Study the Effect of Wave Directionality on Dynamic Nonlinear Behavior of Jack-Up Subjected to Wave and Earthquake Loading

Abstract

Sea waves are random in nature as they propagate with different frequencies and in different directions. In literature, there are several studies about dynamic behavior of marine structures under random waves in frequency domain, but little or nothing has been done about wave directionality. In this paper, the behavior of a typical jack-up platform operating in Caspian Sea is studied. In order to model the interaction between spudcan and surrounding soil, it was modeled separately by PLAXIS software. The applied soil properties are based on the field measurements. The results from plastic analyses show that they can differ up to 18 percent compared with those obtained based on the recommendations of API. A 3D model of Iran-Khazar Jack-up was studied using ANSYS software. All elements of triangular legs were modeled by PIPE59 elements and the results obtained from previous step were used to model nonlinear interaction of spudcan footings. The wave and current characteristics are based on the field data. The time-history records of earthquake used in this research are based on Manjil earthquake related to the same area. Nonlinear dynamic analyses including large deflection and material non-linearity was performed. The temporal variation of displacement at deck level was compared under solely wave or earthquake loading and simultaneously acting wave and earthquake loading assuming to be in the same direction and in different directions. It is found that when earthquake loading is applied simultaneously with wave loading in the same direction, displacements are less than wave loading alone. However, when they are applied in different directions, especially when the direction of applied earthquake differs about 30 or 90 degrees with respect to wave direction, displacements become larger. In addition, the effect of wave directionality on the maximum displacement of structure was considered.