MS Thesis R. Hager - Hawaii National Marine Renewable Energy ...

pollution would occur from hydraulic oil spills; fail-safe systems and the use ofbiodegradable oils would reduce this threat (Brooke, 2003).1.3.2 DurabilityStructural challenges include: extreme wave conditions, corrosion, and leakage. Extremewave conditions may cause stroke overextension or slamming (Backer, 2009). Strokeoverextension refers to exceeding the natural motion of the body. Slamming occurs whenthe body re-enters the water after losing contact with the surface, causing fatigue (Backer,2009). These extreme loads can be difficult to predict, and may be up to 100 times the meanload (Clément, 2002). Corrosion also causes fatigue, although ceramic coating offers somesolution to the problem (Drew, 2009). For example, in 2003 the Nissum Bredning had to bebrought back to shore due to a rusty screw, which could have been prevented had stainlesssteel been used (Callaway, 2007). Leakage of pumps or hydraulic fluids may also causeconcern; in 2007 the Aqua Buoy sank due to a pump failure (Callaway, 2007).1.3.3 Frequency IssuesThe frequency of waves are challenging for multiple reasons: a lack of compatibility withelectric generators, a lack of compatibility with bodies, and a narrow bandwidth (Falcão,2010). Generators typically require frequencies 500 times greater than the frequencyprovided by waves (Clément, 2002). For a single body impinged on by a harmonic wave,resonance between the body and wave is optimal. However, unless the body is large (lengthsgreater than 10m) or supplementary weight is added, wave frequencies are typically too lowto resonate with the body (Falcão, 2010). For small bodies, the resonance bandwidth is15