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Using appropriate solution techniques for transformer inrush current transient study is of great prominence owing to the inevitable inclusion of differential equations leading to complicated analysis procedures. This study aims to propose an analytical-numerical method to accurately analyze the three-phase three-limb core-type transformer inrush current in different cases considering the nonlinear behavior of the iron core.

The proposed method focuses on acquiring equations for inrush current and also the magnetic core flux by the application of a simulation-based iterative approach. In this regard, multiple integral equations are solved taking the time intervals into account. Then several derivations and integrations of matrix terms are substituted into the obtained results so as to simplify the solution process.

The method provides notable enhancements in computation time and also excellent qualities of accuracy compared with conventional numerical methods.

The proposed method is simulated for two three-phase transformers via MATLAB software. The obtained simulation results have been also compared with experimental tests.

Actually, the analytical-numerical method is capable of computing higher number of iterations in a shorter time efficiently, while making use of the conventional numerical procedures may not result in expected convergences. The simulation results of the proposed analytical-numerical technique illustrate a close agreement with the experimental test, and hence, verify the method preciousness.

During its operation, vibrations in jacket platform due to high operational and environmental loads could reduce its productivity and endanger its safety. Tuned mass damper (TMD) is one of the vibration control devices commonly used in buildings to reduce their response. Basically, TMD is a device attached to a structure as a mass on properly tuned spring and damping elements. The purpose of this paper is to study the utilization of TMD to reduce the wave-induced vibration of platforms.

First, the study investigates the optimum TMD parameters to reduce the response of the platform. The effectiveness of the optimized TMD in reducing platform response due to wave load is then analyzed. Finally, the reduced response of the platform is attributed to an extension of platform service life. Cyclic load in the form of vibration is related to cyclic stress on the tubular connections on a jacket platform, which causes fatigue failure. Fatigue performance of a platform is quantified by its fatigue life, which should be at least twice the intended service life.

It is found that the optimized TMD cannot be achieved by simply adjusting the TMD damping as high as possible. The results show that the response of controlled platforms by properly tuned TMD could be reduced by about 20 percent from the uncontrolled case.

Although vibration of a structure can be reduced by increasing its stiffness, this approach is considered not economical since it increases the cost for material construction. With the falling of oil prices, an optimization of jacket platform structure is considered necessary to reduce its construction cost. TMD can be used as an alternate solution, even though the only recorded utilization of TMD on a jacket platform is known in Sakhalin Platform in Russia. This fact gives a motivation to carry out more studies about TMD application on jacket platform, as this device is already used in numerous of inland structures.