Calculation of Second Order Drift Forces on a FLNG Accounting for Difference Frequency Components

The usual procedure for calculation of slow drift forces is to simplify the quadratic transfer function by representing the difference frequency components in terms of the zero difference results. In this way the second order problem is much simplified as well as the computational effort. However this approximation has some limitations and in particular for the slow drift oscillations problem it may be important to consider correctly the difference frequency components. The paper presents an analysis of the slow drift exciting forces on a FLNG (Floating Production Storage and Offloading system for production of LNG). The complete second order forces are computed by WAMIT V6.1s which is based on a boundary element method. Results of the horizontal second order drift forces are presented for several difference frequency values. Deep water and two shallow water depths are considered, since water depth seems to have an important effect on the quadratic transfer functions.