To standardize and simplify the design process of offshore structures, the designer wave load is developed. The load can accurately predict the extreme response of offshore structures by the static analysis under the realistic wave conditions. The designer wave load can be calculated by multiplying modification factors and the wave force based on Morrison's equation with the mean wave height and mean wave period.
The dynamic response of offshore structures subjected to random wave forces can be computed by the random vibration analysis (Malhotra and Penzien 1970a), and its maximum expected value and probability of the first passage of a barrier level during its lifetime can be examined (Cartwright and Longuet-Higgins, 1956, Vanmarcke, 1975). However, it is obviously inefficient to carry out dynamic analyses repeatedly during design process and also troublesome to combine dynamic responses with other design responses due to design loads. Since it implies the necessity of the simple design methods to evaluate extreme responses of the offshore structure, this paper describes the fundamental theory of the designer wave load, which evaluates the maximum expected responses of an offshore structure by the static analysis (Taniguchi and Kawano, 1998). By knowing the wave height and the wave period, the Morrison's equation enables to calculate wave force on the vertical column immersed in the sea. Therefore, the maximum expected wave force on the vertical column can be determined by the probability analysis based on the spectral form of wave force. To calculate the maximum expected wave force t&n the known parameters such as the mean height and the mean wave period, the modification factors for them are defined. At the time, since the drag and inertia forces do not reach their maxima at the same time and the same frequency, the modification factors for them are defined respectively.
