Random Vibration Analysis Of Subsea Pipelines Under Spatially Varying Ground Motions Considering Structural Uncertainty

With the rapid development of marine oil and gas exploitation,dynamic response analysis and reliability assessment of subsea pipelines under earthquakes have become increasingly prominent.Difficulties of these problems mainly include the following aspects.Firstly,ground motions induced by earthquakes have significant spatial variations due to the large spatial scale of subsea pipelines.Secondly,there are nonlinearities in the analysis model,which are the dynamical unilateral contact effect between pipelines and the seabed during vertical ground motions,and large deformations of subsea pipelines during horizontal ground motions,respectively.Thirdly,structural uncertainties are introduced inevitably in the finite element discretization of subsea pipelines for some complex local features,such as the corrosion,welding,attaching organism,and so on.Lastly,ground motions are more likely to exhibit strong randomness during an earthquake due to the natural random factors of soil and complex propagating mechanisms of earthquake waves.Therefore,this thesis conducts relevant research on subsea pipelines from aspects including random vibration response analysis,dynamic reliability estimations and structural uncertainty modeling.The details can be summarized as:1)An efficiency random response and sensitivity analysis method is proposed for multi-span continuous beams with random structural parameters subjected to stationary/non-stationary ground motions with spatial variations.For the case of stationary random earthquakes,the proposed method can provide explicit formulations of random responses by using analytical frequency response functions of multi-span continuous beams.It is free from modal reductions and calculation of the quasi-static solution,hence has higher accuracy and efficiency compared to the conventional quasi-static displacement decomposition method.For the case of nonstationary random earthquakes,the proposed method is implemented in the frequency domain with the help of the Fourier transform.Therefore,it avoids step-by-step time integrations in each frequency point of the power spectrum density functions and has significant advantages in computing efficiency to classical time domain methods.In addition,the proposed method gives explicit formulations of random responses of multi-span continuous beams and facilitates the sensitivity analysis with respect to random structural parameters tremendously.2)Combing the linear complementarity method and subset simulation,a high efficient method is proposed for the reliability estimation of subsea pipelines under spatially varying ground motions with considering the unilateral contact effect between pipelines and the seabed.The dynamic contact of subsea pipelines and seabed during earthquake is considered as a linear complementarity problem,while based on the subset simulation,the small failure probability of subsea pipelines under earthquakes is expressed as a product of a series of intermediate events with larger conditional probabilities.Therefore,the proposed method improves the efficiency of the reliability estimation of subsea pipelines significantly and has a uniform calculating formulation.On the basis,influences of structural and earthquake factors on the reliability are studied through some numerical examples,while the corresponding influence mechanisms are summarized.The key factors affecting the earthquake reliability of subsea pipelines are identified and studied.3)Based on the nonparametric uncertainty theory,a computational method is presented for random response analysis of subsea pipelines with considering the modeling uncertainty and nonlinearity.The present method deals with the modeling uncertainty of subsea pipelines with random matrices globally.Compared to the traditional parameter uncertainty modeling method,it’s more suitable to represent uncertainties resulting from the simplification of complex local features(corrosion,welding,attaching organism,etc.)in the finite element discretization of subsea pipeline.Uncertainty computational models are established for the random response analysis of subsea pipelines under spatially varying ground motions with considering nonlinearities,which are the dynamical unilateral contact effect between pipelines and the seabed,and large deformations of subsea pipelines,respectively.Besides,a composite implicit time integration procedure is introduced in present method and hence the integration instability due to the geometric nonlinearity is avoided.On the basis,propagations of parametric uncertainty and modeling uncertainty in the system are studied.