A wind turbine support structure is literally the foundation supporting a multimillion-dollar asset over its 20+ year design life to generate income.
The foundation of a wind turbine support structure is one of the most important parts of a wind farm; it is literally the foundation supporting a multimillion-dollar asset over its 20+ year design life to generate income. It is also one of the least visible aspects of the system, normally being buried with only a small portion protruding above ground. So, how does one gain an appreciation of the condition of this structural element when you can’t see it? Furthermore, how can one monitor its condition?
This article will discuss the benefits of using structural health monitoring as it relates to wind turbine generator (WTG) foundations, how it can be undertaken and its role in refinancing/sale/life extension studies. The discussion can also be applied to more easily accessible aspects of the turbine support structure, including the tower.
Real world example of degradation in foundation stiffness and subsequent repair
An evolving understanding
The general goal of structural codes is to provide an acceptable probability of failure for the given design life of the structure. Considering that wind turbines and their foundations are relatively new structures and their sizes and the environmental conditions that they are used in are changing rapidly, understanding of their behaviour, likely damages and failure modes, and industry requirements are evolving. As such, the applicable knowledge and standards used at the time of design of the WTG foundations is likely different to the current standards and understanding of foundation failure mechanisms today.
During the assessment of existing structures, often done as part of a technical due diligence review or when issues arise, it can and does occur that potential deficiencies in existing structures are identified. Furthermore, analysis or inspection of the current state of an existing structure may result in the determination that the foundation design does not provide the level of reliability intended by the standard (i.e. a non-conformance with current standards is identified).
While this does not necessarily mean the intended design life won’t be reached, it does indicate that further analysis should be undertaken to better understand the current health of the foundation and the risk to the asset during its lifetime.
Foundation monitoring recommendations from Standards
Current standards related to wind turbine foundation design, such as IEC 61400-6, DNVGL-ST-0126 and DNVGL-ST-C502, provide recommendations for planning, defining inspection programs, types of inspections, determining appropriate intervals between inspections and documenting inspection findings. They also provide guidance on specific items to focus on during the inspection. In addition to this, DNVGL-ST-0126 also recommends that inspections of foundations are used in conjunction with structural health monitoring.
However, the vast majority of the WTG foundation is generally not visible or accessible, which makes inspections challenging.
Why structural health monitoring (SHM) is needed
Structural health monitoring of WTG foundations and support structures provides an insight into the performance of what can’t be visually inspected without destructive testing such as drilling or excavation. While this has been standard practice in many other industries faced with similar challenges, it is not widespread in the relatively young wind industry. As multi-megawatt scale WTGs age, it will become an important factor in maximising the revenue from any wind asset.
In a nutshell, SHM can provide increased certainty about the health of the system and how it is changing over time. Not only can data from long term monitoring be used to detect whether a system is deteriorating, in some cases, the monitoring of deficiencies can be used to determine when intervention is required to address these deficiencies or performance issues.
Often in conjunction with a threshold value, SHM can help to determine if a certain limit has been reached, and in some cases, when it might be reached if it hasn’t already been reached. This information is particularly useful in the planning of significant investment in further actions or retrofits etc.
While there are multiple benefits to having SHM monitoring of WTG foundations, there are of course some limitations associated with it. For example, on its own, it may not detect or provide warning of abrupt or unexpected failure modes. However, in conjunction with other structural analysis and document reviews, these modes may well be identified beforehand and appropriate monitoring designed. Likewise, monitoring for a short or intermittent period of time does not usually provide useful information. Given the influence that environmental conditions can have on the stiffness of foundation/WTG systems, SHM provides the most useful information when implemented over longer time periods in select environments across the wind farm.
Finally, monitoring alone does not change the risk related to WTG foundations – it is a tool to help inform further decisions and to provide confidence about the condition of an asset.
Increasing certainty through structural health monitoring
In the context of the refinancing, sale or life extension of a wind farm asset, the condition of the foundations is often one of the largest unknown factors in the equation. This is where structural health monitoring can really provide valuable input into the analysis.
Depending on the data which has been recorded, it can be used to infer the extent of damage existing in the structure and the current state of the foundation, and hence whether there may be more margin in the foundations than anticipated – a critical factor in determining any life extension scenarios.
Theoretical analysis may result in a higher risk profile as a result of uncertainty in the foundation design or given geotechnical conditions. However, the presence of sufficient SHM data proving no deterioration is present would provide a solid basis to potentially downgrade the risk profile.
The long-term monitoring of WTG foundations well before their original designed end of life is reached also provides an excellent basis for their ongoing monitoring in any life extension scenario. Monitoring a foundations performance against its measured baseline provides the confidence to separate seasonal (or other environmental) changes from actual degradation, potentially allowing the foundation to be operated over a longer period of time than a purely theoretical life extension calculation may allow.
There are 2 main viable approaches to carrying out SHM on a wind turbine foundation. These are highly dependent on the aim of the monitoring campaign and the existing equipment in the WTGs, but can in general be grouped as follows:
- Utilisation of existing measurement equipment installed in the nacelle of WTGs
- Installation of specific SHM monitoring equipment in the foundation, tower and/or the nacelle.
While the first approach is generally the more economical one, its suitability is highly dependent on 2 main factors:
- Whether the existing condition monitoring equipment is suitable and accurate enough for the desired outcomes
- Whether the data from the condition monitoring equipment can be made available to the asset owner/manager for the purposes of monitoring.
This is not always the case. In contrast, the installation of custom SHM monitoring equipment can be designed and engineered specifically for the desired outcomes. The cost of a custom SHM system is typically a tiny fraction of the value of the asset or its annual revenue, however the insights gained can be invaluable.
The vast data sets from each system can then be appropriately manipulated into useful information and reviewed by an experienced engineer to identify trends in the foundation performance upon which make recommendations can be made.
It pays to know your wind asset better
SHM can provide increased certainty about the health of a foundation/WTG system and how it is changing over time. This information can be incredibly useful in understanding the condition of the asset, and in making decisions in the context of a refinance, sale or life extension analysis.
However, it’s important that any structural health monitoring campaign is well designed and the analysis process rigorous to ensure that the resulting information can be relied upon.
DNV GL has implemented structural health monitoring systems in Australia as well as on multiple projects throughout the world. We can assist with the design, implementation and analysis of any structural health monitoring project for wind turbine structures.