“…The structure does not supply resistance to horizontal forces due to wind, ocean currents or waves. However, restoring forces are provided through buoyancy, ballast, guy wires, axial piles, tendons and other devices (Haslum and Faltinsen, 1990;Sellers and Niedzweck, 1992;Bar-Avi and Benaroya, 1997;Patel and Witz, 2013). As a result of this structural concept, the natural frequencies of compliant towers are usually lower than wave excitation frequencies (Islam et al, 2009).…”