- This paper introduces wave directionality into a joint environmental contour description that has been used previously in the study of extreme response behavior for uni-directional random seas. A modified cosine power-spreading model that accounts for the frequency dependence of the various waves in the design seastate was utilized. Data from the Gulf of Mexico was used to evaluate the coefficients needed to describe the coupled frequency dependent behavior of the wave spreading exponent and the mean direction of wave travel. To illustrate the methodology the dynamic behavior of a generic steel caisson platform was studied. A finite element model of the platform was subject to a joint environmental description of directional seas. The time domain simulation yielded the stress levels at specified platform elevations. Based upon these numerical predictions, fatigue life estimates were evaluated using a simplified Palmgren-Miner approach. Results for both uni-directional and multi-directional sea simulations were obtained and are compared.