Examination on low-frequency QTF of a platform

For deep water offshore structures, the second order different frequency force may induce large responses in the horizontal directions. The second order different frequency force is usually computed by the Fourier transformation of the second order quadratic transform function (QTF), which is hard to be obtained due to its complexity, time consuming and the influence of ‘irregular frequency’ in its computation. In practical application Newman’s approximation[5] is widely used, without considering its viability.