In the context of hydropower reservoirs and large-head dams, where water levels may fluctuate by several metres within hours or seasons, conventional fixed-mooring solutions for floating photovoltaic (FPV) arrays often struggle to maintain ideal tension, leading to stress on moorings, increased maintenance and reduced operational reliability. Enter the Tension Buoy from Fred. Olsen 1848 – a self-adjusting mooring buoy that integrates a motorised winch, depth-sensing and real-time tension monitoring to continuously adapt chain length and maintain consistent mooring tension. Designed specifically for variable-water-level environments such as reservoirs behind hydropower dams, the system promises to simplify hybrid solar-hydro deployments by ensuring steady anchoring through draw-down, refill and seasonal cycling.
To explore how the system performs in real reservoir environments, we spoke with the engineering team behind the Tension Buoy. Their insights shed light on the control architecture, load-management philosophy and practical considerations that make the technology suited to hydropower-scale fluctuations and hybrid-plant integration.
How does the Tension Buoy’s automatic winch system detect and respond to water-level changes in real time?
Each buoy is individually equipped with a motorised winch, battery with solar charging and controller with wireless communication. As water level variations are slow, the winch is geared to suit a small motor and a correspondingly small battery.
The main task of the moorings is station keeping of the floating solar island. A GPS receiver located on the island routinely reports position to a centralised controller. In addition, a depth sounder monitors the water level at regular intervals and adjust the mooring lines accordingly to verify mooring line tensions are within expected range.
What are the design parameters for maximum and minimum water-level variation that the system can accommodate?
Accommodating water-level variations is a matter of sizing the corresponding chain length. As variations grow, the weight of chain increases so the buoy size may have to be increased, but there are no maximum or minimum water-level variation limits for the buoy.
Can you elaborate on the load management design – how does the buoy ensure consistent mooring tension under varying hydrodynamic forces such as wind, waves, and current?
The main objective of the mooring is to keep the FPV installation inside its position envelope, which it does by all tensioning buoys working on a collective centralised algorithm. However, monitoring of mooring line tension is also a key feature of the Tension Buoy product, as it will be affected not only by the instantaneous depth, but also by wind, waves and currents. To facilitate this, a load cell is fitted between the winch and the buoy, effectively reporting mooring line tension to the centralised controller.
What materials are used in the buoy’s structure and chain system to ensure long-term durability in freshwater and reservoir environments?
The buoy is made from UV resistant polymer and filled with internal foam. The winch is made from galvanised steel to ensure corrosion resistance while maintaining low cost. Battery and wireless communication are fitted in a sealed enclosure.
How is corrosion and biofouling managed in the Tension Buoy design, particularly in tropical or sediment-rich reservoirs?
Those sites where level of fouling is severe, buoy size and net buoyancy must be dimensioned to handle the additional mass. The added viscous drag on the buoy from biofouling is minimal compared to the tension level on the buoy from the mooring lines. As for fouling on the chain itself, the subsurface roller and top side gipsy wheel removes this during operation.
How does installation and commissioning of a Tension Buoy differ from a conventional fixed mooring system in terms of time, cost, and technical requirements?
Anchors are typically preinstalled with chain held available at surface with surface buoys.
Buoys are towed to position and chain fed into the winch from a work boat. From there, each winch tensions itself based on command from the central controller. This eliminates the need for manual tensioning on installation, eliminating need for heavy duty equipment as well as the need for quality control of tension level during installation.
What maintenance regime is recommended for the automatic winch and its moving parts, and can maintenance be performed without retrieval of the buoy?
Given the relatively slow changes in water depth at a reservoir, the winch can be operated at low frequency, typically minutes per day or less, so operational hours are few over a year.
Except for the lower guide pulley, which is mounted on slide bearings, there are now moving parts that are submerged. Otherwise, all moving parts are accessible from top of the buoy from a surface vessel.
How is system health monitored – is there remote telemetry or condition-based maintenance integrated into the design?
Yes, at each winch, current consumption of the winch motor and mooring line tension is monitored. As these two parameters are correlated, this allows for remote condition monitoring of the whole drive train.
What redundancy or fail-safe mechanisms are in place if the automatic tensioning system fails or loses power?
A number of fail-safe functionalities are available, subject to FPV system size, water level variations and environmental conditions. A back-stop at the end of the return chain will ensure chain is not allowed to slip through the winch and fall to seabed, while the weight of the chain will always provide a certain tension in the mooring line.
In case of power failure at one winch, the communication link will immediately be reported down and as such notify the failure. Water level variations, and hence need for chain length adjustments, are slow, so there is ample opportunity for operators to inspect and repair/replace power system at one winch.
How does Tension Buoy integrate with existing floating PV array mooring layouts – does it require specific array geometries or anchoring patterns?
The only requirements for integration of the Tension Buoy is that it needs to be integrated along the perimeter of the floating PV array, and the chain needs to be allowed a certain minimum angle to vertical plane, typically minimum 30 degrees (meaning horizontal footprint of 50m at 100m max water depth)
Have you conducted pilot projects or trials on operational hydropower reservoirs, and if so, what were the observed performance outcomes?
We are currently doing a joint pilot project with Energias de Portugal (EDP) at one of their largest hydroelectric reservoirs in northern Portugal. The system was built and tested in Norway and has now been successfully installed on the reservoir.
What are the key considerations when retrofitting Tension Buoy systems onto existing hydropower reservoirs with fluctuating heads or drawdown zones?
Operating in fluctuating heads (from water level variations) is the core functionality of the Tension Buoy, so the only considerations is proper dimensioning of the chain length. As for drawdown zones, the buoy can end up and rest on dry land without taking damage and will float back up in position as water level rises again.
How does the system interface with hydro plant operations – is there potential for communication between dam management systems and Tension Buoy controllers to optimise operations?
Absolutely, the Tension buoy controllers are operated over wireless LoRa Node infrastructures, a high range, low power communication infrastructure. Currently, we use encrypted MQTT as communication protocol, but this is adaptable to suit the communication and control system implemented at the hydro power plant.
How scalable is the Tension Buoy for large FPV arrays (tens or hundreds of MW) – what are the engineering or economic limits?
Tension buoy is in principle a mooring point available at the water surface (with small horizontal footprint, as opposed to the long horizontal stretch of typical mooring arrangements for FPV on hydro dams). It replaces long ropes stretches and extensive use ofcostly flexible tension members with standard mooring chains, offering considerable cost savings on hardware cost and cost of installation.
Are there plans to adapt the technology for other variable-water-level applications, such as pumped storage reservoirs or tidal lagoons?
Yes, the Tension buoy is a mooring system suitable for all cases of variable-water-level applications such as reservoirs (incl. Pumped storage), tidal lagoons or shoreline sites with large tidal variations .
