Monday 15 July 2013

Definitions in depth: Oscillating Wave Surge Converters

The term 'Oscillating Wave Surge Converter' (OWSC) is primarily used to describe devices that absorb wave energy by moving in …

pitch, which does seem slightly counter-intuitive. In the standard shipkeeping names for the 6 DoF of an immersed body, the axes are related to the body, so translation along the longitudinal ship axis is called surge, and rotation about the transverse axis is called pitch, whether this occurs in head seas (0° incidence) or beam seas (90° incidence).

Why 'surge' rather than 'pitch'?

However, the default assumed incidence is 0°. In wave energy research, this is such a common assumption that the term surge is associated with translation in the direction of wave travel. The term OWSC came about to emphasise the potential for capturing power from the horizontal component of the wave exciting forces. The existence of the vertical component is more obvious because the free water surface provides a visual indicator of the vertical oscillation. Thus OWSC was used to describe a device excited by the horizontal water particle motions.

Why pitch rather than surge ?

The surge and pitch DoF are both excited in the presence of horizontal water particle motions. Pitch has an advantage over surge in that (along with heave and roll), there is an inherent buoyancy restoration in this mode. There is a righting pitch moment in the direction of the equilibrium position. Surge motion does not have buoyancy restoration, so a surge oscillator must include a physical spring, which has associated costs.

When motion is with respect to ground, another advantage of pitch over surge is the seabed attachment. For pitch this is a pivot; for surge this is a guide rail. It is easier to provide an end-stop for a pivoting device than for a device on rails. As the pivoting device moves towards its end-stops, it presents a diminishing frontal profile to the waves. A pitching device experiences diminishing hydrodynamic (excitation and radiation) forces as the end-stops are approached, which is not the case for a surging device.

It is interesting to note that a seabed hinged flap does in fact experience surge excitation, in addition to pitch excitation. In fact, it experiences excitation forces in all 6 DoF. If it weren't for the seabed hinge it would be moving in all these DoF. Only the pitch excitation results in motion from which power can be captured. Even though pitch and surge are hydrodynamically coupled, the surge excitation makes no contribution to the pitch moment, as there is no motion in surge. 

Image credit:
'Out for a duck':

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