Limits of Offshore Wind Turbine Installation with the Double Slip Joint

Abstract

In current offshore wind turbine (WTG) tower installations, bolted flange connections are common. These connections are however reaching their limit due to the maximized size of the bolts. Also, bolted connections require the motions between the tower and foundation to be limited during installation, suggesting their workability of installation is low. The Double Slip Joint is a promising and innovative connection between a WTG tower and the support structure, which may remove or reduce these limitations. This research explores the motion and collision limits for WTG tower installation on a monopile (MP) foundation and focuses on the lowering and mating using two types guiding structures, or catchers: the vertical asymmetric catcher and the conical catcher. A first exploration of installation workability is provided as well. A model is developed that simulates the lowering and catcher mating of a WTG tower with a MP. This model includes physical phenomena that affect the motion and collision behavior of the WTG tower. Specific attention is paid to collisions between the catcher and the MP. With the use of finite element analysis, a stiffening non-linearity is observed in the lumped contact element to describe collisions. It is found to be caused by the local deformation of the mating elements. Furthermore, the simulation model calculates 3 degree of freedom in-plane motions numerically of the WTG tower in the time domain using Euler integration. Besides the aforementioned stiffening non-linearity in collisions, other important model features are a time and spatially variable wind load, aerodynamic damping, harmonic horizontal and vertical crane tip motions, crane operator induced crane tip motions and lifting line stiffness and damping characteristics. The model simulates the WTG tower lowering and catcher mating and to determine the installation limits, installation requirements have been defined, aimed at preventing critical events. To be specific, installation requirements aim to prevent slack wires, axial impact, and plastic deformation of the catcher due to contact with the MP. It is also required that the side-lead angle (i.e. the lifting line angle with respect to the vertical) remains below a maximum allowable value. Model simulations are performed for both jack-up and floating installation vessel crane tip excitation characteristics. This research indicates that in both cases, the conical catcher yields higher allowable wind velocities than the vertical asymmetric catcher during installation. This is supported by the findings that the conical catcher has higher allowable contact forces, requires less or no crane operator action during catcher mating and the side-lead angle varies less than with the vertical asymmetric catcher. Another important observation is that the installation limits are often reached through installation requirements related to the motions and collisions of the mating elements. This study shows that applying linear damping to the horizontal WTG tower bottom motions, achievable by (active) tugger winches, the allowable mean wind velocity in installation is increased and this research suggests that the workability of installation increases consequently. The focus in this study is on motion and collision limits in installation as well as on installation requirements. It is recommended for future research to focus on workability and to analyze the entire system consisting of vessel, load, and foundation. As such, wind and waves are considered separately as a source of excitation and vessel-load interaction is incorporated. For 3D analyses, it is recommended to study the control of out-of-plane WTG tower bottom motions and potential inclined collision responses between the catcher and foundation.