A practical formulation for estimating the extreme vector excursion of a floating structure

In the design of a moored floating structure, the traditional approach is to assess the extreme surge and sway responses separately. This paper highlights the importance of considering the extreme lateral excursion, based on the vector sum of the surge and sway offsets, and an approximate formula is developed for this purpose. The method allows for first- and second-order platform motions, as well as mean offsets. The extreme vector excursion is expressed as a simple function of the maximum surge and sway responses, and the correlation coefficient between the two motions. The proposed approximation is benchmarked against more rigorous methods, including time domain simulation. It is found that provided the estimates for the surge and sway extremes are precise, the formula itself does not induce significant errors in the extreme vector excursion. Moreover, it is observed that the prediction tends to be conservative. The practicality of the proposed formulation makes it amenable for incorporation into an existing design code or analysis procedure.