Climate change is triggering an ever-growing demand for renewable energy. The U.S. is still far behind Europe when it comes to offshore wind energy. They have made ambitious plans to reach 30 GW in offshore wind energy by 2030 while currently 42 MW is installed. One of the main challenges in the U.S. is the installation method since a legislation called the Jones act prevents the usage of European installation vessels for shuttling (the conventional method). Building a Jones act. compliant installation vessel is a large investment which comes with risks and long lead times. Feedering is an alternative strategy, but barely any research on it is available. Here, a feeder vessel sails back and forth from the storage port to the (non-Jones act. compliant) installation vessel to supply Wind Turbine Generator (WTG) components. These components need to be lifted from the floating feeder in order to be installed. In the literature, this step is deemed to be the riskiest. However, barely any technical research is available with regards to the lift-off.

In the first thesis of this double degree program, a lift/installation sequence called the direct installation method is deemed to be highly interesting with respect to the logistics and costs. However, this research misses a technical study in order to understand if it is technically reachable to directly install these components. In this offshore engineering thesis, a barge is used as a feeder vessel and tower segments of a 20 MW WTG are chosen as the to-be lifted components. This research focuses on the pre-tension phase before the lift-off. This contains the steps where the crane of the installation vessel is already attached to the tower, pre-tension is building up and the release of the sea-fastening. Here, pre-tension is a percentage of the load that is taken in the crane before the lift-off. This research aims to increase the understanding of whether a tower can be released safely on the floating barge and what can be done in order to realise the idea of a direct installation method.

Frequency, as well as time-domain simulations, are used to investigate the problem. The results show that snap loads occur for pre-tensions up to 10\%. From 30\% and higher, the tower will start toppling. Toppling is initiated due to the inertia of the large tower segment when it is released from its sea-fastening. Toppling the tower is not allowed since this could damage the tower itself, the sea-fastening and/or other components on deck of the feeder. Increasing the limiting wave height is a must in order to make the direct installation method more practicable. This can firstly be done by using more tower segments. Therefore, reducing the size of each segment. Another option is to implement a motion compensation tool that decouples the motions of the feeder and the tower. The third option is to design a seafastening system that reduces the moment after the release, a temporary counteracting toppling system. All in all, can be stated that safely releasing a 20 MW tower segment on a floating barge is highly challenging and more research is required to solve the issues that are found in this research. This is necessary to allow the direct feeder method to be used for future offshore wind installation projects in the U.S.

Climate change is triggering among others a larger demand for offshore wind energy. This leads to new developments of which larger next-generation Wind Turbine Generators is the most relevant. These next-generation WTGs create problems for the carrying capacity of current-day installation jack-up vessels that work according to the conventional installation method (shuttling). These installation vessels can carry less or even nil of these WTGs per trip compared to the current-day WTGs. Fewer turbines per trip would allow the larger current-day installation vessels to maintain their work. However, they have more sailing time since more trips are required. This could also be more inefficient since the Offshore Wind Farms are moving further offshore. The other option within the conventional method is to build larger installation vessels that would carry more WTGs per trip.

Another installation method in the literature called feedering is a potential substitute for the conventional method. Very limited research has been done for feedering as well the practical implementation whereas the conventional method has extensive research and is mostly used in practice. The models in the literature compared two different feeder methods, called, the base port feeder and feeder-ship method, to the conventional method. The models are generic and lack different strategies to find the best feeder solution. The feeder-ship method has the most potential to solve the aforementioned problems and is therefore further investigated to find the best installation strategy, being either the conventional or feeder method.

First of all, the feeder-ship method is evaluated based on practical knowledge and adapted so a base port is used to temporarily store all components that come from the production port. The feeder sails back and forth between the base port and the installation site to supply the installation vessel with WTG components. The installation vessel will stay at the installation site to be as efficient as possible (the least amount of sailing time). This research only looks at the feeder and installation side and leaves the port elements out of the problem. Feeder strategies (within this method) differ in the number of feeder vessels/types and transfer options using either barges or Platform Support Vessels and being indirect (transfer components onto the installation vessel) or direct (install components from the feeder). The installation vessel for feedering is also looked into as being either a current-day or a special feeder purpose installation vessel. Different carrying capacities of the installation vessels are used to create different strategies for the conventional method.

A stochastic Discrete Event Simulation model is created and used in this research to evaluate the strategies. The output of the DES simulation is the project duration per sailing distance and strategy. The costs for this duration is calculated in a costs model from the perspective of the contractor as well as the developer. Besides duration and costs (based on the European market), the strategies are also evaluated on emissions (fuel consumption) since this is an increasingly important element in the industry. A sensitivity analysis has been performed to get a better understanding of which critical parameters the feeder strategies are most sensitive to. The analysis is also used to find the best next-generation WTG installation strategy in Europe which is the main focus of this thesis.