The Pelamis machine is designed to stay out at sea throughout the year and so must be designed to cope with large storm seas. The Pelamis addresses this challenge through a number of fundamental features and through careful design, testing and verification.
Hiding from the Big Waves
The Pelamis’s sleek, streamlined shape and low drag profile minimise loading from hydrodynamic forces such as drag and slamming. In addition, as wave size increases, the machine’s shape allows it to progressively dive under the wave crests. This diving action limits power absorption and thereby loads and motions in large waves.
All wave energy machines must provide force against the waves to make them do work and give up their energy to the machine. Providing such forces requires something for the power take-off mechanism to push against – a source of reaction. Unlike the vast majority of concepts that react against the seabed, Pelamis reacts against itself. This concept of self-reference means that the buoyancy forces caused by the waves are reacted against by buoyancy forces elsewhere in or on the machine itself. This has big advantages for the mooring system because the machine does not have to be rigidly anchored to the seabed but can be moored by a slack system, lowering the forces experienced by the mooring and the machine.
Wave Curvature not Height
The Pelamis responds to the wave curvature not to its height. Waves can only reach a certain steepness before they naturally break, which inherently limits the curvature of an individual wave. It is therefore an advantage to use water surface curvature rather than elevation as the driving mechanism of a wave energy machine as the range of motion is much smaller and well defined.
As an example consider a heaving wave energy system – this has to operate efficiently in 1-2 metre high seas but withstand waves of up to 30 metres in height! With the Pelamis there is little difference between the wave curvatures of a 1-2 metre sea when compared to a 30 metre wave – in fact the waves are less steep.
Designing for Survivability
The Pelamis machine and mooring system are designed to withstand the ‘100 year storm’. To do this a detailed wave resource assessment is first carried out to calculate the extreme conditions at the project site. This information is then fed into the design process, where numerical modelling, tank and component testing is carried out to model the behaviour and loading of the machine and moorings. Finally, the design calculations are independently verified to relevant oil & gas codes and standards.