Finite Element Modeling of Self-Loosening of Bolted Joints

A three-dimensional finite element (FE) model with the consideration of the helix angle of the threads was developed to simulate the second stage self-loosening of a bolted joint. The second stage self-loosening refers to the graduate reduction in clamping force due to the back-off of the nut. The simulations were conducted for two plates jointed by a bolt and a nut and the joint was subjected to transverse or shear loading. An M12×1.75 bolt was used. The application of the preload was simulated by using an orthogonal temperature expansion method. FE simulations were conducted for several loading conditions with different preloads and relative displacements between the two clamped plates. It was found that due to the application of the cyclic transverse load, micro-slip occurred between the contacting surfaces of the engaged threads of the bolt and the nut. In addition, a cyclic bending moment was introduced on the bolted joint. The cyclic bending moment resulted in an oscillation of the contact pressure on the contacting surfaces of the engaged threads. The micro-slip between the engaged threads and the variation of the contact pressure were identified to be the major mechanisms responsible for the self-loosening of a bolted joint. Simplified finite element models were developed that confirmed the mechanisms discovered. The major self-loosening behavior of a bolted joint can be properly reproduced with the FE model developed. The results obtained agree quantitatively with the experimental observations.

The Evolution of a Range of Service Experience Rationalised Sealing Materials

With the benefit of hindsight some of the initial non-asbestos offerings were doomed to be commercial failures and since then evolution, in the true Darwinian “survival of the fittest” sense, has resulted in the demise of those offerings. In parallel with this process of evolution, the sealing material testing tools that are available have improved. Consequently, the relationship between a material’s contents, its structure, test data and its service potential is far better understood than in previous times. This improved understanding has brought, and will continue to bring, enhanced performance benefits to the end user and increased security against unplanned shut down due to service failures. This paper reviews the evolution process, highlights the classes of materials that have proven to be successful and tries to indicate why, in terms of the properties of the material, they have been successful. The salient features of the content and structure of these materials are also discussed. The paper also speculates about the way in which further consolidation of the range of sealing materials may come about as the process of evolution, driven by cost, performance and environment pressures, continues.

An Experimental Investigation of the Effects of Clamped Length and Loading Direction on Self-Loosening of Bolted Joints

An experimental investigation was conducted to study the effects of clamped length and loading direction on the self-loosening behavior of bolted joints. Specially designed fixtures were used for the study. The experiments mimicked two plates jointed by an M12×1.75 Class 10.9 bolt and a nut. The joints were subjected to cyclic external loading. A constant preload of 25 kN was used for all the experiments conducted. During an experiment, the relative displacement between the two clamped plates, δ, was a controlling parameter. The reduction in clamping force, the applied transverse load, and the nut rotation were measured cycle by cycle. The relationship between, Δδ/2, the amplitude of the relative displacement between the two clamped plates, and, NL, the number of loading cycles to loosening is referred to as self-loosening curve and was obtained for different clamped lengths and applied load directions. Similar to a fatigue curve, an endurance limit can be identified from the self-loosening curve. It was found that increasing the clamped length can enhance the self-loosening endurance limits in terms of the controlled relative displacement of the two clamped plates. However, the load carrying capability was not influenced significantly due to the thickness of the clamped plates. For a given bolted jointed structure, an angle of the external load from the pure shearing direction resulted in an increase in self-loosening resistance.

Stress Analysis and the Sealing Performance Evaluation of Pipe Flange Connections With Gaskets Subjected to Internal Pressure and External Bending Moment: Effects of Scatter in Bolt Preload

It has been well known that a scatter in axial bolt forces of pipe flange connections tightened by the torque control method is substantial. It is necessary for evaluating the sealing performance of the pipe flange connections with the gaskets subjected to internal pressure and external bending moment to know the contact gasket stress distributions due to the scatter of the axial bolt forces in the connections tightened by the torque control method. This paper deals with the leakage of the pipe flange connections with a spiral wound gasket subjected to internal pressure and external bending moment tightened by the torque control method. The scattered axial bolt forces were measured in the experiments. The contact gasket stress distributions at the interfaces between pipe flanges and the gasket were calculated under the measured axial bolt force by using elasto-plastic finite element method (FEM) taking into account hysteresis and non-linearity in the stress-strain curves of spiral wound gasket. The effects of the scatter in the axial bolt forces tightened by the torque control method on the gas leakage were also examined by using the actual pipe flange connections under internal pressure and external bending moment. By using the calculated contact stress distributions and the results of the leakage tests, the sealing performance was evaluated. It is found that the sealing performance is worse in the actual pipe flange connection than that evaluated by PVRC procedure.

Fastener Tightening Beyond Yield

The effect of fastener tightening beyond yield on the amount of clamp load loss, due to the application of a separating force, is investigated for a system in which the bolted joint remains within its elastic range. After the initial assembly, the joint is subsequently subjected to a tensile separating force, which increases the tensile stress in the fastener further into the plastic range. Simultaneously, the separating force reduces the clamping force. Upon the removal of the separating service load from the system, the system reaches a new equilibrium point between the fastener tension and the joint clamping force. At the new equilibrium point, the tension in the fastener is reduced due to its plastic elongation. The reduction in fastener tension translates to a partial, yet permanent, reduction in the clamping force. Excessive loss of the clamp load is a failure mode that may lead to joint leakage, loosening, or fatigue failure. Additionally, the loss of the clamp force reduces the mean stress, which may significantly affect the fatigue performance of the system under subsequent cyclic loads. A discretized non-linear model is established in order to describe the fastener behavior, and to determine the clamp load loss due to the permanent set in the fastener. The effect of two non-dimensional variables on the amount of clamp load loss is investigated. The first variable is the fastener-to-joint stiffness ratio, and the second is the ratio of initial fastener tension to its yield strength. Analytical results are presented for a range of stiffness ratios that simulates both soft and hard joint applications.

Joint Data Management System Joint Integrity: The Essential Elements

Leaking joints are a main cause of hydrocarbon releases on United Kingdom Continental Shelf (UKCS) offshore sites. The consequential costs of shutdowns and repair can be very high. There are other significant risks, notably to occupational safety, major incident safety and the environment. Fundamental to joint integrity is the competence of the personnel involved. Leak data indicates that poor joint make-up is a major cause of leaks and a review of the causes confirms that the current skills and practices do not give leak-free joints. Therefore the most important element of a management system is to have competent people working on joints. A competence assurance process should be established where the level of training, assessment and experience required is dependant on the potential severity of a release. The results of this should be that that all joints are made up by personnel with an appropriate level of competence. Control of the competence of people working with joints is the most important factor in preventing leaks. There are many ways to influence the integrity of a pipe joint, particularly during design, procurement, fabrication and any intrusive work. A Management System should include details of best practices that are available, with a guide to when and where they should be used, and clarification on tightening methods. Most of these best practices already exist either as industry or company documents but may not be used effectively. • The management system should improve both their visibility and their use, and ensure capture and transparency of all specific historical joint data. Each operator should positively and effectively manage the integrity of bolted joints. It is expected that this will be built on a process of continuous improvement. The essential elements of such a management system are: • Ownership: There should be an identified owner of the management system, responsible not only for its implementation and ongoing maintenance, but also for communicating its aims and objectives throughout the organisation. The owner should state the expectations for the system and monitor its effectiveness. • Awareness: Everyone with an influence on joint integrity in the organisation should be aware of the management system, its objectives, expectations and effects on day-to-day working. Good awareness needs to be maintained. • Tools: A set of implementation tools is required to ensure that the expectations can be met. These should include risk assessment, competence management and control of the practices used. These are discussed in more detail later in this document. • Records and Data Management: The certainty of a successful joint being made up increases if historical data exists on the activities carried out in the past. Recording traceable data encourages best practice at the time of the activity, and will provide useful planning data for the next time the joint is disturbed. • Learning: Learning from incidents is important. A management system should include the means for gathering relevant data, which should be collected by operations engineers or technicians, and periodically reviewed to establish trends, performance and improvements. • Measurement: Easily monitored, but meaningful, performance standards should be put in place at launch to quantify the contribution being made by the management system and evaluate user satisfaction. Examples include: • The number of recorded leaks during testing and start-up; • Percentage leak reduction attributable to the use of the management system.

Creep Modeling in Bolted Flange Joints

Bolted flanged connections are used extensively in the petrochemical and nuclear industries. Under high temperatures, their leakage tightness behavior is compromised due to the loss of load as a result of creep of not only the gasket material but also the bolt and the flange materials. The relaxation of the bolt load and the corresponding loss of the gasket contact stress are not easy to assess analytically and consequently there is no established design calculation procedure. The objective of this paper is to present an analytical method that is part of the SuperFlange program [1] and is capable of predicting the load relaxation in a bolted joint when subjected to flange, bolt and gasket creep. The proposed method is validated by comparison with 3D FE models of different size flanges. In some cases, the relaxation caused by the flange and bolt materials is shown to be significant.

Determination of Critical Temperature of Compressed Non-Asbestos Fiber Sheet Gaskets

Temperature and pressure limits of compressed non-asbestos gaskets have always been of interest to end users, as well as the gasket industry. Since flanges up to class 300 cover the majority of industrial applications of compressed fiber gaskets, a proposed procedure has been developed to clearly document the effect that temperature has on leak rate of the gasket at the maximum internal flange pressure. Different types of non-asbestos compressed gaskets were tested and results showed that for each material, there is a critical temperature after which leakage increases significantly.

Effect of Aging Time on Sealing Performance of Non-Asbestos Gasket at Elevated Temperature

Tests were concluded by a testing apparatus for ROTT/HOTT, which could continuously control the gasket temperature up to 450°C, under either the stress controlled condition or the strain controlled condition, while the conventional tests were conducted under the stress/strain mixed condition with the temperature fluctuation. The sealing performance of asbestos and non-asbestos gaskets was examined. For the non-asbestos SWG (Spiral wound gaskets) tested by the HOTT, the values of Tp at temperature 210°C and 300°C are distributed over 103∼104, while the value of Tp at 420°C is shifted to the relatively lower level of 102∼103. The non-asbestos SWG has same sealing performance as the substitute for the asbestos SWG between 210°C∼420°C. The PHOTT (Preheated Hot Operational Tightness Test, proposed in this study) is also carried out to compare the evaluation of sealing performance at 300°C with that at 210°C. The difference of sealing performance between the HOTT and the PHOTT at 300°C appears. The PHOTT is unavailable as a simplified substitutive test for the HOTT.

The Reduction of Bolt Load in Bolted Flange Joints Due to Gasket Creep-Relaxation Characteristics

It is known that the bolt loads of gasketed flanged joints decrease in some degree after the assembly due to the creep-relaxation characteristics of gasket. If the bolt loads are greatly reduced, the emission from flanged joints may increase and serious accidents shall occur if things come to the worst. The effect of the creep-relaxation characteristics on the bolt loads of gasketed flanged joints must be clarified. A gasketed bolted joint, in which two hollow cylinders and a gasket is tightened by a set of nut and bolt, is used to clarify the effects of the creep-relaxation characteristics of gaskets on the reduction of bolt loads in this paper. We tried to clarify the phenomena and necessary modeling and experiments were carried out. It is pointed out that the reduction of bolt load cannot be overlooked when stiffer bolts are used in gasketed joints.