Engineering Critical Assessment Research Articles

Structural Integrity – Historical developments through millennia

In this contribution, historical aspects through millennia of structural integrity development are presented, starting from the Great Pyramids in antient Egypt. Special attention is paid to the history of bridges, made of wood and stone in antient and medieval time, then from different kinds of iron and steel following the first Industrial revolution and finally by concrete, be it reinforced or pre-stresses, in modern times. The focus of this contribution is on bridges restoration, reconstruction and preservation as part of the world heritage. Two case studies are described, the old stone bridge in Mostar (Bosnia and Herzegovina) and a historical iron bridge in Transylvania (Romania). In the second case structural integrity analysis was performed in the form of Engineering Critical Assessment, to provide a solution for retrofitting the historical riveted steel bridge. This paper is dedicated to the memory of Prof. Stojan Sedmak (1929-2014), one of the fathers of the fracture mechanics in Southeast Europe. Motivation for our research is the paper published 12 years ago on the significance and applicability of structural integrity assessment, Sedmak et al (2012), which was based on the presentation at the Tenth Meeting “New Trends in Fatigue and Fracture” (NT2F10) in Metz, France, 2010.

Structural Integrity of Welded Joints: An Investigation of the FPSO Module Stools

The main goal of the present work is the investigation of a construction and integration of topside modules for an FPSO in Brazil. Most of the topside module structure manufacturing was subcontracted to a third-party company in Europe. During assembling work undergoing at the construction site, several structural details nonconformities were reported by the EPC Group quality control team, e.g., misalignment of stiffeners at the diamond plate and stool connections, geometry irregularities, and unexpected thickness changes on the connection of diamond plates with the correspondent I beam flanges of the pancakes, unexpected bevel buttering to correct joint imperfections which presented several weld flaws in it. Topside structures are subjected to cyclic loading during operation, hence, subjected to fatigue loading. The latter is of concern since the structural integrity of the welded joints must be met for the entire life of the FPSO. The EPC Group required an independent appraisal of the welded joints and structural details nonconformities. They also requested an Engineering Critical Assessment (ECA) on critical joints to be carried out to determine if there might be any flaws compromising the structural integrity of the topsides. Finally, ECA recommended mitigation actions, which could contribute to new topside module integrations for FPSO in Brazil.

Engineering critical assessment (ECA) for monopile foundation of an offshore wind turbine subjected to pitting

An engineering critical assessment (ECA) framework for the monopile foundation of offshore wind turbines subjected to pitting is proposed in this paper. In the framework, a finite element (FE) analysis is conducted based on a 3D model of pitting, and the extended finite element method (XFEM) is utilized to predict the propagation behavior of pitting cracks. The critical size of pitting cracks is then assessed by a failure assessment diagram (FAD) in terms of the BS7910. Compared with the traditional ECA, the proposed ECA framework is more effective in assessing the effects of pits on crack propagation and thus the excessive conservatism of fatigue failure assessment for pitting structures is avoided. An engineering application example is conducted to demonstrate the capability of the proposed ECA framework. The influence of the pit depth and the initial location of the pitting crack on the foundation's fatigue life are further investigated. The results show that the fatigue life of a monopile foundation can be significantly decreased by pitting, and fatigue life can be decreased by almost 30% even a pit with a depth of 2.3 mm.

Crack Analysis Due to Fatigue Load During Subsea Pipeline Installation

Most of the subsea pipelines in Indonesia are installed using the S-Lay method with the pipelay barges equipped with mooring spreads, tensioners, and stinger. During the subsea pipeline installation, static loads occur due to the pipeline configuration from the firing line of the pipelay barge up to the seabed. The pipe will experience axial tension and bending moment in two critical areas: overbend and sagbend. In addition, fatigue loads occur during subsea pipeline installation due to environmental loads (i.e., currents and waves). Defects that are found after welding will grow due to these fatigue loads. Crack analysis with a fracture mechanic approach known as Engineering Critical Assessment (ECA) is carried out by considering the fatigue load due to significant wave height variations for 0.5m, 1.0m, and 1.8m. BS 7910 is used as a standard reference to determine the allowable defects criteria for external and internal flaws. The depth of the defect (a) is simulated from a depth of 1mm – 3mm. The analysis found that the allowable defect length is decreased by 12.7% - 25.0% from a significant wave height of 0.5m to 1.8m for the external surface flaw. While for an internal surface flaw, the allowable defect length is decreased by 5.9% - 13.6% from a significant wave height of 0.5m to 1.8m. These results can be used as a basis for subsea pipeline installation contractors to perform fatigue load sensitivity and optimize the allowable defects based on the actual wave load at the site.

Engineering Critical Assessment of Embedded Flaws in Undermatch Pipeline Girth Welds

Engineering Critical Assessment of Embedded Flaws in Undermatch Pipeline Girth Welds

Probabilistic fracture assessment of snake laid pipelines under high pressure/high temperature conditions by engineering critical assessment

Offshore pipelines are the most effective way to transport of oil and gas. High Pressure/ High Temperature fluid in the pipeline can cause axial expansion, which this expansion is limited by pipe-soil interactions or end connections of pipes that keep them in position. Due to that, axial stresses are created, that can cause pipeline buckling. According to DNV-RP-F110, global buckling is not a failure mode, but it may indicate ultimate failure modes like local buckling, fracture, and fatigue. The purpose of the present study is to investigate the second mode (fracture) by using Engineering Critical Assessment. In order to perform Engineering Critical Assessment, a snake laid pipeline under High Pressure/High Temperature conditions is simulated, and pipe-soil interaction is considered using spring elements. The fracture response of pipelines with semi-elliptical cracks, the influence of loading (pressure and temperature), and crack geometry are investigated. Using Response Surface Method, a polynomial model for Crack Tip Opening Displacement is introduced. Results showed that temperature is the most influential parameter. To consider the uncertainties in geometry, loading, and material and present more accurate results, probabilistic methods such as First Order Reliability Method and Second Order Reliability Method are used. It is concluded that Second Order Reliability Method can give more conservative results.

Fatigue crack growth rate parametric study on subsea X65 pipeline steel girth welds in H2S/CO2 environments

Steel catenary risers, subsea flowlines and pipelines that are used to transport oil and gas products are susceptible to corrosion fatigue in environments containing CO2 or H2S, termed sweet and sour respectively. Fatigue crack growth rates (FCGRs) in such environments are generally enhanced compared to that in air due to crack tip dissolution or the embrittling effect of ingressed hydrogen generated from the corrosion process. Several factors influence the FCGR including partial pressure of CO2 or H2S, pH value, microstructure, temperature and loading frequency. Although there are reports in the literature on the effect of each of the parameters on FCGRs, data on the combined effects, for example, frequency and temperature are not well documented. The work conducted in this study focussed on the effects of temperature and loading frequency on FCGR of X65 pipeline steel welds in sweet and mildly sour environments. The results demonstrate the importance of the selection of the appropriate test temperature and loading frequency in FCGR laboratory tests in order to avoid use of non-conservative or unduly conservative results in engineering critical assessment (ECA). Furthermore, the influence of low concentrations of H2S and the presence of corrosion inhibitor on FCGRs of carbon steels was investigated. The dominant corrosion processes under specific testing conditions are proposed.

Structural integrity assessment for deep-water subsea pipelines

Deep-water subsea pipelines often have to face highly challenging service conditions such as high pressure/high temperature (HP/HT). Structural integrity assessments accounting for corrosion fatigue (CF) and low-cycle fatigue (LCF) therefore are crucial from the perspective of structural safety and cost efficiency. In this paper, engineering critical assessment (ECA) and structural integrity issues of subsea pipelines associated with the HP/HT service conditions are discussed and a new specified ECA scheme is developed. The development includes two parts. In the first part, CF is addressed in 3 levels, i.e., deriving the corrosion-crack correlation model for hydrogen embrittlement (HE) influenced subcritical fatigue crack growth (SFCG), establishing the synthetic SFCG model for CF and finally utilizing the transition stress intensity factor (SIF) for achieving reasonable assessments of CF in seawater. In the second part, 2 levels of work related to LCF is performed, i.e., building up the energy principles based SFCG model for high-cycle fatigue (HCF) and adapting it to LCF. In each subpart, advancement in relevant studies is reviewed with reference to current industrial codes. The work is examined and its effectiveness is demonstrated with applications.

Effects of Welded Pipe-cutting on the Residual Stress Re-Distribution and Engineering Critical Assessment

Welded rolled up steel pipe plates undergo more roundness deformation than seamless pipes. The main reason is that the existing residual stress after welding was re-distributed by the cutting process. The objective of this research is to investigate the effect of the constraint on the cutting process through finite element analysis (FEA) to minimize deformation. To include the effect of the cutting simulation, the external constraint was used as the mechanical restraint, and a welded pipe without constraints was used for comparison purposes. The results show that the cutting deformation and residual stress were both significantly reduced under the effect of external constraints. Moreover, the method of reducing conservatism in engineering critical assessment (ECA) was proposed using residual stress re-distribution.

Elastic surface crack interaction and its engineering critical assessment within the framework of fitness-for-service standards

In real industrial conditions, it’s common to witness the interaction of multiple cracks such that their stress fields and crack driving forces are disturbed. For coplanar cracks, a greater hazard is expected because of the amplification phenomenon. FFS standards deal with this by proposing interaction rules and idealizing a single bonding flaw for the FAD methodology to be used. The goal of this paper is to analyze the effectiveness of this standards methodology considering twin surface semielliptical cracks on a plate under mode I loading on the interaction range using FE analyses. Results confirm that the amplification phenomena due to the interaction are higher on the interacting crack tip and progressively higher as the coplanar horizontal distance decreases. The loss of constraint was observed to decrease as the coplanar horizontal distance decreased, but little change was observed regarding its parametric angular position. A higher amplification was found at the coplanar horizontal distance on which crack interaction is to be considered meaningful, which indicates inconsistency regarding the interaction criteria used on FFS standards. To conclude, the engineering critical assessment of the bonding flaw proved to be over-conservative as the remaining operational life was observed on the assessment of the interacting flaws.

Mode Mixity in the Fracture Toughness Evaluation of Heat-Affected-Zone Material Using SEN(T) Experiment

Abstract Single-edge-notch-tension, SEN(T), specimens have been found to provide a good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than single-edge notch bend, SEN(B) (bend-bars), and compact-tension, C(T) specimens. This lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Additionally, the SEN(T) specimen eliminates concern of material anisotropy since the crack growth direction in the SEN(T) specimen is the same as in a surface-cracked pipe. While the existing recommended and industrial practices for SEN(T) have been developed based on assumption of homogenous or monomaterial across the crack, their applicability for the evaluation of fracture toughness of the heat-affected zone (HAZ) were evaluated in this investigation. When conducting experiments on SEN(T) specimens with a prescribed notch/crack in the HAZ, the asymmetric deformation around the crack causes the occurrence of a combination of Mode-I (crack opening) and mode-II (crack in-plane shearing) behavior. The extent of this mode mixity is dependent on the relative difference between the material properties of the adjacent girth weld and pipe base metals, as well as the amount of crack growth in the experiment. This mode mixity affects the measurement of the crack-tip-opening-displacement (CTOD) and evaluation of fracture mechanics parameter, J. The CTOD-R curve depicts the change in toughness with crack growth, in a manner similar to the J–R curve methodology. Observations also show a mismatch in the length of the crack growth that is measured on the fracture surface, attributable to the material deformation differences across the two adjacent materials (weld and base metals). This paper discusses the experimental observations of Mode I and Mode II behavior seen in experiments conducted on SEN(T) specimens with a notch/crack in the HAZ and as the crack propagates through the weld/HAZ thickness. The paper addresses the issues related to and the changes needed to account for such behavior toward the development of recommended practices or standards for SEN(T) experiments of weld/HAZ. The effects of mode mixity in HAZ SEN(T) experiments is critical to the development of crack growth resistance, CTOD-R and J–R curves employed in engineering critical assessment (ECA) of pipelines.

Thermo-mechanical analysis of the repair welding residual stress of AISI 316L pipeline for ECA

Through-thickness residual stress distribution is a fundamental input parameter for engineering critical assessment (ECA) according to BS 7910 Annex Q. In this paper, we investigated the through-thickness residual stress distribution at a weld toe/weld centerline. Firstly, we performed a simulation of the thermo-mechanical coupled-welding residual stress with/without repair welding of a thick wall pipeline. To further investigate the effects of component radius to wall thickness ratio(r/t) and heat input on the welding stress profile, we developed a welding pass size merging strategy on a mechanical basis. At least ten weld layers were utilized in finite element analysis(FEA) to improve efficiency, without affecting the through-thickness residual stress distribution. The influence of the r/t ratio on the through-thickness residual stress distribution was determined and described as membrane stress and bending stress. Hoop and axial bending stresses at a weld toe decreased gradually with the r/t ratio increased. In contrast, the hoop membrane stress at a weld toe gradually increased with the r/t ratio increased. Moreover, a similar trend was observed at weld centerlines. Analytical equations for predicting the through-thickness residual stress were established for both original welding and in-depth repair welding processes. It was shown that the residual stress distribution after the repair welding was more efficiently improved when a smaller repair depth was selected, i.e., the improvement was adversely related to the repair depth. The residual stress for the original and the repaired welds was expressed as a logarithmic function in terms of axial bending stress, hoop bending stress, and hoop membrane stress.

Engineering critical assessment of steel shell structure elements welded joints under high cycle fatigue

Existing flaws in critical parts of structural elements may lead to failures of subassemblies and, in case of lack of redundancy, even to the entire structure collapse. Considering the case of cyclic loading – wind load (including extreme phenomena as storms or blizzards), in which some of these type of structures have failed/collapsed, the post factum assessment, indicated the principal damage causes, i.e. existing welding flaws in combination with corrosion (surface flaws). The paper presents the fitness-for-purpose analysis, i.e. Engineering Critical Assessment, applied during design, in order to assist in the choice of welding procedure and/or inspection techniques. The evaluation is made with BS7910/2013 standard using level 2 failure assessment diagrams for possible discovered flaws, considering a real case assessment – a new designed steel monopole type antenna tower structure.

Engineering critical assessment of RPV with nozzle corner cracks under pressurized thermal shocks

Nozzle corner cracks present at the intersection of reactor pressure vessels (RPVs) and inlet or outlet nozzles have been a persistent problem for a number of years. The fracture analysis of such nozzle corner cracks is very important and critical for the efficient design and assessment of the structural integrity of RPVs. This paper aims to perform an engineering critical assessment of RPVs with nozzle corner cracks subjected to several transients accompanied by pressurized thermal shocks. The critical crack size of the RPV model with nozzle corner cracks under transient loading is evaluated on failure assessment curve. In particular, the influence of cladding on the crack initiation of nozzle corner crack under thermal transients is studied. The influence of primary internal pressure and secondary thermal stress on the stress field at nozzle corner and SIF at crack front is analyzed. Finally, the influence of different crack size and crack shape on the final critical crack size is analyzed.

Magnification Factor Mk and its Parametric Solutions for Fatigue Crack Growth Assessment of Flaws at Weld Root Toes in Girth Welds

Abstract To account for the effect of the stress concentration factor (SCF) at the weld toe on stress intensity factor range ΔK, a magnification factor Mk is used. The existing Mk solutions given in BS 7910 were developed for the fatigue assessment of flaws at the toes of fillet and butt welds and may not be suitable for assessing flaws at single-sided girth weld root toes, where fatigue cracking often initiates and the weld width is relatively small. Finite element (FE) modeling was performed to determine a 2D Mk solution for engineering critical assessment (ECA) of a flaw at the weld root bead toe. The weld root bead profile was uniquely characterized by five variables including weld root bead width, weld root bead height, hi-lo, weld root bead angle, and weld root bead radius. Following a parametric sensitivity study, defect size, weld root bead height, and hi-lo were identified as the governing parameters. A total of 6000 FE simulations was performed, covering different combinations of weld root bead height and hi-lo. A series of automation scripts were developed in the python programming language and the Mk solution for each type of defect model was developed and provided in a parametric equation. The accuracy of the 2D Mk solutions was confirmed by the experimental data. It was found that the methods and Mk solutions currently recommended in BS 7910 and DNVGL-RP-F108 are inappropriate for assessing a flaw at a girth weld root toe.

Development of a novel fatigue damage model for Gaussian wide band stress responses using numerical approximation methods

A significant development has been made on a new fatigue damage model applicable to Gaussian wide band stress response spectra using numerical approximation methods such as data processing, time simulation, and regression analysis. So far, most of the alternative approximate models provide slightly underestimated or overestimated damage results compared with the rain-flow counting distribution. A more reliable approximate model that can minimize the damage differences between exact and approximate solutions is required for the practical design of ships and offshore structures. The present paper provides a detailed description of the development process of a new fatigue damage model. Based on the principle of the Gaussian wide band model, this study aims to develop the best approximate fatigue damage model. To obtain highly accurate damage distributions, this study deals with some prominent research findings, i.e., the moment of rain-flow range distribution MRR(n), the special bandwidth parameter μk, the empirical closed form model consisting of four probability density functions, and the correction factor QC. Sequential prerequisite data processes, such as creation of various stress spectra, extraction of stress time history, and the rain-flow counting stress process, are conducted so that these research findings provide much better results. Through comparison studies, the proposed model shows more reliable and accurate damage distributions, very close to those of the rain-flow counting solution. Several significant achievements and findings obtained from this study are suggested. Further work is needed to apply the new developed model to crack growth prediction under a random stress process in view of the engineering critical assessment of offshore structures. The present developed formulation and procedure also need to be extended to non-Gaussian wide band processes.

Engineering critical assessment and variable sensitivity analysis for as-welded S690 steels

Engineering critical assessment (ECA) or fitness-for-service (FFS) is a fracture mechanics based approach that is increasingly utilised for assessing the structural integrity of welded steel structures considering various degradation mechanisms which may occur during service. However, if sufficient number of inputs and/or accurate input values are not available, the ECA result is often considered overly conservative and for such cases adds potentially unnecessary obstacles to design and maintenance solutions High strength steels such as S690, S890 and S960 are increasingly used in the offshore and lifting applications, thus to enhance our confidence in using such steels, it is essential to develop better understanding into the relationship between the information from ECA, fracture toughness that can be currently achieved in industry and the required toughness from rules and regulations. This study explores the influence of different variables on critical fracture toughness requirement of S690 steel structures. In addition, the correlation between all the studied variables are investigated to provide quantitative indications of the most important variables in a static ECA process for high strength steel applications. Quantified sensitivity and correlations are obtained for the six studied variables. Industrial fracture toughness data of welded S690 with section thickness up to 160 mm are collected. This provides a snapshot of currently achievable fracture toughens of S690, in comparison with the ECA results and current rules and standards.

A strain-controlled fracture assessment for submarine thin-walled pipes with V-groove welds and circumferential embedded cracks

Abstract In the offshore industry, oil and gas are commonly transported with thin-walled clad pipes. Reel-lay is a widely used method to install clad pipes. During the installation and service process, engineering critical assessment (ECA) is usually required to guarantee safety. Most of the ECA methods pertain to surface cracks. The DNV-RP-F108 standard indicates that an embedded crack can be transformed into a surface crack in evaluation, and the result is still conservative. However, the evaluation can be over-conservative at times, meanwhile, the effect of the weld material as well as groove shape is not considered. In this work, a more direct assessment of welded clad pipes with a circumferential embedded crack is proposed based on a modified reference strain approach. A strain-controlled boundary condition is applied, and various pipe and weld geometrical shapes are considered in the analysis matrix. Two plugins based on Python script are developed to circularly create a lot of numerical models. Hundreds of regression analyses based on the linear relationship between normalized J integral and nominal strain are implemented. An analytical solution of the peak J integral of the embedded crack is proposed, and the solution is proved to be close to FEA.

An improved crack driving force estimation approach for stress-based engineering critical assessment of reeled pipes

In the study, an improved reference load solution is proposed to determine the reference stress and the J-integral for pipelines subjected to reeling, where the axial strains during installation are typically within the range of 1.5%–3.5%. Accordingly, the reference stress can be directly correlated with the un-cracked remote stress of the component through the loading-independent parameter, λ. The effects of cracked geometries and material properties on the reference stress are systematically investigated, and an empirical formula for the reference stress is provided. The proposed method is validated against various J estimation schemes in the literature. It is concluded that the present reference load solution is able to provide an improved and simplified J-integral estimation for the considered geometries over the full strain range of interest.

A modification of reference strain approach for thin-walled submarine pipelines under large-scale plastic strain and internal pressure

Thin-walled pipelines are widely used in submarine oil and gas transportation. During the process of reel-lay and service, pipelines are subjected to large-scale plastic strain and internal pressure. Reference strain approach takes advantage of natural strain-controlled boundary condition directly. However, it is only reliable for small cracks by the assumption of equality between the reference strain and nominal strain. The effect of internal pressure is not considered in traditional failure analysis diagram (FAD). In this study, a theoretical solution of reference strain considering the influence of internal pressure is proposed. A corresponding failure assessment curve (FAC-New) considering the influence of internal pressure is also constructed based on the derived nominal strain. The proposed FAC-New is in contrast with FAC-R6. It is proved that FAC-New presents a more explicit classification of the influence of internal pressure in Engineering Critical Assessment (ECA).