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Abstract When the sinusoidal buckling of suspended tubular strings is transforming to helical buckling, the deformation changes drastically, which makes the tubular strings extremely unstable. The quantitative description of this buckling configuration in previous theoretical studies is not precise enough. In our current study, dynamic relaxation method is applied to carry out numerical simulation of buckling transition of suspended tubular strings. To obtain stable buckling configurations, larger Rayleigh damping is used to attenuate the dynamic effect. With the increase of axial compression load, the lateral buckling of the suspended strings suddenly becomes spiral shape with two contact points. As the string length increases, the position of the lower contact point remains almost at the same position. While the upper contact point gradually moves upward, and the helix angle between the two points increases and gradually tends to be constant. The maximum contact force is located at the lower contact point, the maximum shear force is located at the bottom of the strings, and the dimensionless distance between the location of the maximum bending moment and the well bottom is about 1.0. These loads, including the maximum contact force, maximum shear force and maximum bending moment, gradually decrease with the increasing string length and tend to be constant. The dimensionless critical load is reduced from 4.41 to 3.81 with the effect of string length. The critical load calculated in this paper is the minimum value when helical buckling just occurs. The results obtained in the current study have wide engineering applications, particularly in failure analysis and prevention of helical buckling. Highlights • Stable buckling configuration is obtained by using dynamic relaxation method. • Quantitative description of buckling deformation, shear force, bending moment and contact force are obtained. • The critical load obtained is the minimum one when the helical buckling just happens. [ABSTRACT FROM AUTHOR]

Abstract A novel design of composite drilling conductor (CDC) was carried out for improving the vertical and lateral bearing capacity. Bearing characteristics of CDC was investigated when subjected to lateral and longitudinal loads based on the ultimate subgrade reaction method and pile foundation bearing theory. Theoretical models for lateral displacement and vertical load were established. Finite element analysis (FEA) was conducted, and bearing capacity and influencing factors of the composite structure were studied. Through laboratory experiments, the interaction between the structure and the soil as well as the regularity of soil deformation and stress variation in the surrounding soil were studied during installation, vertical and lateral load application. The results showed that the vertical bearing capacity increased with increasing depth of the bucket foundation under the combined effect of the composite structure. Meanwhile, the depth of the embedded point of the composite conductor increased with an increased bending moment when subjected to a combined load with a constant vertical force. When the bending moment on the structure top was constant, the lateral displacement of the structure as well as the depth of the embedded point would increase with increasing vertical force. Furthermore, the bending moment change on the top of the CDC was more significant than the vertical force regarding the lateral displacement. It is also showed that the internal bucket foundation produced an "elastic plugging" effect when the CDC was subjected to vertical loading, while soil pressure transmission occurred from the upper soil to the underlying strata soil. When subjected to lateral loads, the earth pressure on the bucket sidewall was redistributed, and the bucket foundation produced a lateral supporting effect on CDC. The composite structure effectively reduced the conductor vertical and lateral displacement and improved the lateral and vertical bearing capacity. These results provide useful theoretical references for further development and optimization of the CDC structure. Highlights • Composite design of bucket foundation and drilling conductor was developed. • The interaction of composite structure and soil was studied by laboratory experiment. • The theoretical analysis on lateral displacement and vertical load was conducted. [ABSTRACT FROM AUTHOR]

During dual-casing sidetracking, it is found that the hardness values of productive casing, cement sheath, protective casing and rock stratum are different from each other, and the contact surfaces between the drill bit and the whipstock, productive casing, cement sheath, protective casing and rock stratum are also different. In consideration of the actual conditions of contact between the drill bit and the whipstock, productive casing, cement sheath, protective casing and rock stratum During dual-casing sidetracking, we built a mathematic model for describing the moment of dual-casing sidetracking drill bit, and then, on the basis of field engineering cases, obtained the curves by using the MATLAB software for describing the relations between the moments and horizontal moving distances of drill bit under three different working conditions. We thereupon discussed, in the view of metal cutting mechanics, the cutting force and torque of drill bit during the sidetracking, verified the strength of sidetracking drill bit, and further provided the technical suggestions for avoiding the bit jamming and breaking accidents During dual-casing sidetracking. Although the mechanics analysis model of the window drill does not fully explain the bit damage seen in the field, but the related analyses and calculations can be used as references for determining the parameters for both design and drilling of dual-casing sidetracking drill bit. [ABSTRACT FROM AUTHOR]

Helical buckling of tubular strings with connectors is divided into two sub-problems: inter-helical buckling and intra-helical buckling. The suspended section of the tubular string is depicted by beam-column model and the continuous contact section is depicted by helix. Further introducing the continuous conditions, boundary conditions and periodic conditions, the deflection curves of tubular strings can be determined. On the basis of energy stability condition, the critical helical buckling loads are deduced by introducing the potential energy factor. The contact forces on contact points and continuous contact sections and maximum bending moments on tubular strings are calculated. The above studied are further applied into a case study. At last, the supporting effect and boundary effect caused by connectors are introduced and the relationship between inter-helical buckling and intra-helical buckling modes is clarified. The results indicate that for connectors of smaller diameters, the supporting effect plays the most important role and inter-helical buckling is easier to initiate. However, for connectors of larger diameters, the constraint effect is dominant and intra-helical buckling is more likely to occur. Increasing connector diameter and decreasing the length between adjacent connectors can increase critical helical buckling loads. Both the contact forces and maximum bending moments in the intra-helical buckling mode are larger than that in the inter-helical buckling mode, so intra-helical buckling should be avoided in the optimal design of connectors on tubular strings. [ABSTRACT FROM AUTHOR]

To explore the rock-breaking efficiency and vibration characteristics of polycrystalline diamond compact bit in heterogeneous rock, this paper analyzes the typical vibration characteristics and failure modes of polycrystalline diamond compact bit. Then, the transverse combination of rocks was innovatively used to simulate a situation of soft-hard interbedded formation, forming three kinds of transversely combined heterogeneous rock samples with different degrees of heterogeneity. We conducted a series of laboratory rock-breaking experiments and the experimental results indicate that rock heterogeneity greatly impacted vibration acceleration, lateral bending moment, rate of penetration, and drilling trend. With the increase in weight on bit and rotation speed, the rate of penetration gradually increases. However, the tangential, axial, and radial vibration acceleration amplitude of the bit all increase simultaneously, which implies that the vibration impact generated by rock-bit interaction increased. The difference in the cutting depth of the drill bit in the heterogeneous formation causes low efficiency. The stronger the heterogeneity, the lower the rate of penetration. Rock heterogeneity, especially the rock properties of the combined rock samples, significantly impact acceleration and lateral bending moment. As rock heterogeneity increased, the bit acceleration increased significantly, intensifying the bit vibration; the lateral bending moment increased slightly, but its fluctuation intensified. The strength difference of heterogeneous rock causes eccentricity, the harder rock exerts greater force on the bit, causing the bit to deviate from the original trajectory. The greater the difference, the greater the eccentricity, consequently, the more the bit shifts to the softer side. Adjusting rotation speed and depth of cut control may be feasible solutions to solve the problem of low drilling speed and high vibration in heterogeneous formation. It is hoped that the findings in this paper will be helpful to explore a reasonable way to reduce vibration while maintaining high efficiency. • Typical vibration characteristics and failure modes of PDC bit are analyzed. • Transversely combined heterogeneous rock samples are innovatively formed and used. • Rock heterogeneity greatly impacts efficiency, vibration and drilling trend of PDC bit. • Adjusting rotation speed and depth of cut control may be feasible solutions. [ABSTRACT FROM AUTHOR]

To explore the rock-breaking efficiency and vibration characteristics of polycrystalline diamond compact bit in heterogeneous rock, this paper analyzes the typical vibration characteristics and failure modes of polycrystalline diamond compact bit. Then, the transverse combination of rocks was innovatively used to simulate a situation of soft-hard interbedded formation, forming three kinds of transversely combined heterogeneous rock samples with different degrees of heterogeneity. We conducted a series of laboratory rock-breaking experiments and the experimental results indicate that rock heterogeneity greatly impacted vibration acceleration, lateral bending moment, rate of penetration, and drilling trend. With the increase in weight on bit and rotation speed, the rate of penetration gradually increases. However, the tangential, axial, and radial vibration acceleration amplitude of the bit all increase simultaneously, which implies that the vibration impact generated by rock-bit interaction increased. The difference in the cutting depth of the drill bit in the heterogeneous formation causes low efficiency. The stronger the heterogeneity, the lower the rate of penetration. Rock heterogeneity, especially the rock properties of the combined rock samples, significantly impact acceleration and lateral bending moment. As rock heterogeneity increased, the bit acceleration increased significantly, intensifying the bit vibration; the lateral bending moment increased slightly, but its fluctuation intensified. The strength difference of heterogeneous rock causes eccentricity, the harder rock exerts greater force on the bit, causing the bit to deviate from the original trajectory. The greater the difference, the greater the eccentricity, consequently, the more the bit shifts to the softer side. Adjusting rotation speed and depth of cut control may be feasible solutions to solve the problem of low drilling speed and high vibration in heterogeneous formation. It is hoped that the findings in this paper will be helpful to explore a reasonable way to reduce vibration while maintaining high efficiency. • Typical vibration characteristics and failure modes of PDC bit are analyzed. • Transversely combined heterogeneous rock samples are innovatively formed and used. • Rock heterogeneity greatly impacts efficiency, vibration and drilling trend of PDC bit. • Adjusting rotation speed and depth of cut control may be feasible solutions. [ABSTRACT FROM AUTHOR]

During the subsea well construction and workover, a subsea well is connected to the drillship through the blowout preventer (BOP) and the drilling riser. Environmental loads acting on the drilling riser and the drillship are transferred to the wellhead, conductor, and casings, which can cause fatigue issues at critical points such as at the casing couplings. The main function of a subsea wellhead system is to perform the interface between the BOP and the well, ensuring its integrity throughout its operational life. Factors such as the increase in the weight and height of the BOP on 6th generation platforms contributed to the increased risk of failure due to fatigue. The system in this work is represented by a 3D finite element model including internal cement, conductor, and surface casings. Although simplifications were adopted in the modeling, the essence of the well physical behavior remains. The work addresses the influence of the wellhead stick-up on the maximum stress in the conductor below the seabed, comparing analyses of two wellhead stick-up and including horizontal displacement resulting from the integration of the platform/riser dynamics. These horizontal loads are transferred to the top of the wellhead system as a bending moment and shear force. A time series containing riser horizontal loadings is utilized to obtain the von Mises stress. This information is used to count cycles with the rainflow counting technique. The number of cycles resulting for the two wellhead stick-up is compared and demonstrates how wellhead stick-up can be influenced by an increase in dynamic loads acting on the wellhead top. An influence can be observed on the maximum stress point of the conductor located below the seabed. From these results, it was possible to conclude the wellhead stick-up can influence the depth of a critical point in the conductor below the seabed. • Subsea wellhead ensures integrity to the well throughout its operational life. • Loads acting on the drilling riser can cause fatigue at the subsea wellhead. • Subsea wellhead exposed to a failure reduces the structural integrity. • Wellhead stick-up effect can be observed at maximum stress point below the seabed. [ABSTRACT FROM AUTHOR]

With the advancement of technology, various types of downhole tools have been developed. It is necessary to ensure that downhole tools can be tripped to the predetermined well depth through a drilling and completion string without any damage. However, mathematical models to predict the downhole tool deformation under axial loading are lacking. In this paper, an eccentric tube deflection model is established according to the geometric characteristics of downhole tools. First, six contact cases of an eccentric tube under axial loading in a curved wellbore are considered. The model solves the eccentric tube deflection in the different contact cases through the beam-column model and energy method. Second, using this model, the critical axial load in the contact cases is examined along with the deflection, bending moment, and contact force of the eccentric tube. The results show that the deflection, bending moment, and contact force of the eccentric tube are related to the contact case. The bending moment and contact force of the eccentric tube in wrap contact under axial compression are higher than those in other contact cases. In particular, when the eccentric tube is subjected to wrap contact under axial compression, the maximum bending moment and contact force increase sharply, and this aspect can help evaluate the accessibility of the downhole tool. Finally, this paper proposes a new method based on the eccentric tube deflection model to analyze the accessibility of a completion string in the context of eccentric downhole tools. The effectiveness of the proposed method is demonstrated through a field case. • A deflection model of eccentric tube in curved wellbores is established. • Critical axial load, bending moment and contact force are obtained. • The influence of offset on deflection, bending moment and contact force of eccentric tube cannot be neglected. • A method for evaluating the accessibility of a completion string with eccentric downhole tools is proposed via a field case. [ABSTRACT FROM AUTHOR]

Buckling of concentric tubular strings in pipe is an essential problem in the fields of medical, biological, and nanomaterials, especially in petroleum engineering. Completely different from the buckling of the classic Euler compression columns, the buckling of concentric tubular strings involves geometric nonlinearity and contact nonlinearity, and has more complicated geometric deformations and buckling configurations. In this paper, a finite element method combining the spatial beam element and the two-layer contact gap element is proposed, and a virtual transient dynamic method is introduced to solve the static buckling of concentric tubular strings. The internal concentric tubular strings are discretized into spatial beam elements, and the contact relationships between tubular strings are described by two-layer contact gap element. Meanwhile, the larger damping is used to attenuate the dynamic response of the system, so as to find the static part of the transient dynamic solution. Appropriate damping and element length are determined by numerical simulation. The structural characteristics and contact states of eight post-buckling configurations are studied. The helix pitch values of specific configurations are in good agreement with the theoretical solutions. In addition, the distribution of bending moment, shear force and contact force in various post-buckling configurations is studied. For the full helical buckling, it is found that there is only one non-contact section in the transition section of inner tube and two non-contact sections in the transition section of middle tube. The research results of this paper provide a theoretical basis and reference for further study on the buckling of multi-layer concentric tubular strings. • Finite element method and transient dynamics method are used to solve the static buckling of concentric tubular strings in pipe. • Eight post-buckling configurations as axial compression force increases for concentric tubular strings in pipe. • Transition section includes only one non-contact section for inner tube while includes two non-contact section for middle tube. [ABSTRACT FROM AUTHOR]

The SB Oilfield in China is characterized by ultra-deep wells with a depth more than 7000 m. During drilling the large wellbore in upper section, stabilizer failure occurred frequently. Three out of seven ultra-deep wells occurred with a fracture accident at the box connection of stabilizer. These accidents have caused great economic losses to the oil fields. However, the cause of this failure cannot be explained by previous research results. In this paper, we present an innovative method, which combines the analyses of drillstring dynamics and localized stress of tool joint, to study the failure mechanism of the stabilizer box connection and theoretically explain the cause of failure. Firstly, numerical simulation model of drillstring dynamic was carried out, from which we analyzed the dynamic loads on drillstring with focus at the stabilizer. Then an ABAQUS model was established to study the mechanical characteristics of the stabilizer thread connection. Finally, the vibration signal was decomposed by Fourier analysis, the fatigue frequency ratio and vibration frequency spectrums were obtained. On this basis, the fracture mechanism of stabilizer box connection was revealed. The results showed that the unreasonable drilling parameters could cause severe impacts between drillstring and borehole. These impacts were prone to induce high-frequency bending moment at box connection of the stabilizer by noting that large variable cross-sections of drillstring were used in SB Oilfield. The high-frequency stress acting on thread caused the failures of stabilizer. The maximum bending moment at the box connection of stabilizer reaches 196.49 kN m. It results in a maximum difference of von Mises stress being 407.40 MPa at the first engaged thread with a frequency of 27.06 Hz. This leads to a fatigue frequency ratio of 64.58. Such a large fatigue frequency ratio means that the box connection is subject to a large load with a high frequency, which eventually causes the failure of stabilizer. It suggests that the proposed method, based on the analyses of drillstring dynamic stress and localized stress variation of tool joint, is an effective way to study the failure of drillstring stabilizer. • An innovative method is presented to study the failure mechanism of box connection of stabilizer. • The macro dynamic load analysis and local dynamic stress analysis of tool joint are used together. • The fatigue frequency ratio has been proposed as an evaluation criterion. • The analysis results are consistent with the field observation in SB Oilfield of China. [ABSTRACT FROM AUTHOR]

A new model depicting the buckling configuration of a tubular string in a three-dimensional curved wellbore is proposed. During modeling, the toolface angle is considered, and the sinusoidal buckling and helical buckling of the tubular string are considered on the basis of alignment with either the lower curved side or the outer curved side of the wellbore. After modeling, the energy method is used to solve the model, and the critical buckling load, which is related to the relationship between the buckling configuration and the axial load, can be obtained. The critical buckling loads calculated using the model presented in this paper are consistent with the measured data from published experiments and the trends of the He and Kyllingstad model. The results of the new model also show that with an increasing wellbore curvature, the influence of the toolface angle on the critical buckling load and the characteristic parameters of the buckling configuration (wavelength, amplitude, and pitch) decreases. Based on the expressions for the quantitative relationships between these characteristic parameters and the axial load, procedures for calculating the contact force and the bending moment are given. The results for the contact force show that the calculated values from the Johancsik et al. model and the Mitchell model are the average values calculated using the procedures presented in this paper for tubular strings under sinusoidal buckling and helical buckling, respectively. The calculation results show that the influence of the toolface angle on the bending moment of a tubular string under helical buckling is less than that under sinusoidal buckling. By using this model, not only can the critical buckling loads, contact forces and bending moments of tubular strings in three-dimensional curved wellbores be calculated, but the trends of these calculated values with the buckling configuration can also be identified to guide the selection of the control parameters for tubular strings in the field. • A buckling model for a tubular string in a three-dimensional curved wellbore is established. • The critical buckling loads and the parameters (wavelength, amplitude, and helix pitch) can be obtained by the model. • The critical buckling loads calculated by the model are consistent with the values measured in published experiments. • Based on the buckling model, the contact force and bending moment of the tubular string can be determined. [ABSTRACT FROM AUTHOR]

Oil wells typically have production tubing installed within casing strings. In the scenario of buckling, the casing string is commonly simplified to be rigid at the designing stage (Mitchell, 2012). It hasn't been fully explored by literature yet how the neglected casing string displacement impact the buckling mechanism. An improved analytical model in this paper based on Mitchell's study (2012) addressed this issue in vertical wells. The post-buckling configuration typically includes two scenarios: sinusoidal and helical buckling. This paper investigates both scenarios of concentric tubular buckling and develops an analytical model based on the minimum energy theory. The author has verified the new model with well-recognized literature before the application. Computations are also conducted to investigate the prediction improvement than previous models. This new model can find broad applications in both onshore and offshore completion/production operations. To the knowledge of the authors, there are only two existing models so far for concentric string buckling, namely Christman's model (1976) and Mitchell's model (2012). The authors find by case study that (1) Christman's model tends to overestimate the stiffness of concentric string system. (2) Mitchell assumes an unlikely transition configuration before helical buckling, where the concentric string system is independent of the wellbore. As a result, Mitchell's model ignores the wellbore restraint impact on buckling in the transition mode. The author brought up a sinusoidal buckling model for the concentric strings in the transition mode. The analytical model is derived based on the minimum energy theory. This model can evaluate the buckling induced pitch, bending moment, bending stress and total length change. The author also verified the new model with well-recognized literature by Lubinski (1962). In the end, computations investigate the prediction improvement than previous models. • There is very limited research on concentric tubular buckling regardless of its wide application onshore and offshore. • Christman (1976) overestimates the dual pipe stiffness assuming minimal inner pipe motion and evenly distributed loading. • Mitchell (2012) assumes an unrealistic dual pipe buckling configuration under small buckling force. • This new model addresses the deficiencies and provides a more reliable prediction for practical use. [ABSTRACT FROM AUTHOR]