FOUNDATIONS: Integrated foundation monitoring
An important tool for reducing risks and costs in offshore wind is integrated, intelligent monitoring of the foundation structures. If well implemented an integrated monitoring approach will continuously track the ‘state of health’ of a foundation and identify in a very early stage the onset of structural problems.
This allows early, low cost repairs, optimal preparation of the repair operation, but also a follow-up of the repair measures themselves. As such a significant risk reduction is achieved over 20 to 25 operational years, the foreseen life span of a wind farm.
In the offshore wind industry however a disconnection exists between the development phase and the operational phase. In the development phase budgets are limited and monitoring is only a minor concern, often overlooked.
It is however in the operational phase that the costs of the insufficient or inappropriate monitoring have to be borne. The related amounts are a multiple of the initial investment that would be required to implement an intelligent monitoring solution. The disconnection between development and operational phase thus also contains a huge potential for reducing the total cost of energy.
Identification of monitoring areas
A proper remote monitoring solution begins with identifying those components and/or phenomena that need to be tracked. Criteria that play a role are the risks related to potential issues with the component, and the amount of information that can be gathered from the phenomenon.
For corrosion the following phenomena are of interest, both as a direct immediate follow-up and as with regard to their evolution in time:
• Degradation of organic coatings at the exterior part of the foundation, such as splash zone and higher parts, as this represents the onset of corrosion damage and the associated material loss.
• The degree of protection offered by the Cathodic Protection (CP) system in place at the foundation, as overprotection may result in hydrogen embrittlement, thus structural issues, while insufficient protection may lead to unforeseen corrosion activity.
• The occurrence of specific, mainly localised forms of corrosion such as pitting, water-level corrosion, MIC, etc., as they give rise to stress concentrations or other structural problems.
• Composition of isolated bodies from water as this has a significant influence on the type of corrosion damage that can occur and also on how to deal with it.
For structural integrity, the following should be observed:
• Natural resonant frequencies of the structure, mainly regarding their relation to the frequencies of ambient excitations such as wind, waves, etc.
• Damping characteristics in the different vibrational modes of the structure, to verify with design assumptions.
• The strain levels at those locations experiencing the highest stress concentrations (onset of permanent deformation).
• Strain (static as well as dynamic) in bolts in new foundation types or flanges subjected to potential damage during installation.
• RMS amplitude of vibrations, also connected to the vibrational modes
• Fatigue cycles on vulnerable locations, mainly welds.
• Stress field in the foundation and/ or transition pieces, including the resulting deformations and the evolution thereof.
• All linked with directions of the loads (wind and waves) as well as location of secondary steel structures.
Further structural issues deserving attention:
• The integrity of the grout in grouted connections, for monopile foundations and near the seabed in jacket foundations.
• Dynamic behaviour of free hanging cables in the foundation, coupled with tower dynamics and water movement.
• Inclination of the tower.
• Scour around the foundation and the evolution thereof.
Monitoring set up for optimal data interpretation
For a correct, unambiguous interpretation of the obtained data it is essential to measure the environmental parameters in the foundation (oxygen, dissolved oxygen, pH, potential, temperature, pressure. Etc.) to source SCADA and met data such as wind speed and direction, RPM, pitch and yaw angle, tides, wave heights.
For each of the phenomena described, a dedicated monitoring setup containing a specific set of sensors can be designed and installed. From experience we know however that a significant overlap is to be expected and that an intelligent combination of sensors can result in an integrated follow-up of almost all these phenomena at once. Moreover, isolated monitoring setups next to each other cause loss of value in many other domains:
• The reduced ability for an efficient remote follow-up of the monitoring units themselves. Timing-issues such that it becomes very hard, almost impossible, to correlate the recorded data with each other.
Overlap in recorded data, resulting in an enhanced consumption of bandwidth on the fibre optic link and storage space.
Multiple points of contact, resulting in an administrative overload.
• Difficulties with scaling, e.g. when expanding an existing setup using additional sensors.
• Inefficient management of backup power and backup storage due to the overlap of remote ‘logic units’.
Solid monitoring solution
In order to obtain real, advanced information on the state of the offshore structures, the acquired data from which it will be derived needs to be of sufficiently high quality. To obtain this, multiple elements need to be taken into account.
Design The design consists of not only selecting the required sensors, but also where to locate them. This needs to be done in close collaboration with the designer and/or developer as decisions taken in the design process influence the decision on where to monitor and how frequently. Next to the sensor selection the design also guides the architecture of the monitoring units themselves: what kind of data transfer to use, local backup power and backup storage, backup communication, remote management, timing of acquisition, etc.
Sensors Next in line is selecting the exact sensors to use. Next to their functionality, the choice is guided by sampling interval, accuracy required, environmental conditions the sensors need to operate in, expected operational life, ease of replacement, ease of maintenance, stability, etc.
Logic units The choice of the logic unit is guided by many of the choices made during the Design and Sensor phase. The type of sensors (digital, analog 0-10V, analog 4-20mA), the amount of sensors, their location, the accuracy required, all guide the blueprint of the logic unit. Moreover this needs to be in line with the requirements put forward regarding data management, backup power and storage as well as remote management.
Data logging This covers logging of all data acquired offshore. Elements of relevance are backup storage in case communication lines are disturbed, but also initial treatments, down sampling and matching with predefined alarm levels.
Data transfer All data is prepared for transfer to the onshore data storage (the next 2 elements in the chain). The main data transfer is depending on the communication lines available, but will most often be fibre optic-based.
Data storage The data storage is the place where all data obtained at multiple foundations in the farm will be stored for the long term. Long term storage is essential as some phenomena will only occur after long (multiple years) periods of time. In storage emphasis lies on preventing data loss, but also on availability. This is coupled with the next phase.
Reporting In reporting the acquired data is used to generate reports for use by the operator and appointed parties. Reports can be monthly overviews of the situation, but also more profound analysis of cross-correlations. They cover both the evolution of one specific structure, but also comparison between different structures. Long-term trends are reported, but also relations with regards to predefined threshold values. If issues are detected, advanced analysis based on long datasets can be required. This will involve sourcing long datasets from the data storage. A last type of reports is relevant in the phase of lifetime extension. At that point it can be decided to continue operating the present farm or to repower existing structures. What better means is there to demonstrate the condition of the structure than reports based on long-term follow up of the structure.
Alarms The alarms are the last shackle in the chain, and are to be generated in two physical locations: one is on the structure itself, when threshold values are exceeded. The other location is the onshore data storage, arising from a more profound analysis of acquired data. These structure-based issues are however not the only use of alarms. Also issues regarding data acquisition, transfer and storage need to be brought to the attention as early as possible. This should however not be the concern of the operator, so these alarms are directed towards the provider/maintainer of the monitoring system such that the normal situation can be restored as fast as possible.
This follow-up of the monitoring process itself thus becomes an inherent part of the O&M process of the farm. In case of advanced monitoring, the resulting knowledge will be used to guide the O&M activities (planning and contents), but servicing the monitoring setups will be part of the O&M process itself. The amount of additional transfers needs to be limited however by implementing advanced remote management features and using almost maintenance-free sensors and monitoring hardware.
The approach proposed here is universal: foundation types, turbines and installation methods may become more and more uniform, but these are not the only factors contributing to the state-of-health of a foundation structure. Wind regimes (thus loads), temperatures, water movement, soil type and soil movements are more difficult to control, as they are inherently governed by nature. Also variations in material properties, quality of construction and components play a relevant role. All result in an inherent variability that can only be taken up by implementing an integrated, intelligent continuous monitoring system.
The value of integrated monitoring
Proper installation and commissioning of the sensors and monitoring setups is another essential part for obtaining the desired information from an advanced monitoring package. Therefore dedicated training of the operator staff and a follow-up by continuous remote (onshore) assistance is vital.
Depending on foundation type, location and experience of the operator, the layout of the monitoring setup may differ. This does however not result in additional costs as the same concept applies and our setups all are based on interchangeable building blocks. A final result of the approach offered is in maintaining the setups state-of-the-art. The remote management schemes implemented to the setups have made it possible to continuously upgrade the setup from the shore. As such every setup located offshore can, at minimal cost, be made more intelligent during the 20 or 25 years of operational life, based on the knowledge gained during the long-term follow-up of the farm, but also from advanced analysis performed on the data as well as experience in other farms.
The integrated monitoring approach will allow reducing costs in multiple ways:
• As inspections provide only data in discrete time points, the information content obtained is also limited, resulting in the need for additional trips for further interpretation. Continuous monitoring results in continuous data sets. Interpretation is much more straightforward and more trips offshore can be avoided
• In reality a limited number of inspections will still be needed to complement the data obtained. Using the monitored data and knowledge of the setup, inspection trips will be however more targeted and better prepared and can be guided from the monitoring centre to obtain the highest value possible.
• Due to integrated monitoring and trend analysis, deviations of the behaviour from the predicted behaviour will be detected in a very early stage. As multiple parameters are monitored an accurate image of origin and result can be assembled.
This allows early, minor, thus relatively cheap interventions to be set up and stabilize the issue. The same monitoring setup will be used in the follow-up stage to assess the long-term performance and efficiency of the repair.
• An advanced monitoring setup will allow making the transition from planned maintenance to condition based maintenance. This will reduce the total cost of maintenance operations, contributing further to a reduced cost of electricity.
• As deviations from the normal or expected behaviour are captured in a very early stage, chances are high that the situation can be restored to normal within a very short timeframe. This results in a significant reduction of the risk associated to the foundation structures. The lower the risk, the greater the negotiating power you have to lower insurance fees. Adding up again to a potential reduction in cost of electricity.
• The same risk reduction is also a benefit in the financing phase. More and more external, risk-averse sources of financing are sought in the offshore wind sector. As proper monitoring significantly lowers the uncertainty regarding the long-term structural integrity, this will be reflected in the premiums asked by external parties to invest in the farms.
• At a certain stage in the life of an offshore wind farm, the permitted period to operate the farm will come to an end. At that point the operator may choose to apply for an extension of the permit or for a new one, coupled with a repowering project. This process will be based on an extensive report on the state of the foundation. Imagine how much more straightforward this process can be based on 20 years of continuous data and the associated knowledge, compared to having to set up a dedicated inspection campaign offshore.
• Last but not least: savings related to future constructions. As the advanced monitoring leads to a profound knowledge of the in-operando behaviour of the foundations, the origin of unexpected phenomena can be fully analysed and subsequently remedied in future designs. Continuous learning is yet another factor in the process of permanent reduction of the cost of electricity.
The above has taught us that an integrated, intelligent monitoring solution represents a very efficient means for cost reduction. If taken up from the development phase and making use of the important concepts introduced above, the risk related to the management of offshore foundation structures can be kept at a minimum for the foreseen life of 20 years and beyond.
With thanks to Yves van Inglegem, Founder of Zensor