Support structures for far and large offshore wind farms

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Public Summary

Offshore wind is a relatively young and rapidly growing industry. The design of the majority of the existing offshore wind turbines (OWT) resembles that of the design of the first large scale offshore wind farm Horns Rev, offshore the coast of Denmark. The basic building block of such an OWT are a monopile foundation(MP), a transition piece(TP) with a vertical positioned grouted connection, a turbine tower and a turbine. Of the 2488 installed offshore turbines at the end of 2014, 78% are supported by the same foundation type: the monopile.

The current dominance of the monopile might disappear in the future though. So far the monopile has been applied at offshore wind farms in relatively shallow waters(<35m) in combination with turbines up to 5MW. The turbine sizes and corresponding weights have continued to grow since the first offshore wind farm. Next to the increase in size and mass of the wind turbines, offshore wind farms are placed further offshore and in deeper waters. These two developments will lead to monopiles that are longer, have larger diameters and are heavier. The weight and diameters of these piles are slowly reaching the industry limits in terms of fabrication and lifting capacities of dedicated wind installation vessels.

Besides these challenges of the monopile the future of this type of support structure was uncertain after a discovery that the grouted connection was settling at the majority of the installed monopiles. This discovery has triggered research into the cause of and solutions to this problem. These investigations, which included several joint industry projects, resulted in a revision of the Det Norske Veritas offshore standard for the design of wind turbine structures in 2011. The revision recommends that axial loads should be transferred in an alternative way through the connection between the foundation and the turbine tower. One of the proposed solutions is that the pile should have a small angle relative to the vertical to prevent slippage.

In this project the overall goal is to improve the monopile design in order to make it more cost efficient and extend its use into deeper waters. To that end first a survey on possible improvements of the monopile was performed. Secondly, the effect of secondary steel, such as a boat landing and/or J-tubes, on the design of monopiles was investigated using of guidelines of various design standards. Lastly, most of the work was done to investigate the use of the slip joint as an alternative to the grouted connection.

The survey on the monopile was performed using the designs of the monopiles of the Princes Amalia wind farm. The survey showed that these designs were conservative and that with current knowledge and on the edge design the mass of these monopile could have been reduces with 20%. Using the FLOW Cost Model, around 4% cost reduction is envisaged.

Furthermore, the effect of secondary steel, such as a boat landing and/or J-tubes, on the design of monopiles was investigated using guidelines of various design standards. It is shown that determination of the hydrodynamic coefficients depends on the code used and can influence the outcome of the wave load calculation up to 30%. Using the design of the monopiles of the princes Amalia Wind farm it was shown that these secondary steel items can result in a wall thickness increase of 8-12%.

An interesting solution to the current grout problem is a slip joint. A slip joint consists of two conical sections made of steel, one attached to the top of the foundation pile and the other being the bottom of the transition piece. In this way there is a steel-to-steel connection without the use of grout.

Although the slip joint connection has been used successfully in the past in onshore wind turbines it has not been used offshore yet. Expanding this experience to the offshore environment is a challenge but if this is done successfully, a competitive and promising alternative for the grouted connection will be put forward. One of the challenges for the slip joint is to ensure a proper fit of the cones in a controllable way.

This proper fit can be obtained by introducing vibration on top of the transition piece during installation. This new method for connecting joints is investigated in this project both numerically and experimentally.

To capture the basic physics of the slip joint subjected to vibrations a simplified numerical model is developed. The results give insight into the basic behaviour of the cone and into the influence of the frequency and amplitude on its movement.

Subsequently, the problem is considered from an experimental perspective with the aim to assess experimentally the effectiveness of the vibration assisted installation. These experiments investigate the validity of the first model and proves the principle of creating movement with the use of vibrations. The obtained data reveals that the use of vibrations at specific frequencies allows the slip joint to move. This ability of vibrations to let the cone move can be used to move the cone downward during installation and also upwards for dismounting of the slip joint. The results of the experiments show that these movements are obtained using forces and vibration frequencies well within current industry capabilities. With a detailed experimental modal analysis on specimens used in the experiments, insight is gained into the structural modes which are effective in enabling the sliding. Lastly, a finite element model of the slip joint is developed which can be used to identify an optimal shaker that allows sliding for offshore installation and dismounting purposes of slip joints.

The use of the slip joint has potential cost and risk reductions: Groutless connections will solve the current grout capacity issues, save on grout material and create a reduction in diameter and required steel of the transition piece in comparison to the grouted connections. A potential benefit is the reduction in installation time by removing the grouting process of +/- 8 hours and the required healing time of the grout that is around 28 days. After the healing the joint can be loaded and the turbine can start producing. Secondly, the use of the slip joint will allow monopiles of more than 1000 tons to be composed of a number of lighter sections of very large diameters. In this way the applicability of the monopile can be stretched to deeper waters and larger turbines. Another benefit of the lighter sections is that the bulk of the offshore wind lift vessels can be used for installation of monopiles. The use of the slip joint therefore solves the grout issues and provides the industry with a way to continue applying their preferred support structure: the monopile.