Connector Response of a Grid Pattern Multibody Very Large Floating Structure Subject to Wave Loading

Master thesis (2022)

Authors

H.P. van der Werf Mechanical Engineering

Contributors

A. Grammatikopoulos Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering (supervisor 1)
C.L. Walters Ship Hydromechanics and Structures - Mechanical, Maritime and Materials Engineering (supervisor 2)
Martijn Hoogeland TNO (supervisor 1)
Noud Werter TNO (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2022 Haico van der Werf
Fluid-structure interaction Multibody dynamics Offshore energy VLFS Offshore Floating Solar
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Published Date

23-09-2022

Language

English

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Faculty

Mechanical Engineering

Abstract

The energy transition requires us to explore all options for generating non-fossil energy. Companies are starting to invest in technologies such as offshore floating PV systems (OFPV) to avoid congested urban population centres. OFPV structures are likely to consist of many small, simple, flexibly connected floaters. The entire structure must be able to survive extreme offshore conditions. The OFPV response in various sea states is heavily influenced by the connector design.
In this thesis, a 3D boundary element based numerical model is used which was developed by Tuitman [2]. The model is expanded to output the forces and moments experienced by the compliant connectors which have linear stiffness in 6 degrees of freedom. After successful verification and validation, three case studies are presented which are a three floater serially connected model and a 3x3 and 4x4 grid connected model. Various sea states and wave headings are analysed to show the effect on dynamic behaviour of a compliant connector. The time domain-based approach is used to capture nonlinear Froude-Krylov and hydrostatic forces. The remaining hydrodynamic terms are linearised by solving in the frequency domain. The connector response is linearised by using a finite stiffness matrix but the forces are solved in the time domain to include the effects from the nonlinear hydrostatic terms.
The results show that the resonant response of the structure and connectors is critical in determining the floater motions and loads in the connectors. Additionally, the stiffness of the connector influences the natural frequencies of the structure. The forces and moments in the connectors of the grid are much more varied than the serially connected structure because of the complex interaction of the floater hydrodynamics and connector resonance. The floater motions and connector response becomes less evenly distributed for oblique seas and when sea states match the natural frequency of the structure. The grid experiences bending in multiple directions which results in large connector loads at some locations on the structure. When expanding to a larger 4x4 grid there is a different distribution of connector loading than the 3x3 grid because of the extra degrees of freedom. The aft connectors experience lower loads than connectors facing the waves due to a shielding effect.