Optimization of the installation sequence of an Offshore Wind Farm Monopile Installation Template

Abstract

Heerema Marine Contractors is assigned to execute the installation of 64 monopiles for the He Dreiht Offshore Wind Farm provided by Energie Baden-Württemberg (EnBW). The installation of the monopiles is done by means of a template structure. This template is specifically designed for this project and is installed on the seabed temporarily. The initial design of the template includes large mudmats to ensure the required stability during the full monopile installation sequence. Analysis shows that this initial template design does not perform well with regard to the template installation sequence. Additionally, when lifting the template through the wave zone, the large horizontal surface area of the mudmats yields significant hydrodynamic forces acting on the template in vertical direction. .
Therefore, the main objective of this thesis is to generate and develop alternative foundation concepts to optimize the template installation sequence. The foundation concepts considered in this research are a mudmat, suction bucket, push-in pile and helical pile foundation concept. Using the mudmat as a foundation concept yields a foundation area of 924 m^2 in order to ensure stability on the seabed, independent of the soil type. The suction bucket foundation concepts results in a horizontal foundation area of 390 m^2 for sand, or 117 m^2 for a clay seabed. The push-in pile and helical pile foundation concepts developed for a sand seabed, result in an area of 306 m^2 and 319 m^2. For a clay seabed respectively a foundation area of 75 m^2 and 88 m^2 is required.
The operability of each foundation concept is determined by means of a hydrodynamic analysis of the template when lifting it through the wave zone. The operability is the amount of time the template can be lowered through the wave zone, expressed in percentage based on 24 hours. A simplified method stated by DNV is used to determine the hydrodynamic loads acting on the foundation in heave direction. For the considered homogeneous sand and clay seabed, the mudmat foundation concept proved to have the lowest operability. The suction bucket concept scored second, and the push-in pile and helical pile foundation concept showed the best performance on operability for both types of seabed.
To evaluate which concept yields the most optimal template installation sequence, a multi-criteria analysis (MCA) is performed. This MCA also takes the installation time, construction costs and the risk of damage into account as criteria. Taking all these criteria into account, it is obtained that the mudmat foundation concept does provide the best performance when considering the template installation sequence, based on the assumptions stated throughout this thesis. Even though the mudmat foundation concept has the lowest performance of all foundation concepts when lifting the template through the wave zone, this concept overall performs better than the alternative foundation concept due to a short installation time on the seabed, low construction costs and low damage risks. The suction bucket foundation concept shows an average score on all criteria. This resulted in the second preferred option. The push-in pile and helical pile foundation concepts show a good operability, however, these concepts score low on the construction costs, installation time and damage sensitivity. As a result, these concepts show the lowest performance considering the template installation sequence.