http://hdl.handle.net/2268.2/10910 Thu, 05 Mar 2026 22:09:46 GMT 2026-03-05T22:09:46Z http://hdl.handle.net/2268.2/25050 Title: Investigations on inner Structures of the Propeller Blade referring Additive Manufacturing Abstract: Propeller blades in maritime propulsion systems must withstand complex hydrodynamic loads, torsional stresses, cavitation, and varying static and dynamic pressures, while delivering high thrust, torque, and efficiency. Traditional manufacturing methods (casting and milling) limit designers to solid geometries, precluding the use of advanced hollow or lattice internal structures that reduce weight and material usage. These limitations are overcome by additive manufacturing (AM), especially Wire Arc Additive Manufacturing (WAAM), which makes it possible to produce complex, lightweight designs with integrated internal features in a near- net-shape. In this work, unique internal configurations, derived via topology optimization and biomimetic patterns (honeycomb, gyroid, and bone‑inspired lattices), are investigated for propeller blades operating in diverse environments, from open water to ice‑prone seas. Emphasis is placed on selecting suitable AM materials (e.g., high‑nickel aluminum bronze) and on developing robust WAAM‑milling hybrid processes to ensure consistent bead geometry, material integrity, and compliance with DNV certification guidelines. Finite element analyses validate that optimized internal structures, in which different configurations of internal structures achieve favorable stiffness‑to‑mass ratios while respecting fatigue and strength criteria. This research outlines both design methodologies and manufacturing parameters necessary to realize next‑generation, additively manufactured marine propellers. Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2268.2/25050 2025-09-01T00:00:00Z http://hdl.handle.net/2268.2/25049 Title: Requirements Evaluation for the Implementation of Offshore Electrolysers in Green Hydrogen Production: An Approach for the Marine Industry Abstract: The marine industry is increasingly evaluating offshore green hydrogen production as a pathway to decarbonize key energy-intensive sectors. However, offshore electrolysers face unique technical, economic, and operational challenges that require adaptation to withstand harsh marine conditions. This thesis evaluates the requirements, feasibility, and techno-economic performance of integrating offshore electrolysis and wind resources. It combines a comprehensive state-of-the-art of electrolyser technologies and floating offshore foundations with a European case study and a techno-economic framework. The state-of-the-art identifies key strengths and limitations of the main types of electrolysis technologies, as well as the current market trends and gaps of floating platforms concepts, connection schemes, demonstration projects, and regulatory frameworks. It highlights that PEM offers the greatest offshore readiness due to its compactness, pressurized hydrogen output, low minimum load, and fast dynamic response despite its reliance on precious metals. The case study applies a multi-criteria site-selection which identified Viana do Castelo as the optimal Atlantic location for a 500 MW offshore hydrogen project. It is based according to market demand, infrastructure and resource availability for the 3 configuration schemes considered. Experimental testing and hydrodynamic modelling of a semi-submersible platform with PEM modules confirm an acceptable performance under marine motions with mitigation and operational strategies, including ramping controls, buffering, marine-grade protection, and integrated water treatment. Finally, a techno-economic evaluation estimates LCOH of 4.00 €/kg for centralized onshore, 4.57 €/kg for centralized offshore, and 4.82 €/kg for decentralized offshore electrolysis at the case study site. A sensitivity analysis shows that wind availability appears as the dominant cost driver. Export distances over ~120 km favor HVDC or hydrogen pipelines due to electrical losses in HVAC, while port distance increase O&M significantly until SOVs become viable at ~150 km. Water depth does not influence results significantly for the deep-water range considered. Centralized onshore electrolysis remains the most cost-effective for near-shore sites with strong wind and proximity to shore, while offshore configurations gain advantage when long export distances or space limitations occur. Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2268.2/25049 2025-09-01T00:00:00Z http://hdl.handle.net/2268.2/25048 Title: Analysis of Quadratic Transfer Functions in Operability Studies for Heavy Lift Operations Abstract: Current industry standards for vessel motion analysis commonly considers only 1st order diffraction analysis. This assumes that external excitation only occurs at the incident wave frequencies and thus natural modes outside of the wave frequency range would not be excited. However, in some cases, research have shown that 2nd order effects could still induce resonances despite being outside of the wave frequency range. The lifting of a monopile by an offshore heavy-lift vessel (HLV) could be one such case where, due to an upwards shift of the vertical center of gravity (VCG) as the monopile is lifted, the natural frequency of roll falls below and outside of the incident wave frequency range. This thesis presents a calculation and analysis of the effects of including quadratic transfer functions (QTFs) in a heavy-lift analysis. In the beginning of the thesis, a comparison of QTFs is performed between two different software NEMOH and Orcawave for the simple geometry of a floating cylinder. This is done to validate the calculation results of Orcawave with NEMOH, whose calculation results have been verified in some publications. Then, mesh studies are performed on the vessel lid, vessel hull, and free surface to study their effects and convergence. The first part of the main work of the thesis covers radiation-diffraction analyses performed on a heavy-lift vessel using Orcawave to obtain, analyse, and compare the 1st and 2nd order transfer functions of two loading conditions. An attempt to compare 1st and 2nd order loads for a given sea state in presented, as well as a rough attempt to compute and compare the 1st and 2nd order significant roll motions in a given range of incoming wave periods. The second part of the main work of the thesis uses Orcaflex to perform time-domain simulations of the heavy lift. This allows modelling of flexible and multi-bodies and solves the complete equation of motion including both 1st and 2nd order excitations naturally. Separate simulations omitting and including QTFs are performed. The resulting motion response is analysed in both time and frequency domain to obtain the most probable maximum (MPM) roll motions and identify excitation of the natural modes. Two operability studies omitting and including QTFs are then carried out to obtain limiting wave heights by comparing the MPM roll motions against allowable limits. The results are analysed and compared to observe the effect of QTFs on operability of the heavy-lift. Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2268.2/25048 2025-09-01T00:00:00Z http://hdl.handle.net/2268.2/25047 Title: Calibration and validation of analytical heat source models for hybrid laser beam welding and GMAW applications in naval structures Abstract: Welding technology plays a crucial role in the construction of ships and offshore structures, where the selection of an appropriate welding process directly affects production efficiency, structural reliability, and overall lifecycle costs. Gas metal arc welding is widely applied in shipyards due to its high flexibility; however, it involves a comparatively large heat input, leading to extensive heat-affected zones, distortion, and residual stresses that increase rework and maintenance costs. In contrast, laser beam welding offers high welding speeds and reduced heat input, but its industrial application is limited by the requirement for very small joint gaps and tight tolerances. Hybrid laser-arc welding (HLAW) combines the advantages of both processes, overcoming their individual drawbacks and thus providing an efficient solution for the production of such structures. However, despite the progress made in recent decades, there is still no analytical model available that is capable of realistically describing the welding process in all its complexity. In order to better capture and understand the influence of temperature distributions arising from HLAW, an analytical model is indispensable. Such a model would make it possible to investigate and predict characteristic welding features, most notably the fusion zone and the surrounding heat-affected zone. Beyond describing the weld geometry itself, the development of a reliable analytical model would also provide a valuable tool for analyzing thermally induced distortions and residual stresses, which strongly influence component quality in practice. Compared to numerical approaches, analytical models offer the decisive advantage of significantly reduced computational costs, which enables fast predictions and preliminary assessments. In this work, an analytical model for the prediction of welding-induced temperature fields in finite-thickness plates under convective boundary conditions is extended by introducing a second heat source to approach HLAW applications. Both the arc and laser contributions are represented by Goldak’s double-ellipsoidal heat source model. To reproduce keyhole mode welding characteristics, a vertical offset of the laser heat source is introduced. The heat source parameters are calibrated against experimental fusion zone geometries using a dedicated optimization framework. Finally, the results are validated through finite element analysis, demonstrating good agreement of the analytical model for describing HLAW conditions. Mon, 01 Sep 2025 00:00:00 GMT http://hdl.handle.net/2268.2/25047 2025-09-01T00:00:00Z