Wind turbine installation vessels (WTIVs) are ships that are specifically designed to install offshore wind turbines. These WTIVs have four or six large truss-like legs that are lowered towards the seabed by means of jacking systems. These jacking systems are regulated by control systems to ensure the leg is lowered with a constant velocity regardless of external disturbances. The bottom of these legs are outfitted with spudcans, which have a conical shape to allow for penetration into the seabed. When these spudcans have settled in the seabed, the hull is lifted above the waves. Consequently, wave-exciting forces on the hull are prevented and the motions of the hull are near-zero so that the on-board crane can perform the installation operations with minimised disturbances.

The objective of this thesis is to develop and analyse a model that is able to describe and simulate the dynamics of these jacking systems in great detail in response to external loads and the dynamics of the WTIV. The control systems of the jacking systems are included in this model to simulate and evaluate the interaction between the control system and the dynamics of the WTIV. Two conventional control systems are considered: the Volts-per-Hertz (V/Hz) and the direct torque control (DTC) method. In the process of lowering the legs and subsequent platform lifting, a transient phase can be identified during which the spudcans are penetrating the seabed. Due to the periodical motions of the ships, multiple impacts with the seabed are expected. Additionally, the jacking systems and the leg undergo a change of load direction as initially the leg is in tension and the jacking systems are generating power, and afterwards the leg is in compression and the jacking systems are consuming power. This thesis is focused on this seabed penetration phase as this phase introduces complicated dynamics. In literature, no model is available that has the abilities to simulate the WTIVs and its jacking systems with control systems in such level of detail.

This research gap is addressed by developing such a simulation model. This model is written in Python and developed using finite element (FE) techniques and solved using numerical time integration. Seabed characteristics are derived using a detailed coupled Eulerian-Lagrangian (CEL) FE models. Multiple control strategies are simulated and evaluated, each differentiating how the velocity and torque setpoints of the jacking systems are calculated. From the simulation model, it is found that in order to achieve load sharing between jacking systems, torque and velocity require to be independently controlled which only the DTC method has the ability to. Furthermore, each of the jacking systems should be provided with its own power supply. Best performance and stability was achieved when each chord of the leg is given a common torque and velocity setpoint, which is equivalent to a common torque and setpoint per leg in reality. Moreover, load sharing can be improved without a control system by increasing the relative stiffness ratio between the chord and the mechanical contact between rack and pinion. ... Read more

According to the European Environment Agency (2016, pp. 137–140) annual mean river flow and the frequency of fluvial floods will have increased by 20% before the year 2100, in North-western Europe. It had been postulated in media in reports (De Ingenieur, 2014; „MIRT-verkenning Grevelingen”, 2012; Slootjes et al., 2010; Slootjes, 2013; Lammers, 2014) that because of this, large pumping stations are required in the Rhine-Meuse delta in the Netherlands. To investigate this postulation, a simulation model in Python was created that describes the Rhine- Meuse delta as four separate water basins with flow exchanges and boundary conditions (astronomical tides and river inflow). From this simulation model it was concluded that every 86–137 years, flood flow rates of the rivers are such, that the design maximum water level is compromised. The acceptable flooding risk is only once every 2.000 years, so this situation is unacceptable. Dutch engineer answered to the postulation and invented a high-capacity pump called the „Deltapump”, with a capacity ranging 170–200 m3s-1. Moreover, a conceptual design for a pumping station was created. After a conceptual design creation, verification calculations and a cost-to-merit evaluation, a pumping station with 28 Deltapumps in total, based on the conceptual design of Schut, was created. This pumping station is integrated within the Haringvlietdam and is covers an area of 420 × 190 m2. Its capacity, dependent on water levels in the Haringvliet, ranges 4.900 to 5.250 m3s-1, making it by an extremely large margin, the biggest pumping station in the world. Its costs, expressed as Net Present Value, are estimated at € 915 million by the year 2100, 70% of which covers the mechanical components of the pumping station and 30% the civil components. After the flood risk analysis and the pumping station design, it was posed that, whilst the pumping station itself has advantages—better capacity per unit width and less costs per unit capacity, it is not a cost-effective method to prevent flooding in the Rhine-Meuse delta. Calculations and the simulation show it only requires operation once every 92 years. It would therefore seem more cost-effective, and a permanent solution, to upgrade all dykes and dams along the Rhine-Meuse delta, so that more water can be stored. This should be investigated in future reports. ... Read more