This thesis focuses on modelling a gravitational launch of gravity based foundations (GBF's) for offshore wind turbines. This is an extension of the Blyth Offshore Demonstrator project by BAM Infraconsult. To become more competitive in the offshore wind energy market, the construction of self-buoyant GBF's installed using the `float and submerge' technique needs to be optimized. GBF's will need to be constructed on land, which calls for a way to `launch' the GBF's. Another study suggests the use of semi-submersibles or immersion structures, which are very expensive. Due to the GBF's high strength and stability, it might be suitable for a gravitational launch. Although these launching methods have been widely applied on steel ships and jackets, the application for reinforced concrete structures is rare, especially using slipways with an abrupt ending. A conceptual design is needed to test the technical and economic feasibility of this launching technique. For the purpose of making a conceptual design, the launching process must be modelled. The goal of this thesis is to develop models that can be used in a conceptual design phase to easily determine the optimal shape and dimension of the slipway used to gravitationally launch GBF's. This thesis contains three modelling methods: (simple) mathematical models, physical scale model tests and a Computational Fluid Dynamics (CFD) model in ANSYS Fluent. Simple mathematical models were derived to model each phase of the launching process. Translational movements when the GBF is in full contact with the slipway were described using a simple force balance, both for a dry and a partially submerged slipway. For the kinematics as it tips over the slipway edge, equations of motion were derived and solved. The main limitation of this set of equations of motion is that hydrodynamic forces are not included. For the kinematics of the freely floating structure, equations of motions were solved and hydrodynamic components were determined analytically from literature and numerically using ANSYS Aqwa. Physical scale model tests were performed to (1) validate and calibrate the mathematical models, (2) determine the most favourable slipway geometry, and (3) develop more insight into the GBF behaviour during a gravitational launch. The tests were conducted at a 1:100 scale. For calibration of the hydrodynamic components, free decay tests were performed. In the abruptly ending slipway launch tests, slipway inclination (9°, 14° and 21°), freeboard height (positive, zero and negative), and initial GBF velocity (high and low) were taken as variables to investigate their effect on the severity of GBF dynamics. Severity of the dynamics was parameterized by the maximum GBF rotation measured around the horizontal axis during the launch (pitch). A large GBF rotation means large motion amplitudes thus violent movements, which are undesirable.
Comparisons to mathematical models show good agreement in most cases after a calibration of the hydrodynamic components. The full launch procedure was not modelled accurately by the mathematical models. An empirical formula was developed relating the slipway variables to maximum GBF rotation, which provided more accurate results for a larger range of variables. A lower freeboard and a steep slipway inclination was always favourable. No such trend was observed for the initial velocity. Most favourable slipway geometry tested had a slipway inclination of 21° and a slipway ending below the water surface (negative freeboard), resulting in a maximum GBF rotation of 29° around the horizontal axis.
A CFD modelling strategy was proposed and used to compare to the physical model tests. The CFD model was set-up in 3D using a dynamic mesh and a three degree of freedom solver to compute all acting forces on the moving body. Despite a coarse mesh with insufficient quality, the solution converged and showed numerical stability, also for large motion amplitudes. Depending on the scale, the CFD model showed good agreement to the physical scale model tests. The CFD model has a high potential in terms of the range of initial conditions, flexibility in structure shape and dimension, and amount of output data.
The simple mathematical models (force balance, equations of motion and an empirical formula) are sufficiently accurate to analyse key differences between slipway alternatives to make choices in the conceptual design phase, after a calibration using physical scale model tests. Due to a long computational time and time consuming improvements, the CFD model is more suitable in a more detailed design phase, where it could be very valuable.
A first estimate of most important launch requirements was made using the mathematical models for abruptly ending slipways and showed technical feasibility. More research should mainly focus on improving the CFD modelling strategy and on making a conceptual design for a specific location to further investigate economic and technical feasibility.