Mass Damper and Buoyancy Functioning of a Submerged Tank on the Response of Fixed offshore Platforms

By increasing demand for oil in recent years، explorations from deep offshore fields are feasible. In such deep waters، even fixed offshore structures may have considerable movements under design loads، while having less displacements of the platform is often requested. Many innovative concepts have been proposed to minimize responses of structures under environmental loads in recent decades. In a tension leg platform، the buoyancy force causes tension in the tendons، which is changed by platform movement and produces a lateral stiffness to reverse the platform into its initial position. The amount of generated additional stiffness depends on the platform displacement and buoyancy forces. Fixed submerged tanks may be used in design of a compliant platform in deep water to reduce transfer weight of the structure to the support and to decrease the effects of legs buckling. However، the tanks should be located in an appropriate water depth to minimize the effect of wave forces. In order to decrease the response of fixed offshore platforms in deep waters، an innovation concept is presented. In this concept، a submerged tank is tied up to the platform in an appropriate location acting a buoyancy force to the system. This force adds tension force to the legs which may reduce the required chord diameters and/or eliminate some braces. However، the added mass of the tank due to wave action has considerable effect on dynamic behavior of the system. In addition، the vertical buoyancy force of the tank generates a resistance moment in the system when the tank oscillates. This resistant moment depends on the location of the tank and time. In this paper، considering the effects of the tank on the platform responses، solutions for reducing platform displacement are investigated. Analyses have been carried out by taking into account the large deflection and nonlinear geometry effects for which a MATLAB program has been developed featuring the following capabilities: Calculation of wave forces based on the Morison equation for jacket members and the Froude-Krylov method for the tank. Taking into account waves and structure interaction. Non-linear analysis of the structure considering large deformations effects. Dynamic analyses results showed that the tank acts as a weight damper under wave actions. In this case، the added mass has also contribution on the inertia force. So، there is an optimum stiffness for each mass. For dual mass damper and buoyancy functioning of the tank، the stiffness should be defined in such a way that the performance of the tank would be appropriated in both consequences. Results of analyses on a case study platform show that the performance of the tank on reducing the platform responses is much better for the dual mass damper and buoyancy functioning comparing to only the mass damper functioning.