Workability of a Floating XL-Monopile Installation

Abstract

In order to meet the climate goals to minimize the global temperature rise, an accelerated growth of renewable energy sources is necessary. Within the renewables market, the offshore wind sector takes a big part and has experienced a rapid growth over the past years. Around 80% of offshore wind turbines have a monopile foundation, (a large diameter tubular support structure). In present time, monopiles are installed by using either a jack-up or a floating vessel. Due to the trend in the offshore industry that wind turbines increase in size and that wind locations move to deeper waters, an installation with floating vessels may become the most cost-effective option. Jumbo Maritime has designed a new vessel to expand its offshore fleet: HLCV Stella Synergy. This heavy lift crane vessel will be used to install monopiles. Multiple installation methods are under consideration and this thesis focuses on the method with mooring lines to provide station-keeping during the installation. Due to the footprint of the vessel relative to an earth-fixed position, a motion compensated pile gripper is used to maintain upright position of the monopile. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. During the early driving phase (see Figure 1.1) of the installation, the monopile has limited interaction with the soil and acts as an unstable inverted pendulum. The upright position of the monopile is maintained by the gripper frame. The forces applied by the gripper frame on the monopile are reaction forces on the vessel. These reaction forces on the vessel can cause an increased vessel footprint. A workability assessment is performed for a moored floating monopile installation, and the influence of the gripper frame reaction forces on this workability is analyzed. Considering nonlinear phenomena in the installation system, such as a gripper frame control system and viscous drag forces, a time-domain simulation model is made in AnySim XMF for which a hydrodynamic database is used as input. The provided hydrodynamic database is based on the results of a scale model test in a water basin, for one specific loading condition. The provided hydrodynamic database is also compared with an AQWA calculation. The reaction forces from the gripper frame are determined with a MATLAB/Simulink model which generates a gripper force time series based on the behavior of the monopile and the gripper frame due to environmental loading. This gripper force time series is applied to the vessel as an external force during the time-domain simulations. During these 3 hour simulations, the vessel is subjected to co-linear environmental conditions from two different incoming directions, while the vessel is moored to the seabed. The workability is calculated for sea conditions in the North Sea. In a second simulation model, the same simulations are performed but without the gripper frame force time series applied to the vessel to determine its influence on the total workability of the installation. The gripper frame forces have no significant influence on the workability of a monopile installation. In beam waves, the workability is limited by excessive roll motions for longer waves and by insufficient station-keeping from the mooring lines for higher waves. The loading condition addressed in this research proves unfavorable due to its low roll natural frequency. Investigation is needed whether this loading condition is representative of a loading condition which is likely to occur during a monopile installation. In head waves, the system performs satisfactory and is mildly limited by excessive roll and pitch motions.