This thesis focuses on the going-on-location (GoL) operation of jack-up vessels in the offshore wind energy industry. The GoL process involves the transition of a jack-up vessel from a free-floating state to a state where it’s elevated above the waterline and pinned to the seabed. The workability of these vessels is significantly affected by the impact loads experienced during the GoL process, particularly when interacting with stiff seabeds and non-negligible sea states. In current methodologies, certain parameters like wave height and wave period set the limitations to when GoL process can proceed. For safety reasons, the GoL process is halted if these conditions are exceeded. Instead of solely depending on external factors like wave height an wave period, the focus transitions towards how the vessel dynamics relate to the impact forces it encounters during the GoL. To address this limitation, a comprehensive framework has been developed that combines hydrodynamic, structural, and soil-spudcan interaction elements to evaluate impact forces during the GoL process. The framework offers a flexible and case-specific configuration. It allows for easy modifications, integration, and
replacement of components and input parameters. This case-specific arrangement is advantageous due to the wide range of jack-up vessels and environmental variations. In model implementations adhering to the framework, the jack-up vessel is represented as a multi-body structure, in contrast to the conventional rigid-body representation often employed. Within the multibody approach the spudcans, the legs, and the vessel are described by separate bodies each with its own properties. The primary focus of this research is on the dynamic soil-spudcan interaction process, which has not been extensively covered in existing standards. The soil-spudcan interaction model is to determine the instantaneous force acting on the spudcan as it contacts the seabed during GoL. By integrating elasto-plastic soil behavior into the
soil-spudcan interaction element, the model encompasses descriptions of soil resistance to spudcan penetration and lateral displacement, taking into account memory and potentially stateful characteristics. A simulation model, adhering to the framework, has been developed, integrating hydromechanical, structural,
and soil-spudcan interaction submodels within the Orcaflex environment. Three distinct simulation scenarios are examined: an undisturbed vessel (free-floating), a disturbed vessel (full GoL), and a pinned vessel (elevated
jack-up). The disturbed vessel scenario, which includes a full GoL process, has exhibited consistency in both undisturbed vessel simulations, where the vessel is the free-floating stage, and in pinned vessel simulations,
where the vessel is in the pinned stage. The impact phase is situated between these two boundary cases, and the framework effectively represents simulation models within its scope. In addition, simulations with varying sea states are performed for regular and irregular sea states. Simulations involving varying regular wave patterns suggest that the maximum downward spudcan velocity (DSV) is a critical parameter influencing the magnitude of impact forces on the spudcans. For irregular waves, the simulations indicate that the maximum impact forces are more closely related to the pinned vessel scenario, as
the maximum impact occurs towards the end of the impact phase.
In conclusion, this thesis has effectively described the behavior of jack-up vessels during the impact phase of the GoL process. For any model utilizing the framework, the GoL process can be simulated, and the results
can be analyzed to assess workability. Furthermore, the study proposes a potential correlation between vessel dynamics and maximal impact forces, a relationship that could potentially guide on-board decision-making
processes. The enhanced understanding of the interaction between the spudcan and the seabed, along with the comprehensive framework, contributes to improving the decision-making process for executing the GoL
operation of jack-up vessels in the offshore wind energy industry. ... 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

Offshore contractors use special cable laying vessels (CLV’s) in order to install subsea power cables, which transport offshore generated energy. Workability studies are performed to evaluate whether operations can be performed or not. These workability studies are subsequently a combination between operability assessments (performed in OrcaFlex months prior to operation) and forecasted weather windows. During the operability assessment, eventually limiting sea states are defined which are expressed in significant wave height (Hs), wave peak period (Tp) and incident wave angle relative to the vessel heading (a). If the forecasted weather windows exceed the predefined limiting sea states, operations cannot be executed. These current workability assessments are based on 1D JONSWAP spectra. This thesis contains an exploratory study of implementing a motion-based forecasting (MBF) method into workability studies, based on 2D wave spectra. This is done by focusing on the motions at the chute of the CLV, as this is where the subsea power cable leaves the CLV. Before MBF can be implemented, however, at first a specific limiting motion predictor needs to be identified that covers the cables integrity limits. Furthermore, this limiting motion needs to be quantified as well in order to compare this essentially composed new cable limit with MBF predicted chute motions. Subsequently, all cable integrity criteria need to be converted into this limiting motion to preserve the subsea power cables integrity. For normal cable lay operations, eventually all mechanical properties and cable limits are covered by three main cable integrity criteria. These are minimum bending radius (MBR), minimum bottom tension (BT) and maximum top tension (TT). By means of modelling a lot of various environmental loading conditions, the research described in this thesis shows that the chute z velocity is eventually indicated as a proper limiting motion predictor. Out of all investigated chute motions, chute z velocity (which represents the vertical velocity of the chute) shows the best correlations with the aforementioned cable integrity criteria. To determine workability by implementing motion based forecasting, this limiting chute z velocity needs to be quantified. The chute z velocity needs to account for all three main cable integrity limits, hence these limits are essentially converted into one single chute z velocity limit (which is essentially a new introduced cable limit). To quantify this limiting chute z velocity not the entire relation is of interest, but only the range close to breaching the cable integrity limits. Therefore, a percentile line method is introduced which is constructed on the 90th-percentile values per bin. Based on this research, it was found that binwidths must not exceed values of 0.5m/s to obtain robust results. However, no minimum amount of data points per bin was established as this varied for all assessed subsea power cables. Finally, workability studies of the current 1D JONSWAP (based on a single Hs −Tp sea state) method was compared to a 2D superimposed JONSWAP accounting for both swell and wind wave contributions. This comparison indicates that when determining workability based on more detailed 2D spectra, the workability study provides more smoothed results. This means, the higher chute z velocity peaks from the 1D total sea state are downscaled when simulating a 2D spectrum, whereas the lower chute z velocity peaks from the 1D total sea state are upscaled when implementing a 2D spectrum. The obtained motion-based chute z velocity forecasts indicate that optimization is possible by splitting the total sea states into both swell and sea waves contributions and accounting for wave spreading. ... Read more