Floating Jacket Installation

Abstract

Contrary to a one-off jacket installation where a contractor can wait for favourable weather conditions, the installation of support structures for offshore wind turbines is of repetitive nature. This repetitive nature leads to a need for a better understanding of the behaviour during installation of various jacket geometries. This need brings the research objective of this thesis to "Development a parametrized motion prediction model for jacket installation.". A 2D numerical model of a jacket type support structure installation in the side-lead direction has been developed in Python with two ways of calculating the forcing. Both of these methods rely on the Morison equations. The first calculation method uses bins along the length of all jacket members. Current, waves and the jacket's motions result into a flow perpendicular to the member center lines. This flow is used to determine the drag and inertia loads exerted by the water. The second method uses an equivalent stick model. This is a cylinder with a varying diameter over its height, representing the jacket structure. Strictly horizontal water particle velocities and accelerations due to current, waves and structural movements are used. This, in combination with the reduced number of bins leads to a reduction in computational cost. To find the characteristics of the system a sensitivity study is performed. Based on environmental and operational measurements taken from five jacket installations, the input for simulations were set in the goal of reproducing the measured response. The environmental and vessel measurements did not satisfy the level of accuracy needed, leading to a mismatch in results. In most cases the jacket structure design for a project is a given, meaning that jacket weight and location of center of gravity are set. These properties have a big impact on the dynamic system, nevertheless, the rest of the system still gives opportunities for increasing the workable limits. Two effective examples of this is the weight of the jacket lifting tool and crane height. A stick model implementation of the hydrodynamic forcing gives results comparable to the member evaluation implementation. The natural frequencies from the FFT taken of the free vibrations match well. However, a small difference in damping is observed. The equivalent stick model provides good results for quick estimations at low computational cost. The response of three jacket designs is studied using a base case jacket for reference. Implementing a slender design with smaller member diameters results in lower jacket inclinations for comparable sea states. This comparison also holds as the significant wave height is increased. A coarser jacket design with bigger member diameters results into a higher response amplitude. However, the difference in response between the base case jacket and the coarse design fades as the significant wave height increases. The third considered design has suction buckets with a large diameter. These buckets make the system very susceptible to wave loading when lowered through the upper layer of the water column where the highest water particle velocities and accelerations are present. Concluding from the analyses, predicting the jacket motions during the installation phase with the numerical model with the equivalent stick implementation is most effective. The results are suitable for quick estimations. The numerical model with implementation of the member evaluation is preferred if more detailed results are needed.