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Wave Dragon versus The Storms

Wave Dragon versus The Storms

During the last week the survivability of the Wave Dragon prototype was once more successfully tested in two major storms. Both storms reached 28-29 m/s mean wind speed and gust of 38 m/s.

Crash Test Series

As a part of the prototype testing we have been running a series of survivability tests to verify Wave Dragon’s predicted survivability characteristics in real sea conditions. The ‘Crash test series’ (as it has become affectionately known) has been designed to get “hands on” experience of the Wave Dragon’s behaviour in all possible fault situations. Initially, we have turned down all onboard systems one at a time and then all of them. As survivability can depend on the actual floating position of the Wave Dragon platform these system-shut-downs and total black-outs have been tested in “normal” situations as well as in most unfavourable situation, e.g. in strong heeling or pitching situation combined with closed turbines and totally floated reservoir.

Last week, however, nature offered us the opportunity to really put the Wave Dragon to the test, with the Danish winter storms gusting up to 38 m/s. The decision was taken to place Wave Dragon in her most vulnerable position and see how she got on.

This entailed Wave Dragon being at:

  • Max floating level = max wind and wave load
  • Closed turbines = reservoir flooded
  • All control system closed = no ability to adjust heel or trim

The result was that even when left in the most vulnerable position and battered by, not one but two major storms, Wave Dragon suffered no damage at all and the mooring did not move an inch.

Concerns about Survivability

The one concern most commonly raised about the Wave Dragon technology is the mooring issue. Many people question the possibility of mooring such a large structure and its ability to survive storms. This concern is of course fed by the 2005 stranding of the prototype. This stranding was however the result of a broken load cell and a missing redundancy chain. There will not be a load cell in the main mooring on a future Wave Dragon device, and as last week shows Wave Dragon is more than capable of surviving stronger storms than that of 2005 as it now is deployed at a more exposed position with higher waves.

In fact mooring a large and low-profile device like the Wave Dragon is relatively easy compared to other offshore wave energy converters such as buoys and surging devices.

Devices that absorb energy by getting into resonance with the waves and thus convert this energy by wave induced motion are fully exposed to the often extreme loads from the individual waves and these loads are fully transformed into a force in the mooring line. This is not the case for a Wave Dragon device. This is due to the fact that Wave Dragon:

  • Is designed to stay calm and not rely on extracting energy from wave motion
  • (And thus) Is slack moored which greatly reduces the forces in mooring lines
  • Is very large which by itself (geometry and inertia) greatly reduces mooring forces

From a recent design of the CALM buoy mooring system for the Welsh Demonstrator project we know (see figure below) that the resulting force response in the Wave Dragon mooring lines is a around 1/4 to 1/5 of the max mooring load (100 yrs storm).

Risk Assessment – Survivability

Any mechanical and electrical systems are at risk of breaking sooner or later. It is thus vital that a wave energy converter’s survival does not depend on an active system to survive.

The excellent survivability characteristics of the Wave Dragon device are no surprise. In fact – surviving any environmental conditions (100 years events) with any system apart from the mooring out of work – has been one of the key parameters behind the development of the Wave Dragon technology.