The rules of ASME B&PV Section VIII Code provide (i) allowable tensile stress at elevated temperatures based on time duration of 100,000 hours; (ii) external pressure charts which do not account for reduction of buckling strength due to creep under short or long-term loads. For short term operating conditions for which component design governed by an allowable tensile stress based on long term (100,000 hours) creep properties is unrealistically conservative and may be non-conservative in the case of very long load durations (>100,000 hours). At elevated temperatures, the allowable compressive stress depends on magnitude of load and duration. The newly released code cases, Case 2951 ‘Alternative Approach for Operating Conditions (Including Occasional Loads) in the Time-Dependent Regime’ and Case 2964 ‘Allowable Compressive Stress in the Time-Dependent Regime’ provide procedures for elevated temperature design of pressure vessels considering load durations. In this paper, for successful implementation of these code cases, the key variables are reviewed and the inherent assumptions made in the development of these code cases are highlighted. Also, the code cases are applied for design of typical pressure vessel components and the results are compared with other relevant procedures which consider load duration into account.

Pipeline design codes recognize the potential risks posed by gas pipelines by relating the factors which affect the probability of failure to consequences in particular locations. To maintain more or less uniform risk level, ASME B31.8 Code adopted risk based concepts indirectly through location classifications by specifying different design factors (DFs). Engineering Critical Assessment (ECA) of onshore natural gas pipelines in accordance with pipeline-specific methods such as API 1104 allow much deeper defects in higher class pipe, which is in contrary to the basic design concept. This is due to the lack of consideration for the higher consequences in the higher classes even though they were considered at the design stage. To ensure the failure probability within a target value, generic fitness for service standards such as API 579-1/ASME FFS-1 and BS 7910 recommend partial safety factors (PSFs) to key variables. However, there is no correlation between the design factors used during the pipeline design stage and the PSFs used for the ECA. To achieve the basic intent of design code i.e., to maintain risk level as per location classifications, this paper proposes to use PSFs based on class location. Few case studies are presented to demonstrate the proposed methodology.

The Pressure-Area method is recently introduced in the ASME Boiler and Pressure Vessel (B&PV) Code, Section VIII, Division 2 to reduce the excessive conservatism of the traditional area-replacement method. The Pressure-Area method is based on ensuring that the resistive internal force provided by the material is greater than or equal to the reactive load from the applied internal pressure. A comparative study is undertaken to study the applicability of design rules for certain nozzles in shells using finite element analysis (FEA). From the results of linear elastic FEA, it is found that in some cases the local stresses at the nozzle to shell junctions exceed the allowable stress limits even though the code requirements of Pressure-Area method are met. It is also found that there is reduction in local stresses when the requirement of nozzle to shell thickness ratio is maintained as per EN 13445 Part 3. The study also suggests that the reinforcement of nozzles satisfy the requirements of elastic-plastic stress analysis procedures even though it fails to satisfy the requirements of elastic stress analysis procedures. However, the reinforcement should be chosen judiciously to reduce the local stresses at the nozzle to shell junction and to satisfy other governing failure modes such as fatigue.

Low alloy heavy-walled hydroprocessing reactors in the refining industry, which operate at elevated temperatures and high hydrogen environments, need to be designed for protection against brittle fracture especially during start-up and shutdown. While developing minimum pressurization temperature (MPT) envelopes for these reactors, both temper and hydrogen embrittlement effects must be considered. However, there are no specific rules to evaluate loss of toughness due to hydrogen embrittlement in the design Codes / fitness-for-service Standards. Earlier, Japanese steel and pressure vessel manufacturers (JSPVM) developed a methodology to estimate MPT envelope for Cr-Mo steel pressure vessels. Recently, a state-of-the-art fracture mechanics based methodology is published in Welding Research Council (WRC) Bulletin 562 to determine MPT envelopes for equipment operating in high-pressure hydrogen environments. Meanwhile, API RP 934-F draft provided guidance on developing MPT envelops for heavy wall Cr-Mo vessels. In this paper, a brief summary of the JSPVM, API and WRC approach is presented with flow charts. A case study of establishing the MPT envelopes for a typical hydroprocessing reactor using WRC, JSPVM and API approach is presented and the results are compared and discussed. The advantages and potential savings of using recent WRC approach are highlighted.

In the mating phase of float-over operation, the topsides deck load from the vessel is transferred onto the jacket either by ballasting the vessel or by the combination of ballasting and hydraulic jacking system. During this phase of operation, the topsides and jacket experience impact loads through the contact points in a short duration of time. To evaluate the impact loads and to capture the transient effects precisely, a non-linear time domain hydrodynamic analysis is required. To obtain the design loads, generally the numerical jacking simulation is initiated at the time instant of maximum wave height when the jacking system is used. However, the conservative response may also depend on the relative velocity between the jacket and topsides legs. In this paper, a series of non-linear time domain as well as linear frequency domain hydrodynamic analyses are performed to evaluate the impact loads between 9000 tonne integrated topsides deck and a 4-legged jacket in a water depth of 50 m during float-over mating operation. The simulations are performed using MOSES software. The float-over hardware such as LMUs (leg mating unit), DSUs (deck support unit), Jacks, Fenders and Mooring lines are modelled as appropriate linear / nonlinear springs. The principle of the mating operation is considered through a combination of vessel ballasting and jacking operation. This paper discusses about random wave seed selection, effect of vessel response and wave headings on the impact loads of LMUs and Jacks/DSUs.

Mechanical damage in subsea pipelines in the form of local dents / buckles due to excessive bending deformation may severely threaten their structural integrity. A dent / buckle has two significant effects on the pipeline integrity. Notably, residual stresses are set up as result of the plastic deformation and stress concentrations are created due to change in pipe geometry caused by the denting / buckling process. To assess the criticality of a dent / buckle, which often can be associated with strain induced flaws in the highly deformed metal, integrity assessment is required. The objective of this paper is to evaluate the severity of dent / buckle in a 48" subsea pipeline and to make the rerate, repair or replacement decision. This paper presents a Level 3 integrity assessment of a subsea pipeline dent / buckle with metal loss, reported in in-line inspection (ILI), in accordance with Fitness-For-Service Standard API 579-1/ASME FFS-1. In this paper, the deformation process that caused the damage (i.e. dent / buckle) with metal loss is numerically simulated using ILI data in order to determine the magnitude of permanent plastic strain developed and to evaluate the protection against potential failure modes. For numerical simulation, elastic-plastic finite element analyses (FEA) are performed considering the material as well as geometric non-linearity using general purpose finite element software ABAQUS/CAE 2017. Based on the numerical simulation results, the integrity assessment of dented / buckled subsea pipeline segment with metal loss has been performed to assess the fitness-for-service at the operating loads.

Skirt to bottom head attachments of coke drums experience severe thermo-mechanical cyclic stresses, causing failures due to low cycle fatigue. Accordingly, many skirt attachment designs have evolved over a period of time starting with simple conventional weld build up design, improved weld build up design, integral forged attachment design and others. The objective of this paper is to compare thermo-mechanical fatigue life of three different skirt attachment designs using elastic-plastic fatigue assessment methods of ASME Section VIII, Division 2. A transient thermal analysis model is first developed incorporating appropriate boundary conditions. The time-dependent variable heat transfer coefficients at the inner surface of the coke drum, which change with the operation stages and the levels of oil filling and water quenching, are determined based on the field measured thermocouple temperature data on the outer surface of the coke drum. Sequentially coupled elastic-plastic transient thermo-mechanical stress analyses of coke drum skirt attachments are carried out using both Twice Yield and cycle-by-cycle methods. The effective strain ranges and the fatigue life of three different skirt attachment designs are calculated and compared.

External pressure charts of ASME Boiler & Pressure Vessel Code do not account for reduction of buckling strength due to creep under long-term loads at elevated temperatures. This restricts the design of ASME boiler and pressure vessel components in the creep range. Although the design factors and equations for creep buckling are available in various other literatures, the external pressure charts in the creep range are not available at present time. External pressure charts are developed for most commonly used elevated temperature materials (Carbon & 1.25Cr-0.5Mo and 2.25Cr-1Mo low alloy steels) in the refinery and petrochemical industry up to a temperature of 550 °C. API 579-1/ASME FFS-1 isochronous stress-strain curves from Omega model, which does not include the effects of plasticity and primary creep, are used. Thus adjustment factors are proposed based on available isochronous stress-strain curves which considers the plasticity and primary creep effects. The developed external pressure chart for 2.25Cr-1Mo steel at 538 °C is validated with ASME B&PV Code Case 2676. The loads and load combinations to be considered in the design and examples illustrating the application of the developed external pressure charts in the creep range are presented. Codes and Standards: Recent Developments in ASME Codes and Standards.

A number of process reactors in refinery and petrochemical industry are constructed using low chrome alloys which are operating in the creep range and are in cyclic service. ASME B&PV Section VIII Code provides allowable stresses at elevated temperatures for design of reactor components, which are controlled by creep properties. However, fatigue design rules and fatigue exemption rules are not applicable, precluding construction of reactors using low chrome alloys at temperatures above 371°C (700°F). ASME B&PV Section III Division 1 - Subsection NH (ASME-NH) Code considers cyclic failure modes at elevated temperatures and provides creep-fatigue interaction rules and damage limits. In this paper, creep-fatigue damage under elevated temperatures is investigated for process reactor components using elastic analysis method of ASME-NH Code for a defined representative load cycle. Also the complexities in application procedures of ASME-NH rules with thermal and structural analysis results are described in detail.

Reliability analysis of mooring lines requires an accurate prediction of extreme responses for large number of sea states even for a short-term based approach. In deep water, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been shown to give more accurate results but at a higher computational expense. Therefore, efficient computational tools are required for reliability analysis of mooring lines for deep water floating systems. Enhanced decoupled dynamic analysis method, in which the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring line, is an efficient method and provides results comparable in accuracy with the fully coupled dynamic analysis procedures. This paper presents the application of enhanced de-coupled dynamic analysis method for reliability assessment of mooring lines for deep water floating systems. For reliability analysis of mooring lines, the methodology presented in Ding et al. [5] is adopted. Reliability analysis of a critically loaded mooring line for a deep water classical spar floater under extreme environmental loads is performed using environmental contour approach. Mooring line tension time histories under various storm conditions are calculated using enhanced de-coupled dynamic analysis. The uncertainty in the predicted maximum mooring line load due to different storm events, variability in met-ocean conditions and numerical models is considered. Probability of failure and the corresponding reliability index of the mooring line are calculated. The impact of variability in predicted mooring line load, line capacities and factors of safety on mooring line reliability are studied. It is seen that enhanced de-coupled dynamic analysis, which predicts the mooring line loads as accurately as coupled dynamic analysis with lesser CPU time, can be used more efficiently for reliability assessment of mooring lines for deep water floating systems. Copyright © 2012 by ASME.

Efficient dynamic analysis of mooring lines and risers is necessary for deepwater floating systems that typically consist of a number of mooring lines and risers. In deepwater, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been proposed which can account for the coupling effects and consider most of the nonlinearities present in the problem. These methods have been shown to give more accurate results compared to traditional de-coupled analysis methods although they tend to be computationally more expensive. If the system has a large number of mooring lines and risers, it becomes very difficult and impractical to perform time domain coupled analysis. A number of efficient methodologies have therefore been proposed in the past to balance the accuracy of results with computational efficiency. Such methods include the frequency domain approach, combination of frequency and time domain methods, and combination of coupled and uncoupled analysis methodologies. Enhanced de-coupled dynamic analysis is an efficient method and is similar to the traditional de-coupled dynamic analysis method except that the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring lines. In this paper, dynamic analysis of mooring lines for a deep water classical spar floater under random waves is performed by using the enhanced de-coupled dynamic analysis method and the response statistics are compared with results obtained from coupled dynamic analysis. The spar is modeled as a rigid body with six degrees-of-freedom and the mooring lines are modeled as finite element assemblage of elastic rods. All major non-linearities and the dynamic interaction between spar and its mooring lines are considered while determining the tension time histories. Hinge connection is assumed at the fairleads. At every time step of the integration of equations of motion of the spar, a series of nonlinear dynamic analyses of the mooring lines is performed using a subcycling technique. From the analyses, it is found that the enhanced de-coupled dynamic analysis provides results comparable in accuracy with the results obtained from coupled dynamic analysis in terms of predicting the response statistics, but requires only one third of the computational time. Therefore, enhanced de-coupled dynamic analysis can be used for accurate prediction of mooring line dynamics for deep water floating systems. Copyright © 2011 by ASME.

The traditional design methodology for pressure reinforcement of openings in pressure vessels is usually accomplished utilizing the equivalent area replacement. This methodology is specified in Divisions 1 and 2, Section VIII of the ASME B&PV code. It is found that in some cases the stresses at the nozzle shell junctions exceed the allowable limits for stress, even though the code requirements of area replacement rules are met. A parametric study was carried out to arrive at simplified formula for stress concentration factor in thin radial nozzle shell junctions under internal pressure loading. An axisymmetric method equivalent to 3D Finite Element Analysis (FEA) was used. Simplified formula for stress concentration factor is expressed in terms of non-dimensional parameters. It is derived based on data generated for maximum membrane stress intensity at the junction. The proposed formula provides a good approximation of the membrane stress intensity at the nozzle shell junction and allows the designer to incorporate the FEA results in the initial design stage.