Numerical Application of a Stick-Slip Control and Experimental Analysis using a Test Rig

In 2017, 2019 and 2021, an Operator drilled 3 ultra-deepwater exploration wells in offshore West Africa. The water depth for well A is around 2900m, well B is around 2800m while well C is circa 2100m. The objective of these wells is to evaluate the potential hydrocarbon accumulation in the subsurface area. Besides the usual challenges being wildcat wells and located far offshore, the wells drilled through pre-salt formations. Hole enlargement due to salt dissolution is also a concern apart from salt movement to close the hole owing to its plastic nature. Furthermore, the lack of offset wells for determining wellbore behavior also caused another challenge. Experiences from the nearby wells showed many signs of wellbore instability such as tight spots and losses. The common causes of wellbore instability are unbalanced stress dependent on stress change and rock strength, and shale-fluid interaction due to hydration of clays. A geomechanical model was constructed and detailed salt creep modelling was developed. Well A was drilled until the final target depth with minimal wellbore instability and encountered no losses at all while drilling. Well B which was drilled later, also drilled to the final target depth with very minimal wellbore instability. Well C was drilled with longer salt exposure around 490m. The mud weight selected for all wells also successfully minimized salt creeping. These are strongly evident while drilling and running in casing, no tight spots were encountered. Both wells successfully drilled through salt formation with very minimal salt dissolution. Through the application of geomechanics and systematic drilling fluids design reduce potential non-productive time (NPT) related to salt plastic movement that could be faced by Operator eventually saving Operator's drilling hours.

Testing and gathering data from a well to evaluate the potential of a new field is a common practice in the oil and gas industry. However, performing this task in a subsea environment to test a well in ultra deep water presents significantly greater challenges to the operator and service providers. Since testing in the depths required for the subject well had not previously been attempted, there were concerns of problems that might occur in maintaining response times and control of the test equipment. This paper will present the innovative solution that was devised to perform the evaluation testing in the deepest offshore producing well in the world (1,853 meters or 6,079 feet) from a drill ship with conventional subsea equipment. The equipment, the well, location conditions, and procedures that were used to safely and efficiently perform the evaluation testing will be discussed. To date, this has been the deepest well in which this function has been successfully performed. From the well testing data gathered, the Petrobras Reservoir Engineering and Geology Departments were able to determine production potentials, which indicated that this was the most important discovery of the last few years, as well as to obtain information that would be invaluable for field development. It is anticipated that the field will have an estimated oil production of 180,000 BOPD and 4.8 mm m3 of gas by the year 2,000.

Drilling a wellbore can result in several types of vibration that lead to inefficient drilling and premature failure of drill string components. These vibrations are subdivided based on their operating direction into lateral, torsional, and axial vibrations. Especially in hard and dense formations, high-frequency torsional oscillations are found in the bottom-hole assembly (BHA). These critical vibrations are induced by a self-excitation mechanism caused by the bit–rock interaction. Self-excitation mechanisms are regenerative effects, mode coupling, or a velocity-dependent torque characteristic at the drill bit. To increase drilling performance and reduce tool failure due to high-frequency torsional oscillations, the critical vibration amplitudes localized at the bottom-hole assembly need to be minimized. Increasing the damping of self-excited systems to affect the energy output during vibration is a common approach to mitigate self-excited vibrations. In drilling systems, the achievable damping is naturally limited by the small installation space due to the drilled borehole diameter. Therefore, alternative methods to influence vibrations are necessary. Applying parametric excitation in self-excited systems can result in a parametric anti-resonance and therefore in an energy transfer within different modes of the structure. This allows, among other benefits, improved utilization of the structural damping. In this article, the influence of additional stiffness–based parametric excitation on self-excited torsional vibration in downhole drilling systems is investigated. For this purpose, a finite element model of a drill string is reduced using the component mode synthesis and analyzed with the goal to mitigate torsional vibrations. The multiple degree of freedom drill string model is investigated regarding the additional energy transfer due to the parametric excitation. Robustness of various parameters, especially with regard to the positioning within the bottom-hole assembly, is analyzed and discussed. Additionally, the problem of multiple unstable self-excited modes due to the nonlinear velocity-dependent torque characteristic in drilling systems is addressed.

The offshore industry's current trend is driving towards ultra deepwater wells, combined with deeper total depths. In order to be able to set and run larger and heavier casing strings with hook loads approaching or exceeding 2.5 million lbs, an innovative heavy duty landing system has been developed and put on the market. Such systems must be capable of landing heavy loads while ensuring slip crush resistance. It includes an extended slip with enhanced design (coupled with a new greasing system), a 2.5 million lbs landing carrier and a pipe with a thick walled high strength material section, a light weight tube, and a rotary shouldered connection. One of the current operational challenges with existing heavy duty landing strings is linked to the positioning of the slips, in particular when using extended slip designs. The limited length of the slips area requires additional time for proper positioning and set up. To address these issues, a new slip proof section design has been developed: the combination of an extended crush proof section and the absence of a heat affected zone in that particular area decreases running time while completely eliminating the risk of setting the slips on a weak point. The "Spiri-Reinhold pipe crushing" formula has been used to design this system. However, considering the approximation of this model's results, crushing performance has been assessed using the Finite Element Analysis, as well as physical tests performed with a specifically designed pulling unit. These full scale test results have been compared to the ones obtained with both the Finite Element Analysis and the standard "Spiri-Reinhold pipe crushing" formula.

The offshore industry's current trend is driving towards ultra deepwater wells, combined with deeper total depths. In order to be able to set and run larger and heavier casing strings with hook loads approaching or exceeding 2.5 million lbs, an innovative heavy duty landing system has been developed and put on the market. Such systems must be capable of landing heavy loads while ensuring slip crush resistance. It includes an extended slip with enhanced design (coupled with a new greasing system), a 2.5 million lbs landing carrier and a pipe with a thick walled high strength material section, a light weight tube, and a rotary shouldered connection. One of the current operational challenges with existing heavy duty landing strings is linked to the positioning of the slips, in particular when using extended slip designs. The limited length of the slips area requires additional time for proper positioning and set up. To address these issues, a new slip proof section design has been developed: the combination of an extended crush proof section and the absence of a heat affected zone in that particular area decreases running time while completely eliminating the risk of setting the slips on a weak point. The "Spiri-Reinhold pipe crushing" formula has been used to design this system. However, considering the approximation of this model's results, crushing performance has been assessed using the Finite Element Analysis, as well as physical tests performed with a specifically designed pulling unit. These full scale test results have been compared to the ones obtained with both the Finite Element Analysis and the standard "Spiri-Reinhold pipe crushing" formula.

Excessive drill string vibration leads to loss of the drilling process e ectiveness and premature damage to the equipment. Therefore, understanding the system dynamics is essential. The complex behavior of drill strings attracted considerable attention in specialized literature. Due to the drill string slenderness, torsional vibration is present in most drilling routines, eventually reaching the stick-slip phenomenon. This torsional vibration results from the nonlinear interaction between drill-bits and rocks. Despite the complexity of the bit-rock interaction, researchers often treat the relationship between torque and bit velocity as a dry friction force in a slender system. A large number of dry friction models is used to describe the interaction between drill bits and rocks although a proper model is required for a precise interpretation of systems with friction. This contribution utilizes data from a test rig that is capable of reproducing full-scale system torsional behavior with simple brake devices to introduce friction into the system, disturbing the rotating motion. This test rig is modeled as an actuated torsional pendulum for the numerical investigations of the experimental friction. This dissertation proposes a friction model based on experimental data and analyzes various frictional phenomena observed, including hysteresis. The experimental results are used to identify the proposed model parameters. Lastly, it compares numerical simulations with the experimental results and validates the proposed friction model.

Torsional vibrations experienced by drill strings can be detrimental to drilling operations. With a goal of understanding torsional vibrations experienced by drill strings and determining means to attenuate undesired vibrations, the authors have studied the effect of adding a sinusoidal modulation to a constant rotation speed of a drill string. A combination of modeling, analysis, and experiments is used to explore the influence of this rotation input modulation on the system response. The drill string is modeled as a modified Jeffcott rotor, which is described by a system with three degrees of freedom. Considering the case of forward whirling of a rotor in continuous contact with a stator, the equations of motion are reduced to a single degree-of-freedom nonlinear oscillator describing the torsional motions. In order to understand the fast time scale and slow time scale components of the motion, the method of direct partitions of motions is used to determine an approximate response to the nonlinear oscillator. The obtained results of the analysis illustrate that with the sinusoidal modulation of the rotor drive speed, the equivalent torsion stiffness can be enhanced and the character of the friction force at the contact can be made smooth. The analyses helps bring forth the stabilizing influence of the added sinusoidal input to the rotor drive speed. Over the considered parameter ranges, the numerical results obtained with the full three degree-of-freedom model and the reduced single degree-of-freedom model are found to be in agreement with each other. Furthermore, the results from these models are found to compare well with those obtained by using the method of direct partition of motions. Experiments with a laboratory scale drill-string arrangement are to be carried out to validate the analytical and numerical findings and further explore the effectiveness of the drive speed modulation on the rotor dynamics. Introduction Slender rotating structures are used in many engineering applications. Drill strings are long rotating slender structures, which are used in drilling operations. A schematic of a rotary drill rig is shown in Fig. 1 (e.g., Liao, Balachandran, Karkoub, and Abdel-Magid, 2011). Drill strings experience different types of vibrations (axial, torsional, and lateral vibrations) that may lead to detrimental failures of a drilling system. Drill-string vibrations have attracted the attention of many researchers, and many models have been developed to understand them. Since drill strings have a large length-to-diameter ratio, typically, the first torsional natural frequency and first lateral natural frequency are close to each other. This frequency proximity and the nature of the system allows for coupling and energy transfer between the associated vibration modes. Here, as a step towards developing further understanding, a drill string is modeled as an extended planar Jeffcott rotor with gravity acting normal to the rotor. Due to the planar motions, no gyroscopic effects are considered. The extended model, which was earlier considered in the work of Vlajic, Liu, Karki, and Balachandran (2014), is shown in Fig. 2. The model is described with three degrees of freedom (DOF), namely, x and y to account for lateral motions, and theta to account for torsional motion. A large number of research efforts have focused on controlling drill-string vibrations by using different feedback control algorithms, which need measurements along the drill string. In this work, the authors propose a different control approach to mitigate whirling motions during continuous rotor-stator contact. This can be compared to a situation of a drill string being in continuous contact with the borehole. The approach is open loop in implementation and this approach is based on adding a high frequency input to the drive speed of the drill string. Given the open-loop nature, the method does not depend on any measurements along the drill string or rotary table, which could be attractive for a practical stand point. Although the current focus is on motions of a rotor with continuous stator contact, it is planned to study stick-slip and non-contact cases in the future. System Modeling, Studies, and Results As previously mentioned, the drill string is modelled as a Jeffcott rotor with three DOF. Proceeding along the lines of the group's prior work (Vlajic, Liu, Karki, and Balachandran, 2014), after some approximations for the continuous rotor-stator contact case, the 3 DOF system is reduced to a single second-order nonlinear differential equation governing the torsional motion. In Fig. 3, for a representative case with a constant drive speed, the torsional state histories obtained for the full model and the reduced model are plotted. It is noted that the final state of the motion is captured quite well. To further analyze the response during forward whirling, the reduced-order model was nondimensionalized and an approximate solution was obtained by using the method of direct partitions. This method allows one to separate slow scale motions from fast scale motions. As discussed, in the group's prior work, this method can be useful to examine whirling motions. The results obtained by directly integrating the full model as well as the reduced-order model have been compared with that obtained by using the method of direct partitions of motions. It is seen that the perturbation analyses is able to provide an approximate solution that compares well with the numerical results obtained through direct integrations. Next, this analysis is used to examine the influence of the addition of secondary frequency component. It is seen that the addition of the secondary frequency, a high-frequency one, helps smooth out the friction coefficient variation with respect to the relative speed at contact. This is found to have a beneficial effect on the motion and helps suppress large-amplitude torsional motions. Stability analysis was also conducted to examine this effect. Concluding Remarks A study has been conducted to examine the influence of sinusoidal modulation of rotor drive speed in a system, wherein the rotor experiences continuous stator contact. A single degree of-freedom reduced model is developed to study the torsion response and it is found that the addition of a high frequency input can be beneficial in attenuating rotor motions. This is believed to be useful for developing open-loop control schemes for attenuating forward whirling motions of drill strings. Results obtained from a lab scale experimental arrangement will be used to examine the effect of this drive speed modulation further and they will be reported in the conference presentation. Future studies could build on the current effort to examine cases of backward whirling. Acknowledgment: The authors would like to gratefully acknowledge the support received from the Qatar National Research Fund for NPRP Project 7-083-2-041, to pursue this collaborative work between the University of Maryland, College Park, MD, USA and Qatar University, Doha, Qatar.

Drill string models are of two major types- Conventional and Periodic. Conventional drill string models are widely used due to their simplicity in manufacturing and dynamics analysis. Drill strings are affected mainly by torsional, lateral and axial vibrations. The dispersive or propagative nature of these vibrations can only be analyzed by using periodic drill string models. Drill pipe makes up the majority of the drill string. In reality, at each end of the drill pipe, tubular, larger-diameter portions called the tool joints are located. Since the dimensions of the tool joint are negligible when compared to the drill pipe, conventional drill string models assume drill strings to be uniform structures. The tool joints in drill strings introduce a geometrical periodicity in the structure.A periodic structural component is comprised of a repeating array of cells, which are themselves an assembly of elements. The elements may have differing material properties as well as geometric variations. The periodic structures act as mechanical filters for the wave propagation. A material or geometrical change in the structure causes a disruption to the continuous propagation of vibration waves.There are three types of waves propagating in rotating structures: torsional, flexural (bending) and longitudinal. Of these waves, torsional and longitudinal waves are dispersive while flexural waves are non- dispersive. The proportions of the waves which are dispersed vary according to the Wavenumber. The wavenumber is a property of the wave and depends on the structural, material and geometrical properties of the wave guide. Waveguide is the medium through which a wave travels. The wave number, and hence the dispersion and propagation characteristics of waves can be tuned by varying the material and geometrical periodicity in the drill strings.This paper presents the vibration propagation and attenuation characteristics in conventional and periodic drill string models. The periodic drill string model is developed such that it can be easily scaled to a real drilling rig model. The research also features a laboratory setting which comprises a mini drilling rig and actual drilling samples. These features ensure robustness of the dynamics analyzed for the periodic drill string model.The research findings are analyzed under zero and non- zero borehole drill string interactions. Vibration information is collected from upper and lower parts of the drill string models to compare the propagation and attenuation characteristics. Proximity sensors are used for the data acquisition which is then analyzed using frequency and power spectrum graphs. The time and frequency domain vibration propagation characteristics are investigated for both the conventional and periodic drill string models. Laboratory testing analyzes (1) introduction of stop and pass band regions in frequency spectra by periodic drill string models, and (2) vibration attenuation in torsional and lateral vibration modes. The tests confirm the introduction of stop bands in frequency spectra for the periodic drill strings, while they were absent and had an all pass-band for conventional drill string models.The research opens up a new method of passive control of drilling vibrations- by introducing tool joint designs which could be tuned to efficiently dampen out the harmful vibrations affecting the drilling rigs.

Bit feed principles and operations of slide drilling

Utilizing a universal joint can lead to significant vibration within a driveline system. This study presents a model for analyzing the torsional and lateral vibrations of a driveline connected by a double universal joint. The governing equations of motion are derived, and the Runge-Kutta method computes steady-state responses across a spectrum of input rotational speeds. The focus is to examine the effect of system parameters, including static angular misalignment, load torque, and lateral stiffness. Relative amplification is used to analyze the effects of parameters on system vibration. Results indicated that the second-order component of input rotational speed induced by the universal joint was the factor that caused the vibrations. For the considered system, static angular misalignment significantly impacts both the torsional and lateral vibrations. Increasing the angular misalignment from 15° to 30° results in a threefold increase in lateral vibration amplification, while torsional vibration amplification is increased by nearly two times. The effect of load torque is almost linearly proportional to torsional vibration but is nonlinear to lateral vibration. Thus, lateral vibration is significantly impacted compared to torsional vibration for higher load torque. Changing the stiffness leads to a modification of the natural frequency. Increasing the lateral stiffness shifts the critical speed to a higher speed range, resulting in reduced lateral vibration amplitude. It is demonstrated that a slight fluctuation in angular misalignment due to lateral vibration will not affect the torsional vibration even if both vibrations are coupled. The findings may enhance understanding of how changing system parameters affects vibration.

During the engineering of horizontal well, drill string is in the complicated moving state, strong vibration is the main reason that induces drill string failure. Drill string vibrations usually have axial vibration, lateral vibration, torsional vibration and coupling vibration, the drill string near the bottom of well usually vibrates strongly. As an example to G78-E15, the variety of vibration is analyzed by using ANSYS, the model of vibration is established, the rules of vibration are obtained. The analysis can provide reliable theory basis in order to avoid resonance of drill string.

Real-time monitoring of mechanical specific energy and bit wear using control engineering systems

Large shuttle tanker based drill ships have proven to be the first commercially successful new build design alternative for ultra-deep water drilling application. Historical trends indicate that more cost-effective semisubmersible drilling units will follow the drillship designs in ultra-deep water drilling applications. Several suitable designs are being developed and cover a broad range of capabilities, performance, and cost. In evaluating the most cost effective design alternative for a particular application, it is important to understand the cost sensitivity and operational efficiency of various vessel design variables. Detailed cost, performance, and capability comparisons of new ultra-deep water semisubmersibles designs indicate significant cost sensitivity to variables such as structural complexity, design environment, vessel motion performance criterion and transit speed requirements. Design compromises in these areas may not significantly effect the drilling performance for certain applications but can result in significant cost reductions to the drilling unit. This cost reduction potential must carefully be evaluated against the compromises with consideration given to the duration, flexibility, and total information known about the projected drilling program. As more information and knowledge is obtained about specific drilling programs and consistent long term drilling programs evolve, cost effective "fit for purpose designs" will become justifiable. The evolution of commercially successful rig designs are anticipated to be developed in a conservative step by step process that can be accelerated by increased knowledge and experience. With strong deep water demands, this evolution process may occur with considerable overlap significantly reducing the overall time in which cost effective designs are perfected and proven. Future potential cost improvements in ultra-deep water drilling units will continue to strengthen the economic attractiveness of deep and ultra-deep water exploration and development projects. Introduction Except for a few isolated projects, new construction for the offshore drilling industry has been dominant for the past IS years. During the last few years however, the industry has committed to many major hull conversions and has announced orders for the construction new vessels involving over 30 drilling vessels. Several of these vessels are being built or converted for deep water drilling applications but the majority are being built to explore and develop prospects in ultra-deep water. In this discussion, deep water is referred to as water depths ranging from 1,000 to 2,000 meters and water depths greater than 2,000 meters is referred to as ultra-deep water. For the most part, the trend has followed the patterns set previously when the industry has ventured into deeper water. First existing vessels are upgraded to meet the immediate needs of the operators. Then the new building vessel of choice has once again been the drillship or monohull designs. These vessels are easily adapted to carry the heavier equipment and increased variable loads required for supporting activities in deeper water. The characteristics of the larger monohull shows promise of superior motions and increased operability when compared with the smaller vessels of the previous generations The design of the ultra-deep water mobile offshore drilling units (UDWMODU) is rapidly evolving as Operators become more familiar with their long-term requirements and rig designers optimize the variables to increase overall drilling performance in this new market.

Cementation of wells, especially in deepwater has attracted a great deal of attention after recent loss of well control events and subsequent changes in regulatory requirements with respect to achieving effective zonal isolation, preventing the release of wellbore fluids to the environment and avoiding the need for cement job remediation[1, 2]. Deepwater well construction presents unique challenges for proper cementation. These challenges are driven by the greater complexity of deepwater (DW) wells. There is; narrower pore pressure (PP) and fracture gradient (FG) margin, use of oil based mud (OBM) or synthetic based mud (SBM) drilling fluid that is incompatible with water based cement slurries, additional length for the cementing fluids to travel due to the sea water depth increasing the risk of intermixing between fluids, technology pushed to the limit, and the cost of drilling in deepwater that is much higher than conventional wells. Currently there are two main flow regimes used when planning for effective mud removal and cement placement: effective turbulent flow and effective laminar flow. Turbulent flow is generally considered the preferred method of efficiently removing fluids; however there are a number of requirements to be met in order for turbulent flow to be both safe and effective. Laminar flow is the most widely used due to the ease of placement design and the lower annular velocities required to achieve effective cement placement. Drilling fluids, weighted spacers and cement slurries can all be designed in effective laminar flow, since they exhibit a yield stress. Brines, water, and chemical washes are typically designed in effective turbulent flow. In this paper the authors will discuss the factors to be taken into account when designing cement jobs using fluids in turbulent flow and some of the overlooked benefits of using turbulent flow design for mud removal. Case histories from DW wells cemented in the Gulf of Mexico region and risk factors will be discussed. It will be demonstrated that when designed and executed properly, the use of turbulent flow mud removal with Newtonian fluids can be beneficial in DW cementing operations.

During drilling operations, three type of vibrations can appear in the equipment of rotary drilling systems which are axial, torsional and lateral vibrations. Their influences, either simultaneously or separately, prevent the drilling rig from extracting oil and gas from the target reservoirs. Many researcher have indicated that the most damaging hazards in the borehole are those caused by Stick-slip phenomenon, it is the sever status of the torsional vibrations that happened most time along the tool string. It has been shown that one of the main re-generators of this phenomenon is non compatibility of drilling parameters with the type of the geological formations. Therefore, many studies have dedicated to find the appropriate choice of parameters during drilling but few of them have discussed the effect of their variation during the drilling process. The main objective of this study is to analyse the effects of variation of Drill pipes and BHA dimensions and drill mud viscosity on the severity of Stick-Slip vibrations. Moreover, the obtained results have allowed us to set the optimal values of these parameters in order to minimize torsional vibrations without reducing rate of penetration.