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.
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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.
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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. ... Read more