ABSTRACT Forces on platforms composed of slender members in combined waves and currents have usually been calculated by summing the wave and current velocities before squaring them and applying the appropriate drag coefficient in Morison's equation. Recent measurements at the Lena guyed tower have cast doubt on this standard method of coupling the wave and current kinematics. A data segment recorded during Hurricane Frederic at the Cognac platform provided an opportunity for further verification of various methods of calculating the forces. At the time of the measurements, the significant wave height had declined from a maximum of 28 feet to 23 feet, but the surface current had increased to 3.0 feet/sec. The wave force model used in this study has been described by Rodenbusch and others, and is based on random directional wave theory. Comparisons with other data from Cognac and from the Ocean Test Structure have previously shown that this model provides an accurate means of predicting wave loads on template type structures. The model was first run in its standard configuration with coupled wave and current kinematics. The current and wave were then uncoupled by squaring the velocities before adding them. Alternate methods of stretching the wave kinematics above the mean water level and two-dimensional simplifications of the theory were also tested. The standard method produced the best prediction of the fluctuating thrusts in the two instrumented members. Uncoupling the wave and current kinematics produced forces that were clearly smaller than the measurements. All of the methods under-predicted the mean forces in the members. INTRODUCTION Random directional wave force (RDWF) models are increasingly used in the design of deep water structures. These structures are flexible enough that dynamic analysis is an important part of the design process. The distribution of energy with frequency in the wave spectrum must thus be considered. Forristall et al (1978) have also shown that the random directional wave model provides a more accurate description of the natural environment than regular wave theories do. Structures composed of slender members have a nonlinear response to waves, so Monte Carlo simulations are used instead of linear transfer functions for wave forcing. RDWF models can differ in their treatment of wave kinematics near the mean water level, force coefficients, and the combination of waves and currents. The basic model used in this study was described by Rodenbusch (1986). Three dimensional wave kinematics are calculated from the directional spectrum using Iinear wave theory. The kinematics near the mean water level are modified using the "delta-stretching" algorithm described by Rodenbusch and Forristall (1986). The force coefficients in Monson's equation are taken from the large scale laboratory measurements made by Rodenbusch and Knistrom (1986). The drag coefficient is specified as a function of member roughness and the amplitude to diameter ratio of the flow.