| A series of oil fields on land had been found since the founding of our country,like Daqing oil field and Zhongyuan oil field.At the same time,some oil fields in shallow waters had been found in the Bohai sea near Dongying .With the development of economy of China,the demand for energy is increasing .But the reserves of the oil and gas in the oil fields on the land are becoming lower and lower.As a result, the energy shortage is getting worse,and the exploitation of offshore oil and gas fields is becoming more and more important.At present,the deveiopment of offshore oil field in China mainly concentrates in the Bohai sea,the East China sea and other shallow waters and the jacket platform is the main style.It can be used in the regional where the depth of water ranging from a few meters to several hundred meters.This structure is composed by circular components connected together by welding.Acculative fatigue damage of the platform will occur due to the random load acting on the structure,and the aftereffect is very serious.Many accidents had occurred due to the fatigue damage in the worldwide.For the essentiality of the safety,the assessment of the fatigue life and the fatigue reliability is very important.The phenomenon"stress concentration"can not be ignord when considering the fatigue damage.As the weld shape and its defects,stress concentration will occure and it will make the hot spot stress higher than the stress in other regions. As the importance of the stress concentration for the fatigue life and its reliability,calculating the hot spot stress refinedly is necessary.The designer in the design department considers the fatigue life only when evaluating the fatigue capability in most cases .This method bases on a certaint theory and the result is certain.But there are so many uncertain factors in the process of computing the fatigue damage that everybody can not ignore them ,just like the predigestion of calculation of the wave forces and the errors between the hot spot stress and the actual results,which will make the value of the damage life uncertain.So it is more reasonable to consider some uncertain factors.Long-term states or just a serious condition(ice load)has been considered in the assessment of the fatigue life and its reliability,but,unfortunately,the possibility of the short-term states occurring in the service life of the platform is very great..In the Bohai sea, the extreme wave condition occurs every year .For these reasons, considering the extreme sea states is valuable.Based on an jacket platform ,a numerical model is founded by ANSYS at first.Secondly,the result of the dynamic response of the platform is presented.In this process,the nominal stress time history curves of the elements near weld joint can be presented. Thirdly,the stress concentration factors of the weld joints can be calculated according to the formula which is used to calculate the stress concentration factor given by the specification. Based on these results,the hot spot stress curves can be drawn .The effective stress cycle ranges are counted by the rain-flow counting method.And then,the fatigue life of the tubular joints of the jacket are calculated using S-N curve .At last,the fatigue reliability are calculated.The affect of the extreme sea states will be analysed as the focus. |
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