Throughout the offshore industry large structures installed on the seabed use mudmats as a foundation to provide in sufficient bearing capacity and stability. Due to their size, the mudmats have a significant influence on the hydrodynamic behaviour and installation requirements. It is proposed to change the design of the mudmats from a near solid design to a perforated structure in order to reduce the total loads. When describing the hydrodynamic behaviour of a structure in a body of water, two terms are of importance: the added mass and added damping. For solid flat plates there is much data available to compute the coefficients for models. However the coefficients are influenced by the degree of perforation and the presence of boundaries. Since there is little information available where the presence of a boundary is combined with a perforated structure, it is proposed to conduct experiments to elaborate on the hydrodynamic behaviour of a perforated structure close to an impermeable boundary. A total of six scale models were constructed with a perforation ranging from 0 up to 75%, these models were oscillated in the water with different amplitudes, from 10 mm up to 160 mm. These oscillations were performed at different frequencies, from 0.2 Hz to 2 Hz. These tests were performed in still water and subjected to a uniform current of 5 mm/s and a current of 20 mm/s. All data obtained with these experiments was collected and analysed. The first conclusion drawn from the analysis of the experimental data is that both the added mass as the damping decrease with an increasing perforation. The added damping is found to be larger for high excitation frequencies, however the difference decreases for increasing perforation ratios. The added mass is smaller for high frequencies, but this relative difference does not decrease for larger perforation ratios. Where the damping is not significantly affected by the presence of a current, it is observed that the added mass decreases when a current is present and this effect is larger for lower perforation. The added damping shows linear behaviour when plotted against the amplitude of oscillation, except for experiments where the smallest amplitudes are tested in combination with a low excitation frequency. For the latter experiments a more quadratic behaviour of the damping is observed. The majority of the energy is found at the excitation frequency. For the added damping it is noticed that similar trends are distinguished for the first order and third order, the energy at three times the excitation frequency. At two times the excitation frequency there is only little energy found for the added damping. For the added mass there is no significant trend observed when comparing the first, second and third order energy at the load signal against the perforation. It is however found that there is a similar trend in the dependency of the added mass on the excitation frequency in both the second and third order, although the energy is decreasing for higher orders.

For many types of floating structures, roll motion is the most important wave induced motion. More specifically, roll motion is of high significance for barges, in order to correctly predict the acceleration of the structure caused by roll motion and determine the procedure of sea-fastening and off-loading. The correct forecast of roll motion requires accurate estimation of roll damping. At the same time, the main sources of roll damping are the creation of waves, skin friction and creation of vortices (eddies), due to roll motion. For this reason, the physics of the problem cannot be fully described by a flow model based on potential theory, as it is highly dependent on vorticity and viscous effects. As a consequence, potential theory algorithms underestimate roll damping, which results in over prediction of roll motion and thus in a conservative estimate of workability. It is also important to mention that the vorticity and the viscous effects, lead to nonlinear damping characteristics. More specifically, the roll damping moment is controlled by its odd numbered harmonics, the first of which is dominant. Consequently, it is common to express the damping moment in an equivalent linearized form, equal to the first harmonic. During the last years simulating the flow around the rolling vessel using viscous flow algorithms is becoming more and more popular. Using CFD (Computational Fluid Dynamics) is a cheap and fast way to create a “numerical” wave tank and perform numerical decay tests, forced roll simulations, and roll response simulations in regular waves. Of course the underlying algorithm should be validated, before any commercial or scientific use. In this thesis, roll damping will be estimated by performing virtual forced oscillation tests, and the computed roll damping coefficients will be presented as a function of roll amplitude. The virtual forced oscillation tests have been performed with the open-source CFD software OpenFOAM. Additionally, numerous numerical experiments are performed in order to choose the optimum discretization schemes, turbulence model, mesh and time configurations. Finally, Ikeda’s experimental data is used in order to validate the viscous flow algorithm and the numerical model. Two methodologies have been used in order to determine the nonlinear roll damping. In the first case the free surface is included in the viscous flow model, using the Volume of Fluid method. The second approach disregards any free surface effect and the total damping is calculated as a superposition of viscous (by viscous flow algorithm) and wave (by potential theory algorithm) damping. Both approaches are compared with Ikeda’s experimental data and it is concluded that both methods are able to capture accurately the linearized roll damping coefficients, for various amplitudes. Finally, viscous flow simulations of the full scale barge, in order to calculate the roll damping coefficients, are notably time consuming. For this reason, in order to reduce the computational time, the methodology which neglects the free surface effect, is chosen, as it is a good compromise between time efficiency and accuracy.

The offshore wind industry is entering a new level of maturity. Announcements of bigger offshore wind turbines, the interest in new locations for offshore wind farms in harsher environments and the appearance of zero subsidy bids are proof of a rapid development. The next generation turbines are expected to be significantly larger and heavier compared with the current operating turbines. This poses new requirements for safe and efficient installation, requirements that go beyond the capabilities of existing jack-up installation vessels and equipment. Therefore, to avoid bottlenecks for future development, new installation equipment is needed. The goal of this thesis is to find an efficient way for installation of future offshore wind turbines with a rated power of up to 20MW. The characteristics of these large size turbines were studied by examining the relation between the rated power and the rotor diameter of operating offshore wind turbines. The derived dependencies between the desired power and the required area of the rotor were validated with data from announced turbines. Extrapolating these dependencies has resulted in a prediction for the 20MW turbine of a rotor diameter of 250 metres, a hub height of 160 metres above sea level and a nacelle with a mass of around 1100 tonnes. To be able to develop new concepts for the installation of these turbines, interviews were conducted with industry experts and criteria were derived. Next, upscaling of the equipment of the current jack-up vessel was investigated, already existing concepts were reviewed and new concepts were developed. Based on the set criteria, a suitable installation concept was chosen. The chosen concept eliminates the need for lifting the heaviest component (the nacelle) to the highest height (hub height) by dividing the tower of the turbine into several segments. It consists of a temporary installation frame that can be placed from a jack-up vessel on top of the foundation of an offshore wind turbine. While installing the frame on the foundation with a crane, the nacelle, hub and blades are mounted together on the deck of the jack-up vessel, forming the rotor nacelle assembly (RNA). Then, the RNA together with the first segment of the tower is placed in the frame, skidded sideways and brought up by a built-in jacking mechanism in the frame. It needs to be brought up 45 metres, so the following segment of 40 metres can be skidded underneath. While jacking the first segment, the following segment is placed next to the lift frame and prepared to be skidded. After the skidding of the next segment is finished, the previous segment is lowered on top of the other segment and they are mounted together. While fastening the connection, the jacking mechanism is lowered by recycling the strokes so it can start lifting the next segment. This is repeated until the complete turbine has been installed. When all the tower segments are installed, the turbine can be commissioned and the frame is retrieved. Optimisation of the concept has been performed by highlighting the logistical process regarding placement of the frame, lifting of the turbine and retrieval of the frame. A concept design is presented that can install the future offshore wind turbines with a rated power of up to 20MW. It is able to install the large size turbines faster compared to upscaling the existing installation equipment, it can be used for several turbine sizes and it only requires small modifications on the design of an offshore wind turbine. The concept consists of a jack-up vessel from where the installation is performed offshore. This was preferred over a floating vessel, since movement of the jack-up vessel is reduced significantly when lifted out of the water. For wider applicability of the developed concept, for example on a free floating vessel, (non jack-up), further research is required to reduce motions between the turbine and the foundation.

Tideway is a marine solutions provider for the offshore oil & gas and renewables and among other activities specialized in subsea power cable installation. To optimize the installation of power cables in high currents a better understanding of the dynamic behavior of the cables is needed. During installation of subsea power cables in high ocean currents, significant vibrations might occur. These vibrations are called vortex induced vibrations (VIV) and they are result of the vortex shedding behind the cables. Significant forces may arise due to VIV which can compromise the cable's integrity limits and further introduce problems to the installation procedure. The main objective of this research is to assess the effects of the VIV on the cables and provide solutions to improve workability. In this thesis a coupled wake-structure model was created in order to study the interaction between the vortices and the cable. The model is based on the phenomenological model of the wake oscillator which can capture well the self-exciting, self-limiting behavior of VIV, therefore estimating accurately enough the lock-in region and vibration amplitude. A wake oscillator with acceleration coupling is used and different sets of tuning parameters tuned to 1DOF and 2DOF free vibration experiments are examined. The structure has been modeled in the finite element software OrcaFlex and the wake oscillator model was implemented as an external function in the programming language Python. To confirm the validity of the above mentioned model two experiments were used. Firstly, the model was tested against the Delft flume experiment where the case of a vertical riser is examined and further the São Paulo experiment where the case of a steel catenary riser (SCR) is studied. The model was also compared with the Milan wake oscillator model from OrcaFlex's VIV toolbox. The model showed good agreement with the experimental results and found to be superior of OrcaFlex's VIV model. Next, VIV analysis was performed for a case study. The analysis was done for different current speeds and directions and the effects of the VIV on the tension and the bending radius of the cable were determined. For lower current velocities, lock-in response and standing waves along the cable were observed, while for higher velocities multifrequency response, beating behavior and traveling waves were observed. An important conclusion of this research is that the coupled wake-structure model used in this thesis is suitable for assessing VIV effects on catenary shaped cables. Furthermore, the analysis showed that catenary shaped cables with remaining length on the seabed exhibit smaller crossflow vibration amplitudes than straight cylinders.

Vortex-induced vibration or as well-known as VIV is a phenomenon that has caught the attention of engineers along history. VIV is related to many engineering fields. In the offshore industry, it is of importance for risers that extract oil from the subsoil, and tethered anchors for floating units. The main aim of the thesis is to model the VIV phenomenon through a new version of the wake oscillator model by Qu,2019. This modified model includes a new in-line coupling term that is introduced in the wake equation of motion. Apart from having a new in-line coupling term in the wake equation of motion, the model also includes a coupling term that relates the lift force with the fluctuating force in the in-line direction. In order to analyze the performance of the modified wake oscillator model, it was required to understand the advantages and drawbacks of some models that were implemented before. During the investigation, four main steps were followed: 1. The performance of the equation of The Van der Pol oscillator to model the VIV phenomenon was explored. 2. Analysis and identification of advantages and drawbacks on the wake oscillator model by Ogink and Metrikine,2010, the one without in-line coupling, were conducted. This wake oscillator model was used to model a rigid cylinder elastically supported in free oscillation experiments. 3. The modified wake oscillator model (Qu,2019) was used to model a rigid cylinder in free oscillation experiments and in forced in-line vibration experiments. 4. Performing a sensitivity analysis in the new model. After analyzing the behavior of the new model, it was concluded that adding a coupling term in the wake equation of motion was helpful to improve the phenomenological modelling of coupled cross-flow and in-line vortex-induced vibrations of an elastically supported cylinder in fluid flow during free oscillation experiments. However, for the case of forced in-line vibration experiments, discrepancies between the model results and experimental measurements were observed. A more in-depth study on the simulation of forced vibration experiments is recommended to further improve the wake oscillator model.

Offshore pipelay is the operation of laying a pipe from a surface vessel to the seabed. As the suspended pipe approaches the seabed, it bends downwards forming the sagbend. The sagbend is controlled by the vessel position and a tensile load is applied to counteract the weight of the pipe. During the engineering phase of previous HMC projects, compression was observed in the sagbend, for both shallow and deep water projects. Clients and regulatoratory authorities are hesitant to allow compression, limiting workability. DNV-GL implies that compression can be accepted if it does not induce other failure modes, like local buckling. A global stability assessment should be performed to identify if the resultant deformations are acceptable. HMC is intrigued to discover what the constraints are regarding global buckling during pipe-laying. The objective is to develop a methodology to calculate an allowable compressive load, increasing the operational window. During pipe-laying, the vessels motions are transferred to the pipe, resulting in a time varying loading that can exceed the applied tension in the sagbend area, leading to compression. This can induce two different mechanisms of stability loss. Either the lateral vibrations can be excited, amplifying the amplitudes of the lateral motion, or tension can change into compression in a segment of the pipe for a part of the wave cycle, leading to a pulsating compressive loading. Focus is given on the instability onset due to the compressive pulsating loading, named as impulse dynamic buckling, as dangerously high strains can occur in that case. The system can exhibit higher load bearing capacity than the quasi-static Euler buckling load, when the loading is dynamic, due to inertia and drag. Depending on the project requirements, different installation vessels are utilized. The vessels response under wave excitation should be taken into account for the pipe-laying analyses. For that reason, a specialized finite element software, Flexcom, is used. The ability of Flexcom to accurately predict the onset of impulse dynamic buckling is validated by comparison with a simplified analytical model. A curved simply supported Euler-Bernoulli beam is utilized for validation, based on the theory of moderately large displacements. A dynamic buckling criterion and an applicable methodology, capable of predicting the onset of impulse dynamic buckling based on Flexcom’s post-processing results, is proposed. Constant velocity loading is applied to demonstrate the effect of loading velocity in the dynamic buckling load. Harmonic displacement loading is applied to validate the proposed dynamic buckling criterion. Two previous HMC projects are utilized as base cases to evaluate the proposed methodology. Finite element analyses in Flexcom is performed and the critical parameters regarding dynamic compression generation are identified. Based on the proposed dynamic buckling criterion, global stability assessment is performed for the base cases, against regular and irregular wave excitation. The critical vessel motions for the onset of impulse dynamic buckling, are identified. A sensitivity study on the driving parameters is performed. Following a limit state design approach for the base cases, results demonstrate that compression could be tolerated during pipelay operations, providing that other failure modes are covered. Hence, the limit of the local buckling criteria can be fully exploited despite the occurrence of compression.

It is well known that for tall cylindrical structures under wind action, such as wind turbine towers, vortex induced vibrations (VIV) may occur. During the installation of offshore wind turbine towers there are different stages of transport where we might deal with VIV. The main objective of this thesis is to explore the possibility of modeling the VIV of flexible cylindrical structures by means of an existing wake oscillator model. Sub objectives are to investigate the influence of the deck stiffness and vessel motions on the VIV. And also, the effectiveness of strakes as mitigation for VIV is investigated. From the literature study it is found that an existing wake-oscillator model with acceleration coupling is a good way to model the VIV of a rigid cylinder that is free to vibrate in water. This wake oscillator model, which was tuned to the experiments in water, is then tuned to an experiment where the rigid cylinder was free to vibrate in air. These free vibration experiments were all done in the sub critical flow regime. In the next step the model is extended to model the VIV of flexible cylindrical structures. In an attempt to validate, this model is used to simulate the cross-flow amplitude of a in situ chimney on whom measurements were done during VIV. Due to the size of the in situ chimney, the Reynolds number at which VIV occurred suggests that we are in the super critical flow regime. However, the measured Strouhal number had a frequent value of 0.21 which is common for the sub critical flow regime. From the simulations it was found that by making use of the force coefficients from the super critical regime good agreement is found between the simulated and measured crossflow amplitude, while adopting force coefficients from the sub critical regime, the model significantly over predicted the response. Finally, the model is applied in a case study with a wind turbine tower. A total of four cases are investigated. From this case study it was found that a stiff deck will result in VIV at higher windspeeds, but will also increase the loads. The VIV of the tower is hardly influenced by the vessel motion since the frequencies of the two are far apart. According to the model, covering the top one third of the tower with strakes significantly reduce the VIV, but with a penalty of increased in-line bending load on the deck.

The main objective of this dissertation is to compare the small-scaled model test by the flume with the full-scaled model test carried out by the towing tank and to find out the differences between the test results and the test facilities to give proper suggestions for the design and operation on small-scaled testing facilities. The methodology used in this dissertation is carried out on the small-scaled model test in the flume. Three main steps are carried out, the first is comparing the test results of the flume and the towing tank for both the stationary test and the free vibration test, the second is looking for the theoretical explanations of the differences between the test results, the third is finding out the advantages and disadvantages of the small-scaled model testing facility compared with the full-scaled model test results which have been carried out by the towing tank in the previous research. After the model test, the wake oscillator model is also used simulate the real model test and find out the influence of different parameters on the test results. The main conclusions of this dissertation is that for small-scaled model test carried out in the flume, the free vibration tests match the full-scaled model well, which can be used to evaluate the vibration suppression efficiency of the strake model. In the stationary test, the small-scaled model provides higher drag coefficient due to the difference of Reynolds number. In the numerical model, the parameters which may influence the oscillating amplitudes are the tuning parameters and the different series of tuning parameters are given according to the spring stiffness.