Capture width of the threefloat multimode multi

Authors: Peter Stansby Efrain Carpintero Moreno Tim Stallard

Publish Date: 2015/04/02

Volume: 1, Issue: 3, Pages: 287-298

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Abstract

A moored multibody line absorber is an attractive option for offshore wave energy conversion Laboratory studies have been undertaken to determine capture width with multimode excitation and heave resonance for the threefloat system M4 where the adjacent float spacing is about half a typical wavelength giving antiphase forcing The floats increase in diameter and draft from bow to stern and the bow and mid float are rigidly connected by a beam A hinge with a damper above the mid float absorbs power from the relative rotation between the bow/mid float and the stern float The resonant heave frequency for each float is different Antiphase surge forcing between mid and stern floats is substantial while there is no hydrostatic stiffness producing resonance This represents a hydrodynamically complex system and the laboratory experiments indicate high overall capture widths in irregular waves across a range of peak periods without damping optimisation With different spectral peakedness and directional spread the capture width is greater than 20  of a wavelength based on the energy period across a range of peak periods typical of an offshore site for floats with a rounded base The maximum capture width was about 37  of a wavelength with rounded base floats having rounded rather than flat bases increased energy capture by up to 60  by reducing energy losses due to drag For floats with flat bases comparisons with a geometrically scaled device five times larger and with similar magnitudes of equivalent damping showed similar capture widths as a proportion of wavelengthWave energy is a massive renewable energy resource but effective conversion has proved difficult largely due to its irregular nature First we give some general background Point absorbers are single devices that move in heave pitch or surge or some combination Resonance amplifies power generation such that the theoretical maxima in terms of capture width length of wave crest converted are 1/2pi 1/pi and 1/pi wavelengths respectively eg Falnes 2002 Examples of heaving point absorbers are Archimedes Wave Swing Polinder et al 2004 and CETO Caljouw et al 2011 examples of pitching devices are PS Frog McCabe et al 2006 and Oyster Cameron et al 2010 Response to resonance is generally narrow band although this is normally broader for pitching devices and may be broadened through latching control eg Babarit and Clément 2006 To be effective most devices are designed to function as arrays either separately tethered to the bed or from a fixed platform eg Manchester Bobber Lok et al 2014 Stallard et al 2009 WaveStar Hansen and Kramer 2011 or from a floating platform eg Langlee Lavelle and Kofoed 2011 In another form of point absorption wave motion is transmitted to an air column driving oscillatory air motion through a turbine usually a Wells turbine rotating in one direction eg Masuda et al 1995 and Ocean energy 2014 Another concept is based on line absorption with Pelamis Yemm et al 2012 a wellknown example Here the device consists of a number of longitudinal cylindrical segments aligned with the wave direction connected by hinges at which power is taken off A segment is typically half a wavelength long so forcing in pitch is maximised This device is driven principally by buoyancy The device is floating with a mooring and is usually about two wavelengths long This has the potential to exceed the capture widths of a single point absorber A different form of line absorber known as Anaconda Farley et al 2012 has the form of a flexible submerged tube designed so that a bulge of water in the tube forms due to the wave pressure and travels at the wave speed effectively in resonance There are several good reviews available eg Cruz 2008 Drew et al 2009 Falcão and de 2010 Falnes 2002 and general hydrodynamic theory for wave–body interaction is developed in Mei 1989Some basic principles are apparent Wave energy resource is greatest offshore which implies relatively deep water deployment greater than 20 m say and hence floating wave energy converters which require mooring Single devices are of limited value for largescale generation Resonance is desirable to optimise power generation but this is a narrow band process for a single mode and geometry In terms of engineering practicality floating moored systems are relatively easy to deploy and maintain relative to fixed supporting structures Power takeoff systems may take various forms with hydraulic systems quite mature although accessibility above water level is desirable for maintenance

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