Conference programme

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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Felix Schubert RES, United Kingdom
Felix Schubert (1) F
(1) RES, Kings Langley, United Kingdom

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Felix Schubert is a Senior Turbine Engineer in RES's offshore department in Kings Langley, UK. With an educational background in general and aeronautical engineering, he has been working in the energy sector for over 7 years and for 5 of these in the wind industry. For the past 3 years he has primarily fulfilled technical and commercial roles in a number of offshore wind projects during their construction and operation. His work focuses on WTG construction and O&M package activities during the contract negotiations, project planning and execution phases. Recent projects include Lynn and Inner Dowsing (post-warranty O&M), Lincs (construction & operation) and Race Bank (construction planning and contract negotiations).


Poster Download poster (7.19 MB)


Time vs. Yield Based Production Warranties


For offshore wind farm projects, long term availability or yield warranties are a key requirement (5 year contracts are typical, 15 year contracts are not unheard of). There are two principal means by which the production of wind turbine generators (WTG) is assessed, measured and warranted: the time based or the yield based production assessment. Choosing and negotiating the right production warranty is particularly important for the success (and profitability) of offshore wind farm projects due to the high cost and weather risk of maintenance work. Furthermore the warranty should offer the right protection from revenue loss balanced against the cost of the service agreement. However, most importantly it must encourage the right relationship between the contracting parties and the right behaviours to work towards optimal project performance, rather than partisan scope delivery.


The presentation provides a comparison of the ‘pros’ and ‘cons’ of each of the two performance methodologies for a generic project in its first 5 years of operation when the performance warranty provider and the service contractor are (effectively) the same party. Furthermore, as every project is unique, a number of topics are being discussed which are particularly relevant for performance assessments and the negotiations and management of an appropriate performance warranty.

Main body of abstract

Fundamentally, yield and time based performance assessments and related warranties have very similar aims, however the way in which either methodology goes about achieving these differ in many ways. Both approaches fundamentally aim to accurately measure the wind farms performance relative to the ideal ‘no downtime’ scenario. This assessment is then used to determine compensation (and often also incentive) payments between the contracting parties giving the asset operator confidence in the WTG technology and assuring any investors that the project is bankable. The approaches differ in the data which is analysed and how representative and readily available this data is. Data on WTG uptime and downtime is easily gathered, but WTG curtailment is less easily captured by a time based methodology and so this approach does not always translate into a representative estimate of lost performance. On the other hand, while actual WTG yield can be measured directly, theoretical yield (and the resulting yield loss) has to be calculated based on the wind speed and a WTG yield model, neither of which is always accurate.
While time based warranties have been an industry standard for many years, yield warranties have much less of a track record. The combination of these factors tends to result in time based warranty levels being slightly higher than those for a yield based warranties.
More recently, the yield warranty concept has become a popular new approach following the realisation on some projects that the availability warranty has not delivered the desired result where both the operator and the service contractor work jointly towards optimising asset performance. Possibly because yield warranties look attractive when viewed at a high level; however the full implications on daily project activities can be difficult to understand unless analysed in detail.


The similarities and differences between time based and yield based performance assessments are complex and have different implications on every project. Choosing the right approach for a project requires a clear understanding of the objectives, detailed analysis of the options and level-headed negotiations with the warranty provider. Most importantly, the full impact of the decision on the way data is gathered and analysed, routine maintenance work is planned and commercial relationships will develop needs to be fully understood.

Learning objectives
- Understand the principle similarities and differences between time and yield based warranties;
- Identify some of the key considerations for a project when choosing between the two; and
- Gain awareness of the challenges and opportunities which either approach can bring.