In this paper attention is paid to low frequency wave drift forces on a semi-submersible moored in irregular head waves and to the low frequency horizontal motions caused by the drift forces. The wave drift forces are computed based on three-dimensional potential theory using the direct integration method. Time records of the low frequency drift force in irregular waves are computed using the second order impulse response function technique. Results of this method are compared with results of the direct summation method. The comparison shows that use of the second order impulse response function leads to numerically accurate results. A computed low frequency drift force record is compared with a measured drift force record. The comparison shows that the predictions are qualitatively correct but some 30 to 40 per cent lower than measured data. Results of time domain simulation computations of the low frequency surge motions in irregular head seas are compared with measurements. A discussion is given regarding differences between computations and measurements.