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Real-time risk assessment using time series: A case study of a knockout drum XXXXXXXXX[0000−0000−0000−0000] XXXXXX xxxxxx@xxxxxx.com Abstract. Industries are increasingly connected with the rise of the In- dustrial Internet of Things. Gathering and preserving data about equip- ment operation and production processes are now essential, enabling real-time analysis and decision-making to enhance efficiency. At the same time, the development of digital twins makes it possible to evaluate these optimizations in a virtual environment and a secure manner, aiming to analyze their effectiveness without compromising the actual production environment. We can apply these techniques with artificial intelligence models to perform risk analysis of increasingly complex production sys- tems. Machine learning models can be trained using historical data, cap- turing the relationships between variables and the characteristics of the context in which the equipment is inserted. Thus, these models can help prevent unwanted events through real-time inferences, providing infor- mation to operators to support decision-making. The present work aims to present a technique for risk assessment through time series. Through a case study of a knockout drum, we built a digital twin and trained a neu- ral network to infer the behavior of the liquid level in this drum. In this way, operators can make preventive decisions to prevent the fluid level from reaching the maximum allowed and prevent events that compromise the safety of the operation. Our technique demonstrated satisfactory re- sults, avoiding unscheduled stops in the natural gas flow system in the cases analyzed in our tests and showing robustness in scenarios where the sensors present noise in their readings. Keywords: Digital Twins · Risk Analysis · Time Series. 1 Introduction Industries, in general, may operate equipment in high-risk environments, requir- ing a high level of safety and process maturity. Failure to use this equipment can lead to severe accidents, impact worker safety, and cause financial losses to the organization. In this context, Digital Twins (DTs) are powerful tools for risk mitigation and operational performance optimization. By creating a virtual model similar to real-world equipment, DTs allow for the simulation of different operational scenarios and analysis of equipment behavior in real-time. 2 XXXXXXXXXX et al. Through simulations of risk situations, it is possible to use DTs to collect data on the behavior of system components during the occurrence of some undesired event, even if it has never occurred in the real-world production environment of that organization. Machine learning models can be used to analyze this infor- mation, identifying patterns that indicate potential failures at early stages and enabling decision-making to prevent the event from occurring. In this work, we propose using DTs in conjunction with machine learning models for monitoring and risk analysis of knockout drums in the oil and gas industry context. A knockout drum is used to efficiently separate a gas stream’s liquid and gas phases before boilers use it. When liquid accumulation occurs in the drum, a trip is adopted as a mitigation barrier, which aims to temporarily deactivate the gas supply to subsequent processes so that the risk of explosion is mitigated until the liquid level is normalized. In the context of an oil refinery, a knockout drum can be used before boilers that generate steam. Therefore, liquid contaminants in the generator’s burner can cause explosions with significant impacts [10]. Our work’s main objective is to propose a risk analysis technique using DTs and a machine learning model capable of predicting liquid accumulation in this equipment. This approach seeks to provide important information to operators to assist in the decision-making process, avoiding the need to activate safety systems that can cause setbacks in production and ensuring the continuity of operation safely. 2 Related works We searched the literature for studies that propose using algorithms for risk analysis in industrial equipment. We grouped these works into two categories: those that apply probabilistic models to estimate the occurrence of events and those that use artificial neural networks to identify patterns and build inference models. 2.1 Probabilistic models Probabilistic models are often used during the risk analysis stage. These models are essential for quantifying and understanding the various factors that influ- ence the safety and reliability of industrial equipment. By focusing on applying Bayesian networks, specifically, probabilistic models prove to be effective in rep- resenting the conditional dependency relationships between observed variables. The ability of Bayesian networks to handle uncertainties makes them a promis- ing choice for risk assessment, in addition to their graphical nature, which allows for an intuitive visualization of the connections between system elements, facil- itating the identification of critical points and the decision-making process. Developing a Bayesian network aimed at risk analysis requires identifying all possible risks associated with the equipment and the relevant variables that may exert influence. This process requires extensive knowledge about the operation Real-time risk assessment using time series 3 of the equipment under analysis; however, it is possible to find some works in the literature that seek to assist in this mapping process. In [8], the authors present a mapping technique of a bow-tie diagram that allows its conversion into a Bayesian network, helping to identify which safety barriers are critical and possible accident scenarios. In the work [1], the authors explore creating a Bayesian network for the event of liquid overflow in an offshore burning system. Knowing the risk factors of the system, the authors schematized the bow-tie diagram of the process, identifying the causes, safety barriers, consequences, and their impacts, with liquid overflow considered the top event. The probabilities of the primary events that served as a basis for calculating the probabilities of safety barriers were extracted from the works [7,5]. This work demonstrates that Bayesian networks are robust tools for probability calculation in risk analysis. Developing a Bayesian network aimed at risk analysis requires identifying all possible risks associated with the equipment and the relevant variables that may exert influence. With the advancement of the digitalization process in industries, probabilis- tic risk assessment techniques tend to become increasingly dynamic. In [2], a real-time probabilistic risk assessment method for the petrochemical industry is proposed based on data monitoring. The technique combines probabilistic risk analysis with a Dynamic Bayesian Network (DBN) to integrate prior knowledge and online data in estimating the risk of basic events and accidents. The method does not require historical event data or expert knowledge to define the condi- tional relationship between data monitoring and prior risk. Experiments showed that this method could capture real-time variations in the risk of primary events. The solutions developed with DBN are robust and flexible, as they can handle different types of events and data. 2.2 Artificial Neural Network Techniques that use artificial neural networks represent a robust approach to artificial intelligence. These models are designed to learn complex patterns and hierarchical representations from large datasets. The generalization capability of these models is an essential feature for risk assessment in industrial processes, as it allows for the dealing of unprecedented scenarios based on previous behaviors. However, the effectiveness of these models is directly dependent on the quality and quantity of training data and the use of appropriate optimization and reg- ularization techniques, making the development and learning process of these modelschallenging. Strategies incorporating historical analysis through time series stand out as effective options for real-time risk assessment. They enable understanding of variations and trends in historical data, allowing the projection of future scenar- ios based on various factors. Thus, leveraging previous information recorded by sensors makes it feasible to estimate operational conditions for a specific future period. In [9], a model for predicting oil-walled equipment failures using the Feed Forward Neural Network is presented. These pieces of equipment can experience 4 XXXXXXXXXX et al. problems due to sand accumulation, corrosion, pressure variation, and other fac- tors. Therefore, to ensure the performance of the equipment and avoid costs asso- ciated with losses, it is essential to identify the origin of these failures early. The proposed model proved effective in predictions, surpassing the results achieved by models such as Random Forest and Decision Tree. These results indicate that FFNN-type neural networks can represent a practical choice for the failure prediction problem. Some types of recurrent neural networks have also gained prominence due to their efficiency in dealing with time series. In [11], a method for predicting equipment operating status at a power station through historical data using LSTM cells is presented. This method achieves a lower RMSE value than the ARIMA model, proving the capability of LSTM networks in handling time series analysis tasks. Depending on the challenge requirements and the nature of the data, net- works that use LSTM layers can also be combined with other layers, such as GRUs. This strategy can increase the model’s generalization capability, making more robust predictions. Thus, in [4], a method for predicting abnormal operat- ing conditions is presented, combining physical knowledge in a neural network model, meaning the model only receives physically essential features for solving that problem. An LSTM-GRU-based model for multivariate time series was con- structed to perform the prediction, integrating two RNNs to predict the future trend of the critical variable data. The proposed method is applied to a case study involving an abnormal condition of crude oil with water in a catalytic cracking unit, showing superior prediction performance compared to other models. 3 Time series based risk assessment Operational or maintenance failures can prevent the proper flow of liquid, re- sulting in its accumulation inside the drum. This scenario represents a risk to the stability of the process, as the accumulation can result in the activation of the trip as a safety barrier. The trip is an action taken to interrupt the gas flow, preventing the liquid from exceeding the maximum allowed level and entering the pipes, extinguishing the flames of subsequent equipment fed by the clean gas from the drum. If the flames are extinguished, gas can accumulate inside the equipment, which could cause an explosion if there is a conducive environment. Although necessary, activating this mitigation barrier causes a temporary halt in production, as the gas supply to the equipment is interrupted for a certain period. For the development of this study, we created a DT of a knockout drum in AVEVA Dynamic Simulation, which allows us to simulate various behaviors to collect the necessary data for training and evaluate the model and analysis technique. The simulator enables sending sensor reading values to external soft- ware and receiving external commands from other software to control system components. This communication was carried out through the OPC standard. Real-time risk assessment using time series 5 Figure 1 presents a diagram that illustrates the modules developed for col- lecting the training dataset. These modules communicate with the DT through an OPC server, transmitting data to the simulation and storing the results. The goal is to generate random situations, risky or not, to capture information about the vessel’s behavior during these situations. This information is later used in the training of the neural network model. Fig. 1. Training data generation architecture diagram The solution’s architecture can be visualized for real-time risk prediction in the diagram presented in Figure 2. Through an OPC server, the digital twin sends the values collected by the sensors, which go through a preprocessing stage aimed at normalizing the values and selecting only the variables expected by the model. Then, these values are directed to the predictive model, which makes predictions with the goal of displaying the expected future behavior. These predictions are presented to the operator, who can follow them and decide on the automatic control of the gas source that feeds the vessel. Fig. 2. Prediction architecture diagram This can deactivate the flow of contaminated gas from the refinery and ac- tivate the flow of gas from the external company, providing contaminant-free 6 XXXXXXXXXX et al. gas as soon as the risk of a trip is detected. Thus, the activation of the trip is avoided, as the liquid present in the vessel stabilizes while the gas continues to feed the subsequent processes. This modularization allows improvements and updates to be implemented with less effort, optimizing the continuous enhancement process. Furthermore, asynchronous communication between components is essential for applications in real-world industrial environments. This feature is due to the large amount of data constantly generated in these environments, making it essential for efficient data management. 3.1 Digital Twin of a knockout drum In the petrochemical industry, dynamic simulators are essential. They replicate the behavior of systems and processes, allowing for accurate analyses and reli- able predictions in different scenarios. Through these simulations, it is possible to assess the impact of variables, test different configurations, and optimize op- erations, resulting in cost and risk reduction as well as increased efficiency and productivity. Through a risk analysis of a knockout drum, we identified some scenarios that could lead the vessel to an undesired state. Based on the Bow-Tie diagram developed in the work [1], we constructed an analysis to trace possible threats that could result in excess liquid in the vessel. The identified threats were leak- age in the drum, which can disturb the balance of internal pressure; the high flow entering through the gas pipeline, where liquid accumulation can occur if the drainage system is not prepared for sufficient flow; failures in level transmit- ters, this information being essential in decision-making processes; operational dysfunction in processes prior to the vessel, which can cause gas contamination. We used AVEVA Dynamic Simulation software to implement a detailed model of a knockout drum, including the necessary instrumentation for observ- ability and process control. In the simulated scenario, the drum receives a flow of natural gas from other refinery processes. After removing impurities in the gas, the drum feeds the boilers for thermoelectric power generation, which supplies steam and electrical energy to the entire refinery. Figure 3 presents the model of the drum developed in the simulator. The model was based on the work [1] and adapted to meet the risk scenarios identified in our analysis. The knockout drum V1 has a diameter of 2.5 meters and a height of 3 me- ters. SRC1 is the source of natural gas from the refinery. Table 1 presents the chemical compounds present in this gas. For simulation purposes, contamination is performed by the presence of water in its composition, which can be variable, allowing us to evaluate the system’s behavior in high-liquid flow scenarios. The gas has a temperature of 40◦C and a pressure of 3.5kg/cm 2 . The SRC_CONCESSIONÁRIA and VALVE6 control the flow of gas from the external company. This gas has the same composition as the refinery gas but is free of contaminants,as the external company performs a prior treatment to eliminate them. Activating the flow to use the clean gas from the external company ensures the temporary continuity of natural gas supply to the drum Real-time risk assessment using time series 7 Fig. 3. Knockout Drum model in AVEVA Dynamic Simulation Table 1. Composition of natural gas Component kg-mol Nitrogen 0.0079 Carbon dioxide 0.0006 Methane 0.9782 Ethane 0.0095 Propane 0.0026 Butane 0.0013 Water Variable until the liquid level is normalized, preventing an unplanned process shutdown. The transmitters TR1, TR2, and TR3, operating redundantly, measure the liquid level inside the drum. A voting system was implemented to ensure measurement reliability. The liquid height readings are carried out simultaneously, and any divergent values are discarded. The resulting final value is a reference for opening valve VALVE4, which allows the internal liquid to flow. The valve VALVE5 is used to simulate leakage scenarios of the drum’s content. This valve does not require control systems as it serves only as an artifact to assist in the validations. 3.2 Dataset generation We simulated some scenarios to observe the DT’s behavior and collect data from the drums’ sensors to create our training and testing set. The evaluated scenar- 8 XXXXXXXXXX et al. ios were the risk scenarios described in subsection 3.1. For this, we developed a module that communicates with the virtual model, assigning values to the con- trol variables according to the failure scenarios to be simulated. The controlled variables at this stage were leakage valve opening rate, noise insertion in the readings of level transmitters TR1, TR2, and TR3, and control of the amount of liquid in the natural gas flow. This module consists of an input generation algorithm that generates random values for each simulation variable. The entry of liquid through the gas pipeline is simulated through random numbers between 0.0 and 10.0 that are generated at each data reading, simulating the dynamic variation of the volume of liquid present in the gas. The definition of the values in question aims to simulate an intense flow of liquid in the vessel. This choice is justified by the fact that the liquid will represent about 91% of the final product after normalizing the gas composition by the simulator. Randomness is also applied to the simulation of noise in level transmitters and in the opening of the leakage valve, with a 5% probability for each event. The probability values were defined through tests; the algorithm was run with different probability configurations, and the results obtained were analyzed. These values aim to ensure the representativeness of the data from each risk scenario in the final training and evaluation set. Table 2 describes the tested probability values and the results achieved. These values were applied in a routine until a set of approximately 135,000 records was generated. Table 2. Comparison between the probability values of the DT inputs Transmitter noises Leakage Prob. Quantity Percentage Quantity Percentage 0.01 3036 2,23% 985 0,72% 0.02 6023 4,43% 2037 1,50% 0.05 13984 10,30% 4955 3,65% 0.1 26984 19,88% 10034 7,39% 0.4 78343 57,72% 39840 29,35% 0.5 87511 64,48% 49941 36,80% After analyzing the results obtained, the value of 0.05 was chosen because it presents a balance between the number of occurrences and fidelity to the real behavior of the system. The values 0.01 and 0.02 resulted in an insufficient num- ber of occurrences for robust analysis, which could compromise the validation of the simulated scenarios due to a lack of sufficient examples. On the other hand, probability values higher than 0.05 generated an excessive number of oc- currences, distorting the natural dynamics of the system. This distortion could lead to the creation of an artificial model, compromising the reliability of the re- sults. Therefore, the choice of 0.05 as the probability value is based on its ability to generate sufficient occurrences for reliable analysis without compromising the model fidelity. Real-time risk assessment using time series 9 We built a module that reads the states of the simulation variables and stores them in a structured format, recording the behaviors of the virtual model dur- ing the algorithm execution. This structure facilitates the processes of treatment, training, and evaluation of the model, allowing a deeper analysis of the simu- lation results. The simulation was run for 65 hours, generating a dataset with 12 trip events distributed among 135,000 records. Each record is composed of 56 attributes. This information allows for a better understanding of the drum’s behavior in different scenarios and operating conditions, supporting validation, optimization, and model improvement. 3.3 Risk prediction model The process of defining the structure of the model is a fundamental step in the development of a model based on artificial intelligence. Therefore, it is necessary to understand the characteristics of the problem and what answers are desired. The problem presented in this work consists of analyzing the behavior of the liquid level in the knockout drum over time, considering adverse situations such as sensor failures, high liquid flow, and leaks. Thus, analyzing time series to identify patterns and trends is a basis for predicting future behaviors. LSTM layers stand out for their ability to handle long-term dependencies, a key feature for dealing with time series. In [3], the authors present a study introducing the use of the Seq-2-Seq technique in the industrial context for energy consumption modeling. This technique follows an encoder-decoder model and uses LSTM layers to map the input sequence into a fixed-dimension vector. Then, another LSTM layer is employed to decode the target sequence from this vector [6]. The results demonstrated this approach’s effectiveness in sequence analysis, allowing for the detection of patterns and precise information inference. Thus, the neural network model proposed in this study implements the Seq-2- Seq technique, using LSTM layers as encoder and decoder. Figure 4 illustrates the architecture of the proposed model. Initially, the input sequence’s values were normalized using the MinMax tech- nique. In the encoding stage, the LSTM layers process the sequence, capturing the temporal characteristics and dependencies of the data. The result of this en- coding is a vector representing the model’s internal state. In the decoding stage, this vector is used as input for another LSTM layer, which generates an output sequence. Finally, this sequence is processed by two fully connected layers, which are responsible for generating the final output vector with the dimension of the prediction horizon. Dropout layers are used to prevent the model from being overfitted. 3.4 Model training To evaluate the model’s generalization capability and estimate the ideal values of its hyperparameters, we conducted a series of training sessions to verify the model’s behavior in different scenarios and identify the best configuration. We used the ADAM optimizer with a learning rate of 0.001 to train our model. Each 10 XXXXXXXXXX et al. Fig. 4. Model architecture experiment was run for 100 epochs. To maintain test consistency, we set the seed value to 0. We used the TimeSeriesSplit technique for cross-validation. As input, the model received a time series extracted from the simulation. Each series comprised 30 sets of values obtained from sensor readings, followed by the next 15 values of the dependent variable. In this process, we adopted the strategy of reducing the number of features sent to the model, selecting only the variables influencing the liquid level. Table 3 presents the evaluated hyperparameters, the tested values, and the best results obtained. From this analysis, it was possible to identify the ideal configuration of hyperparameters for the proposed model, optimizing its perfor- mance for the problem at hand.Table 3. Analysis of model hyperparameters Hyperparameter Best value Tested values LSTM units 64 [16, 32, 64, 128] Dropout rate 0.2 [0.0, 0.2, 0.4] FC units 16 [8, 16, 32] Batch size 128 [32, 64, 128] The Seq-2-Seq model receives as input a time series formed by the features of a specific period and a set containing the values of the dependent variable, where the definition of its dimensions directly influences the assertiveness of the model’s predictions. To determine the best dimensions, we conducted some experiments, which are presented in Table 4. Real-time risk assessment using time series 11 Table 4. Analysis of the dimensionality of time series Input Output RMSE 15 15 0.0491 30 15 0.0327 30 30 0.0739 When the input is too short, relevant information can be lost, compromising the model’s ability to capture complex temporal patterns. This feature is espe- cially problematic in time series that exhibit long-term dependencies. Similarly, when dealing with very long outputs, the model may face difficulties in maintain- ing relevant contexts over time. Moreover, very small outputs may not contain enough details, resulting in simplified and less accurate predictions. As indicated by our experiments, the model that uses a time series composed of data from the last 30 readings and makes predictions for the following 15 units of time showed the best performance, demonstrating a balanced relationship between input and output dimensions. We evaluated using hyperbolic tangent and ReLU as activation functions for the LSTM layers. The hyperbolic tangent is a function that incorporates non-linearity, allowing the network to learn more complex relationships between input and output data. The ReLU function has better computational efficiency and avoids the problem of gradient vanishing. We conducted two experiments to assess the performance of the two activation functions on our training set. The results are presented in Table 5. Table 5. Results of the activation function analyses in the LSTM layers. Activation function RMSE ReLU 0.0623 Hyperbolic tangent 0.0327 Based on the collected results, the hyperbolic tangent was chosen as the ac- tivation function in the LSTM layers. This choice was made due to its better performance than ReLU, demonstrating greater flexibility in capturing corre- lations between variables. This feature, in turn, increases the neural network’s ability to learn more sophisticated representations. 4 Experiments and results We conducted two evaluations to analyze the functioning of the technique pro- posed in this work. Our objectives were to understand the efficacy of our model and ensure that the proposed solution is applicable in various situations to pre- vent liquid accumulation in the drum. 12 XXXXXXXXXX et al. The first evaluation focused on collecting information about the model’s per- formance in preventing liquid accumulation. For this, we compared the operation of the knockout vessel in two distinct scenarios: with and without the use of the technique presented in this work. To conduct the evaluation, both simulations were run for a period of 24 hours, and we used the same generation module described in subsection 3.2 to generate the inputs for our simulation. During the experiment that did not incorporate our technique, four trips were identified, resulting in the interruption of boiler operations due to the temporary suspension of the gas supply. In the experiment where our technique was used, no trips were observed, thus ensuring stability in the gas supply to the boilers. Fig. 5. Liquid level behavior during trip prevention tests In the graphs presented in Figure 5, the blue line indicates the real-time liquid level reading, and the green line indicates the prediction generated by our model. Figure 5 (a) shows the simulation’s behavior without applying the proposed technique. We can observe that the liquid level reached the maximum limit, and the trip was triggered to interrupt the gas supply to the boilers. In Figure 5 (b), with the technique in operation, it can be seen that, as it approaches the maximum level, the refinery’s gas source is automatically switched to the treated gas from the external company. This situation ensures a continuous gas supply to the boilers and provides more time for liquid drainage. In our second evaluation, we explored the performance of the model’s pre- dictions in the presence of noise, transmitter failures, and leaks. This analysis aimed to understand how the proposed model reacts to adverse situations and how different types of failures influence its estimates. To assess the accuracy of the inferences, we ran each simulation for 5 minutes and stored the average difference between the actual liquid level value and the value estimated by the proposed model. Figure 6 (a) demonstrates the excellent accuracy rate of the proposed model in predictions, even with the presence of leakage. The lines in the graph remain Real-time risk assessment using time series 13 Fig. 6. Liquid level behavior under evaluation scenarios close, indicating a minimal discrepancy between the actual and inferred values. The average error in this scenario was approximately 0.04. The impact of transmitter failures on inference accuracy can be seen in Figure 6 (b). A slight difference between actual and inferred values is observed, as all transmitters provide incorrect information. This failure interferes with the voting system, leading the model to infer values with less precision. Tests revealed that the predicted values were always slightly higher than the actual values, where the average error was approximately 0.09. This indicates that the proposed technique still showed promising results in preventing trips because even with transmitter failures, the switch of gas source occurred before the liquid reached its maximum level. The insertion of noise did not significantly impact the accuracy of the in- ferred values, as demonstrated in Figure 6 (c). The average error value in this scenario was 0.008. This is possible because the model receives input information from different components of the simulation. Thus, the importance of the input attributes is distributed, and if some value is incorrect, the impact on inference is smaller. However, if many pieces of information are incorrect, the error rate may increase, as shown in the scenario of Figure 6 (d). In this scenario, failures in level transmitters and noise in sensors were simulated. Throughout the execution of the simulation, a gradual increase in the disparity between the actual values 14 XXXXXXXXXX et al. and the predicted values were observed, indicating that our technique did not achieve satisfactory results in this scenario. The model’s ability to maintain its accuracy in most of the presented risk scenarios ensures the stability of the proposed technique in the production pro- cess. This indicates that even in abnormal situations, the model is still capable of providing good estimates, allowing corrective measures to be taken to avoid the need to trigger the trip. 5 Conclusion This work presented a real-time risk assessment technique for preventing liquid accumulation in a knockout drum. A DT was constructed to collect training data and evaluate the risk scenarios identified in our Bow-Tie analysis. We propose a set of modules with an algorithm for generating the training and testing data sets. To perform inference, we present a recurrent neural network architecture using LSTM layers utilizing the Seq-2-Seq technique to extract patterns from time series and make inferences about liquid-level behavior. This research represents a significant advancement in risk analysis associated with equipment and processes in the oil and gas industry. Artificial neural net- works offer an approach that eliminates the need for experts and allows real-time inferences. The presented model stood out for its ability to produce consistent and reliable results, even when the data providedby sensors are noisy. Further- more, this real-time risk assessment technique can be used in other industrial contexts if necessary modifications are made. The source code and the dataset are available at: https://anonymous.4open.science/r/analise-risco-7BD6 6 Future works The technique proposed in this work has great potential to enhance process safety. However, future research is necessary to understand its capabilities better and maximize the results obtained. A promising area of investigation would be to explore the impact of using a more extensive set of variables on the technique’s effectiveness and precision. Moreover, incorporating other variables inferred by different machine learning models may increase the robustness of the results. Disclosure of Interests. The authors have no competing interests to declare that are relevant to the content of this article. References 1. Alauddin, M., Khan, F., Imtiaz, S., Ahmed, S.: A probabilistic risk assessment of offshore flaring systems using bayesian network. In: Khan, F.I., Siddiqui, N.A., Tauseef, S.M., Yadav, B.P. (eds.) Advances in Industrial Safety. pp. 121–131. Springer Singapore, Singapore (2020) https://anonymous.4open.science/r/analise-risco-7BD6 Real-time risk assessment using time series 15 2. 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