Floating wind is a proven concept to harvest wind energy in deeper water. Most technology of floating wind is transferred from the oil and gas industry. However, a commercial wind farm can consist of 50 wind turbines, and an efficient transport method is needed. Wet-towing is an efficient transport method, but it introduces fluid-structure interaction and should be carefully investigated. For wet-towing operations, the expected drag load is of interest, so a vessel with the right capacity can be chosen. Vortex shedding can occur since most semi-submersible platforms consist of several cylinders. This can cause vortex-induced motion and cause undesired motions such as sway motion.

While previous studies have examined fluid-structure interaction of semi-submersible platforms, two key gaps remain. First, most are conducted at model scale, where the lower Reynolds number leads to different flow regimes and may not reflect full-scale behavior. Second, many vortex-induced motion studies use single-phase simulations with a double-body assumption, neglecting the free surface and its influence on wake dynamics. These gaps highlight the need for full-scale, multiphase simulations to capture realistic wet-towing hydrodynamics.

This study carries out full-scale multiphase simulations to investigate three key aspects. It examines the influence of towing configuration, specifically heading and draft, on the hydrodynamic performance of the platform. It evaluates the impact of the free surface on vortex-induced motion of semi-submersible platforms. Finally, it compares the accuracy and efficiency of drag prediction and vortex behavior across three modeling approaches: full-scale multiphase, full-scale single-phase, and model-scale multiphase simulations.

The results indicate that 180 degree heading is preferable to 0 degree, as it produces a more stable wake, lower drag, and avoids the constant offset lift forces observed in the 0 degree case. A shallower draft of 10 m is also favored, as both the free end and free surface effects help suppress coherent vortex shedding, reducing the risk of vortex-induced motion and lowering drag. Comparing models, full-scale single-phase simulations exhibit vortex shedding, while multiphase simulations develop a steady wake. This discrepancy arises from the unrealistic symmetry constraint at the free surface in single-phase cases. Finally, while model-scale simulations scaled by Froude’s method can offer a conservative estimate for drag, they lack the fidelity needed to capture accurate vortex behavior.

These findings highlight the importance of using appropriate CFD models when evaluating towing strategies, and provide a foundation for optimizing semi-submersible transport operations in future floating wind projects. ... Read more

Subsea rock installation is an offshore engineering process where rocks are placed on the seabed as protection of cables and pipelines and as scour protection. The inclined fallpipebis a new piece of equipment specifically designed to install rocks close to submerged structures. This thesis investigates the processes of the rock flow in and below the inclined pipe. This is done with a research question in two parts: What is the rock behavior in the inclined fall-pipe, and what is the behavior below the inclined fall-pipe? The research is questions are supported by sub-questions on the influence of pipe angle, production and rock size.

To answer the research questions model tests have been carried out at a scale of 1:15 in the Dredging Lab at TU Delft. The tests were performed with varying pipe angles, production rates and rock sizes. The tests have been analyzed with special focus on velocity, flow behavior, touch down offset from the pipe, and the spread of the rocks. For analysis, video recordings of the tests have been used and the tests have been analyzed with Particle Image Velocimetry software PIVlab.

The results of the tests reveal that the velocity of the rock flow mostly depends on the pipe angle and production rate, and for a lesser part on the rock size. Steep pipe angles increase rock velocity, increasing production leads to a higher average velocity, and smaller rock sizes increase the velocity.

The spread of the rocks and the offset from the pipe are influenced the strongest by the stand-off (SOD) distance between the pipe and the bed. In the tests the SOD was determined by the pipe angle. To compare the tests, they were also analyzed at the same height below the pipe. The results show that the spread of the rocks is only influenced by the height above the floor. The offset is influenced both by the angle and the production. The influence of the production is only visible at lower angles. The increase in production means an increase in velocity and the rocks falling further away. More horizontal pipe ... Read more

Due to urbanization, improved living standards and electrification, approximately five times more raw minerals are necessary in 2050 compared to 2018. In deep oceans, the seafloor contains these minerals in the form of polymetallic nodules. Nodules are about the size of golf balls that grow throughout the ocean at depths between 3500 m and 6000 m. They contain a wide variety of metals, such as manganese, copper, nickel, cobalt. Nowadays, for large-scale applications, hydraulic lifting is almost exclusively considered for vertical transportation through the water column. However, there is little research available about using other techniques instead. To tackle this knowledge gap, this thesis studies the feasibility of transporting the nodules using a concept of mechanical lifting. The concept used in this thesis consists of two alternating containers that are lowered and hoisted by lifting and guidance wires. Due to the conditions, such as the large depth, the environmental characteristics and the positioning and heading of the vehicles, there are technical uncertainties regarding mechanical lifting. Risks include the yaw rotation of the container, which might result in rope entanglement and wearing of the ropes. This thesis presents a study into the yawing stability of the concept of mechanical lifting for the vertical transportation of polymetallic nodules, which is a crucial factor to operate reliably.

The research question is answered by performing an experimental test and a CFD analysis. The experimental tests include the dynamics of the system while testing various configurations and is validated by an analytical integration in time and a CFD simulation at model scale. The CFD analysis takes away the uncertainties and unknowns: the drag force, the yawing moment and the fluctuation magnitudes and frequencies. The CFD analysis is performed using the open-source software OpenFOAM and simulates multiple configurations. The results of the simulations are compared to the restoring moment by the guidance wires, by transforming the excitation moments into static and dynamic responses of the system. The CFD model is validated by testing the model with a 2D cylinder and 3D sphere, and by performing a mesh convergence study. The CFD simulations are validated by literature. With the obtained drag forces, the energy consumption is calculated.

From the results, it can be concluded that the system can stably be transported at 2 m/s, as the static and dynamic responses are well within the safety limits. The largest response occurs in the middle of the water column, as the rotational stiffness is the smallest at that location. The dynamic response is smaller compared to the static response, as the high frequent fluctuations (f > 0.075 Hz) are damped. Rope entanglement will not occur during normal operation at 2 m/s. However, critical situations due to incidental events can arise, including a winch failure, friction or a sudden high current. This has not been evaluated in this research and therefore stability cannot be guaranteed. As lowering at 3 m/s with an inclined system and including the current results in a static maximum yawing rotation larger than the safety limit, the stability cannot be guaranteed for operating at 3 m/s. ... Read more

A Cutter Suction Dredger (CSD) is a vessel that is used to cut hard soils with precision. It is moored stiffly through a spud pile while the soil is being cut. This, in combination with wave loads, causes the whole system to be dynamically challenging and nonlinear. Previous attempts to model a CSD in operation have been limited to the frequency domain. However, in order to be able to include these nonlinear effects occurring during the operation of a CSD, a time domain analysis is necessary. The hydrodynamic forces in the time domain analysis will be calculated using Ansys AQWA. The external forces which occur during operation will be calculated by a custom script written in Python. The script consists out of 3 modules; a spud module, a winch module and a soil module. The spud module is responsible for calculating the mooring forces on the spud pile in order to maintain its position. The spud has been modeled as a beam with 3 pinned supports. This linear model has been expanded to include the option to take the presence of a flexible spud carriage into account. This means that if the buffer of the flexible carriage is activated, the stiffness of the system changes accordingly in order to reduce the loading on the spud with increasing displacements. The winch module is responsible for determining the force required to achieve the swing around the spud at the desired velocity. Within the module, the application of a PID controller ensures that the tension in the side wires is adjusted dynamically in order to maintain a stable swing velocity throughout the whole process. Tuning the controller correctly is critical to avoid unnecessary tension peaks while at the same having a sufficiently quick response to sudden changes. The soil module is responsible for calculating the reaction forces on the CSD as a consequence of the cutting of the soil. The soil is characterized by the specific energy characteristic, which enables the module to be applied to all types of soil, from soft clay to hard rock. The 3D force vector on the cutterhead is then calculated using the volume cut and the rotational torque of the cutterhead. Furthermore, the module keeps track of where the soil has been cut, adjusting the new height as the cutterhead passes. This enables to realistically create a time series where the volume cut during each timestep will vary due to the oscillations of the cutter. The final result is a model in 7 degrees of freedom which can dynamically respond to the nonlinear reactions caused by cutting soil, wave loads, mooring through a flexible carriage and varying side wire forces. This model is now ready to be used to investigate the effects of different combinations of soil types and wave conditions. ... Read more

After years of using fossil fuel, the transition to renewable energy sources need to be made to limit the increase in temperature and to support the future energy demand. To support and speed up the transition phase from fossil fuels to renewables it is necessary to decrease the costs. Offshore Wind Turbines are widely used for the production of renewable energy and several Offshore Wind Turbine projects are planned for the future. Most of the Offshore Wind Turbines are founded by monopile, large steel tube, to support the wind turbine. Nowadays, these monopile be installed by either jack-up vessel or moored floating vessel. However, these installation method come with a major drawback: the installation procedure is time consuming. A new installation method is propose to reduce the installation time. This thesis focus of the feasibility to install the monopile with a dynamically positioned (DP) vessel. The
required station keeping situation is faster achieved with a DP vessel. Due to the footprint of the DP vessel relative to an earth fixed position, a vessel motion compensated pile gripper is used to maintain the upright position of the monopile and to decrease the interaction forces between vessel and monopile. Adding the monopile to the vessel is an off-design condition for the DP controller. During the early hammering phase of the monopile, the monopile have limited interaction with the soil and is unstable. The upright position is maintain by the gripper frame. The forces from the gripper frame on the monopile are reaction forces on the vessel. Beside these forces, environmental forces are acting on the monopile and via the gripper frame acting on the vessel. The forces on the vessel could lead to unstable behavior and/or increased vessel footprint. A simulation model is build to investigate the behavior of the DP vessel during the operation. A industry used DP simulator and a simulation model of the Bokalift1 is used. A model of a typical shallow water and deep water monopile is build. A hydraulic based gripper frame is simulated with an inclination controller and a induced vessel motion controller which need
to maintain the upright position of the monopile. The inclination controller is tuned with a higher bandwidth compare to the bandwidth of the DP controller to prevent motions of the monopile is the same frequency range as the linear motions of the vessel. The forces from the gripper frame are fed into the Kalman filter of the DP controller. This is done to prevent a drift of the vessel when the gripper frame starts acting on the vessel. In all simulation cases with governing environmental conditions, the vessel could maintain stable behavior. The rotations of the monopile are in the same frequency as the first order wave forces on the vessel. This relative high frequency motions to not significantly amplify the position of the DP vessel. However, despite the fact of feeding the Kalman filter, a larger drift is observed in case of the large, deep water monopile in the operation stage when the gripper frame force is introduced to the vessel. This increase the requirement on the envelope of the gripper frame. The requirement on the gripper frame is given in terms of power, force and envelope based on governing environmental conditions. The requirements on the gripper frame are assumed to be within an acceptable magnitude. The operation seams to be promising in the future. ... Read more