Research On Non-linear Analysis Method Of Jack-up Unit’s Structure In Deeper Sea

With the increasing strength of offshore oil exploration and development，the Jack-up unitis gradually applied in the deep water and severe environmental sea area. The key point ofapplying the Jack-up unit in such condition is reflecting the physical process between the unitand environment accurately. This paper mainly studies the dynamic assessment model,boundary problem and the scientific analysis method, which can be applied in the deep seaand severe offshore condition for the Jack-up unit. The purpose of this paper is seeking thescientific and efficient method to do the nonlinear analysis for the structure、foundation andwave load.With the increase of water depth, both the leg length and size of the Jack-up unit willgreatly increase. Comparing with the offshore fixed unit, the natural period of the Jack-up unitincreases a little，which is gradually approaching the main range of the wave period.Therefore, dynamic response problem of the Jack-up unit becomes very obvious. The keypoint of improving the Jack-up unit performance and reducing the oil and gas developmentcost in the deeper sea is to solve the problem of the structure nonlinear analysis method forthe Jack-up unit in the deeper sea (water depth exceed90meters). The follow aspects havebeen researched deeply:(1) Research on structure performance analysis method for Jack-up unit in the deeper sea.The comparison has been made between the traditional design wave method and moreadvanced random analysis method for the application in the performance analysis of marinestructure. The applicability and limitation of those two methods using in the deeper sea (waterdepth exceed90meters) have been studied deeply. The dynamic analysis method suitablefor Jack-up unit in the deeper sea is summarized, which can reduce the unnecessarilyconservative in the traditional analysis method and improve the overall performance of theunit. The general guiding thought of performance analysis and4dynamic analysis methodsfor the Jack-up unit are also proposed in this paper. The basis for the key techniques researchand system research of the Jack-up unit nonlinear structure in the deeper sea is provided. Theunit model analysis is carried out on the basis of Jack-up unit for3kinds of leg structure,which are the international mainstream at present. The natural period of the Jack-up unit inthe deeper sea has been acquired to validate the correctness of this paper. (2) Footing fixity research of Jack-up unit. Footing fixity research is always the focusproblem in improving the performance of the Jack-up unit. Footing fixity problem has littleeffect on the dynamic performance of the unit, which is can be ignored for this reason.However, with the water depth increasing to the deeper water, the fixity problem becomesprominent extraordinarily, which has great influence on the unit dynamic response analysis. Inorder to reduce its effect on the unit performance and improve the overall performance of theunit, it is necessary to solve the fixity problem with the suitable methods. The comprisingresearch has been carried for the Footing-soil interaction curves, which is obtained from theanalysis method and numerical analysis method. Using the model C theory, which is takenthe Footing-soil strain hardening law into consideration, the research is also carried out on theelastoplastic boundary problem of Footing-soil. Its result can determine the fixation stiffnessof the Footing and soil、simulate the combined vertical torque and horizontal load andimprove the design boundary between the Footing and soil. This method can simulate theinteraction process between the Footing and soil more truly, which can improve the dynamicperformance of the Jack-up unit in the deep-shallow water greatly.According to the NewWave theory, the research on dynamic nonlinear problem ofJack-up unit is carried out by using the structure performance research method stated above,which is taken the Footing-soil boundary of strain hardening theory into consideration.Usually the regular wave assumption is adopted in calculating the interaction between thewave and structure, taken the Airy and Stokes wave for example, in which the wave height isgenerally measured by the significant wave height. This method considers all wave energy isconcentrated in a wave frequency. As a result of neglecting the real wide spectrum characterof sea wave,the calculated structure dynamic response is not representative.Basic concept of the NewWave is stated in this paper. New ultimate wave model is alsointroduced, which is taken the randomness、spectrum and nonlinear of the wave intoconsideration. The new wave model can not only be used as worst design standard for harshmarine structural calculation, but also simplify the calculation of Second order wave forcegreatly and save a lot of time cost. The method and steps of introducing the NewWave into thehydrodynamic calculation between wave and structure is sated. Based on the NewWavetheory, the nonlinear dynamic response analysis of the Jack-up unit is carried out for three different kinds of leg structures finally. Comparing with the nonlinear dynamic responseanalysis results obtained by the international traditional design method, the important positionof the analysis method and boundary problem in the Jack-up unit design for the deep-shallowwater is summarized, which can improve the structure design of the Jack-up unit in the deepersea greatly.