Systematic Accuracy Assessment of Two Frequently Used QTF Approximations in the Case of a Rectangular Barge

This paper presents numerical results of the monodirectional second order forces acting on a rudimentory FPSO hull shape. The exact calculation of second order loads through potential theory is computationally challenging and requires the evaluation of a slowly-convergent free surface integral. In practice, this integral is often neglected as it is widely assumed that it does not contribute significantly to the overall load and few commercial seakeeping software propose this option anyway. An even rougher and widely used approximation consists in expressing the second order loads occuring at a non-zero difference frequency from the mean drift loads. It is called the Newman’s approximation; it enables serious CPU time gains, but at the cost of a worse accuracy. The object of this paper is to investigate on a simple case the accuracy of each of these two approximations (without the free surface integral and Newman’s approximation) and the influence of parameters such as the water depth and the sea state wave length. Depending on the resonant period of the considered mooring system, it may give some insight on a good compromise between CPU time and accuracy when choosing the way of determing the second order loads.