Mating Control of a Wind Turbine Tower-Nacelle-Rotor Assembly for a Catamaran Installation Vessel

ABSTRACT

The assembly and installation costs of an offshore wind farm can approach 20% of the capital expenditures; therefore, time efficient installation methods are needed for installing offshore wind turbines. This study investigates the feasibility of a novel wind turbine installation concept using a catamaran. The catamaran is designed to carry wind turbine assemblies on board and to perform installation using lifting grippers. The installation of a rotor-tower-assembly onto a spar foundation is considered with a focus on the mating process of a tower-nacelle-rotor assembly. The spar foundation has been pre-installed at a representative site in the North Sea, and the catamaran has thrusters regulated by a dynamic positioning system. Numerical modelling of various components of the concept are introduced. Time-domain simulations of the system are performed in irregular waves, and the relative motion and velocity between the tower bottom and the spar top are analysed during the mating process. It was found that the active heave compensator can effectively reduce the relative heave velocity and the risks of structural damage during the mating process.

INTRODUCTION

The offshore wind industry has witnessed continuous growth in the past decade. To improve the cost effectiveness of offshore wind power, there is a trend to design larger wind turbines for greater water depths. Various types of supporting structures have been proposed for offshore wind turbines (OWTs). Generally, for water depth less than 40 meters, monopile, gravity-based, and jacket foundations are the most commercially competitive (Pieda and Tardieu, 2010). For water depths greater than 100 m, floating foundations including spar, semisubmersible, and tension leg platforms are viable solutions, although their commercialisation is still at a preliminary stage because of costs.

Offshore installation is expensive. According to a recent study (Moné et al., 2017), the assembly and installation cost contributes approximately 20% to the capital expenditures of a bottom-fixed offshore wind farm. The installation costs are partly due to the rental of installation vessels and weather-restrictive nature of traditional marine operations (e.g. significant wave height ≤ 2.0 m). The turbulent wind condition is another factor that poses constraints. To avoid delays during offshore installation and to increase profit margins of the offshore wind industry, innovative and cost-effective methods for installing OWTs are desired. For instance, Sarkar and Gudmestad (2017) suggested an installation approach using a floating vessel with a floatable subsea structure for installation of monopile-type wind turbines. Guachamin-Acero et al. (2017) proposed another method for installing bottom-fixed wind turbines based on the inverted pendulum principle. Yet, these installation methods are not readily applicable to floating wind turbines.