• Nedzad Dedic
4. term , Design of Mechanical Systems, Master (Master Programme)
The constant improvement of offshore wind energy generation has resulted in more efficient, but also larger and heavier offshore wind turbines. Different foundations for these have been developed for different sea depths and one of the most popular is the monopile foundation, suitable for water depths up to 30 m. It in-volves steel tubing connected by means of high strength grout. This technology is known from offshore oil- and gas platforms, primarily used to carry vertical loads. However, the primary loads related to wind turbines are the wind and wave induced vibrations and consequently overturning moments. Design guidance for such connections is standardized by Det Norske Veritas, who have developed several parametric design formulae. Nevertheless, these solely comprise formulations for vertical load transfer, while horizontal load transfer is advised to be addressed numerically or/and experimentally. The overall objective of this work is to generalize the structural behaviour in the grouted connection sub-jected to a static horizontal load based on a specific connection used at Gunfleet Sands Offshore Wind Farm, DK. This is done by deriving an approximate analytical solution and verifying it by a corresponding numeri-cal solution. The analytical solution is based on derivation of governing field equations that describe the mechanical response of the grout. This is done by considering the connection as a 3D boundary value prob-lem and approximating the displacement boundary conditions, whereby the problem is reduced to a dis-placement formulation. Although the static effects are of interest, it is chosen to derive this as an elastody-namic formulation in terms of the equations of motion. This leads to a coupled formulation of the problem, which is resolved by introducing an uncoupled expression of the displacement field, based on potential func-tions. Substitution of this field leads to a problem given by two wave equations. The solution to these is con-ducted by first considering a 2D formulation and gradually advancing into 3D. The approximate boundary conditions are finally applied, thus solving for the displacement field and thereby the entire boundary value problem. Subsequently, the obtained solution is compared to a corresponding Finite Element Analysis (FEA), carried out in FE computer program ANSYS. Furthermore, the assumptions made in the two analyses are compared to results from a similar but more advanced FEA, carried out by an external source. The outcome of the analytical analysis is a parametric computer program which approximately yields dis-placements, strains and stresses at any point of the geometry. The program results are verified by the corre-sponding FEA with a reasonable agreement. The errors involved are not entirely negligible at all points in the structure, but very reasonable considering the approximated boundary conditions. Additionally, it is dem-onstrated that the elastodynamic formulation can be applied to elastostatic problems, provided that the ap-plied dynamic disturbance is sufficiently low. Hereby, the statement that considering elastostatics as a limit of elastodynamics is a complete nonsense [Olsson, 1984] has been disconfirmed.
Publication date2009
Number of pages77
Publishing institutionAalborg Universitet, Institut for Maskinteknik
ID: 17644486