Reliability Based ECA Flaw Acceptance Criteria and Safety Factors of Risers and Flowlines

2017 ◽  
Author(s):  
Yohann Miglis ◽  
S.-H. Mark Chang ◽  
Xinhai Qi
Keyword(s):  
High Stress ◽  
Fatigue Performance ◽  
Life Extension ◽  
Unstable Fracture ◽  
Safety Factors ◽  
Acceptance Criteria ◽  
Project Cost ◽  
Critical Flaw ◽  
Fatigue Loads ◽  
Girth Welds

Risers and flowlines are an integral part of deepwater oil and gas field developments around the world. Risers, which serve as the interface between floating platforms and subsea flowlines, are subjected to low-stress high-cycle fatigue loading due to platform motions and vortex induced vibration (VIV). Flowlines are increasingly required to withstand high-stress range fatigue due to high pressure and high temperature (HP/HT) conditions causing lateral buckling along the flowline. Risers and flowlines are generally made by steel tubulars which are joined by girth welds for most subsea applications. Therefore, the quality of the girth welds is critical to the fatigue performance of risers and flowlines. Fatigue design of risers and flowlines is based on the SN fatigue approach. However, that approach does not address the potential for weld flaws to affect performance. Fracture mechanics based engineering critical assessments (ECAs) provide the technical basis for Non-Destructive Evaluation (NDE) and critical flaw acceptance criteria (FAC). The FAC should address maximum allowable flaw sizes at the fabrication stage to ensure that initial girth weld flaws do not grow excessively and cause unstable fracture or through wall failure over the entire service life. Where there is variability and/or uncertainty, ECAs use conservative assumptions and safety factors. However, as HP/HT developments are becoming more common, FAC resulting from ECA tend to be smaller. The specification of FAC plays an important role in the success of the project in terms of quality, cost and schedule. A more stringent FAC will have more weld rejections, which results in slower fabrication and higher cost, or may even become too small to be detected using automatic ultrasonic testing (AUT). In addition, weld repair will adversely affect the quality and increase probability of failure as girth weld failures are often found initiated at weld repairs. The result is questions about assumptions and safety factors applied. As part of this reliability-based assessment, this paper considers two design examples to address reliability based ECA flaw acceptance criteria and safety factors of risers and flowlines. The first example is a deepwater steel catenary riser (SCR) subjected to fatigue loads due to vessel motion, wave fatigue and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads. This paper offers valuable insights into a balanced approach for inputs selection in ECA by deriving a reliability based FAC and comparing it with the approach outlined in DNV-OS-F101 (Reference 1). It demonstrates FAC can be significantly increased by using reliability based ECA, and as such it will result in faster fabrication and reducing the project cost and schedule. This is of particular interest when considering fatigue performance and life extension of risers and flowlines, asset integrity management, and their relationships with project cost and schedule. Instead of the fit-for-purpose ECA which calculates fatigue life with known girth weld flaws, this paper discusses how to determine allowable initial flaw sizes to satisfy riser and flowline fatigue requirements by deriving a probability density function of the critical flaw acceptance criteria using a reliability based Monte-Carlo approach. This paper provides an approach which is beneficial not only for detailed design but also for tendering purposes during the very early stages of projects, with less conservatism.

Avoiding Local Brittle Zones in Offshore Pipeline Girth Welds for Reeling Installation Involving Large Plastic Strain

2013 ◽  
Author(s):  
Jens P. Tronskar ◽  
Vebjørn Andresen
Keyword(s):  
Fracture Toughness ◽  
Plastic Strain ◽  
Strain Curve ◽  
Structural Steels ◽  
Coarse Grained ◽  
Unstable Fracture ◽  
Large Plastic Strain ◽  
Acceptance Criteria ◽  
Offshore Pipeline ◽  
Girth Welds

Pipelines for reeling are designed to tolerate the large plastic strain associated with the reeling installation process based on widely accepted strain based design principles for subsea pipelines as described in Det Norske Veritas (DNV) Offshore Pipeline Code OS-F101: 2012 [1]. Engineering Critical Assessment (ECA) to develop flaw acceptance criteria for automatic ultrasonic testing (AUT) for girth welds subject to large plastic strain shall according to DNV-OS-F101: 2012 [1] and DNV RP-F108 [2] be carried out in accordance with BS 7910 [3], at assessment Level 3B, with amendments and adjustments described in Appendix A of DNV-OS-F101 for strain-based loading. This is a tearing analysis using the material specific failure assessment diagram (FAD), the material stress-strain curve and the fracture resistance J-R curve (or CTOD-R curve) for the HAZ or WM. It is therefore essential that the pipeline girth welds exhibit maximum load behavior and large tearing capacity to enable development of workable and practical flaw acceptance criteria for the girth welds on the stalks. Welds in offshore structural steels are known from the early 80s introduction of low carbon-manganese micro-alloyed steels, to occasionally exhibit low fracture toughness associated with so-called local brittle zones (LBZ) in the HAZ. Similarly, in the 90s LBZs were found in pipeline seam welds welded at high arc energies. Presence of such microstructures may have a dramatic effect on the coarse grained HAZ CTOD fracture toughness properties causing unstable fracture in the CTOD tests and CTOD values below 0.1 mm at test temperatures of 0°C and below. Recently low CTOD critical fracture toughness values due to pop-ins and unstable fracture initiation in the HAZ have been experienced for pipeline girth welds for reeling and investigation confirmed these were caused by LBZs. This paper makes a comparison with the situation experienced earlier for welds in structural steels and pipeline seam welds, to understand the factors influencing the LBZ formation, and to show how such problems can be avoided. To avoid LBZs formation in the girth welds is imperative for reeling installation, where the large plastic strain associated with reeling installation affects every girth weld.

Structural Integrity for Nuclear Power Plant Against Excessive Load on Design Extension Condition

2013 ◽  
Author(s):  
Daigo Watanabe ◽  
Kiminobu Hojo
Keyword(s):  
Nuclear Power ◽  
Structural Integrity ◽  
Safety Factors ◽  
Design Code ◽  
Acceptance Criteria ◽  
Integrity Assessment ◽  
Current Design ◽  
Factor Design ◽  
Excessive Load ◽  
Load And Resistance Factor

This paper introduces an example of structural integrity evaluation for Light Water Reactor (LWR) against excessive loads on the Design Extension Condition (DEC). In order to assess the design acceptance level of DEC, three acceptance criteria which are the stress basis limit of the current design code, the strain basis limit of the current design code and the strain basis limit by using Load and Resistance Factor Design (LRFD) method were applied. As a result the allowable stress was increased by changing the acceptance criteria from the stress basis limit to the strain basis limit. It is shown that the practical margin of the LWR’s components still keeps even on DEC by introducing an appropriate criterion for integrity assessment and safety factors.

Crack Growth During Full Scale Reeling Simulation of Pipes With Girth Welds

2004 ◽  
Author(s):  
Oddvin O¨rjasaeter ◽  
Olav Jan Hauge ◽  
Guy Ba¨rs ◽  
Per Egil Kvaale
Keyword(s):  
Crack Propagation ◽  
Crack Growth ◽  
High Strain ◽  
High Stress ◽  
Strain Range ◽  
Full Scale ◽  
Small Scale ◽  
Final Fracture ◽  
Lack Of Fusion ◽  
Girth Welds

Installation of pipelines by reeling has proved to be an effective method. However, the pipe bending results in very high stress and strain and cannot be handled by conventional design rules, as stated in design codes, e.g. [2]: High strain crack growth must be assessed according to specific case-by-case selected criterions. In the present work the performance of 10” and 12 3/4” pipes with typical weld defects is studied — from initiation of cracks at notches to final fracture. Information was obtained from several sources: full scale cyclic bending of pipes, FE simulations, and small-scale tests. The plasticity during reeling operations results in substantial non-linear behavior due to varying cross section properties, cyclic creep, and different material response at tensile and compression side of the pipe. Hence, a full scale reeling simulation must be carefully planned and include sufficient tolerances. Critical cracks in pipe girth welds initiate mainly from the surface (undercuts, lack of penetration, or lack of fusion), but potentially also internally (lack of fusion or large pores). Various configurations of these parameters were investigated in full scale pipe tests. It was possible to verify both crack propagation during the reeling cycles, and the point of final fracture (for ECA verifications). In pipe design on must assure safe conditions for both reeling operations and for later in-service loading. Proper design tools must be available. Several methods for high strain crack growth analysis were considered and also compared to small-scale specimen data. Conventional strain-life methodology failed to predict the crack propagation accurately. A new approach including a tensile strain range parameter offered promising results.

An Integrated Approach to Welding, AUT and ECA for Pipeline Construction

2014 ◽  
Author(s):  
David Horsley ◽  
Jing Ma ◽  
Jan van der Ent ◽  
Casper Wassink ◽  
Martin Fingerhut
Keyword(s):  
Integrated Approach ◽  
Final Outcome ◽  
Critical Element ◽  
Unified Approach ◽  
Acceptance Criteria ◽  
Pipeline Construction ◽  
Girth Welds ◽  
Design Options ◽  
The Impact ◽  
Welding Productivity

An integrated approach for the development of welding, inspection, and alternative weld flaw acceptance criteria, as used for girth welds during pipeline construction is presented. Welding is typically the pace limiting step during pipeline construction and is critical element of pipeline integrity. As such it is vital that it be completed efficiently and with high quality. Each of these three elements is vitally important to welding productivity and quality. At the core of the approach is the coordination of the three elements such that they are developed in concert. By this coordinated effort, all design options are considered leading to optimization of the final outcome. The approach is described by providing an example alternative weld flaw acceptance criteria, and giving the logic pertaining to choices of welding setup, AUT setup, the standard used for design and construction, and the impact of choices within these three elements on the final outcome. The paper illustrates the importance of a unified approach on weld productivity and quality.

Using ECA in Structural Integrity of Submarine Pipelines for Optimalization of Intervention Work

2010 ◽  
Author(s):  
Ragnar T. Igland ◽  
Trond Lamvik
Keyword(s):  
Structural Integrity ◽  
Bending Moment ◽  
Pipeline System ◽  
Unstable Fracture ◽  
Axial Capacity ◽  
Detail Design ◽  
Management Scheme ◽  
Residual Capacity ◽  
Girth Welds ◽  
Intervention Costs

The paper deals with the design methodology to define the design loads and determine the maximum allowable size of girth weld defects. The motivation for this work is reduced intervention costs obtained by opening all free spans as these are governing for rock infill volumes. 20–30% reduction of the intervention work is obtained. Structural integrity of the pipeline related to the interference with fishing gear is an important design scenario. Trawling in free span, pull-over loads with clump weight as an ALS condition is the main issue. REINERTSEN observed during detail design a lack of acceptance criteria for ALS conditions in the DNV OS-F101 design code, Ref. [1] for interference between trawl gear and subsea pipelines with low D/t ratio. Curvature in the trawl pull-over point as a function of time is found approximately constant while trawl load is increasing. The membrane forces carry most of the trawl load a few seconds after the trawl impact while bending moment decreases. This is in accordance to the philosophy that the strain and the curvature will be nearly constant for increased loading. The global load bearing mechanism is membrane and less bending. This means that we have control on the strain and that the pipeline system maintains its stiffness against loading for this high axial capacity of the flowline. These observations leads to a deformation controlled trawl load approach where an ECA of the flowline can be used to document structural integrity. Engineering Criticality Assessment (ECA) analysis is applied to evaluate the integrity of the flowlines with respect to risk for unstable fracture in girth welds due to impact from trawl equipment. The fatigue load effects from installation, temporary and operational phases are included in the ECA analysis. Geometric effects and external/internal pressure are included using the tailormade softwares LINKpipe, Ref. [7] and Crackwise4, Ref. [8]. The residual capacity of the flowlines is calculated with emphasis on fatigue during operation after the trawl pull-over. The fatigue life should be within the inspection interval, reflecting the Integrity Management Scheme.

An Innovative ECA Approach For Reeled CRA Welds And Its Validation Programme

2021 ◽  
Author(s):  
Richard Jones ◽  
Dr Thurairajah Sriskandarajah ◽  
Dr Daowu Zhou ◽  
James Hymers ◽  
Kieran Munro ◽  
...  
Keyword(s):  
High Stress ◽  
Crack Tip ◽  
Test String ◽  
Test Rig ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Defect Growth ◽  
Girth Welds ◽  
Stress Fatigue ◽  
Crack Tip Opening

Abstract This paper presents an innovative defect growth ECA methodology for pipeline girth welds and its validation programme, applied specifically to reeling ECA of pipelines with under-matched strength welds. The ECA method is a tear-fatigue approach that accounts for the blunting limit in JR curves during pipe spooling and reel-lay. Fatigue crack growth may occur by low cycle high stress fatigue and by tearing, but the latter only if the crack tip opening displacement exceeds the blunting limit. Conventional ECA with BS7910 is limited because the weld's strength needs to be over-matched. Alternative industry methods for the application of FEA to under-matched strength welds are computationally more intensive than the presented innovative approach. Fatigue crack growth for low cycle high stress fatigue is calculated using Paris’ Law in the approach but, if the crack tip opening due to the tearing mechanism is less than the blunting limit then tearing growth is zero. With the innovative method, if the crack tip opening displacement exceeds the blunting limit then the tearing defect growth is included. Hence, the method is a combined tear-fatigue approach. Welded pipe strings were fabricated from pups composed of clad material; i.e. carbon backing steel pipe with a 3 mm layer of corrosion resistant alloy (CRA) on the inner circumference. Each test string was approximately 10.5m long and fabrication was from a mix of six 0.5m length pups in the central zone of each string and two longer end pups. Three girth welds included EDM notches for test purposes which simulated planar flaws. The notches were on the extreme tension fibre, as the test string gets pulled to the reel former in a reeling test rig. Full scale reeling simulations involved pulling the test strings up to 6 times to the reel former in a reeling test rig. Measurement of defect growth associated with the EDM notches was by scanning electron microscope (SEM), from specimen segments extracted from the test strings. Predictions of defect growth were by finite element models in combination with pipe-specific data that was the outcome of an associated small-scale test programme. Validation of the ECA-by-FEA approach is by a predictive best estimate study, for which there is excellent agreement between the measured values and the calculated defect growths. The ECA-by-FEA approach is conservative for project work, as shown by a high estimate study and an offset blunting limit study. Early development of the ECA approach was for small diameter CRA pipelines during the execution of the Guara-Lula project (Sriskandarajah et al, 2015). The presented full-scale tests, innovative defect growth measurement by scanning electron microscope and the FEA and defect growth calculations were full validation of the approach, with pipe strings that had outer diameter of 323.9mm.

Fracture Toughness Testing and Validation of Pipeline Girth Weld Flaw Acceptance Criteria for Sour Service Under Large Strain

2011 ◽  
Author(s):  
You You Wu ◽  
Wen Guo Yuan ◽  
Tse Ven Steven Chong ◽  
Jens P. Tronskar
Keyword(s):  
Fracture Toughness ◽  
Oil And Gas ◽  
Large Strain ◽  
Axial Strain ◽  
Laboratory Data ◽  
Local Buckling ◽  
Acceptance Criteria ◽  
Service Environment ◽  
Girth Welds ◽  
Sour Service

Fracture toughness is one of the most important input parameters for assessment of pipeline girth weld failure capacity. For many new subsea pipeline projects there is a need to develop flaw acceptance criteria for pipeline installation considering the operation phase which may involve the transport of sour oil and gas and where the pipeline is exposed to large axial strain due to local buckling. Engineering Critical Assessment (ECA) performed using laboratory data based on conservative KISSC testing gives small acceptable flaw sizes which may be below the workmanship criteria for pipeline laying. DNV has conducted extensive research based on the requirements of DNV-OS-F101 and DNV-RP-F108, aiming to establish a method to develop J-R curves applicable for ECA of pipeline girth welds in sour service environment and a methodology to validate the ECA by segment testing in a laboratory-simulated sour service environment as per DNV-RP-F108.

Case Studies on ECA-Based Flaw Acceptance Criteria for Pipe Girth Welds Using BS 7910:2005

2007 ◽  
Author(s):  
Mohamad J. Cheaitani
Keyword(s):  
Case Studies ◽  
Pipe Wall ◽  
Single Point ◽  
Welding Residual Stress ◽  
Practical Case ◽  
Acceptance Criteria ◽  
Specific Level ◽  
Failure Assessment ◽  
Girth Welds ◽  
Selection Of

The use of an engineering critical assessment (ECA) approach to derive flaw acceptance criteria for pipe girth welds has become common practice. It allows the maximum tolerable size of weld flaws to be determined on a fitness-for-purpose basis, offering substantial advantages over the conventional workmanship approach. BS 7910:2005 is widely used to derive ECA-based flaw acceptance criteria for pipe girth welds. It offers a flexible assessment framework within the context of the well-established failure assessment diagram (FAD) approach. However, it can be relatively complex to apply and it may lead to assessments that are more conservative than codified pipeline-specific procedures. This paper illustrates, through practical case studies on assessing the significance of circumferential girth weld flaws, some of the options available to the user of BS 7910. The case studies cover the selection of the FAD (generalised or material-specific, with and without yield discontinuity), tensile properties (specified minimum or actual values); fracture toughness properties (single point CTOD values including δ0.2BL and δm, or full CTOD resistance R-curve), and welding residual stress (assumed to be uniform through the pipe wall with a yield strength magnitude, or considered to have a through-wall distribution associated with a specific level of welding heat input).

The Qualification and Continued Evolution of Reeled Steel Catenary Risers

2009 ◽  
Author(s):  
Fin Gray ◽  
Brett Howard ◽  
Alexandra Pieton ◽  
Ramon Gallart
Keyword(s):  
Full Scale ◽  
Fatigue Performance ◽  
Track Record ◽  
Mechanized Welding ◽  
Steel Catenary Risers ◽  
Weld Fatigue ◽  
Fit For Purpose ◽  
Girth Welds ◽  
The Impact ◽  
Welding Processes

Technip began qualification of reeled Steel Catenary Risers (SCR) back in 1997. Industry had raised concerns at that time over the plastic straining cycles that are intrinsic to the reel lay method and the impact these could have upon the service fatigue life of the girth welds. The qualification programme, therefore, included comparison of reeled welds against virgin welds for a suite of fatigue and mechanical testing including full scale fatigue and fatigue crack growth tests. Reeling was shown to have no discernable impact for the fatigue performance level sought when a controlled SCR fabrication process was adhered to. This provided sufficient confidence that the technology was fit for purpose and led to successful fabrication and installation of the first reeled SCR in 2001. Since then more than 25 have been installed in the Gulf of Mexico, with most projects including full scale weld fatigue test qualification following reeling simulations. This paper includes the following: (a) a summary of the philosophy adopted for qualification, fabrication and installation of a reeled SCR, (b) presentation of the reeled SCR track record and evolution of the technology to include mechanized welding processes (c) a look at ongoing developments targeting even higher fatigue performance, and (d) discussion on the development of fracture mechanics techniques that provide further confidence in the concept and can be used to derive appropriate weld acceptance criteria.

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