| In recent years,"Energy Crisis"happened more and more frequently, which made people pay increasing attention to offshore natural resources, especially oil and natural gas. However, daily production of most fertile oil fields passed their peak yields, which makes it urgent to explore new reservation so as to settle the problem. It is reported that Shengli oil field which is located in Bohai Bay, has large area of virgin fields for exploitation. However, a large portion of them are"marginal fields"and located in muddy soft foundation. This demands the developer design offshore platform that could not only be applicable for the soft foundation, but also satisfy the economy of the whole project. Therefore, traditional jacket platform which consumes more piles and steel bar cannot meet the requirement of these small oil fields. Instead, new movable platform with lower cost should be designed and adopted.In this paper, a new concept of gravity platform with skirt plate is introduced as single bucket foundation, the dimensions are designed and its rationality is also analyzed according to daily yields of the oil field. When put into practice, the platform will firstly be exposed to working loads- daily oil loading exerted on the caisson bottom in vertical direction which changes evenly with time. Moreover, the Bohai bay is located on active seismic belt area where random seismic loading may affect the safety of the platform. In addition, relatively high latitude would cause the water freeze and expose the platform to ice blocks and/or ice clusters; The harsh sea wave occur everywhere in Bohai Bay, all of which would lead "structural resonance". In consideration of the economic importance, environmental significance and all possible factors mentioned above, tests should be performed to determine the static and dynamic stability, both in geotechnical meaning and structural meaning.The content of this text includes the following four parts:(1) The background and research progress about bucket foundation and gravity platform respectively are introduced separately, the current proceedings both home and abroad are summarized and the problems to solve are put forward.(2) Based on the practical soil conditions and productivity of oil fields in Bohai Bay, a new concept of platform- gravity platform with single bucket foundation is introduced and designed, then the dimensions and fabricating materials are initially quantified so as to control the cost and expand the profits.(3) A scale ratio of 1/25 is chosen for laboratory reduced-model tests. First, characteristics of soil samples selected from Bohai Bay are quantified and the foundation soil in reduce-scaled model test is formed with the original soil. Second, according to the seepage principle, total stress transducers and pore pressure transducers are pre-buried in the soil so as to monitor the stress state, and then the model test data is converted back to the prototype state through MATLAB programming. Thrid, referring to numerical simulation, a series of Drucker-Prager yield criteria and Mohr-Column strength criterion are employed respectively, so as to judge the credibility of the model data and determine the yield strength. Fourth, based on the conclusions reached above, the oil capacity of the caisson is quantified as 1000 tons.(4) The structural stability of the platform under possible environmental loadings is analyzed through finite element method. First, response of the platform under random seismic loading is simulated with 2 groups of amplitude-modulated actual seismic record and 1 group of artificial earthquake wave. It is seen from the analysis that in contrast with the horizontal response, the vertical response is slight. It can also be concluded from the time-history displacement and maximum nodes displacement that under random earthquakes, the deformation of the platform is mainly in accord with first and second vibration modes; In the vertical direction, absolute displacement response of the column nodes decreases with the height when the height is smaller than 12m; when greater than 12m, it increases with the height. Second, the ice loading in the winter is simulated with simplified concentrating mass model and/or ice induced vibration model of ice cluster. Through finite element analysis, we can see that dynamical amplification effect is obvious under cyclic loads induced by ice cluster and the displacement response is much bigger than the concentrated mass model, thus the ice induced vibration should be adopted for analysis. Third, the possible response of the platform under specified wave spectrum is also analyzed, which is concluded that"structural resonance"may happen under specified sea wave spectrum, this should be avoided in engineering practice. |
PDF Full Text Request