainty in these estimated inputs is, and on top of that: how big the effect on the predicted DP capability is. To be able to quantify how certain a DP capability calculation is and which input data contribute most to the output uncertainty, in this thesis the input uncertainties for both static and fast-time dynamic calculations are investigated. Each method is subdivided in three different design stages which are: conceptual design, preliminary design and as built design. For the static calculations all three design stages are evaluated but for the fast-time dynamic simulations only the as built stage is considered.

The static calculation part of the analysis consists of determining the input uncertainties, the input sensitivities to the output and finally calculating the output uncertainty. The method used for this calculation assumes that either the relation between input and output is linear or can be linearised at the point of interest. The input uncertainties are calculated using historical data of the Bibby Wavemaster 1 which is the vessel used as case study throughout this thesis and is specifically designed for the purpose of servicing offshore wind farms. It is observed that the input uncertainties of the main dimensions of the vessel are clearly reducing when moving through the design stages. Furthermore it is concluded that the environmental coefficients of wind, waves and current are the most uncertain, even in the conceptual design stage where input parameters of the main dimensions of the vessel vary the most. When considering the sensitivity in the three design stages no big changes were observed, meaning that in all design stages the main dimensions of the vessel are most sensitive to the output. Finally the uncertainty in the output was evaluated were it was observed that for the as built stage still a standard deviation of 4% uncertainty of the output is present, resulting in a calculated 99.7% confidence interval of either 12% too high or too low.

In the dynamic calculation part only the as built stage is considered. Again the uncertain parameters are defined but due to the PID controller in the dynamic model some new input parameters are now present. The gains of this PID controller are assumed to be uncertain and are therefore taken into account during the dynamic uncertainty analysis. Due to a limitation in the aNySIM licence bought by Damen it is impossible to change the wave coefficients which causes their uncertainty not to be taken into account. Since the dynamic simulations are considered to have strong non linearities and possibly even discontinuities due to thruster saturation, the calculation method used for the static part is not applicable anymore. Therefore it is decided to use Monte Carlo simulations to quantify the uncertainty in dynamic DP calculations. Due to the large computational time required to perform large amounts of Monte Carlo simulations with aNySIM, a machine learning method is used to capture the dynamic behaviour of the vessel. A small number of simulations performed by aNySIM is required to train the model which are selected using the Sobol design of experiments technique. This technique optimises the choice of the simulation points to make sure the complete space of possible inputs is covered. By using the machine learning model to obtain an output of a dynamic simulation only a fraction of a second is required instead of 17 minutes when using aNySIM. By running the Monte Carlo simulation on the created machine learning model it was observed that the 97.7% confidence interval for offset can either be calculated up to 8.7% too low or too high whereas the prediction for heading up to 23.1% too low or too high when compared to the base case. It is concluded that using DP for the purpose of people transferring by means of a "Walk To Work" bridge, uncertainties should be taken into account to reduce both safety and contractual requirements risks.

Plastic pollution in the marine environment is an increasing problem with severe impacts on ecosystems and economies around the globe. The Ocean Cleanup (TOC) Foundation develops a floating barrier able to intercept, concentrate and extract plastic from the marine environment. TOC has conducted several experiments and numerical studies to determine the plastic capturing efficiency of its system. One of the phenomena leading to degradation of capturing efficiency is wave overtopping. During a wave overtopping event, the system is not able to properly follow the waves. This allows water to wash over the system and possibly lead to plastic, which was retained by the system, escape into the open ocean. This issue is amplified by the use of a stiffer barrier than the original concept that was developed by TOC.
To model and quantify plastic loss due to wave overtopping, the ideal approach would be to use a nonlinear 3D CFD method including hydro-elasticity of the barrier structure. Given the size of the problem and the number of conditions that need to be simulated to characterize the design space of the system, the use of such a method is computationally expensive and therefore unrealistic. Therefore, the objective of this work is to propose an alternative method. A method is presented which aims to quantify overtopping volumes by coupling a hydrodynamic solver to a 2D CFD solver. This thesis work will present the method in three parts.
Part I develops a hydrodynamic model to predict the first order (linear) motion response to wave excitation of a 2D cylinder with a spring attached. It is proposed that this model approximates the motions of a 2D cross-section of a long flexible cylinder subject to random wave excitation. The model’s output is a time trace of the motion response, which allows for coupling to a 2D CFD solver. It is found that the spring constant is dependent on the wave characteristics of the ambient wave field. Also, the ability of the 2D cylinder to follow waves is sensitive to the spring stiffness introduced in the model.
Derivation of the spring constant is handled in part II. Here, A model to approximate bending effects into a linear spring is set up. The ambient 3D wave field surrounding the capturing system is modelled and wave statistics are derived. Based on these wave statistics, spring constants are derived for given physical and environmental configurations. From the results it is found that for the wave cases 1 to 4, values of the equivalent spring stiffness coefficient are significant enough to influence the motion response of the floater. For case 5 and 6, values for the equivalent spring stiffness are relatively low and its influence on the response is expected to be negligible.
Part III handles coupling of the hydrodynamic model to the CFD solver ComFLOW. In the cases with low wave steepness, derivation of overtopping statistics has been achieved and results show that overtopping performance can be assessed by performing the steps taken in this research project. In these cases, the wave field is generally behaving linear and the motion response obtained from the hydrodynamic model can be coupled to the CFD solver. Results show that wave height and the applied spring stiffness are governing parameters for overtopping performance. It was also found that in some cases, non-linearities are introduced in the wave field. In the cases where non-linearities occur, the motion response derived by the hydrodynamic model deviates from the motion response that one would expect.

The Offshore Industry is one of the most challenging industries. Offshore vessels must survive all weather conditions over its lifetime. In heavy storms, waves and ship motions become so large that water flows onto the deck, It is referred to as Green Water and is a serious challenge that needs to be considered in designing offshore vessels. The methods for making green water prediction are limited. More data needs to be collected to ensure safe navigation and operation.

ComFLOW is a numerical tool in the field of Offshore Engineering used to study these non- linear and complex green water phenomena. ComFLOW is a Navier-Stokes solver with Volume of Fluid(VOF) for free surface motion, It has been used for simulation of dam break flows as a model for green water flow on deck and falling objects in calm waters. In this research, it is used to generate dam-break waves along with simulations involving regular and irregular waves leading to water on deck. It is used to evaluate whether dam break model is a complete model for studying green water loading on deck. A grid convergence study for ComFLOW is conducted along with a verification of ComFLOW for 2 dimensions. Optimum grid cell size is advised for accurate and stable results in ComFLOW for 2 dimensions. Dam break simulations are performed on an idealized deck to observe changes from that of a regular domain boundary.

Wave prediction and it’s impact on the topside facilities due to regular and irregular waves is the objective of this research. Absorbing boundary conditions are used to prevent wave reflections. Non-breaking waves are generated for irregular waves. Impact of water on the structure on deck is of primary importance. The wave height is an important parameter in dealing with such green water events. The role of wave height along with the distance of structure on deck is studied in this research. The input parameters such as dam height and distance of the structure from dam height are varied. Localized impact pressures are considered and changes in impact pressure are observed. A structure in regular and irregular waves is also compared with a dam break model. The main conclusions are:
• Variations in dam break loads compare well to theory.
• The dam break is not a complete model for assessing green water loads.

The activities of humans in the marine environment are on a steady rise; this trend follows the exposure of offshore structures and vessels to harsh environmental conditions. On the other hand, there is continuous demand for lighter, faster and more efficient vessels. Technological advancement in composite materials and better design methods invariably leads to more sophisticated computational tools. In these tools, the inherent flexibility of the structure is involved in the calculation of its
loading and response – introducing the aspect of fluid-structure interaction. However, more than fluid-structure interaction, hydroelasticity which is a subset of fluid-structure interaction allows for deformation of the structures due to fluid. Current industry practice neglects these deformations and all structures are designed as rigid bodies. These practices lead to overestimation of the frequency which translates to the mass and how stiff the structure is. Design based on hydroelasticity
allows for knowing the true behaviour of the structure; thus, a coupling of fluid and structure yields a true appreciation of these designs. The objective of this study is to formulate and solve analytically the coupling phenomenon behind fluid-structure interaction. An experiment designed by Lampros Rizos in 2016 which modelled fluid-structure interaction consisted of a set-up with a cylinder immersed in a larger cylindrical tank. The immersed cylinder having a flexible circular membrane (structure) located at its bottom. The cylinders were filled with non-viscous liquid and exhibited free surface. The designed experiment was decoupled and a systematic build up in understanding the dynamics of all components – structure and fluid were solved. Firstly, the dynamics of the structure when dry (In vacuo) was idealized in 2-D and 3-D. Secondly, the system was coupled and solved
in 2-D and 3-D when structure is located at the top and bottom of a single cylinder individually. The use of a series solution: Fourier-Cosine and Fourier-Bessel series solutions were used for 2-D and 3-D cases respectively. Along with these series solutions, the fluid governing equations were employed to solve for the coupled frequencies. Lastly, the entire submerged structure was analysed and solved. The hydroelastic or coupled frequencies and vibration patterns are determined for lower angular and radial modes for which the influences of various parameters are investigated. A trend was observed – that for lower modes, the hydroelastic frequencies are lower than individual membrane natural frequencies at low frequencies. At higher frequencies, the coupling effects are larger making the coupled frequency higher than uncoupled frequencies. There is a strong relationship between the tension of the membrane and the hydroelastic frequencies. An increase in the tension causes an increase in the hydroelastic frequencies. Also, the hydroelastic frequencies increase as the fluid depth increases. More so, the increase in fluid depth tends to converge towards the uncoupled fluid free-surface frequency. The comparison of the analytical solution with experimental and numerical solutions does not match perfectly and thus, further works to improve this study are recommended.

Structures at sea, which can either be floating or bottom-founded, are always affected by several types of dynamic loads from wind, waves and currents; with floating offshore structures subjected to more motion and loading from these loads. Hence, necessitating more in-depth hydrodynamic analyses. Increasing size and weight of these floating structures together with industry demands have led to unsuitability of existing models – which make use of rigid body (no hydroelasticity) assumptions. This, in addition to other reservations about current practices, has led to the need for more realistic formulations of the interaction between fluids and our offshore structures, taking hydroelasticity into account. Extreme wave impacts, which are responsible for severe damage of offshore structures, have for a long period been only studied with experimental methods, as non-linear nature of the complex wave kinematics make solutions not straightforward. More recent advancements have seen the development of numerical techniques and programs, based on the Navier-Stokes equations, to better understand extreme waves and impacts on offshore structures.

This thesis focuses on the topic of experimental validation of fluid-structure interaction (FSI) in ComMotion – an advancement in the ComFLOW program (an improved Volume of Fluid (iVOF) Computational Fluid Dynamics (CFD) code) to capture motion of flexible structures in the presence of free surface effects and a two-way coupling between the fluid and the structure. This has led to the need to provide reliable data for validation purposes. The research is conducted in continuation of the studies carried out by Rizos (2016) in which an electromagnetic drive linear motor produced noise and variable amplitude enforcement obtained in the analyzed results, amidst other uncertainties in the measurement techniques used. The current methodology principally makes use of a different motion technique – rotary to linear motion to obtain more reliable data.

Firstly, rotary to linear motion was chosen (motivated from the internal combustion engine of an automobile) and then verified computationally with analytical derivations. The experiments were then designed, with the initial step being to verify the said linear motion. A sensitivity of the motion signal to weights in the system was carried out. Further, the experiment designed made use of a latex rubber membrane, applied with a clamped boundary condition at the bottom of a partially-filled rigid cylindrical structure. Tension existing in the membrane, due to water columns above it, was analytically obtained, as complete knowledge of system parameters was crucial. Linear oscillatory motion at the end of the converted rotary motion was applied to the fluid-structure system. Motion signals were obtained – rigid body measurements, to subtract them from the membrane motion measurements obtained. Motion measurements were acquired with laser triangulation displacement devices and a force transducer was used to monitor applied forces in the plunger during the experiments. The output of the transducer can offer validation data to the FSI solver.

Membrane measurements were taken at half radius at a 30° step over a 180° span, and at the centre. This was in a bid to capture maximum deflections of the membrane, as interest was to obtain the coupled frequency mode shape. Finally, the obtained membrane motion data were analysed to prove their reliability, and to draw conclusions from them. Volume (of liquid), frequency and polar dependence analyses were carried out. Recommendations were subsequently outlined, for future research.

Due to a low restoring force as well as lack of damping, the roll motion of a
vessel is usually the motion with the highest amplitude of all degrees of
freedom. Therefore, in history many have tried to stabilize rolling ship
starting with Watts in 1883. He described the damping of the roll motion by
having a group of men running to the "high" side of the vessel. Since then,
a great deal have been written about the subject of roll damping for
stationary vessels. The principle however has been the same ever since, a
shift of mass to counter the roll motion. Water tanks, carts on a curved track and sliding masses, all tend work with this principle. In 1904 Schlick introduced another device to damp roll motions was introduced, the gyroscope. By controlling the precession angle, the roll motion could be damped. Following from an increase in computational power more advanced control strategies could be applied, leading to a renewed interest in the subject as well as different methods to model the vessel for controller design.

In the current paper, the performance of a gyroscope is compared with the performance of a moving mass, modeled as a double pendulum, for dynamic performance of a showcase vessel, the Huisdrill P10000. Results of this analytic study are promising, a reduction of more than 80% for both systems was
achieved at the natural frequency of the Huisdrill. The decay time after a roll amplitude of 5.7 degree was reduced from about 250 seconds to 60 seconds for the gyroscope controller model and 70 seconds for the double-pendulum controller model. A static heel control system is modeled based on the double-
pendulum equations. This proves to be able to compensate for the heeling moment depending on the weight at the crane tip as well as the weight and distance available to compensate. Combing the static an dynamic approach is possible with the note that there should be sufficient space for the mass to deviate after compensating for static heel.

In irregular waves the 4-DoF model shows good performance for the showcase of the Huisdrill P10000. The RMS value for the roll-roll motion was decreased by 65% For other degrees of freedom, similar reductions were obtained. From this it can be included that for the given seastate, significant reductions can be achieved using the ballast train. In the time domain, it was shown that great reductions in most probable maximum of reduction of roll amplitude were achieved for time periods close to the natural frequency of the vessel. For no time period, reductions in performance were measured. It was found that
the most probable maximum roll angle linearly increased with the wave height, as well as the most probable deviation did.

Interpreting the results of this study, it can be stated that both the gyroscopic control system as well as the double-pendulum system are able to introduce great reduction in dynamic roll motions. The main benefit of using the double-pendulum system is its ability to compensate for the heeling moment induced
by the swiveling crane as well as compensating for the dynamic motions that a vessel will undergo offshore.

It is recommended to do a simulation of a non-linear time-domain model or to perform model tests for verification of the results found in this report. The gap between static and dynamic control should be filled by a master control which is to be designed and run in the time domain.

There lies an opportunity for significant cost savings in the installation of subsea cables in offshore wind farms which is why the current work proposes a state-of-the-art method for monitoring the cable during installation. The proposed method enables offshore crew to look through the water with the use of augmented reality. To this end a real-time numerical model of the subsea cable dynamics is developed. The model is created in programming language C# and visualization in augmented reality is done using the game-engine Unity. This development environment is uncommon for engineering and scientific purposes and has been selected based on its ability to visualize model results in augmented reality, its extensive graphics library, its capability to process real-time in- and outputs and its free availability. Relevant physics are analyzed on contribution to global cable geometry and tension for the case of shallow water cable laying, resulting in an equation of motion which is sufficiently accurate for representing the physical phenomena occurring during cable lay. An assessment of the measurements required by the model during operation is made. The main challenge of modeling in real-time is that the average model time step is limited by available computational power, in turn limiting the axial stiffness range which can be modeled without the occurrence of numerical instability. Discretization is done using a lumped mass method. It is shown that cable dynamics can be modeled in real-time using an explicit method and that overcoming the associated limitation on axial stiffness does not lead to inaccurate results. The developed numerical solution is verified using OrcaFlex, which is typical software for dynamic analysis of offshore marine systems. An augmented reality interface is developed, including color codes indicating the structural state of the cable. The current work enables the visualization of the real-time model in augmented reality. Potential legal issues related to the implementation of augmented reality during offshore cable lay are outlined during a collaboration with Tilburg University. Successful practical implementation of the proposed innovation is associated with promising opportunities.