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.

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.

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.

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.

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.