Analysis of natural oscillations of the offshore fixed platform

Offshore fixed platforms are the main hydrotechnical structures for the development of marine oil and gas deposits. They are designed for drilling wells, the oil and gas extraction and preparation, the pumping water into a layer, the wells repair, the preparation of raw materials for the transportation.Structures for development of marine deposits are the most numerical group of offshore structures. At the same time, such structures are one of the most difficult and largely problematic. To avoid the threat of the resonance that is accompanied by the sharp increase in the oscillation amplitude it’s very important to know values of periods and frequencies of natural oscillations of the structure. In this article the impact of the flexibility of the pile foundation on values of dynamic parameters of the offshore fixed platform with the depth of immersion 11 m were analyzed.The analysis of natural oscillations of the platform on piles was performed using its two discrete finite-elements models: the flexibility model,wherethe interaction of soil basis and piles was implemented by using of finite elements that simulated the elastic connection between joints,and the rigid model,where piles were rejected and the platform was considered to be rigidly attached to the absolutely immovable rigid body.For every model first twelve basic form of natural oscillations were analyzed. In first two models of natural oscillations of the platform took part basically only the flexible connection mast, in the third form of natural oscillations took part basically only the flambeau consoletherefore values of frequencies and periods of oscillations didn’t depend on the flexibility of the supporting foundation of the platform. In all other forms of natural oscillations of the platform taking into account the flexibility of the supporting foundation changed values of dynamic parameters of 1,25-1,5 times that indicates the need to take into account this factor when analyzing of stress-strain state of members of the oil production structure.