Structural Capacities of Flanged Joints

Subsea oil and gas production equipment such as risers, manifolds, trees, wellheads, tubing hangers, tubing heads, blow out preventers, etc., are predominantly designed to criteria of API 6X, API 6A, API 16A, API 17D, API 17G, ASME VIII-2, ASME VIII-3, and NORSOK U-001 design codes. These industry codes provide design criteria to verify failure modes such as plastic collapse, fatigue failure, ratcheting, local strain failure, brittle failure, etc. Among these failure modes, plastic collapse and fatigue failure can be considered as the primary failure modes. A detailed review of plastic collapse design criteria from these codes and standards is discussed in this paper. These codes allow elastic (linear), limit load (elastic perfectly plastic), and elastic-plastic finite element analysis (FEA) to estimate the structural capacity. Some of these codes allow geometric nonlinearity in limit load FEA and some are not. All elastic-plastic analyses are solved with geometric nonlinearity. For this study, four components are considered which represent many subsea equipment. A pressure vessel (cylindrical shell) with internal pressure, open-end vessel (cylindrical shell) with external pressure, load shoulder, and pipe (cylindrical shell) bending are considered. Three materials with yield to tensile strength ratio of 0.53, 0.79, and 0.89 are considered in this study. A detailed review of plastic collapse criteria design criteria from these codes and standards is discussed in this paper for the four cases above.

In a bolted flange joint, metal-to-metal contact type gasket takes over only part of the bolt load to achieve seating stress, and the additional bolt load is transmitted to the metal-to-metal contact to compensate for the unloading effects due to internal and external loadings. Due to this advantage, flange joints with metal-to-metal contact type gaskets are gradually used in chemical industry, nuclear power industry, etc. A three-dimensional nonlinear finite element model is developed to highlight the complex behavior of the flange joint with metal-to-metal contact type gasket under combined internal pressure and thermal loading. Despite of the common perception that the gasket stress in the metal-to-metal contact type gasket stays constant, reduced gasket stress is concluded due to flange rotation and joint thermal expansion.

Austenitic stainless steel is widely used for boiler and pressure vessels, however the actual stress-based method to determine the allowable stress limits the full usage of plastic metals such as austenitic stainless steel that with low ratio of yield to tensile strength. The parts of ASME and European codes of pressure vessels have been attention of it and some measures have been taken to increase the allowable stress by decreasing the design factors or change the specified yield strength from 0.2% proof strength to 1.0% proof strength etc.; the other important method is to increase the yield strength by pressure strengthening. A series theoretic computation and experimental analysis were carried out based on the systematical survey of these codes including Chinese GB150, ASME VIII-1, ASME VIII-2, EN13345 and EN13458 etc, and the result showed the difference of allowable stress among these codes and indicated that Chinese GB150 is most conservative with high strength margin, whereas EN13458 attendance with pressure strengthening is the best solution to increase the allowable stress as well, which increased the allowable stress 99.3% than that of GB150.

A floating-type joint applies the entire bolt load to the gasket to generate sealing contact stress, and is prone to leak due to the bolt load loss. Different with the floating type one, a metal-to-metal contact type joint applies only part of the bolt load to the gasket to achieve seating stress, and applies the additional bolt load to the metal-to-metal contact to compensate for the unloading effects due to internal and external loadings. Due to this advantage, metal-to-metal contact type joints are gradually used in chemical industries, nuclear power industries, etc. A flange joint together with a metal-to-metal contact type gasket forms the metal-to-metal contact type joint studied in this article. A three-dimensional nonlinear finite element model is developed to highlight the complex behavior of the flange joint under bolting-up, pressure loading and thermal loading conditions. Despite the common perception that gasket stress in a metal-to-metal contact type joint stays constant, reduced gasket stress is concluded due to flange rotation and joint thermal expansion.

The performance of a flanged joint is characterized mainly by its ‘strength’ and ‘sealing capability’. A number of analytical and experimental studies have been conducted to study these characteristics under only internal pressure loading. However, with the advent of new technological trends for high temperature and high-pressure applications, an increased demand for more complex analysis is recognized. The effect of steady-state thermal loading is a well-recognized problem and makes the analysis more complex. To investigate joint strength and sealing capability under combined internal pressure and different steady-state thermal loadings, a comparative three-dimensional non-linear finite-element analysis of gasketed and non-gasketed flange joints is carried out and their behaviour is discussed. To determine the safe operating conditions or actual joint load capacity, both the flange joints are further analysed for different internal pressures and temperatures.

The 2004 edition of ASME B31.3, Process Piping, includes strength reduction factors that apply to welds operating in the creep regime for the material. These factors apply to allowable stresses for sustained loads such as weight and pressure. This paper describes the background for the rules.

From Winter meeting of American Society of Mechanical Engineers; Detroit, Michigan, USA (11 Nov 1973). The probability of failure corresponding to specified levels of allowable design stress was calculated for pressure vessels designed in accordance with the ASME Boiler and Pressure Vessel Code. The analysis was performed for maximum shear stress failure and for cyclic stress failure. The significance of such failure prediction is ddscussed and a rationale for selecting an allowable stress is presented. Examples are presented that demonstrate the estimation of vessel failure probability as a function of load variation, strength variation, and design safety factor. (auth)

Investigation on allowable compressive stresses in pretensioned concrete members at transfer

Piping systems of energy industries in oil & gas play a critical role in meeting the increasing global energy demand. A great portion of these pipelines is located in high seismic-prone areas. Such systems have been found to be quite vulnerable to seismic events. Current seismic design approaches to piping systems are mainly based on the allowable stress method, even though more modern design methods are currently available for buildings or nuclear power plants; for example, the Performance-Based Earthquake Engineering (PBEE) framework has not been applied yet to piping systems and relevant structures. In this respect, both information about the quantification of limit states for pipes and adequate non-linear structural models for seismic analysis of piping systems and relevant structures are very limited. One of the key ingredients of PBEE approach for the assessment of the seismic vulnerability of existing structures is the evaluation of fragility curves, namely the probability of exceeding a certain level of damage for a given seismic intensity measure (IM). However, the contributions in the literature on this delicate aspect are very limited. This paper deals with such a problem by using a very popular method, namely the Cloud Analysis, originally developed as a method for probabilistic seismic demand analysis of civil structures. This method is here applied to a typical piping system for process plants. For this purpose, the structure is properly modelled, especially support structure and pipe, including pipe fittings like elbows and bolted flange joints. Using natural accelerograms selected from the PEER database and in accordance with given hazard conditions, the probabilistic seismic demand analysis is performed adopting different engineering demand parameters (EDP) consistent with the damage states expected in the pipes and fittings and in the support structure. According to the results of experimental tests campaign performed in the past by some of the authors on flanged joints, and elbows, different damage states (leakage, yielding, rupture) have been identified and related to the corresponding EDP and the corresponding probability of exceeding has been determined by assuming a lognormal distribution of the response. The analysis intends to recognise the most probable damage condition in a refinery piping system subjected to a seismic input.

Welding of commercially pure (CP) Titanium is challenging due to various factors like Incompatibility of Titanium with other materials, high reactivity, sensitivity to impurities, clean environment etc. Fundamental concern during Titanium welding and cooling is oxidation in weld zone (Weld and Heat affected zone) caused due to inadequate protection from atmospheric contamination of weldment. ASME Boiler and pressure vessel codes like ASME Section VIII Div. 1 - UNF-95, ASME Section VIII Div. 2 (Para. No. 6.2.8) are silent on assessment of weld zone oxidation levels and only specify Bend test requirement on weld Test plates for Category A or B joints. Other ASME code like ASME B31.3 for process piping are silent on requirement of Titanium Welding Test plates and weld zone oxidation. Intent of this paper is to provide insight on various levels of weld zone oxidations formed during CP Titanium welding and exhibit how various oxidation level influences mechanical properties in Titanium weld. Paper provides comparative analysis of Mechanical properties (Tensile, Bend and Hardness) achieved against various levels of weld zone oxidation. Paper concludes why assessment of weld zone oxidation/coloration after each weld pass (As welded/unbrushed condition) is critical aspect during welding and how the various levels of oxidation affects the mechanical properties of commercially pure Titanium welds. Paper also exhibits the need for incorporating the weld zone oxidation/coloration assessment requirement in ASME Boiler and Pressure Vessels, ASME Welding and ASME Piping codes like ASME Section VIII Div 1, ASME Section VIII Div 2, ASME Section IX, ASME B31.3.

Bolted flange joints are extensively used in pressure vessels and piping equipment and rotating machinery. Achieving a uniform bolt preload during the assembly process is particularly important to satisfy tightness in applications such as oil, gas, fossil, and nuclear industries. However, this task becomes very difficult due to the need of retightening the bolts because of elastic interaction and bolt cross talk. The risk of leakage failure under service loading is consequently increased because of the scatter of the bolt preload. This article presents an analytical model based on the theory of circular beams on the linear elastic foundation that simulates the elastic interaction present during the tightening of bolted flange joints to reduce the number of passes while achieving bolt load uniformity. As such, a novel methodology that optimizes tightening sequence strategies is suggested to obtain uniform bolt tension while avoiding yield under a minimum number of tightening passes. In this regard, based on the target preload, the load applied to each bolt in each pass is suggested. The developed approach is validated both numerically using finite element method and experimentally on a NPS 4 class 900 welding neck flange joint using the criss-cross tightening and sequential patterns.

We have provided a comparative analysis of the current international computing standards (European Standard EN 13445-3; ASME-Code, Section VIII, Division 2; British Standard (PD 5500: 2009)) that take into consideration only the static loading of flanges and bolts, if the temperatures of the flanges, bolts and sealing gasket are equal to each other and lower than creep temperature. The paper has put forth relations for completion the calculation method for flange joints in situations not taken into consideration by standards, namely: � static loading if flange temperature and bolt temperature are different; -thermal transient loading; � creep stress; � fatigue loading in the general case of a sequence of blocks of normal stresses. Furthermore, relationships have been proposed for the calculation of the maximum allowable difference between a flange and a bolt so as to ensure both the tightness and the mechanical strength of the flange joint.

The performance of a bolted flange joint is characterized mainly by its ‘strength’ and ‘sealing capability’. A number of numerical and experimental studies have been conducted to study these characteristics under internal pressure loading conditions alone. However, limited work is found in the literature under conditions of combined internal pressure and axial loading. The effect of external, axial loading pressure being unknown, the optimal performance of the bolted flange joint cannot be achieved. Current design codes do not address the effects of axial loading on structural integrity and sealing ability. To study joint strength and sealing capability under combined loading conditions, an extensive experimental and numerical study of a non-gasketed flange joint was carried out. Actual joint load capacity was determined at both design and test stages with the maximum external axial loading that can be applied for safe joint performance. Experimental and numerical results have been compared and overall joint performance and behaviour is discussed in detail.

The realization of a hydrogen transport network requires the requalification of existing high‐pressure natural gas pipelines and the design and construction of new hydrogen pipelines. Therefore, appropriate design codes are needed, which consider the detrimental effects of gaseous hydrogen on the mechanical properties and ensure a robust, but not too conservative, lifetime concept. For the assessment of hydrogen pipelines, the ASME B31.12 is understood as a benchmark standard. Recently, the technical rule DVGW G 464 (2023) was published in Germany. Both standards consider a fracture mechanics‐based concept, where the pipeline design lifetime is assessed by the hydrogen‐assisted fatigue crack growth of a pre‐existing defect until a defined lifetime criterion is reached. This article compares both concepts within a use case study, where influencing factors on the lifetime predictions are identified and discussed. Relatively similar results are obtained for typical loading scenarios, whereby the ASME B31.12 predicts more allowable cycles compared to the DVGW G 464 mostly arising from a different assumed crack length. To reduce the conservatism, the focus in the design and assessment of hydrogen pipelines should be placed on a detectable small initial crack size and the measurement of material‐specific crack growth curves resulting in pronounced lifetime enhancements.

A methodology is presented for Level 1 and 2 Fitness-for-Service (FFS) assessments of process and power piping subject to random vibration loading. The intent is to provide a basis for random vibration assessment based on concepts from spectral fatigue, which is simplified to the degree that a non-specialist can conduct the FFS assessment with little prior knowledge of the subject matter. The proposed Level 1 FFS assessment is based on extension and generalization of established industry screening curves for piping vibration. The measured (overall) RMS vibration level is compared against allowable values given by the curves evaluated at the average crossing frequency of the measured vibration. The proposed Level 2 FFS assessment utilizes an allowable cyclic stress. The allowable stress is a function of the target design life as well as the average crossing frequency and kurtosis of the measured vibration. Both approaches are illustrated by example.