The increasing deployment of floating offshore wind turbines has driven the development of novel installation concepts, for which the dynamic behaviour during critical load-transfer and release operations must be well understood to ensure safe and reliable offshore execution. This study examines the safe release of a fully pre-tensioned Tension-Leg Platform (TLP) with an installed wind turbine from a submerged installation deck (ID) within Allseas Engineering’s Windchanger installation concept. In this approach, a Heavy Transport Vessel (HTV) transports the assembled TLP wind turbine offshore, submerges the ID, tensions the tendons, and eventually releases the TLP from the deck. The release is a critical operation as hydrodynamic forcing and the mechanical interactions within the coupled HTV–ID–TLP system can generate abrupt changes in motion and load if not properly controlled.

This release sequence in this thesis is divided into three phases. In the pre-release phase, the ID remains coupled to the HTV, and different stiffness–damping configurations are tested to assess how possible passive softening can reduce the vessel motion transmitted to the TLP. At the release, the physical connection between the ID and the TLP is removed, causing the remaining support to shift to the tendon system. This moment produces a transient response that is strongly dependent on the wave phase at the instant of release. In the post-release phase, the ID is lowered to establish a positive clearance gap that prevents the deck from re-contacting the TLP under continued vessel motion.

To investigate these dynamics, a hybrid modelling approach is used. First, a linear 9-degree-of-freedom model is developed using the Euler–Lagrange formulation, capturing heave, pitch, and roll motions of the three bodies. It includes tendon stiffness, hydrostatic restoring, radiation damping, and an adjustable HTV–ID interface. This model is used to study modal properties, frequency-response behaviour, and sensitivity to interface softening. Second, a nonlinear time-domain model is implemented in OrcaFlex to simulate realistic hydrodynamic loading, tendon behaviour, and the complete release sequence under both regular and irregular waves.

The results demonstrate that applying softening reduces the transfer of HTV motion to the TLP in the pre-release phase, particularly in heave motion, and can therefore improve operability. The linear 9-DOF model identifies the dominant coupled heave–pitch and roll modes of the three-body system and demonstrates how softening influences its natural period and amplitude, explaining the trends observed during the release. The frequency-response analysis further shows that motion transfer at the release-relevant frequencies is strongly affected by the HTV–ID interface settings. In regular waves, the release response depends strongly on the wave phase. In irregular-wave simulations for moderate sea states, the tendon loads remain smooth at the release instant with no snap effects, and the turbine top shows only a brief acceleration peak immediately after release due to the sudden change in support. These responses remain within acceptable limits, and the results show that tendon loading is driven mainly by the pre-release motions rather than by the release itself. After the release, lowering the installation deck provides the required clearance, with beam-sea conditions being the most critical due to larger roll motions.

Overall, the results show that submerged release within the Windchanger concept is technically feasible in mild sea states, provided that the release moment is selected at a favourable timing and vessel motions are effectively controlled. Interface softening is not strictly necessary, but it improves operability by reducing pre-release motion transmission. The hybrid model approach forms a basis for defining operational limits and supporting procedural planning. Future work should refine hydrodynamic modelling, explore the practical implementation of softening methods, and develop operational guidance for offshore execution. ... Read more

This study investigates the development of sustainable offshore fish farms in Sisal, Yucatán. Local fishermen face seasonal restrictions on fishing due to environmental regulations. Given the socioeconomic dependence of the region on fishing, the community of Sisal has been experiencing increasing instability of livelihoods. Offshore fish farming has emerged as a potential solution to this challenge, offering an alternative income source outside the traditional fishing season. However, previous industrial attempts to introduce fish cages failed due to a lack of local engagement and inadequate design, leaving Sisal residents sceptical. To address these past issues, this research seeks to design affordable, durable, and locally accepted fish cages that meet the unique environmental and social conditions of Sisal. Valuable insights were gained from fish farms in Celestún, a nearby village with successful communityled offshore aquaculture. Celestún’s approach, using smaller, manageable, and collectively funded cage, has proven to be both economically and socially beneficial. This makes it a relevant model for Sisal, though Sisal’s steeper coastal gradients and greater exposure to maritime forces require adaptations to ensure durability and long-term success. The research follows a multi-step methodology, beginning with interviews with local fishermen and experts to understand their needs and preferences for cage design and placement. These insights were integrated with environmental data on wave height, wave period, and current speeds collected through field measurements and the ERA5 reanalysis dataset. Using this input, an Multi-Criteria-Analysis (MCA) was conducted to determine the optimal offshore location for the fish farms. To determine the structural needs for fish cages under Sisal’s conditions, the research used ProteusDS simulation software [14] to model various cage dimensions, mooring tensions, and layout configurations. Key findings indicate that positioning the fish farms at 8 kilometres offshore is optimal for long term success. At closer distances to the coast, water quality decreases, resulting in higher maintenance requirements and compromised fish health. Greater distances increase installation costs and operational costs due to higher fuel demands. With the optimal location established, the research follows with the determination of key design parameters essential for the structural integrity of the fish cages near Sisal. An extreme value analysis of an ERA5 dataset was performed to estimate the 20-year return level for the wave height, resulting in a design wave height of 4.19 metres. This value was adjusted for local conditions using a scaling factor derived from the comparison between local and ERA5 data, resulting in an adjusted design wave height of 3.40 metres. A power-law regression was then applied to establish the relationship between wave height and wave period, estimating a design wave period of approximately 8.01 seconds corresponding to the adjusted wave height. For the current analysis, the 95th percentile of current speeds was examined, determining a maximum design current speed of 0.50 m/s near the surface. Furthermore, analysis of wave and current directions revealed that extreme waves predominantly come from the north to north-east directions (340° to 20°), while the strongest currents flow toward 70° and 250°, indicating eastward and westward flows. The optimal cage design determined through simulations includes a cage diameter of 12 metres and a net depth of 4.7 metres to withstand Sisal’s environmental forces. Additionally, distinct mooring-tension configurations were tested in the ProteusDS software, including Concept 1 (a single-cage setup), Concept 2 (a two-cage configuration with four mooring anchors), and Concept 3 (a three-cage arrangement with three anchors). Each concept required specific anchor weights and dimensions to endure the high wave and current forces at this location. Orientation adjustments were also incorporated to reduce tension, aligning each cage setup with different wave and current directions, thereby optimizing structural reliability. Future fish cage designs should include adaptive anchoring and precise orientation to enhance stability and involve the local community for sustainable, long-term success. The study concludes that, to achieve long-term viability, fish cages in Sisal must be affordable, easy to maintain, and capable of withstanding local environmental conditions. Future recommendations include deepening community involvement, implementing enhanced safety and resilience measures, and refining cost analysis to foster broad acceptance among local fishermen. By ensuring that the fish cages are both economically viable and environmentally sustainable, this project aims to secure a stable income for Sisal’s fishing community, thereby improving their quality of life while reducing pressure on marine ecosystems ... Read more