PO099

Offshore Wind Foundation - State of the Art Review, and onwards to 20MW Wind Turbines!

Narasimhan Sampathkumar, Trevor Hodgson, Andrew Thompson, Una Brosnan, Emma Conway
Atkins Ltd, UK

Abstract

Going forward, the global offshore wind market is entering a new phase of development - one where turbines in the 7.0-8.5MW range are the norm and where projects are becoming more complex in terms of size, water depth and distance from shore.  There is a strong desire and demand from all parties (including wind farm owner and investors alike) to make offshore wind more competitive than other alternative energy technologies, by achieving a Cost of Energy (CoE) lower than €70/MWh.  Improvements in design efficiency and cost reduction must be accelerated in the coming years to achieve this.

This presentation initially provides a state of the art review of offshore wind foundation design, by firstly providing a brief overview of the development of offshore wind foundations for the last 10 years, and then the potential future trends around the world between 2017 and 2025 (including for both fixed and floating structure).  It describes how offshore wind foundation design must evolve to meet the demand of the industry.  It will then identify the main criteria that can be used in the decision making process for foundation design selection. 

As part of this presentation, a review of fixed jacket foundations design analysis methodology will be discussed.  The paper will also highlight the importance of wind turbine-foundation coupled analysis options for the refinement of the design. 

The final part of the presentation will provide estimated weights and costs of fixed jacket offshore foundation designs for predicted development of 10- 20MW rated wind turbines.   

Method

Extensive market research has been carried out on the current offshore wind foundation markets.  Based on Atkins extensive experience in the offshore wind foundation design, an in-depth in house knowledge has been gathered and analysed. The key design drivers are identified and opportunities of standardisation of design across the wind farm are also discussed and highlighted.

A review of the coupled analysis including sequential and fully integrated analysis has been discussed, and improvement options are proposed.

To determine the effect of using 10-20MW wind turbine with a fixed jacket foundation, quasi-static analysis by utilising turbine sweep envelope and estimated damage equivalent loads have been carried out to determine the fixed offshore foundation weight and cost.  The cost is estimated based on market research. 

Results

 Whilst it is not surprising the use of monopile foundations will continue to be the main trend for shallow water depth (0-40m), the shift to use larger turbine (8MW+) will favour the use of jacket foundations.  The use of Gravity Base foundations is gaining momentum for offshore wind farms with challenging ground conditions.  Whereas, the use of floating structures is a great option for water depth greater than 60m.

The results showed that a saving of jacket weight of up to 10-15% can be achieved for fixed jacket foundations. This was done by reviewing the requirements for installation, fabrication, and design parameters such as corrosion allowance and local jacket joint design. 

The use of 10-20MW foundations may become a challenge from offshore installation perspective. 

Conclusions

Since the cost of foundations can represent up to 30-50% of the total wind farm development cost, foundation design is an important area to focus on to achieve the target CoE, and this can be achieved through innovative and continuous improvement of foundation design across a number of areas. For the next few years, the general trends of the offshore wind foundation market will demand greater efficiency in order to achieve a COE lower than €70/MWh.

The use of 10-20MW foundation may become a challenge from offshore installation perspective.  The use of horizontal installation and load out may become the norm if the installation vessel are not able to accommodate the lift height and weights.

Objectives

The learning objectives of this presentation are as follow: