The Power Take-Off Unit is the core of the Fred. Olsen wave energy conversion technology. Comprised of a high cycle winch, a belt drive gear box and an actively controlled generator, the PTO unit is generic, scalable and can be fitted to any floating structure to produce electrical power through damping of wave induced motion.
BOLT Lifesaver can accommodate five PTO units, though currently only three units are built and installed.
A PTO unit operates completely independently and obliviously of the other PTOs. This provides a redundancy for maintained operation in the case of a PTO failure.
Some key parameters for the PTO unit:
|Nominal sea state
|Avg. electrical power output @ nominal sea state
|Generator nominal power rating
|Bill of material
Key features of the PTO unit are:
- Fully electric
- Actively controlled mooring line tension through winch drum torque control
- High cycle winch line (several million cycles between service)
- Compensation for angles between winch line and winch drum arising to pitch/roll- and drift of the floating structure
- Compact and modular design
Energy is available from relative motion between the PTOs and the ocean floor. The component connecting these two reference points, is the PTO winch. The winch line is moored to the seabed at one end and wound around a drum at the other. As the PTO is displaced, the winch line is tensioned and inflicts a torque on the drum. Through a gear box, a generator provides a torque that results in a controlled winch line tension. As the PTO travels back down from a wave crest, the generator operates as a motor to wind the winch line back in with a controlled tension.
Horisontal drift as well as pitch and roll motion of the floating structure, gives rise to a constantly changing angle between winch line and drum. A guiding system helps compensate for this angle and ensures good alignment as the winch line is guided onto the drum.
Even the most sophisticated rope products that is exposed to bend under tension will see internal abrasion between strands and have a predictable lifetime of only a few hundred thousand bend cycles. Considering a typical wave site has around five million waves per year, Fred. Olsen found that a new winch line had to be developed in order for the technology to be viable.
Collaboration with a world leading manufacturer of transmission components, was initiated in 2010 to develop a winch line with increased capacity for bend over sheave load. Following six years of extensive rig- and in-sea testing, a patent pending winch has been developed with lifetime of several million bend cycles.
Regular, off-the-shelf gear boxes requires routinely lubrication, are compact and inaccessible for maintenance and have poor resistance to offshore marine environment. In collaboration with Gates Corporation, a high ratio, belt based gear box has been developed that has low maintenance requirements, good accessibility for maintenance and high resistance to corrosion.
Absorbed energy is the winch line tension force multiplied by drum rotation. Hence, net energy is energy produced on upward motion minus energy spent winching line back in.
Generator, gear box, winch line and the floating structure makes up a complex system of dynamically couple masses.
This prompts the need for a carefully implemented generator control algorithm to extract maximal amount of energy while reducing system oscillations and force excitation.
The generator control algorithm employed by the PTO unit is the result of extensive simulation and in-field parameter study. Velocity proportional damping and early force saturation has been a fundamental part of the Fred. Olsen generator control algorithm.
An expert of a production series with pull out tension 30kN (max. capacity 100kN per PTO), pull back tension 10kN (defined with negative signage) is provided below. Green graph is power consumed by PTO when winding back winch line, while yellow grap is produced power. Net delivered power is the sum of the two.
Detailed description of the Fred. Olsen WEC Power Take-Off development and design is found here: