Floating installation of offshore wind turbines in a single lift

Abstract

The wind energy industry is booming. In 2017 the installed wind capacity has overtaken coal as the second largest form of power generation in Europe. Costs of wind farms are driven down, and even the first offshore wind-farm contract without subsidies is a fact. Installing the Wind Turbine Generators (WTGs) more efficiently increases the profit margins in this highly competitive industry. Nowadays, WTGs are installed using jack-up vessels, but there is little known about the installation of large wind turbines using a floating vessel in offshore sea states. Installation using a floating vessel solves the problem of increasing water depths, critical leg-retrieving forces, the need of more powerful jacking systems, and the time-consuming jacking process. By installing the WTGs in a single lift, the mating operations can be avoided, resulting in shorter offshore installation times.

Though due to the high crane tip, already small vessel motions result in significant WTG motions. The turbine manufacturers only allow minimum accelerations and impact velocities, since the components are sensitive and damaged easily. The objective of this thesis is to identify the dynamical response and coupling of vessel-turbine, to decrease the motions of the WTG and to translate this into a lift and landing system. The focus is on studying influencing parameters, rather than to assess the workability of the proposed concept. The Bokalift 1 is used as a case study, to explore the possibilities of installing WTGs with assets of Boskalis.

An OrcaFlex model is built to simulate the dynamic behavior of the turbine and vessel under regular and irregular wave loads. Both the free-lift and landing phase are studied, with the use of modal analyses and time-domain simulations in OrcaFlex. The hydrodynamic properties of the vessel are calculated in AQWA. A dynamic model is also derived analytically, using the Lagrangian formalism, to validate the modal analysis of OrcaFlex and to study the coupling between the turbine and the vessel.

Resonance is found by the excitation of two lifting configuration modes in the longitudinal plane, namely the pendulum and double pendulum mode. For the transverse plane significant coupling was found between the turbine and the vessel, resulting in a modal shape which is a combination of the pendulum mode and the roll mode of the vessel. A second peak of resonance is caused by the excitation of the double pendulum mode for the transverse plane.

The motion of the WTG, as a consequence of the resonance of the double pendulum mode, is related to the natural period of this mode relative to the RAO of the vessel. Time-domain simulations in OrcaFlex showed that significant motion reduction of the WTG is possible, by lowering this natural period below the wave period excitation range of the vessel. A modal analysis of the analytical model described that the natural period is also a function of the rotational stiffness of the lifting configuration, resulting in an avoidance of resonance when adding enough rotational stiffness. Resonance of the roll mode cannot be avoided without changing the hydrodynamic properties of the vessel. Motion reduction by the usage of tugger line damping is also studied. Tuggers placed under an angle towards the vessel also proved to avoid resonance of the double pendulum mode. An improved setup could increase its potential to damp out the resonance caused by the roll mode as well.

The proposed landing system is modelled in OrcaFlex to assess the performance under irregular wave loads. Time-domain simulations proved to reduce the vertical velocity of the WTG up to 97% and below 0.01m/s. However, the current guiding system still allows for rotations of the WTG. These rotational motions could be significant due to the double pendulum mode and could lead to local vertical impacts of the bottom edge of the tower on the foundation.
Based on the obtained results, it can be concluded that both the horizontal and vertical motions of the WTG can be reduced significantly during the free-lift and landing operation from a floating vessel.