Srinivas Sriramula

Reliability of complex chemical engineering processes

Abstract This paper presents a stochastic performance modelling approach that can be used to optimise design and operational reliability of complex chemical engineering processes. The framework can be applied to processes comprising multiple units, including the cases where closed form process performance functions are unavailable or difficult to derive from first principles, which is often the case in practice. An interface that facilitates automated two-way communication between Matlab® and process simulation environment is used to generate large process responses. The resulting constrained optimisation problem is solved using both Monte Carlo Simulation (MCS) and First Order Reliability Method (FORM); providing a wide range of stochastic process performance measures. Adding such capabilities to traditional deterministic process simulators provides a more informed basis for selecting optimum design factors; giving a simple way of enhancing overall process reliability and cost-efficiency. Two case study systems are considered to highlight the applicability and benefits of the approach.

A framework for reliability assessment of ship hull damage under ship bow impact

Ship collision analysis outcomes are generally used in computational models to derive damage distributions. However, damage is usually assessed after the collision energy has been fully absorbed by structural members rather than at the onset of outer hull fracture. Furthermore, the deformation behaviour of ship structural members under load depends on uncertainty modelling through material, geometric, and structural considerations, captured in an appropriate reliability framework. To consider these significant missed opportunities in understanding the probability of ship structures meeting their performance targets during collisions, a novel stochastic framework is proposed in this paper. For efficient reliability computations, a plate resistance model is developed for hull damage assessment at the onset of failure. Stochastic modelling capabilities of Python scripting are interfaced with Abaqus® to compute the stochastic response. The reliability computations show that the probability of hull fracture increases as the hull deformation progresses, with maximum values occurring at the onset of outer hull fracture. The framework outcomes are useful in determining optimal ship structural design capabilities.

Reliability of Profiled Blast Wall Structures

Stainless steel profiled walls have been used increasingly in the oil and gas industry to protect people and personnel against hydrocarbon explosions. Understanding the reliability of these blast walls greatly assists in improving the safety of offshore plant facilities. However, the presence of various uncertainties combined with a complex loading scenario makes the reliability assessment process very challenging. Therefore, a parametric model developed using ANSYS APDL is presented in this chapter. The significant uncertainties are combined with an advanced analysis model to investigate the influence of loading, material and geometric uncertainties on the response of these structures under realistic boundary conditions. To review and assess the effects of the dynamics and nonlinearities, four types of analyses including linear static, nonlinear static, linear transient dynamic, and nonlinear transient dynamic are carried out. The corresponding reliability of these structures is evaluated with a Monte Carlo simulation (MCS) method, implementing the Latin hypercube sampling (LHS) approach. The uncertainties related to dynamic blast loading, material properties, and geometry are represented in terms of probability distributions and the associated parameters. Dynamic, static, linear, and nonlinear responses of the structure are reviewed. Stochastic probabilistic analysis results are discussed in terms of the probability of occurrence, the cumulative distribution functions (CDFs), and the corresponding variable sensitivities. It is observed that using the approach taken in this study can help identify the important variables and parameters to optimize the design of profiled blast walls, to perform risk assessments, or to carry out performance-based design for these structures.

A Hybrid Method for Stochastic Performance Modeling and Optimization of Chemical Engineering Processes

As chemical engineers seek to improve plant safety, reliability, and financial performance, a wide range of uncertaintyladen decisions need to be made. It is widely agreed that probabilistic approaches provide a rational framework to quantify such uncertainties and can result in improved decision making and performance when compared with deterministic approaches. This article proposes a novel method for design and performance analysis of chemical engineering processes under uncertainty. The framework combines process simulation tools, response surface techniques, and numerical integration schemes applied in structural reliability problems to determine the probability of a process achieving a performance function of interest. The approach can be used to model processes in the presence or absence of performance function(s), with or without parameter interactions, at both design and operational phases. With this, process behavior can be quantified in terms of stochastic performance measures such as reliability indices and the associated most probable process design/operating conditions, providing a simple way to analyze a wide range of decisions. To validate the applicability of the proposed framework, three case study systems are considered: a plug flow reactor, a heat exchanger, and finally a pump system. In each case, performance criteria based on the original physical model and the surrogate model are set up. Reliability analysis is then carried out based on these two models and the results are assessed. The results show that the proposed framework can be successfully applied in chemical engineering analysis with additional benefits over the traditional deterministic methods.

Unavailability assessment of redundant safety instrumented systems subject to process demand

Abstract The process industry has always been faced with the challenging task of determining the overall unavailability of safeguarding systems such as the safety instrumented systems (SISs). This paper proposes an unavailability model for a redundant SIS using Markov chains. The proposed model incorporates process demands in conjunction with dangerous detected and undetected failures for the first time and evaluates their impacts on the unavailability quantification of SIS. The unavailability of the safety instrumented system is quantified by considering the probability of failure on demand (PFD) for low demand systems. The safety performance of the system is also assessed using hazardous event frequency (HEF) to measure the frequency of system entering a hazardous state that will lead to an accident. The accuracy of the proposed Markov model is verified for a case study of a chemical reactor protection system. It is demonstrated that the proposed approach provides a sufficiently robust result for all demand rates, demand durations, dangerous detected and undetected failure rates and associated repair rates for safety instrumented systems utilised in low demand mode of operation. The effectiveness of the proposed model offers a robust opportunity to conduct unavailability assessment of redundant SISs subject to process demands.

Development of an ABAQUS plugin tool for periodic RVE homogenisation

EasyPBC is an ABAQUS CAE plugin developed to estimate the homogenised effective elastic properties of user created periodic representative volume element (RVE), all within ABAQUS without the need to use third-party software. The plugin automatically applies the concepts of the periodic RVE homogenisation method in the software’s user interface by categorising, creating, and linking sets necessary for achieving deformable periodic boundary surfaces, which can distort and no longer remain plane. Additionally, it allows the user to benefit from finite element analysis data within ABAQUS CAE interface after calculating homogenised properties. In this article, the algorithm of the plugin based on periodic RVE homogenisation method is explained, which could be developed for other commercial FE software packages. Furthermore, examples of its implementation and verification are illustrated.

Probabilistic Considerations in the Damage Analysis of Ship Collisions

Ship collision events are often analyzed by following the approach of internal mechanics and external dynamics. The uncertainties in collision scenario parameters, which are used in the calculation of external dynamics, are usually quantified during ship collision analysis. However, uncertainties in the material and geometric properties are often overlooked during the analysis of internal mechanics. Consequently, it may lead to overestimation or underestimation of ship structural design capacity, which could impact on system performance.

Dynamic behaviour of unstiffened stainless steel profiled barrier blast walls

ABSTRACT Performing optimum design, reliable assessment or suitable verification for stainless steel profiled barrier blast wall structures requires dealing with various challenges, stemming from the associated uncertainties in material properties, fabrication, installation, and more importantly variations in the blast load characteristics. In the analysis, assessment, and design of these blast walls, one of the key areas to be appreciated and understood is the dynamic response of these structures. This paper presents a methodology developed for identifying the predominant structural behaviour and characteristics of profiled barrier blast wall structures, using a probabilistic approach. Twenty parametric base models are developed using Ansys and by implementing a Latin hypercube sampling (LHS) approach, the section properties of the models are represented in terms of probability distributions. A number of models are generated stochastically and modal analyses performed to identify the dynamic sensitivity of these models. The corresponding response classification of these structures is evaluated from the load duration and natural periods of the structures. The results of the study confirm that structural response, for the wide range of profiled blast walls analysed, is mainly quasi-static or static, as opposed to dynamic. In fact, dynamic effects are negligible for unstiffened profiled barrier blast walls and structural responses in most cases can be estimated on a static or quasi-static basis. This conclusion would help a competent design engineer to consider a proper dynamic load factor at an early stage of the design, without involving complex advanced nonlinear dynamic analyses.

Impact of common cause failure on reliability performance of redundant safety related systems subject to process demand

Common Cause Failures (CCFs) can compromise reliability performance of safety related systems and hence configurations with identical redundant units receive special attention in many industries, including in automotive, aviation and process applications. This paper introduces a new reliability model for redundant safety related systems using Markov analysis technique. The proposed model entails process demand in conjunction with CCF and established system failure modes such as dangerous undetected failures for the first time and evaluates their impact on the reliability performance of the system. The reliability of the safety related systems is measured using the Probability of Failure on Demand (PFD) for low demand systems. The safety performance of the system is also appraised using Hazardous Event Frequency (HEF) to quantify the frequency of system entering a hazardous state that will lead to an accident if the situation is not controlled accordingly. The accuracy of the proposed Markov model is verified for a case study of flammable liquid storage tank overpressure protection system. It is demonstrated that the proposed approach provides sufficiently robust results for all demand rates, demand durations, dangerous undetected and CCF frequencies and associated repair rates for redundant safety related systems utilised in low demand mode of operation.

Contribution of Axial Soil Resistance in Buckle Initiation of the HPHT Pipelines on Sleepers

The objective of this paper is to assess the impact of soil axial resistance on initiation of the buckles on sleepers. It also covers the effects of history of pressure and temperature increase on effective axial force as well as the incorporation of external pressure in the Finite Element (FE) models. This is carried out for 6″, 8″, 10″ and 12″ pipelines laying on sleepers with different heights for a range of axial soil frictions and mobilisations. Knowing the sensitivity of buckle initiation to soil parameters can help in simplifying engineering analysis by avoiding repetitive simulations for parameters with less importance.To carry out the above, a series of FE models including normal and bi-linear axial contacts between pipeline and sleeper / seabed were built in Abaqus FE package and at the point of initiation of the buckles, the effective axial force was extracted by a Python script. FE models were validated by comparison of the simulation results with analytical solutions and experimental results from published literature.Copyright