The ever-increasing size of offshore wind turbine substructures and the development of wind farms at sites further offshore, with greater water depths and with extremer weather conditions, raise logistical challenges that have never been faced before. Additionally, the offshore wind industry has to deal with governments cutting subsidies, small profit margins and limited practice guidelines, while it is expected to lower the associated levelised cost of energy to a competitive level in the market. Scientific studies have identified room for optimisation in the substructure (the focus is laid on Monopiles (MPs) with Transition Pieces (TPs) and pre-piled jackets) transportation and installation phases. However, no studies that evaluate the performance of strategies for these phases are identified. Hence, the objective of this study is to “generate insights into the complex system of interdependent strategies for the installation of offshore wind turbine substructures, and to identify and quantify cost-reduction opportunities.” The considered strategies are formed by combinations of transportation and installation strategies, which differentiate based on the number and type of the deployed vessels and the sequence in which the operations are performed.

To quantitatively compare the strategies, and to consider stochastic processes (e.g., weather conditions), a discrete-event simulation modelling approach is adopted. To arrive at substantiated conclusions, a framework is followed, which provides a roadmap and rigour criteria for the design, implementation and evaluation phases. First, a conceptual model is developed and face validated. Next, a numerical “base model” is constructed, which describes the most basic strategy. This model is face validated by industry experts and evaluated by parameter variability, convergence and historical data validation tests. It is concluded that the base model is structured according to shared practical experiences, responds satisfactory to parameter changes, requires 35 simulation runs to converge, and has good predictive capabilities. Hence, it is deemed suitable to function as a “template” for the modelling of the other strategies.

The simulation results are evaluated for each of the considered substructures separately. (i) MP – TP installation. In general, assembly-line installation strategies, in which two Heavy Lift Vessels (HLVs) are deployed, are associated with the shortest installation time. The shuttling – assembly-line and the shuttling–alternating (in which MPs and TPs are installed alternatingly) strategies are associated with the lowest costs. Both involve a shuttling transportation strategy, in which the HLV(s) ensure(s) both the transportation and installation of the components. The mooring of barges alongside an HLV in feeder strategies (feeder vessels supply components to an HLV, which stays at the wind farm under development) and the installation of TPs by a relatively small HLV in assembly-line strategies are identified as the main bottlenecks. Reducing these by relatively simple solutions can result in significant performance increases. Lastly, the project start date is found to be a strong determinant of strategy performance. (ii) Jacket – foundation pile installation. The assembly-line strategies are found to result in the shortest jacket installation times as well. However, only the shuttling – assembly-line strategy is additionally associated with the lowest costs. Furthermore, it is found that a separate pile-dredging vessel can help to reduce the time and costs associated with separate phases installation strategies, in which jackets and their foundation piles are installed in different phases. Also for jackets, the barge mooring alongside the HLV is identified to be the largest bottleneck. Reducing this bottleneck can result in significant performance benefits. Lastly, a relationship is found between the performance of jacket installation strategies and the project start date, although weaker than for MP installation.

The developed decision support tool can provide a platform for further research into the logistics of offshore wind and other industries, whereas the obtained results are only valid within the set boundaries. To widen the applicability, it is recommended to perform follow-up studies in which a stochastic mechanical failure component is included, and the sensitivity to the wind farm size and port-to-farm distance is tested. Furthermore, it is advised to extend this study to investigate the potential of the industry adopting a more holistic process or market point of view.

Adapting the size of upending cradles according to the ever-increasing dimensions of offshore wind turbine monopiles is costly and requires structural modifications to the associated Heavy Lift Vessels (HLVs). This study therefore investigates the potential of the Monopile Upending Smart Tool (MUST), which is a platform suspended in the HLV crane, on which a winch is installed. Two grommets of constant length connect the platform with two trunnions attached to the monopile, and the winch cable suspends the bottom of this structure. Unwinding the winch cable allows for in-crane upending of the monopile. This way, the dependency on the size of the available cradle is circumvented. The focus is laid on determining the workability of the application of the MUST in the crane of the Seaway Strashnov and identifying limitations in its design. Moreover, the potential of systems that increase the workability or reduce the limitations is investigated.

The installation of monopiles using the MUST comprises three phases: the barge mooring / lift-off, upending / slewing and lowering / driving phase. Based on the results from a qualitative critical event analysis, the lift-off phase is expected to be limiting. Hence, a hydrodynamic model is developed to quantitatively analyse this phase. Experimental simulations, which are performed in parallel with the model development, result in the preliminary conclusions that multiple pendulum effects influence the results to a limited extent, while viscous roll damping and hydrodynamic interaction effects can be strong determinants of the resulting responses.

The relative z-motion between the lifted monopile and the barge and the barge roll response are identified as governing parameters. To reduce the first limiting factor, a system that allows for instantaneously increasing the vertical clearance between the monopile and the barge is proposed. The effectiveness of this system is tested for two barge loadcases. For the first case, the average workability increase for the optimal heading at a typical location is calculated as 8.3%. For the second, the increase is marginal, as the barge responses are more limiting. Furthermore, is it found that a Passive Motion Compensator (PMC) can reduce the probability of the introduction of snap loads in the winch cable, and therefore allows for system optimisations. A PMC with 10% of critical damping can reduce the required winch capacity with a factor of 2.3 w.r.t. the uncompensated case.

It is recommended to perform follow-up studies into the system performance during upending / slewing and lowering / driving. Also, it is advised to evaluate the effect of a larger barge and a PMC on the workability. To balance the associated investments and workability increases, the logistical models developed in a parallel study can be used. Finally, for iterative calculations, it was found to be beneficial to make an estimate based on a fast simplified model and to subsequently feed the results back into a more detailed, but slower model.