This paper studies the influence of three different calculation methods of the second-order low-frequency (LF) wave forces on the tanker responses and hawser/mooring tensions in relatively shallow water region. The vessel-mooring-riser coupled dynamic analysis computer program HARP is used to simulate the coupled dynamic responses of offloading tanker moored to a SPM (Single Point Mooring). Because the SPM is supposed to be deployed in shallow water and the slowly varying drift motions of the tanker are to dominate the motion responses in typical operational conditions, the accurate calculation of LF wave-force quadratic transfer functions (QTFs) becomes important especially for mooring and hawser tensions. Like common practice, the so-called Newmans approximation and another approximation method without including complicated free-surface integrals are first used to calculate the LF QTFs on the offloading tanker and they are compared with the complete QTF results. Further comparison is performed by calculating the resulting LF wave-force spectra and response time series by using the three different methods. The impact of the three different approaches on vessel surge motions and hawser/mooring line tensions is also addressed.