Existing far field expressions of second order potentials are by no meanscomplete.Hence there has been no exact far field expression of second orderpotentials.In this paper the far field expression for Φd（2） is purposely avoided indeducing the formulae of second order forces and a series of functions ΦdRn areused.The far field expression of is given,which for （x,U,z）∈Σ,φdRn（2）φd（2）.Using these properties formulae for calculating second order diffractionforces are obtained.To calculate the integral ∫∫1/g fΨds it is dividedinto two parts.One is the integral over a finite domain and the function under theintegral is continuous,so the usual approximate integration formulae may be used.The other is the integral over an infinite domain.Using the far field expressionof first order potentials,formulae for calculating the integral to meet givenaccuracies are given.The mooring force in surge direction is used for comparison between numericalpredictions and experimental measurements.The predicted results are checkedagainst the measured value in a specially designed test.In the low frequencydomain of interest,the mooring forces in surge,for calculated and experimentalspectra are in good consistency so long as the damping coefficients is choosenappropriately.
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