Monopiles have long been the most common foundation type in offshore industry. Almost 80% of the existing offshore wind turbines is supported on monopiles. The main reason for that is their inherent simplicity and low cost in comparison to other foundation types. At the beginning of 2011, market research within the industry indicated that it is not profitable to build wind farms supported on monopiles at water depths over 35m. However, in 2012 this projection was revised, indicating that large diameter monopiles could offer greater benefits than it was previously stated.
Development plans released by The Crown Estate include information regarding the construction of a number of offshore wind farms at water depths up to 50 m, i.e. much deeper than ever before.
Furthermore, the new developments in offshore wind industry are focused on implementing higher capacity wind turbines in order to achieve optimized solutions. The main reason for these changes is the apparent need for reduction of the price of electricity generated by offshore wind farms.
Nevertheless, the usage of higher capacity offshore wind turbines and construction at deeper water depths indicate the need of more efficient structural solutions, such as XL monopiles which are capable to withstand greater loads. XL monopiles are monopiles in diameter exceeding approx. 7m. It is important to highlight that the economic viability and technical feasibility of XL monopiles are at question. Lack of sufficient manufacturing facilities, more expensive hire charge of transport vessels and pilling equipment raise concerns about the practicality of that solution. The above leads to an obvious question: Will XL monopiles be a game changer for offshore wind industry?

Project Objectives

The aim of the project is to investigate the XL monopile technology and to identify its advantages and potential drawbacks. The focus of the study was to:

Perform structural analysis of different size of monopiles

Identify the constrains and benefits of the XL monopile design

Combine results from the technical part of the project and use them to carry out a life cycle financial analysis

Calculate the levelised cost of energy as an indication of viability of the technology

The project has been divided into three parts. The first part, which is the structural analysis, aims to examine two different offshore wind turbines: Vestas V112-3.3MW and V164-8.0MW supported on XL monopiles at various water depths. The results from the analysis are the dimensions and weights of the supporting structure in a number of scenarios. The second part is the cost analysis of the scenarios examined in the structural analysis. A 500 MW wind farm has been investigated in order to calculate the levelised cost of energy (LCOE). The technological challenges (manufacturing, transport and installation), along with the environmental issues associated with XL monopiles have been identified. Mitigation measures regarding the environmental impacts have also been examined. The conclusion part summarizes the results and offers recommendations that could help to overcome existing obstacles in order to make XL monopiles an economically viable solution.