Design Guidelines

Through many years of design and operation of WEC systems, FO has developed a set of design guidelines to work towards a commercially attractive system. The guidelines are focused on the FO technology, but are believed to be valid for most WEC systems.

All floating structure should contribute to energy production

This is a lesson learned from the FO3 system, were an expensive support structure held the absorbers. This structure was quite large compared to the point absorber structures, while it did not contribute to energy production. It was decided that as a first cost down move, all inactive structure had to be removed.

Light-weight system
Although ballast in a floating system can be inexpensive, the secondary effects of heavy systems can significantly drive up cost. The system mass is directly proportional to the submersed volume, which in turn drives the drift forces on the system. Hence, cost of the storm moorings will be directly affected, in addition to the required extra support structure, and the cost of structure itself. Mass should be regarded an expensive commodity, and should be kept as low as possible. FO has made it a common practice to evaluate the system on a rated kW/kg basis.

Tight-moored system
Floating systems have to react against some sort of fixed body to produce force, and hence power. The options are to react against a stiff mooring tied to the sea floor, or to have a dual body or multi-body system where body elements react against each other. A common example is to use a heave plate, which is a large viscous friction plate, below the point absorber. Such solutions require the fixed body to have a mass of at least 0,5 x F0 x g, where F0 is the production force and g is the gravity constant. This extra mass must be offset with an equal amount additional buoyancy, which add yet more mass. FO has decided on a path toward tight moored systems to improved the kW/kg rating.

All mooring forces must contribute to energy production
Moorings contribute significantly to the total system cost and the required mooring forces should be kept as low as possible. Moreover, the mooring system should be designed to utilize the force effectively for energy production, and all moorings should be PTO production moorings. However, while in the developing phase, all WEC systems should be equipped with strong secondary mooring systems until the system behavior is fully understood. Backup moorings have proven to be crucial on several occasions in the FO project.

Electro-mechanical PTO
FO initially experimented with hydraulic PTO systems as they perform well with high force and low speed. However, when evaluating cost, efficiency and controllability, hydraulic systems show performance issues due to the large variation in speed, and the low average speed of WECs. Hydraulics also has limited ability to perform active filtering to damp unwanted system behavior, which is common in WECs due to the large speed variations. 

FO has achieved far better performance with electro-mechanical systems that are based on mechanical gear and variable speed drive, as described Chapter 2.

Unlimited stroke length
The WEC has to be designed with a given stroke, which is the length it can follow the wave motion. The difference between the ultimate stroke, often statistically calculated from the 10-year wave, the average stroke during normal production is very large, and it is tempting to design the WEC with a limited stroke that is optimized for production performance. 

However, this requires the system to handle the impulse that occurs when end of stroke is reached. the maximum impulse Jmax can be calculated by Equation 1.4, where vmax is the maximum WEC speed, m is the WEC mass, mi is the mass equivalent of the PTO rotational inertia and ma is the added mass of the surrounding sea water. The shock absorber systems absorbing such an impulse showed to be extensive and FO has chosen to avoid end stops, and instead ensure enough stroke length to accommodate for the extreme waves.

Jmax = vmax (m + mi + ma)

Early saturation in PTO force

There are large variations in speed and force between the largest waves and the most common waves. When designing the PTO, a force saturation threshold must be selected, from where some of the wave power is left untapped. A strong PTO has the possibility to extract more power through advanced reactive control, described in Appendix J, however, when cost optimizing the system, it becomes evident that the PTO machinery is more expensive than the floater, and that the PTO should be designed to reach nominal production already in low waves. An example of this kind of cost optimization is shown in section 7.3, and demonstrates very clearly the importance of early saturation of the PTO force.

Above surface device
WECs may benefit from being mounted subsurface, or to use submersion as a survival strategy in bad weather. However, having subsurface system components complicates access and design, and conflicts with the rapid prototyping principle [30], which calls for fast development iteration cycles. Until the WEC systems are fully developed and matured, the WECs should be above surface.

KISS - Keep it simple stupid
A Wave energy device requires a high level of complexity just to maintain basic functions, and any unnecessary added complexity should be avoided. Especially for the early stages it is much more important to develop the robustness and reliability of the WEC system rather than maximizing energy output.