Lobe-Tendon Anaconda: An evolution of the Anaconda concept

Abstract

The Anaconda is a water-filled flexible tube, floating just beneath the sea surface and aligned with the direction of wave travel. The Anaconda utilises the bulge wave effect. Sea waves, travelling along the length of the tube, excite bulge waves within the tube. When the speed of the ocean waves is close to that of the internal bulge waves, the bulge wave ‘surfs’ in front of the ocean wave and a resonant transfer of energy occurs. A power take-off is located at the stern of the tube which converts the amplified bulge wave into the desired energy vector.

Wave energy converters made primarily from flexible materials have some potential benefits over their more traditional rigid body counterparts. Flexible materials have a built-in compliance, and a lack of end stops, while a WEC that utilises flexible materials also has the potential to be significantly lighter than a rigid body WEC and therefore potentially significantly cheaper, not only for manufacture, but also for transportation, installation and maintenance.

The original Anaconda concept had a circular cross section. Physical model tests on all-rubber tubes and combined rubber – inelastic material tubes validated the power capture but showed that aneurysm was an issue for circular tubes. In addition, the thickness of a circular tube is inextricably linked to its internal excess pressure by the hoop stress, resulting in excessive material thickness at full-scale.

Checkmate Sea Energy have developed a new Anaconda concept that

• Removes risk of aneurysm
• Breaks the link between pressure and tube thickness

The novel Lobe-Tendon Anaconda concept is made up of a virtually inelastic outer tube and many internal rubber tendons that join points on diametrically opposite sides of the tube. When expanded, the tendons are stretched so that the cross-section of the outer tube is circular, but when contracted, the cross-section will form lobes. With the number of lobes equal to the number of tendons.

A 2D analysis of the relationship between the cross-section area and the static excess pressure has been developed to calculate the bulge wave period of a Lobe-Tendon Anaconda. An overview of the method is presented, along with a comparison against experimental results. The 2D quasi-static method can be used to calculate the volume of material required for a Lobe-Tendon. This can be compared with the 2D bulge wave analysis method for an equivalent circular cross section Anaconda. The results show that there is a reduction of over 70% in the material requirements for the Lobe-Tendon Anaconda.

The original numerical theory used by Farley et al to analyse the Anaconda concept show that power capture is dependent on length, cross section area and bulge wave period, and so independent of cross-section shape. This is demonstrated by comparison of published experimental results for circular cross-section Anacondas and new experimental results for the Lobe-Tendon Anaconda which show equivalent power capture across the two iterations of the Anaconda concept.