Subsea Blowout Preventer Research Articles

A digital twin-assisted intelligent fault diagnosis method for hydraulic systems

As the complexity of modern engineering systems increases, traditional fault detection models face growing challenges in achieving accuracy and reliability. This paper presents a novel Digital Twin-assisted fault diagnosis framework specifically designed for hydraulic systems. The framework utilizes a virtual model, constructed using Modelica, which is integrated with real-time system data through a first-of-its-kind bidirectional data consistency evaluation mechanism. The integrated data is further refined using a two-dimensional signal warping algorithm to enhance its reliability. This optimized twin data is then employed to train a multi-channel one-dimensional convolutional neural network-gated recurrent unit model, effectively capturing both spatial and temporal features to improve fault detection. The subsea blowout preventer in lab is used to study the performance of the method. The results show that the accuracy is 95.62 %. Compared to current methods, this is a significant improvement. By integrating DT technology, data consistency optimization, and advanced deep learning techniques, this framework provides a scalable and reliable solution for predictive maintenance in complex engineering systems.

Three-model-driven fault diagnosis method for complex hydraulic control system: Subsea blowout preventer system as a case study

Hydraulic control is a control pattern that uses compression fluid as the energy medium and information medium. Hydraulic system is widely used in the control of industrial systems because of its flexibility and reliability. Hydraulic systems have the characteristics of strong fault concealment, significant sensor delay, and a complex signal transmission mechanism. Hence, it is very difficult to identify the faults of systems under the influence of powerful nonlinear time-varying characteristic. The diagnosis of complex hydraulic control system is a problem facing the current research. In order to cope with the challenges caused by limited sensors and concealment of faults, a three-model-driven fault diagnosis method is proposed for complex hydraulic control system. The three hydraulic control system models with regard to energy, fluid and information are proposed and established to explain the work process of the system from different perspectives. The diagnostic model is completed by establishing a fault reasoning model based on the Bayesian network. A redundant control system for subsea blowout preventer is used as a case to demonstrate the proposed method, and the results show that the proposed method has high accuracy.

Dynamic behavior of risers under nonlinear oceanic environmental loading

The riser system acts as the vital link between the subsea blow-out preventer and the drilling platform. Affected by factors like top tension and marine environmental forces, the riser undergoes deformation and wear, carrying the risk of environmental pollution and financial losses upon failure. Hence, this study examines the riser's dynamic response to marine environmental loading. Initially, the motion differential equation for the riser system under the influence of nonlinear oceanic load is deduced using the principle of minimum potential energy and the variational method for extremum seeking. Subsequently, a nonlinear wave-current load model based on the Morrison equation is established, and the resulting equation is discretized into a finite element model using third-order Hermite interpolation function and the Galerkin weighted residual method. Finally, the dynamic response of the riser is scrutinized employing the Newmark numerical integration method. The study also investigates the impact of both oceanic environmental parameters and drilling parameters on the riser’s dynamic behavior. Comparative analysis of the numerical results reveals that the maximum displacement of the riser occurs at the middle section, whereas the maximum deflection angle is observed at the end of the riser. The periodicity of the deflection angle response is influenced by the position of the riser, showing a trend of decreasing and then increasing from the middle section towards the ends. Notably, the top tension and the velocity of the surface tidal current significantly affect the dynamic behavior of the riser. The findings of this study provide a theoretical foundation for the assessment of riser reliability and the determination of operational parameters.

Fixed Choke Constant Outflow Circulation Method for Riser Gas Handling: Full-Scale Tests in Water- and Synthetic-Based Mud with Gauges and Distributed Fiber-Optic Sensors

Summary Conventional methods of managing gas-in-riser events are not optimal when the drilling riser is isolated from the formation by a closed subsea blowout preventer (BOP) on rigs equipped with mud gas separator (MGS), managed pressure drilling (MPD), or riser gas-handling equipment. The industry is concerned about exceeding the pressure limit of the riser and the flow capacity of the MGS and hence has not been able to reach a consensus on a circulation method for riser gas. This work is an analysis of the first-ever demonstration of the fixed-choke constant-outflow (FCCO) circulation method in synthetic-based mud (SBM) carried out in June 2022. The first-ever demonstration of the FCCO circulation method in water was performed in March 2021. The results from the water tests were discussed in IADC Gas-in-Riser/Riser Gas-Handling Subcommittee meetings, and the new fixed-choke method was named FCCO in November of 2021. The reason for using the acronym FCCO for the new method is that it allows the use of a fixed-choke opening percentage throughout the circulation period by managing the outflow and backpressure by varying only the pump rate. This work includes the comparison of the actual test results from the March 2021 FCCO tests in water with results estimated using a new model. This is followed by a discussion of the results from the June 2022 FCCO test in SBM. Nitrogen gas was injected into the bottom of an annulus 5,200-ft deep, vertical test well (9 5/8×2 7/8 in. casing/tubing) initially filled with water and instrumented with four downhole pressue and temperature gauges, and distributed fiber-optic sensors [distributed temperature sensing (DTS) and distributed acoustic sensing (DAS)] for water tests, and later filled with SBM. We started direct circulation to produce flow out of the annulus through a choke kept at a fixed open position (%) required for a preplanned applied surface backpressure (ASBP). We reduced pump rate if/as necessary to maintain this ASBP to ensure outflow rate within MGS flow capacity. We performed tests at different fixed-choke positions, different average ASBPs, and initial pump rates. We tested constant bottomhole pressure (CBHP) circulation and fixed pump rate methods also for comparison with the FCCO circulation method. The results from the FCCO tests demonstrated better control of outflow compared with the other methods. There is no need to use high ASBP. The use of a high ASBP suppressed the value of peak pressure. Installation of more than one gauge inside the riser enhances safety by allowing real-time influx detection capability, estimation of gas position, gas velocity, and gas void fraction. The FCCO method can be preplanned and easily substituted as the preferred circulation method for staying within the handling capacity of the existing MGS on the rig during gas-in-riser situations.

A Novel Data Assimilation-Based Real-Time State Estimation Method for Gas Influx Profiling During Riser Gas Events

AbstractRiser gas events during offshore drilling operations are hazardous and challenging to control. Therefore, knowledge of the gas influx sizes and distributions in a marine drilling riser is critical for the selection of riser gas handling methods and the estimation of risks of riser gas unloading. An extended Kalman filter-based data assimilation method is developed and evaluated for the real-time estimation of gas influx rates and void fraction distributions in a riser during riser gas handling. Full-scale experiments were conducted in this study for the evaluation of the proposed data assimilation method. An offshore well, which consists of a marine drilling riser and a wellbore below the subsea blowout preventer, was simulated by a 1572-m-deep experimental well. Real-time measurement data, including surface and downhole pressures, pump rates, and liquid outflow rates, were used to estimate the downhole gas influx rates using the Kalman filter. An online calibrated drift-flux model based on data assimilation is used to estimate the distributions of void fractions in the riser over time. The measurement data from a gas flowmeter and the distributed fiber-optic sensing were used to validate the estimation results, and satisfying performance was seen from the presented method. This study proposed a novel data assimilation-based state estimation method by maximizing the use of measurement data of different types from the available managed pressure drilling systems. This method enables the more accurate estimation and prediction of gas behaviors in a riser and helps to facilitate real-time decision-making during riser gas handling.

Fault Diagnosis Methodology of Redundant Closed-Loop Feedback Control Systems: Subsea Blowout Preventer System as a Case Study

In closed-loop feedback control systems, faults are propagated through the feedback link, which eventually leads to the abnormality of the entire system. Generally, it is very difficult to identify the faults of systems under the influence of the closed-loop feedback link. The existence of redundancy improves the reliability of the system. Meanwhile, it also poses new challenges to the fault diagnosis of multiple redundant systems. In this regard, a causality-based method is proposed for the fault diagnosis of closed-loop feedback control system with multiple modular redundancy. The dynamic Bayesian networks for fault diagnosis are established based on sensor data and system parameters. The networks consist of four layers, which are sensors, performances, monitors, and faults, respectively. Furthermore, the conditional probabilities of the fault nodes are calculated by Noisy-OR and Noisy-MAX models. The proposed method can dynamically evaluate system performance and integrate other monitoring information as evidence to assist faults diagnosis and location. A double modular redundant control system for a subsea blowout preventer is used as a case to demonstrate the proposed method, and the results show that the proposed method has high accuracy. The influence of sampling frequency, noise, and redundancy mode on diagnosis results is studied and discussed in the case study.

Analysis of Riser Gas Pressure from Full-Scale Gas-in-Riser Experiments with Instrumentation

Summary Because the use of adaptive drilling processes, such as managed pressure drilling (MPD), facilitate drilling of otherwise nondrillable wells with faster corrective action, the drilling industry should include the effect of gas dispersion, bubble suspension, fluid compressibility, and riser ballooning to avoid the overestimation of riser pressure and to produce more efficient well control methods. The IADC Deepwater Well Control Guideline recommends always addressing riser gas first, before proceeding to control the well in a well control situation. The intent is to remove the risk of gas reaching the surface and the rig floor, putting personnel and assets at risk. However, with the availability of equipment on the rig dedicated to handling riser gas and the fact that the riser is isolated from the wellbore, the atmosphere reduces the level of risk of gas in the riser, whereas the well below the subsea blowout preventer (SSBOP) poses a greater risk. In this paper, we discuss the results from full-scale experiments recently conducted in an extensively instrumented test well (LSU Well-2) and demonstrate that the riser pressures resulting from upward transport or aggregation of riser gas are much lower than the values estimated using the single-bubble model and industry thumb rules. We explain the danger of using an open-top riser top during the monitoring of gas-in-riser and how the situation can get out of control due to the potential dynamic unloading situation. Our research also demonstrates the minimal fluid bleedoff volumes required to reduce pressure buildup consequences of free gas migration in a fully closed riser containing low-compressibility liquid. A differential pressure methodology used in this paper for analysis was also used for detecting the presence, position, void fraction, and lead and tail velocity of the gas column in real time during each of the tests to make decisions during the tests. The results from a successful application of the fixed choke constant outflow (FCCO) method as a new method for circulating out gas from the riser by staying within the gas-handling capacity of the existing mud gas separator (MGS) on the rig are presented. This is the industry’s first test of the FCCO method.

Application of integrated STAMP-BN in safety analysis of subsea blowout preventer

Blowout preventer (BOP) plays a key role in preventing blowout accidents from occurring during offshore or deepwater drilling. The safety of BOP is affected by diverse components and interactions among them. It is vital to identify the crucial failure modes using safety assessment method. To perform a qualitative, quantitative, and systematic safety assessment of BOP, this paper presents an integrated method—using the system-theoretic accident model and process (STAMP)-Bayesian network (BN)—for evaluation of the structure safety and prediction of the failure scenarios. In the proposed method, STAMP is used to identify the failure scenarios by establishing a hierarchical control and feedback model that tallies with the BOP structure, while BN is adopted to calculate the probabilities of potential failure modes based on the outcomes of STAMP. The approach is then explored in a case study where factors regarding humans, components and control systems, and constraints and control relationships among them are all considered. The results demonstrate the feasibility of the integrated method as a way of performing a safety analysis for BOP. This research overcomes the deficiencies of traditional methods that cannot effectively evaluate the interactions of components, thereby providing a reference for safety/risk analyses of complex systems.

Study Investigates Deepwater BOP Technology, Reliability

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 21430, “Deepwater Subsea BOP Technological and Reliability Advancement,” by Mahesh Picha, SPE, Malik Faisal Abdullah, and Ashutosh Rai, Petronas, et al. The paper has not been peer reviewed. Copyright 2021 International Petroleum Technology Conference. Reproduced by permission. Subsea blowout preventer (SBOP) reliability is a major challenge in deepwater drilling and completion operations, accounting for a large portion of major equipment failures and nonproductive time (NPT) costs annually. The complete paper focuses on SBOP technological advancement since the Macondo incident in 2010, with emphasis on reliability, equipment-condition monitoring, and statistical root-cause analysis. The development of new technologies has targeted overall cost optimization of the well life cycle but also has been aimed at assuring SBOP functionality. Introduction Drilling contractors are required to establish minimum standards of redundancy and reliability of their SBOP systems and are required to implement an auditable risk-management process to ensure that their SBOP systems operate above minimum standards. A 2015 study found that 38% of NPT events were related to SBOP issues. Of this 38%, 56% were associated with maintenance, 22% with manufacturing issues, 16% with not following operational procedures, and 6% with training time. Because maintenance-related NPT was the major contributor, a detailed analysis was performed and found that frequency of the maintenance cycle, insufficient detailing of maintenance procedures, a lack of specific tools, insufficient spare parts onboard, and unavailability of technical documentation were responsible.

Ram Performance and Hydraulic Modeling of Subsea Blowout Preventer Control System

Summary A subsea blowout preventer (BOP) system is critical in providing safe working conditions for drilling activities in deepwater oil exploration. However, performance evaluation of the shear ram BOP is still a challenge for the industry. In this paper, two aspects of BOP shearing capability are studied: The required shear force to cut the pipe inside BOP successfully and the time it takes to shear the pipe and close the well. A finite element model is used to calculate the maximum shear force of a typical drillpipe, while an analytical hydraulic model is proposed to calculate the BOP closing time. A case study is presented for estimating the emergency disconnect sequence time in a deepwater offshore scenario. It is assumed that the BOP control system is activated from the surface, employing an electrohydraulic multiplexed control device for closing the BOP and rapidly disconnecting the lower marine riser package (LMRP), thus preventing damage to both riser and wellhead.

Plastic behavior and fracture cross section of coiled tubing in the ram shearing process of subsea blowout preventer

For studying the plastic behavior and fracture cross section of sheared drill pipe are of important aspects for evaluating the rams' shear performance of subsea blowout preventer, the plastic deformation and fracture cross section of drill pipe are analyzed based on the plastic deformation theory and the hypothesis that the deformation process of drill pipe is a four-plastic hinge or an eight-plastic hinge mechanism, respectively, according to different ram structure configuration. The elastic-plastic deformation process of CT90 tube in the simulated shearing process is obtained, the final fracture cross section of CT90 tube is approximately a rhombus circle, which is agreeing with the analytical and experimental results. Furthermore, the structure parameters research shows that ram or pipe structures have limited effect on the peak/terminal value of pipe stress. The major length of rhombus circle increases first and then decreases with the increase of ram's blade mouth thickness, decreases with the increase of ram's V-shape angle and pipe wall thickness, while the minor length shows an opposite trend. The larger pipe outer diameter is, the greater major/minor length of rhombus circle is and the growth rate of major length is significantly faster than that of minor length.

Anchoring and suspending pipe string mechanical behavior of slip ram for subsea blowout preventer

For exploring the anchoring and suspending pipe string mechanical behavior of slip ram for subsea blowout preventer is of critical importance for reliable sealing wellhead in an emergency, the wedge stress theory and thick-walled cylinder theory are applied to analysis anchoring and suspending mechanical behavior of slip ram, respectively. The stress of slip inserts is very tiny in the approaching pipe process and stress concentration occurs at the slip inserts edge when contacting pipe, while the stress of slip inserts is pre-volatile and then gradually stabilized in the suspending process; the mean stress is approximately uniform in each slip inserts at the final moment. The numerical results indicated the wedge stress theory and the thick-walled cylinder theory are applicable to analyze the anchoring stress and suspending stress (mean stress) of slip inserts respectively. The visible damage and plastic deformation occur in the slip or CT90 tube after completing the anchoring and suspending operation whether numerical or experimental results. Furthermore, the sensitive parameters research shows that the slips are damaged in the anchoring stage depends on their structural strength, while in the suspending stage depends on their structural shear strength.

Quantitative risk assessment methodology of installation process for deepwater oil and gas equipment

Proper installation of deepwater oil and gas equipment is a prerequisite to ensure its service life. Deepwater equipment faces many challenges during installation, such as the complexity of the installation process and uncertainty of the environment. Thus, risk assessment is necessary to guide personnel in avoiding and addressing potential problems during the installation of deepwater equipment. This study proposes a quantitative risk assessment method of installation process for deepwater oil and gas equipment based on fuzzy Bayesian networks (FBNs). First, the flow chart of deepwater oil and gas equipment in installation process is translated into a main Bayesian networks (BNs). Second, risk factors in different stages of installation are determined from human, management, equipment, environment and third-party interference, and entire risk assessment model is established. Finally, the FBN model is quantitatively analyzed by introducing fuzzy set theory. An empirical study on the installation process of subsea blowout preventer (BOP) is conducted. The results show that the failure of the subsea BOP installation is a small probability event, and equipment factors have the most prominent effect on the installation process. The sensitivity analysis results are consistent with the quantitative analysis. The corresponding improvement measures are proposed according to the analysis results.

Sour-Service Risers and Accessories for HP/HT Application in the Gulf of Mexico

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 30558, “Development and Implementation of Heavy-Wall, High-Strength, Sour-Service Accessory and Risers for HP/HT Application in the Gulf of Mexico,” by Carine Landier, Jonathas Oliveira, and Christelle Gomes, Vallourec, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission. As oil and gas development in the Gulf of Mexico increasingly requires high-pressure/high-temperature (HP/HT) applications, the need for sour-service (SS) resistance also has grown. To meet these needs, continual innovation and improvement is needed in SS-grade materials from a technical and cost-effectiveness perspective. The complete paper discusses the material properties achieved with several large-diameter, heavy-wall SS pipes. The complete paper presents a detailed, illustrated discussion of the applications for the high-strength SS pipe and its manufacturing process. Applications The authors write that improved materials to meet HP/HT requirements such as those in the Gulf of Mexico are needed particularly for two applications: for risers, which require high-strength, thick-wall sour service; and as a substitute for corrosion-resistant alloy (CRA) with sour carbon material on defined accessories. Vallourec has developed high-strength [125,000-psi specified minimum yield strength (SMYS)] and resistant carbon steel pipes in sizes with outer diameter (OD) up to 23 in. and wall thickness up to 2.5 in. These sizes are common in lower-strength material, but meeting the high-pressure requirements with higher-grade material enables cost savings and eliminates some CRA components. It also enables the use of much-lighter-weight pipe than the 80,000-psi SMYS material that is standard for SS applications in oversize OD and heavy wall. Risers. Most deepwater drilling is performed with classic subsea blowout-preventer (BOP) systems. Access to the well through the BOP is accomplished with low-pressure, large-diameter (19-in. internal diameter) drilling riser pipe. Pipes are supplied in weldable grades (API 5L X65–X80). Large-diameter forged flanges are then welded onto the tubes. Connections are made by multiple bolts. High pressures, required as part of the drilling process, are supplied by small-diameter choke-and-kill lines. This system has served the industry well, but, as well pressures increase, so have cost and feasibility requirements of subsea BOP technology. These costs, driven by the complexity of redundant systems, have driven a desire to explore an alternative solution—a surface BOP with high-pressure drilling riser pipe. Using a surface BOP reduces the complexity and cost of the system significantly because of the ability to inspect it. The drilling riser then carries the pressure to the surface and must be able to contain it. The high-pressure environment that instigated a new solution was based on a 15,000-psi well pressure with NACE Region 2 SS performance. Because of the requirement for weldable grades for attaching the flange as well as SS, the maximum yield strength has been limited to 80,000 psi. At that strength, a very high wall thickness is required to meet 15,000 psi and greater. This becomes very heavy and can be limited by the rig hook-load capacity. Alternatives in weldable grades are nickel-based alloys with SS performance. A full string, however, is prohibitively expensive.

Resilience evaluation methodology of engineering systems with dynamic-Bayesian-network-based degradation and maintenance

Many resilience evaluation methods were proposed to assess system resilience under the influence of specific, constant disasters, and these methods are restricted by fixed external disasters. A novel evaluation methodology combining Markov model and dynamic Bayesian networks (DBNs) is developed, and is suitable for resilience evaluation under the influence of various external disasters. The external disaster model is established by mapping from actual physical models of influencing factors, the failure rate of a single component is obtained from the established disaster model, the system performance is obtained through the combination of Markov model and DBNs, and the resilience value is finally calculated through the integral of the performance curve. An example of power supply and control systems of subsea blowout preventers demonstrates the application of the evaluation methodology.

Quantitative risk analysis of offshore well blowout using bayesian network

Blowout is the most feared and undesired accident during offshore drilling. It is inevitable, but the risk can be maintained to be below the acceptable criteria with effective strategies devised by risk analysis. An application of Bayesian networks (BN) for quantitative risk analysis on offshore blowouts was presented. First, we analyzed the SINTEF offshore well blowout data. 95% of blowout occurred in drilling, completion and workover during offshore drilling. Second, BN was applied to conduct risk analysis of offshore blowout. Based on these data, BN models were built. The prior probabilities with statistical probability method were calculated. The posterior probabilities during blowout were calculated using GeNIe software. The principal risk factors were identified by comparing them with prior probabilities. Shallow gas and abnormal high pressure were the principal risk factors of primary well control failure. Poor cementing and blowout preventer (BOP) failure were that of secondary well control failure. BOP failure is one of the main reason for blowout. Then, the risks of subsea and surface BOP failure were analyzed, combining with BN and Standardized Plant Analysis Risk Human Reliability Analysis Method. According to ExproSoft BOP failure data, the posterior probabilities with the concerning of component failure and human error were calculated. The principal factors were identified. This method provides greater value than the previous models since it can consider the complicated characteristics of geological condition, the whole offshore drilling, completion and workover technologies and operations, surface and subsea BOP common cause failures.

A novel dilution control strategy for gas kick handling and riser gas unloading mitigation in deepwater drilling

Non-aqueous drilling fluids are generally favored in risered deepwater drilling operations for their capability to significantly improve drilling performance. However, their use may increase the difficulty of gas kick detection and handling, due to the high solubility of gas at downhole pressure and temperature conditions. A novel managed pressure drilling (MPD) control strategy is proposed here to handle gas kicks and prevent riser gas unloading (RGU) events in cases of gas influxes in non-aqueous drilling fluids. With the proposed strategy, the dissolved gas kick is diluted to a less severe level after it passes the subsea blow-out preventers by injecting mud into the riser through the booster line. Since the dissolved gas concentration is decreased after dilution, it is easier to mitigate or even fully eliminate an RGU event by applying an appropriate MPD backpressure. This MPD backpressure keeps the kick in solution when it is in the riser, and only allows it to come out of solution after passing the MPD choke.The proposed strategy's effectiveness is demonstrated using multiphase flow model simulations. The modeling approach considers the transient behavior of the two-phase gas-fluid mixture, and is validated against full-scale experimental data. Simulation results for a typical deepwater drilling operation show that the proposed MPD control strategy with riser dilution can greatly improve both the safety and speed of handling gas kicks in deepwater drilling. The improved safety derives from the ability to eliminate RGU events, and the increased speed comes from handling the kick directly in the riser using MPD backpressure and booster line dilution rather than through the lengthy conventional process of circulating it out through the choke line. Sensitivity analysis is conducted to investigate how the performance of the proposed control strategy is affected by influx gas composition, reservoir depth, seawater column height, kick size, and dilution ratio. A main conclusion is that the backpressures required after dilution are within the operating limits of regular marine risers, and therefore the majority of gas influx cases can be handled without the need for high-pressure risers. This allows for the relatively straightforward deployment of the proposed control strategy in the field.

Gas in riser: On modeling gas influxes in non-aqueous drilling fluids with time-dependent desorption considerations

The presence of formation gas in a marine riser above the subsea blowout preventer (SSBOP) can pose great risks to personnel, equipment, and the structural stability of the drilling rig. The offshore drilling industry has been seeking to better understand and simulate gas-in-riser events, to optimize mitigation methods and justify the adoption of new flow control equipment and techniques. This paper presents a modified multiphase model to more realistically describe gas-in-riser events by incorporating time-dependent mass transfer processes occurring in non-aqueous muds (NAMs). A hybrid advection upstream splitting scheme (AUSMV) was used as the numerical scheme. Verification of the model and the numerical scheme were performed using field data from well control events with both NAMs and water-based muds (WBMs). The role of time-dependent mass transfer played during the process, and the proper range of desorption coefficient was evaluated in the case study with NAMs. Furthermore, a comprehensive parametric study was conducted, and results show that the severity of unloading will be significantly underestimated without considering mass transfer in NAMs. Meanwhile, unloading severity can be overestimated if the desorption process is considered as instantaneous instead of time-dependent.

Time‐based replacement policies for a fault tolerant system subject to degradation and two types of shocks

AbstractA single component nonrepairable system suffering from both an internal stochastic degradation process and external random shocks is investigated in this paper. More specifically, the Wiener process with a positive drift coefficient is introduced to describe the gradual deterioration and the arrival number of external shocks is counted with a nonhomogeneous Poisson process (NHPP). Meanwhile, fault tolerant design is incorporated into the stochastically deterioration system so as to protect it from shock failures to some extent and is consummately addressed via a generalized m − δ shock model. From the actual engineering point of view, external shocks are typically classified into two distinct categories in this current research, that is, a minor shock (Type I shock) increasing the damage load on current degradation level and a traumatic shock (Type II shock) resulting in system catastrophic failure immediately. The closed‐form expression of system survival function is derived analytically and is viewed as the generalization of existing reliability function for systems subject to dependent and competing failure processes. Based on which, two time‐based maintenance (TBM) policies including an age replacement model and a block replacement model are scheduled, where the expected long‐run cost rate (ELRCR) in each model is, respectively, optimized to seek the optimal replacement interval. In the illustrative example part, a subsea blowout preventer (BOP) control system is arranged to validate the theoretical results numerically. To compare which policy is more profitable under different conditions, the relative gain on optimal maintenance cost rate of the two TBM policies is presented.

Fracture criteria applied to numerical simulation of blowout preventer ram shearing

The interest in predicting the force required for a subsea blowout preventer (BOP) to cut and isolate the drilling pipe has been growing since the Macondo accident. In previous research, numerical simulation using the nonlinear finite element method combined with the blowout preventer (BOP) shear test has been a common approach. As various fracture criteria exist, the selection and application of a suitable fracture criterion are of great interest to achieve a successful ram shearing simulation. In this paper, the CrashFEM criterion, previously employed in crashworthiness simulation, is first adopted in BOP ram shearing simulation to capture the onset of damage to the pipe. The Johnson-Cook (J-C) and Modified Mohr-Coulomb (MMC) criteria are utilized to carry out a comparative study. The TRIP 690 experimental data are used to derive the fracture parameters. These parameters enable the building of numerical models based on boundary and loading conditions for the BOP shear test. Based on the numerical model, a mesh sensitivity study is carried out. The simulation results using refined meshes show that the shearing forces predicted by the CrashFEM and MMC criteria are generally consistent with the experimental data. The results from the three criteria indicate differences originating from the shape of the fracture limit curve, the damage initiation concerning the equivalent plastic strain accumulation, and the damage distribution in the shearing area. These findings are explained in detail in the paper. A comprehensive application of the CrashFEM criteria for BOP shearing simulation is presented and recommendations are provided about the selection of suitable fracture criteria.