Sea-fastening of Wind Turbine Generators for assembled tower Transportation and Installation

Abstract

Seaway Heavy Lifting is an offshore contractor, involved in the wind energy market. Nowadays SHL is preparing on expanding T&I capabilities to include WTGs, after the experiences with foundations, platforms and met-masts. The company desires to enter in this market with innovative and highly efficient solutions. In order to achieve this goal, Oleg Strashnov HLV, SHL’s flagship, has been selected as the ideal means for this path. Indeed, the potential of such a vessel is seen, by the offshore experts, as possibly being applied to the T&I of WTGs in single pieces, including the nacelle. Current practice is to perform the scope in parts, by means of jack-up vessels, addressing a great amount of the work offshore. Offering an integrated solution would definitely mark a turning point for the market, providing that this new proposal has the economical and efficiency characteristics required. The main challenge for such a proposal, from the structural point of view, is to provide a proper sea-fastening for safe and stable logistical procedures, ensuring structural integrity of both vessel and transported elements. The goal of this thesis is to propose a sea-fastening solution able to meet such requirements. An articulated and detailed path has been followed, from general to more detailed analyses, in order to reach the proposed objectives. Firstly, the definition of the boundary conditions and company’s choices took place, according to the specific needs of the chosen vessel and the solution effectiveness requirements. Current offshore wind energy market situation and trends have been investigated and analysed, in order to find specific WTG designs to use for the study. After the definition of sea state conditions, vessel accelerations have been quantified and the most critical load combination for the WTGs was found. Then, possibilities of sea-fastening arrangements have been analysed, through a conceptual study. In order to deal with the destructive bending moment effects on the WTGs, the concept providing a free moment connection at foundation level has been selected. It was required to provide additional external structures, clamping the WTGs at a certain height, in order to doubly support the transported elements. A parametric study has been carried out to compare the best achievable solution using such a hinged bottom connection, against the best achievable solution adopting a fixed connection. Higher requirements in stiffness, for the external supporting structures, have been found in the case of fixed connection. Among other advantages, important steel savings resulted and it was ensured that final solution did not exceed them. Detailed design has been provided for bottom support, due to its challenges from a structural point of view. Besides rotation, main requirements were vertical and horizontal loads constraint. After a dedicated conceptual analysis, a combination of rubber and steel elements was found to be most effective for the purpose. Linear structural analysis have been carried out, supported by parametric studies. The unusual utilization of rubber for sea-fastening has been analysed and proven to be feasible. Starting from the defined model, additional studies for linear buckling and free vibration analyses followed. Subsequently, focus moved to the intermediate support connection. Where design was less detailed, because of the presence of features more related to mechanical engineering, considerations and suggestions have been proposed. An innovative flexible clamping connection has been designed and proven to be effective; this involved rubber fenders, steel rings and bracings. Finally, the design was carried out for the bottom grillage system, connecting the above sea-fastening to the vessel deck. Practical considerations led to a conservative solution, able to meet the structural integrity requirements and available for further optimisations. Once all the individual parts were designed, adaptation of initial assumptions and solution final assessment were proposed. With its 8050 tonnes of total weight, such a system is proven to provide safe and efficient T&I activities for six WTGs per voyage. Among the selected models, the ALSTOM 6MW is chosen for the analysis, because of its most onerous configuration. Structural feasibility and integrity are assessed through hand calculations and FEAs. Boundary conditions are all met and effective features are finally pointed out. Because of the choice of taking advantage of flexibility and the unusual application of rubber elements, the final solution demonstrates clear innovative features. According to the initial conditions, such a sea-fastening system provides a solution for extremely critical sea-states, probably unlikely to appear in real operations.