Study of Fixed Jacket Offshore Platform in the Optimization Design Process under Environmental Loads

In the structural optimization problem, the aim is to decrease the amount of structural costs and weight, but the safety of platform should not violate the individual limits enforced by offshore codes. The outer diameter and thickness of members are two important variables in the optimization process and their final dimensions should be obtained according to the optimization algorithms such as genetic algorithm. In this process, weight of the jacket is the objective function of optimization problem and constraints are design criteria such as axial and flexural stresses, buckling of members and displacement of offshore structure that should satisfy the limitations imposed by offshore design codes. The drag forces of wave, current and wind on a unit length of structural tubular members of the jacket which are located below and above water surface are directly related to their outer diameter. However, the inertia force of wave is related to the square of the outer diameter. Thus, by changing outer diameter of structural elements during the optimization process, sea environmental forces on these members and their resultant forces on the platform will change. The structural members of the jacket are classified in four main groups including legs, horizontal members, diagonal braces and vertical braces. Each of these groups has different contribution in the optimization process and their degrees of importance are investigated in this research. The results show that horizontal members of jacket have major contribution in the optimization process among other groups. Afterward legs and diagonal braces have the second and third ranks in the contribution percentage respectively. Finally, the lowest contribution in optimization process belongs to the vertical braces.