The direct drive train has the expensive, high-torque and precisely controlled generators which are larger than ordinary ones, increasing the cost. So far, there have emerged some design concept of offshore WTG (Jones et al., 2012; Jones and Chao, 2011). The hydraulic WTG can remove the gearbox and install the generator on the ground, which can reduce the mass on the top of the tower and the cost of installations and maintenance.