Mechanical Modeling and Analysis of Stability Deterioration of Production Well During Marine Hydrate Depressurization Production

Different from oil and gas production,hydrate reservoirs are shallow and unconsolidated,whose mechanical properties deteriorate with hydrate decomposition.Therefore,the formations will undergo significant subsidence during depressurization,which will destroy the original force state of the production well.However,existing research on the stability of oil and gas production wells assumes the formation to be stable,and lacks consideration of the force exerted on the hydrate production well by formation subsidence caused by hydrate decomposition during production.To fill this gap,this paper proposes an analytical method for the dynamic evolution of the stability of hydrate production well considering the effects of hydrate decomposition.Based on the mechanical model of the production well,the basis for stability analysis has been proposed.A multi-field coupling model of the force state of the production well considering the effect of hydrate decomposition and formation subsidence is established,and a solver is developed.The analytical approach is verified by its good agreement with the results from the numerical method.A case study found that the decomposition of hydrate will increase the pulling-down force and reduce the supporting force,which is the main reason for the stability deterioration.The higher the initial hydrate saturation,the larger the reservoir thickness,and the lower the production pressure,the worse the stability or even instability.This work can provide a theoretical reference for the stability maintaining of the production well.

Dynamic Analysis of A Deepwater Drilling Riser with A New Hang-off System

The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to a challenging problem:coupling the dynamics of risers with a new hang-off system combined with multiple structures and complex constraints.To accurately analyze the dynamic responses of the coupled system,a coupled dynamic model is established based on the Euler-Bernoulli beam-column theory and penalty function method.A comprehensive analysis method is proposed for coupled dynamic analysis by combining the finite element method and the Newmarkβmethod.An analysis program is also developed in MATLAB for dynamic simulation.The simulation results show that the dynamic performances of the risers at the top part are significantly improved by the new hang-off system,especially the novel design,which includes the centralizer and articulation joint.The bending moment and lateral deformation of the risers at the top part decrease,while the hang-off joint experiences a great bending moment at the bottom of the lateral restraint area which requires particular attention in design and application.The platform navigation speed range under the safety limits of risers expands with the new hang-off system in use.

深水钻井平台-隔水管系统波激疲劳分析

平台运动是深水钻井隔水管系统波激疲劳的主要来源之一。传统的隔水管波激疲劳分析不考虑或简化钻井平台运动,为了更精确地评估隔水管系统波激疲劳损伤,建立了深水钻井平台-隔水管耦合系统动力学模型及波激疲劳损伤评估方法,开展了动力定位和锚泊定位模式下的隔水管系统动力学响应及疲劳损伤分析,深入揭示深水钻井隔水管系统波激疲劳动力学特性。结果表明,两种定位模式下的平台-隔水管系统受波浪载荷的影响,在常规波浪频率范围内发生波频振动,锚泊定位模式下的平台-隔水管系统还呈现一定的低频特性;两种定位系统下的深水钻井隔水管系统疲劳损伤最大值均发生在隔水管系统顶部,锚泊定位模式下的隔水管系统波激疲劳损伤变异系数较大,动力定位系统下的隔水管系统波激疲劳损伤较大。

Numerical Investigation on Dynamic Characteristics of Casing Deformation During Marine Hydrate Production

Formation subsidence is inevitable during marine hydrate decomposition,and the consequent casing deformation seriously threatens the security of sustainable hydrate production.Owing to insufficient observed data of formation subsidence in field,displacement boundary condition of casing is undetermined.Thus the conventional static methods are inapplicable for the calculation of casing deformation in hydrate production well.The present work aims at proposing an approach to investigate dynamic deformation of the casing during hydrate production.In the proposed methodology,based on the movement theory of hydrate decomposition front,hydrate decomposition process can be simulated,in which hydrate reservoir strength formation subsidence showed time-dependent characteristics.By considering the actual interactions among casing,cement and formation,three models of hydrate production well are developed to reveal the static and dynamic deformation mechanisms of the casing.The application of the proposed methodology is demonstrated through a case study.Results show that buckling deformation and bending deformation of casing reduce the passing ability of downhole tools in deformed casing by 4.2%and 7.5%,respectively.With the progress of hydrate production,buckling deformation will increase obviously,while a little increase of bending deformation will occur,as the formation slippage induced by formation inclination is much larger than that caused by hydrate decomposition.The proposed approach can provide theoretical reference for improving casing integrity of marine hydrate production.

Optimization Design of Fairings for VIV Suppression Based on Data-Driven Models and Genetic Algorithm

Vortex induced vibration(VIV)is a challenge in ocean engineering.Several devices including fairings have been designed to suppress VIV.However,how to optimize the design of suppression devices is still a problem to be solved.In this paper,an optimization design methodology is presented based on data-driven models and genetic algorithm(GA).Data-driven models are introduced to substitute complex physics-based equations.GA is used to rapidly search for the optimal suppression device from all possible solutions.Taking fairings as example,VIV response database for different fairings is established based on parameterized models in which model sections of fairings are controlled by several control points and Bezier curves.Then a data-driven model,which can predict the VIV response of fairings with different sections accurately and efficiently,is trained through BP neural network.Finally,a comprehensive optimization method and process is proposed based on GA and the data-driven model.The proposed method is demonstrated by its application to a case.It turns out that the proposed method can perform the optimization design of fairings effectively.VIV can be reduced obviously through the optimization design.