Wave Induced Forces in Offshore Structures Using Linear and Nonlinear Forms of Morison's equation

Wave-induced forces applied to offshore structures can be estimated by using either the nonlinear or linear form of Morison's equation. Difficulties in using the nonlinear form of Morison's equation arise from the nonlinear relation between the wave force and the response of the structure in the drag-dominated term. The linear form of Morison's equation simplifies the estimation of the wave-induced forces in offshore structures. For this reason, engineering designers working on large-scale offshore structures rely on the modified linear Morison's equation. In this article, a comparative study of the linear and nonlinear forms of the wave-induced force from Morison's equation is presented. A parametric analysis is conducted using a numerical computer model of an offshore structure to shed light on the difference between the wave forces as estimated by the linear and nonlinear forms of Morison's equation. The difference is clarified by showing the response of a model offshore structure when subjected to these wave forces. A modification to the linear form of Morison's equation is proposed, which gives better estimates of the nonlinear wave-induced forces. Response results obtained using the proposed model are validated against those obtained using the traditional linear and nonlinear forms of the equation.