Numerical techniques are extensively used to predict vessel motions and associated contact forces for offshore operations such as lift and floatover deck installations. The accuracy of such predictions however is highly dependent on the comprehensive modeling of sea state conditions, which is often limited by computational power and time constraints. A time-efficient methodology, suitable for modeling large numbers of installation sea states, is developed to alleviate this problem. The methodology is based on the Constrained New Wave model which has been previously used to overcome similar problems. However this has only been for individual, extreme storm conditions. The accuracy, time-efficiency, and practicality of the revised methodology is demonstrated by means of direct comparison of simulation results obtained for a floatover deck installation on the North West Shelf of Australia. The ability to perform a large number of simulations in a time and cost efficient manner is of paramount importance in assessing the system limitations to varying installation conditions, a case that has always been challenging to designers during the development of oil and gas projects. Such flexibility improves confidence in the overall system, necessary for the accurate assessment of the commercial viability of marginal developments.