A new fatigue lifetime assessment approach for offshore jacket structures is presented. It combines a previously developed numerical framework for automated determination of stress concentration factors in tubular joints and a multidimensional finite element modelling approach. The approach is explained based on a case study of an OC4 type offshore jacket. To determine the fatigue life, a directional wave spectrum is combined with the JONSWAP spectrum. The fatigue life of the jacket is assessed for two different sea states. Based on the fatigue analysis the most fatigue critical wave direction is identified. The hot spot stresses in one of the most critical joints are determined and compared to stresses obtained with the Efthymiou equations. The shortcomings of these equations are highlighted and it is shown how the numerical framework can be used to improve the current fatigue design philosophy for offshore jackets which relies on the Efthymiou equations for stress concentration factors in the welded tubular joints.

This study evaluates the influence of volumetric discontinuities on the SENT tearing resistance curve as measured using DCPD and UC techniques. In order to do that, a series of SENT tests was carried out on specimens containing small drilled holes in different locations to simulate the presence of volumetric weld flaws. In addition, porous welds were produced by robotic Gas Metal Arc Welding (GMAW). Welding parameters were fine-tuned in order to obtain specimens with different porosity levels. Afterward, the pores were characterized by means of X-ray Computed Tomography (CT) scans and their position, shape and location were determined. SENT tests were performed in a servo-hydraulic machine using a double clip gauge set up to measure crack opening and DCPD and UC for crack sizing. The results indicated that volumetric discontinuities can influence the accuracy of crack measurement techniques in particular, and the measured resistance curve as a whole.

The strain capacity of pipes under combined loading is an important research topic within strain-based design. While strain capacity equations assume homogeneous pipe material properties, a realistic pipeline shows scatter in mechanical properties. Test and simulation programs have indicated that this “pipe heterogeneity” may reduce the tensile strain capacity under uniaxial loading by a factor up to two. To date, its effect in other scenarios (compressive; combined internal pressure and axial plastic deformation) has received little attention. To investigate these scenarios and compare them with uniaxial tensile loading, Europipe, Salzgitter Mannesmann Forschung (SZMF) and Soete Laboratory, UGent have set up a large-scale test program on UOE pipe X70 (OD = 1219 mm, WT = 17.5 mm), comprising a full-scale pressurized bend test and curved wide plate tension tests. All tested welds joined pipes with nominally equal pipe grade from the same pipeline project, with strongly different actual properties. Optical full field strain measurements by means of digital image correlation reveal local effects of pipe heterogeneity on the strain distribution in the vicinity of the girth weld. All tests showed pronounced degrees of non-uniformity in strain development, in some cases even inhibiting plastic deformation in the stronger material as its weaker counterpart collapses. Implications of the observations with respect to strain-based design are discussed.

Standard pipe corrosion assessments are based on simplifying assumptions with respect to corrosion geometry and focus on pressure based loading. Moreover, when corrosion patches traverse girth welds, validity criteria to their assessment become impractically vague. The integrity of girth welds is additionally influenced by axial stresses, which may act in combination with hoop stress resulting from pressure. In an attempt to address these issues, the authors conducted a detailed assessment on a significant, highly irregular corrosion patch traversing a 12″ natural gas pipeline girth weld. The investigation comprises a full scale uniaxial tensile test and supporting detailed finite element (FE) analyses. Hereby, the model mesh adopts detailed geometrical characteristics resulting from a surface profile scan obtained from stereoscopic digital image correlation. The numerical model is validated based on the uniaxial tensile test, in the sense that plastic collapse and highly complex strain distributions are successfully reproduced. Finally, the FE model is used to explore axial tensile failure in presence of internal pressure.

Engineering Critical Assessment (ECA) guidelines contain amongst others, rules to assess flaw interaction. Major flaw dimensions (depth or height and length) are typically characterized assuming the flaws to be contained entirely within a bounding rectangle through a procedure known as flaw idealization. In (computational) fracture mechanics based calculations, flaws are often assumed to be (semi-)elliptical. This paper investigates the interaction between identical co-planar surface breaking flaws. Two flaw shapes are considered and compared: “canoe-shaped” (quarter-circular ends and constant depth elsewhere) and semi-elliptical. Especially for long shallow flaws, the canoe-shaped approximates the bounding rectangle, whereas the semi-elliptical shape only touches the bounding rectangle at three points (deepest point and two points at the surface). Several flaw dimensions and spacing distances are studied through an extensive parametric study comprising elastic and elastic-plastic finite element simulations. The results, based on Stress Intensity Factor (SIF) and J -integral analysis, show how the flaw shape can affect the degree of interaction. Notably, the inconsistency is less in linear-elastic analysis, but becomes more pronounced at higher (elastic-plastic) loading levels. This work highlights a challenge of comparing analytical and numerical based evaluations of interaction with ECA guidelines.

Engineering Critical Assessment (ECA) of welds is the process of predicting the structural integrity of a structure in the presence of a weld defect under specified loading conditions. Standardized ECA techniques consider the weld to be equal in properties when compared with base metal or use the concept of ‘strength mismatch’ to distinguish the weld from the base metal. In both cases, the weld region is homogeneous. This is a severe approximation from reality, as welds show complex strength heterogeneity patterns. The authors are concerned with techniques to simplify welds in such way that the structural response of the weld is similar to that of the idealized, homogeneous weld. Two approaches are considered: (a) integrating properties along assumed slip lines originating from the defect tip, and (b) assigning All Weld Metal Tensile Tests (AWMTT) to the entire weld region. A plastic analysis procedure suggested by the ASME BP&V code (‘Twice Elastic Slope method’) is adopted to estimate Plastic Load, whose values are compared for the heterogeneous and equivalent homogeneous welds. Finite Element (FE) simulations were performed for Single Edge notched Tensile (SE(T)) specimens. The results put forward the possibilities of weld homogenization while showing its limitations. This will assist in further improvement of weld ECA.

The general aim of a recently finalized European project, i.e. EU RFCS SBD-Spipe, is to generate specific know-how concerning the development and possible use of spirally welded pipes for demanding applications. The demanding applications relate especially to structural integrity issues, both onshore and offshore, requiring good performance under application of large strains resulting in buckling, collapse and/or tensile loading. The outcome of this project can also be used as technical basis for improving standards and guidelines, that address design and safety of spirally welded pipelines. The contribution of Ghent University to this project focusses on the aspects of tearing resistance, defect tolerance and strain capacity of girth welded joints subjected to remote axial tensile load. A numerical and experimental approach is used for the assessment of flaw tolerability and strain development upon tensile loading. Spiral pipes of steel grade API-5L X70 with 36” and 48” diameters have been girth welded using both a manual and semi-automatic welding processes. Curved wide plate specimens have been extracted from the pipes and artificial weld defects have been introduced. The specimens have been loaded in tension up to failure at a temperature of −10°C. This paper reports on the experimental result of a series of curved wide plate tests.

Offshore tubular members are manufactured in a very wide thicknesses and diameters. The design curves considered for the fatigue design of such structures are assessed based on testing small scale laboratory specimens. A correction factor is applied to account for differences in thickness and weld geometry. However crack growth behaviour in tubular elements is complex and it is not totally clear if this can accurately be represented by testing small-scale coupons. Large scale fatigue tests aim to characterize the fatigue behaviour in a more realistic manner. However conducting such tests is very expensive. Hence, a medium scale strip type specimen with dimensions between small scale standardized specimens and a full pipe is designed. It has the same curvature and thickness as the pipe from which it is extracted and a quasi semi-elliptical notch is introduced. It is hypothesized that can account for most of the scale effects. Furthermore it can be tested in conventional test rigs at rather high frequencies. With the objective of quantifying the scale effects between standardized ESE(T) specimens and medium size coupons, a series of crack growth based tests on a high strength offshore steel are carried out. By comparing the corresponding da/dn-ΔK curves it will be possible to evaluate possible scale effects.

Flowlines and risers can be submitted to plastic deformation, sometimes cyclically, due to the installation technique, or sometimes due to exceptional events. In this case, a specific evaluation of defect acceptance in the girth weld is necessary. The present study investigates the possibility to predict ductile tearing during installation when the performed fracture mechanics tests are only high triaxiality specimens and that the effective application requires cyclic loading. A classical analysis is performed using DNV RP F108 to determine the acceptable defect size of for the case in which a pipe is submitted to cyclic loading. In the present investigation, tearing resistance was characterized with SENB specimens. An engineering critical assessment (ECA) was performed considering the size of the expected defects and the amount of plastic deformation to which the pipeline would be submitted. A validation of the ECA was performed by segment tests. While the application of ECA based on the fracture tests would predict ductile tearing with the considered defect, the results of segment tests only revealed blunting for the considered plastic deformation. It confirms the effect that in lower constraint conditions (like in segment tests), SENB test results are overly conservative. The tearing phenomenon was then simulated by the finite element method using two different damage models (Gurson-Tvergard-Needlemann and the Bai-Wierzbicki model) and compared to the experimental results. As the deformation at the crack tip is typically very large, one needs to have knowledge about the hardening behavior in the post-necking region. As this behavior cannot be directly deduced from standard measurements, an automatic identification procedure was developed to determine the post-necking flow behavior of the weld metal and the base material transverse to the weld. As reported in the literature, simplified models like Rambord-Osgood are then inadequate and model including two hardening zones is necessary: one for small deformation and one for large deformation. The calibration of the damage models was only performed on the tearing curve obtained from the SENB experiments, and the segment tests were then “blindly” simulated.

A parametric program designed for modeling tapered, trapezoidal threaded connections is used in combination with Abaqus to investigate the behavior of couplings subjected to static load combinations containing make-up, axial tension and internal pressure. Three criteria are defined and used to quantify the performance of the connection: load distribution, helical gap size between the threads and the amount of global plasticity. From these parameters, the load distribution provides valuable information about the effectiveness of the load bearing characteristics of the thread and can be used to detect possible overstressing of the connection. The helical gap size is used to estimate its tendency to provide a leak tight thread seal, while the global plasticity reflects the total amount of plastic deformation within the connection. During the investigation, the effects of taper angle, load flank angle, wall thickness, size of threads and initial thread clearance are considered. The presented modeling approach consists of three parts. First, the optimal make-up position for a trapezoidal threaded 4.5 inch TN80 connection is calculated using the plasticity criterion. Next, the results for the three performance parameters as a function of both axial tension and internal pressure are discussed. Finally, after investigating the various isolated effects induced by geometric changes, a newly defined, enhanced threaded geometry is suggested and compared with the standardized API-buttress thread.

Different codes are used in the industry for the fatigue design of offshore structures. Because the fatigue phenomenon is very sensitive to different design conditions, standards select those with higher impact and try to cover all other unpredictable factors making use of safety factors. To the authors’ knowledge there is not any comparison of fatigue codes with focus on analyzing the strength and limitations of the design approaches and safety factors included in each one available. This paper has the aim of comparing a number of recognized fatigue design codes for this purpose. Based on this review BS (British Standard) presents the most complete information for offshore design applying the fracture mechanics approach while DNV (Det Norske Veritas) offers the most updated and complete information for fatigue design applying the endurance approach. The safety factors contemplated in the codes are not always comparable and consequently it is not possible to confirm whether one standard is more or less conservative than the others.

The Single Edge Notched Tension (SENT) specimen is a common tool for the determination of tearing resistance in pipeline steels and welds. However, it assumes pure mode I crack tip loading, which is unrepresentative for the evaluation of spiral seam weld flaws. To this purpose, the authors have performed SENT tests using specimens with a tilted notch. This paper evaluates experimental techniques to obtain the tearing resistance of such specimens. Mixed-mode crack opening displacement and ductile tearing are successfully measured using digital image correlation and direct current potential drop, respectively. The latter technique involves a calibration on the basis of a thermo-electric finite element model. Tearing resistance tends to increase with increasing notch tilt angle. Based on the limited number of tests performed, tilted notch SENT testing appears to be a promising tool for the assessment of spiral seam weld flaws.

Weld strength mismatch is a key factor with respect to the assessment of a flawed girth weld. However, it is challenging to assign a single strength mismatch value to girth welds, which are generally heterogeneous in terms of constitutive behavior. The authors have recently developed a method (‘homogenization’) to account for weld strength property variations in the estimation of crack driving force response and the corresponding tensile limit state. This paper separately validates the approach for stress based and strain based assessments. Whereas homogenization is reliably applicable for stress based assessments, the strain based crack driving force response is highly sensitive to effects of actual heterogeneous weld properties. The sensitivity increases with increased weld width and decreased strain hardening behavior. For strain based design, a more accurate methodology is desirable, and large scale testing and/or advanced numerical modeling remain essential.

When assembling tubing strings in the oilfields, threaded connections are used to connect the pipes with each other. The possibility to reuse these connections is often required and a certain degree of leak tightness is required, even without the use of a sealing surface or shoulder. For this reason, the total plasticity within the connection should be limited and relative movement between pin and box ought to be restricted. Within this publication, a finite elements analysis is conducted using a 4.5 inch buttress threaded connection as defined in API 5B together with a connection using the enhanced SR23 buttress thread. In addition, an experimental validation of the make-up stage is conducted by comparing the strains generated during make-up using Digital Image Correlation and infrared monitoring. In order to determine the optimal make-up, values found in literature are compared with a developed method using the magnitude and size of the plastically deformed zones during make-up. In addition, the effect of external axial and pressure loading is examined to identify the effects and critical areas. As a result, it is observed that pressure loading and make-up tend to have similar effects and in order to determine the optimal make-up torque, the pressure ratings of the assembly should be taken into account to prevent overtorqueing the connection. For the case of axial loading, a critical zone is visible near the last engaged thread and excessive loading of this thread can cause premature failure within this zone. Overall, the SR23 connection shows limited, yet visible, advantages over the standard BTC connection as described in literature.

When establishing oil wells, pipe sections are connected by means of threaded couplings. In an effort to minimize the possibility of failure by jumpout, standard buttress connections were introduced. Part of their strength is directly acquired as a result of radial interference during make-up. This paper discusses the results of a numerical study evaluating the effect of make-up on the performance of a standard 4.5 inch API buttress connection when axial tensile force is applied. In order to characterize the structural performance, the load distribution along the coupling length is evaluated, combined with a parameter defining thread separation. The latter is indicative for jumpout and the tendency of creating a leak path throughout the thread helix. From the results it is clear that relative axial displacement within the coupling occurs, even when made up, because of an initial clearance among the load and stab flanks. This clearance may cause a connection to leak through the thread helix when available thread compound cannot heal this leak path. Despite undesirable effects on the sealability and rigidness of this joint, such a clearance is required to decrease frictional forces during make-up while maintaining the desired radial interference.

A key influence factor in the strain-based assessment of pipeline girth weld flaws is weld strength mismatch. Recent research has led to a framework for tensile strain capacity as a function of weld flow stress overmatch. This framework is built around three parameters: the strain capacity of an evenmatching weldment, the sensitivity of strain capacity to weld flow stress overmatch and the strain capacity at gross section collapse. A parametric finite element study of curved wide plate tests has been performed to identify the influence of various characteristics on each of these three parameters. This paper focuses on flaw depth, tearing resistance of the weld, stress-strain behaviour of the base metal, and weld geometry. Influences of these characteristics are mostly found to be limited to one or two of the three framework parameters. A preliminary structure is proposed for equations that further develop the strain capacity framework.

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals’ stress-strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). It can be concluded from further observations that effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Curved wide plate testing is a valuable experimental tool to determine the strain capacity of flawed pipeline girth welds under tension. However, its design, test procedure and analysis are not standardized to date. In an effort to contribute to these three aspects, the authors have executed medium scale (curved) wide plate tests with full field strain measurements and unloading compliance crack extension measurements. This paper discusses specifications, possibilities and limitations of both features and provides representative results. Full field strain measurements and unloading compliance analysis support the validation of a finite element model for curved wide plate testing, and confirm the ability to obtain uniform strain fields in the pipe sections. It is expected that these results may contribute to a future standardization of the curved wide plate test.