ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V004T00A001. doi:10.1115/OMAE2017-NS4.

This online compilation of papers from the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Materials Technology: Environmental Effect on Materials Performance

2017;():V004T03A001. doi:10.1115/OMAE2017-61355.

Hydrogen is about to become a very important energy carrier. It is regarded as clean since its only bi-product is water when it is converted to electric energy in a fuel cell. Hydrogen gas needs, however to be transported to the end user from the hydrogen plant. If the end user is remote from the plant and the total energy demand is high, the hydrogen gas is best transported through a pipeline. Low pressure hydrogen gas pipeline distribution systems already exist onshore and have been in operation for years both in Europe and USA. REINERTSEN has performed a study to evaluate the possibilities for transporting pure hydrogen gas or natural gas/hydrogen mixtures under high pressure through a submarine pipeline from an onshore plant to an end-user either offshore or overseas from where the hydrogen gas is produced. Hydrogen under high pressure is known to have some possible detrimental effects with respect to material brittleness and enhancement of fatigue crack growth which must be carefully considered before a steel submarine pipeline is constructed and installed. The severity of the detrimental effect may be attributed to both the material type and welding procedure, something which must also be taken into the consideration.

This paper is devoted to the possible changes and limitations in material properties caused by pressurized hydrogen gas that must be considered when designing and installing a submarine transport pipeline or converting an existing natural gas pipeline to hydrogen service.

Commentary by Dr. Valentin Fuster
2017;():V004T03A002. doi:10.1115/OMAE2017-61555.

Subsea structures such as pipelines are vulnerable to environment-assisted crackings (EACs). As a type of EAC, corrosion fatigue (CF) is almost inevitable. For such a process, stress corrosion (SC) and hydrogen-assisted cracking (HAC) are the two mainly driving mechanisms. And it was further pointed out that slip dissolution (SD) and hydrogen embrittlement (HE) should be responsible for SC and HAC respectively. Based on such a fact, a two-component physical model for estimating the CF crack propagation rate was proposed. The proposed model was built in a frame of fracture mechanics integrated with a dissolution model for C-Mn steel and a newly established model by the authors accounting for the influence from HE upon crack propagation. The overall CF crack propagation rate is the aggregate of the two rates predicted by the two sub-individual models, and then the crack propagation time is calculated accordingly. The model has been proven to be capable of capturing the features of HE influenced fatigue cracking behaviour as well as taking mechanical factors such as the loading frequency and stress ratio into account by comparison with the experimental data of X42 and X65 pipeline steels.

Commentary by Dr. Valentin Fuster
2017;():V004T03A003. doi:10.1115/OMAE2017-61556.

Use of Single Edge Notch Tension (SENT) specimen geometry to determine the fracture toughness of the pipeline material in sour service environment has earlier been established in OMAE2009-79305(1). Optimization of the testing techniques has been made with use of implementation of Direct Current Potential Drop (DCPD). DCPD allows for the simultaneous monitoring of crack growth throughout the loading process and subsequently the fracture resistance J /CTOD R-curve can be obtained with just a single test specimen. In this paper the implementation of DCPD method in a wet sour environment using the single-specimen procedure is described and the results have been compared with those obtained using the multi-specimen approach and the results showed good agreement. Some other challenges encountered during the testing process are also addressed in this paper.

Commentary by Dr. Valentin Fuster
2017;():V004T03A004. doi:10.1115/OMAE2017-62022.

Laboratory testing has shown that sour brine environments can reduce the fatigue life of offshore steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air. Thus, in order to ensure safe, reliable, and environmentally-friendly deepwater development, the effect of these sour service environments must be properly accounted for in riser and flowline design. However, to ensure that the environmental effect is fully captured, tests need to be conducted at cyclic loading frequencies representative of those experienced in service (typically 0.1 Hz or less), which makes corrosion-fatigue testing very time-consuming and costly. Consequently, there has been a need for predictive models that can reduce the dependence on extensive long-term testing, while at the same time enable existing data to be interpolated and/or extrapolated over a broad domain of relevant mechanical, environmental, and material variables. In response to this need, a Joint Industry Project (JIP) was organized by Southwest Research Institute® (SwRI®) with the objective of developing and validating an analytical model to predict corrosion-fatigue performance of structural steels in sour brine environments. The resulting model is based on the kinetics of hydrogen generation and transport to a fracture process zone (FPZ), where embrittlement occurs in the hydrostatic stress field ahead of the growing crack. The advantage of this kinetic model is that details of the embrittlement process, which are not presently well defined, need not be included since corrosion fatigue crack growth (CFCG) is governed by the rate-controlling process (RCP) in the elemental kinetic steps that supply hydrogen to the FPZ. A general outline of this model is provided here and its validation against independently generated experimental data is demonstrated. The validated model has been implemented in spreadsheet format for convenience as an engineering tool. Due to the fundamental concepts underpinning the model, the engineering tool is shown to be adaptable to predicting CFCG rates in steels exposed to a variety of other environments — including hydrated and dehydrated sour crude oil, moist H2S gas, sweet brine, and seawater — with and without cathodic polarization. An extension of this Phase 1 model from intermediate to lower CFCG rates is currently underway in Phase 2 of the JIP but will not be discussed in detail in the present paper. The primary objective of this paper is to introduce the engineering tool based on the Phase 1 analytical model and demonstrate its functionality in quantifying CFCG rates over wide ranges of mechanical variables (stress-intensity factor range (ΔK), load ratio (Rσ), and cyclic loading frequency), environmental variables (H2S partial pressure, pH, temperature, applied potential), and material variables (yield strength).

Commentary by Dr. Valentin Fuster
2017;():V004T03A005. doi:10.1115/OMAE2017-62181.

To establish flaw acceptance criteria for carbon steel pipeline girth welds that are intended to transport sour crude, wet sour gas and condensate it is important to assess the effect of operating environment and strain levels by performing the fracture toughness/ resistance testing as per DNV-OS-F101: 2013 in a representative simulated service environment or under more severe test conditions. None the less many oil and gas field operators still believe that using workmanship criteria and radiographic inspection will be adequate to ensure a safe future operation of the pipeline meeting the design life requirements under sour or severely sour operating environments. Unfortunately, experience shows that this is a dangerous practice as radiography tends to miss out in detecting the most severe planar defects such a lack of fusion, hydrogen induced cracking and weld root centerline cracks, this is specifically so for narrow J-bevel welds. Hence, DNV GL based on experience from many projects advocates inspection of all sour service pipelines using inspection methods such as AUT with a high probability of detection for planar flaws. Further, the AUT acceptance criteria shall be chosen appropriately with due considerations as workmanship type of acceptance criteria without proper verification may result in non-conservatism in the pipeline girth weld inspection and weld sentencing. This present paper presents some recent project experiences from typical sour service subsea pipeline projects and provide advices representing what is considered current best practice for testing and qualification of AUT systems for sour service projects.

Commentary by Dr. Valentin Fuster
2017;():V004T03A006. doi:10.1115/OMAE2017-62405.

Reinforced concrete is one of the most widely used construction materials for marine structures. Due to the abundance of the aggressive ions such as chloride ions and sulfate ions in the seawater, the reinforcement exposed to the marine and costal environment are exposed to a high corrosion risk. Localized corrosion will occur once the passive film on the rebar is damaged. In this work, the corrosion behavior of the steel in the simulated pore solution containing with both sulfate ions and chloride ions are studied by using cyclic potentialdynamic polarization methods and the corrosion morphologies observed using scanning electron microscope (SEM). The test results show that the initial rebar corrosion is caused by the absorption of the chloride ions in the passive film. The sulfate ions nearly had no effect on the corrosion of the rebar in pore solution and it can further mitigate the pitting corrosion in chloride containing pore solution.

Topics: Ions , Corrosion
Commentary by Dr. Valentin Fuster
2017;():V004T03A007. doi:10.1115/OMAE2017-62695.

It is well established that sour operating environments can give rise to significantly reduced fracture toughness of pipelines made of carbon manganese steel. Fracture resistance of a material is usually defined in terms of a fracture resistance curve, commonly known as an R-curve which is determined by testing pre-cracked specimens under a rising load. Fracture resistance data can be derived by the single specimen method, where crack extension is determined using unloading compliance or the multiple specimen method, where crack extension is measured from the fracture face of each specimen and each specimen is taken to a different load level. The fracture resistance behaviour of API 5L X65 grade pipeline steel determined by testing single edge notched bend specimens in a specific sour environment using both single and multiple specimen test methods is reported. The fracture resistance of the steel was found to be highly sensitive to the loading rates (described by the initial rate of increase of stress intensity factor in the elastic range) applied during the fracture resistance tests. It was possible to identify a loading rate slow enough to provide fracture initiation toughness reasonably close to the expected lower bound toughness. It is possible to produce similar R-curves from single and multiple specimen testing methods (if conditions are otherwise the same). Under comparable loading rates and environmental conditions, side grooved specimens resulted in lower fracture toughness as compared to the toughness determined from the plane sided specimens. It was also noticed that there was a weaker correlation between side grooving and toughness at slower loading rates.

Commentary by Dr. Valentin Fuster

Materials Technology: Fatigue Performance and Testing

2017;():V004T03A008. doi:10.1115/OMAE2017-61183.

Among recently introduced fatigue assessment approaches, the ones based on the notch stress intensity factor (N-SIF) concept appear to be very promising, both because of their theoretically soundness and because of the possibility of straightforward computation of parameters believed to govern the fatigue process. Actually, while such approaches can be considered in between stress-based approaches and crack propagation ones, quantities like N-SIF and strain energy density in a control volume can be evaluated by finite element analysis according to nowadays well-established procedures, mostly already implemented in commercial FEM software. The present study moves from a former one, dealing with butt joints between typical shipbuilding shell plates, and aims at assessing how the misalignment influences the fatigue strength estimation when applying the strain energy density approach. Eventually, the capabilities of such approach in accounting for misalignments is demonstrated and results are found in agreement with magnification factors currently recommended by guidelines and rules when applying other, stress based, fatigue assessment approaches.

Commentary by Dr. Valentin Fuster
2017;():V004T03A009. doi:10.1115/OMAE2017-61201.

The present paper presents a two-phase model for the fatigue damage evolution in welded steel joints. The argument for choosing a two-phase model is that crack initiation and subsequent crack propagation involve different damage mechanisms and should be treated separately. The crack initiation phase is defined as the number of cycles to reach a crack depth of 0.1 mm. This phase is modelled based on the Dang Van multiaxial stress approach. Both a multiaxial stress situation introduced by the acting loads and the presence of the multiaxial welding residual stresses are accounted for. The local notch effect at the weld toe becomes very important and the irregular weld toe geometry is characterized by extreme value statistics for the weld toe angle and radius. The subsequent crack growth is based in classical fracture based on the Paris law including the effect of the Stress Intensity Factor Range (SIFR) threshold value. The unique fatigue crack growth rate curve suggested by Huang, Moan and Cui is adopted. This approach keeps the growth rate parameters C and m constant whereas an effective SIFR is calculated for the actual stress range and loading ratio. The model is developed and verified based on fatigue crack growth data from fillet welded joints where cracks are emanating from the weld toe. For this test series measured crack depths below 0.1 mm are available. The two-phase model was in addition calibrated to fit the life prediction in the rule based S-N curve designated category 71 (or class F). A supplementary S-N curve is obtained by the Random Fatigue Limit Method (RFLM). The test results and the fitted model demonstrated that the crack initiation phase in welded joins is significant and cannot be ignored. The results obtained by the Dang Van approach for the initiation phase are promising but the modelling is not yet completed. The fracture mechanics model for the propagation phase gives good agreement with measured crack growth. However, it seems that the prediction of crack retardation based on a threshold value for the SIFR gives a fatigue limit that is overly optimistic for small cracks at the weld toe. The threshold value has been determined based on tests with rather large central cracks in plates. The validity for applying this threshold value for small cracks at the weld toe is questioned. As the present two-phase model is based on applied mechanics for both phases the parameters that have an influence on the fatigue damage evolution are directly entering into the model. Any change in these parameters can then be explicitly taken into account in logical and rational manner for fatigue life predictions. This not the case with the rule based S-N curves that are based on pure statistical treatment of the bulk fatigue life.

Commentary by Dr. Valentin Fuster
2017;():V004T03A010. doi:10.1115/OMAE2017-61733.

Offshore structures are subjected to irregular loading spectra due to their exposure to waves and wind. The environmental loads cause variable amplitude stress histories on critical spots of the structures. The existing engineering methodology (adopted by most of the national standards) to estimate the accumulated fatigue damage is based on Miner’s rule for crack initiation. Paris rule and its modifications are used for crack propagation prediction. However, Miner’s rule is a linear model and does not take into account the sequence effect of loading blocks with different stress amplitude. On the other hand, the widely used Paris rule does not take into account the load interaction effects (e.g. overload-induced crack growth retardations). The prediction of the crack growth rate and the crack growth direction of mixed mode cracks is an important issue as well. Aim of the present paper is the analysis of the weaknesses of the engineering tools for fatigue analysis, and the demonstration of the advantages of non-linear damage functions and crack propagation models. A review of models for fatigue crack initiation and growth (for mode I or mixed mode loading) developed by the author is presented. Representative results are discussed and commented.

Commentary by Dr. Valentin Fuster
2017;():V004T03A011. doi:10.1115/OMAE2017-61818.

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.

Commentary by Dr. Valentin Fuster
2017;():V004T03A012. doi:10.1115/OMAE2017-62170.

The objective of this research is to explore the correlation between the piezomagnetic response and ratcheting failure behavior under asymmetrical cyclic stressing in X80 pipeline steel. The magnetic field variations from cycle to cycle were recorded simultaneously during the whole-life ratcheting test. Analysis made in the present work shows that the piezomagnetic hysteresis loop evolves systematically with the number of cycles in terms of its shape and position. Corresponding to the three-stage process in the mechanical response, piezomagnetic response can also be divided into three principal stages, but the evolution of magnetic parameter is more complex. Furthermore, the loading branch and unloading branch of the magnetic field-stress hysteresis loop separate gradually from each other during ratcheting failure process, leading to the shape of hysteresis loop changes completely. Therefore, the progressive degradation of the steel under ratcheting can be tracked by following the evolution of the piezomagnetic field. And the shape transition of the hysteresis loop can be regarded as an early warning of the ratcheting failure.

Commentary by Dr. Valentin Fuster
2017;():V004T03A013. doi:10.1115/OMAE2017-62235.

This paper presents the results of fatigue and strength tests for: 1) cruciform joints with the fillet welds performed underwater, and 2) test coupons with one sided cut by Broco® underwater cut. The laboratory testing was conducted in air. The results show that the under-water fillet welds can have root cracking. For large root cracks this results in fatigue performance that is lower than the equivalent in-air welds without root cracks. In the absence of root cracks the DNV [3] approach for cruciform joints is shown to be applicable. The Broco® underwater cut edge cuts performed above the DNV category F3 [3] fatigue performance for the in-air environment, but below the F1 category. Hardness testing on the cut edge heat affected zoned surface showed values as high as 38 HRC. This is typically a concern for hydrogen stress cracking in offshore environments, and thus the using any category above the F3 curve likely is not justified. For in air testing the strength of the underwater welds and the Broco® underwater cut edges showed no appreciable strength reduction vs. what would be expected from conventional flame cut edges.

Commentary by Dr. Valentin Fuster
2017;():V004T03A014. doi:10.1115/OMAE2017-62394.

Fatigue is the most common known problem of drill pipes, since the combination of make-ups performed to connect the pipes and all the external loads, together with the threaded geometry of the connections, will stimulate the appearance of high stress points, cracks and finally promoting considerable economic losses. When threaded connections are used to connect the casing string, the fatigue resistance of the connection will affect the whole integrity of the string, and thus, in most cases, it is lower as the casing body. Generally, fatigue is classified as low-cycle fatigue and multi- or high-cycle fatigue. For Oil Country Tubular Goods (OCTG), a typical high cycle fatigue is represented by drill pipe fatigue in deviated wells. Unlike drill pipe, the casing may be exposed both to low-cycle as well as to high-cycle fatigue. Low-cycle fatigue is a common type of failure when the applied loads induce high stresses in the metallic material. The number of cycles may vary from as low as 10 up to 100. High-cycle fatigue requires a large number of cycles to failure. In order to avoid catastrophic failures, high-cycle fatigue resistance is usually considered to be sufficient if the number of cycles is above 106. The oil business has focused excessively on testing drilling risers and drill pipes under fatigue loads, but when it comes to casing and tubing the experimental approach may require different solutions. Drilling with casing opened the intensive testing of casing connections against fatigue resistance. Moreover, recent papers have shown intensive work on redesigning connections to withstand fatigue. New applications like rotating while running require a rethinking of testing strategy of Casing and Tubing. The following paper focuses on answering the question whether we test enough. The first part compares existing testing facilities, followed by an intensive discussion about the true loads of a casing or tubing connection. Using public testing data, the second part of the paper tries to identify how far the results provided by various types of testing machines can be compared with each other. For example, we found that low cycle fatigue results may not fully reflect the predictions based on extrapolations of high cycle fatigue results.

Commentary by Dr. Valentin Fuster
2017;():V004T03A015. doi:10.1115/OMAE2017-62418.

Identification of the effect of mean stress for fatigue performance of the premium threaded connection for the OCTG pipes, was conducted via full-scale test. API standard material [grade L80-1] was used for the test. The nominal pipe outside diameter and wall thickness were 244.48 mm (9-5/8 inch) and 11.99 mm (0.472 inch), respectively. The fatigue life of the specimens tested with no mean stress was longer than that of the specimens tested with a tensile mean stress. Through-wall cracks were found at the imperfect thread area of the male embodiment, but the crack initiation site depended on the mean stress. However, the primary root cause of the failure on both mean stress levels can be regarded as the fretting fatigue. Fatigue life was also able to be estimated using modified Goodman relation.

Commentary by Dr. Valentin Fuster
2017;():V004T03A016. doi:10.1115/OMAE2017-62487.

There is an ever-increasing demand for subsea transport of corrosive constituents which requires the use of corrosion resistant pipelines. This has generated interest in mechanically lined pipe (MLP) which consists of carbon steel pipe lined with a thin layer of corrosion resistant alloy (CRA), typically stainless steel. The CRA liner is adhered to the backing pipe by means of an interference fit.

MLPs have been traditionally installed subsea using low strain methods such as towing, S-lay or J-lay. More recently, the efficient reel-lay method, typically used for pipelines up to 18” (457.2 mm) in diameter, has also been considered. To prevent damage to the MLP during high strain bending (i.e. wrinkling of CRA the liner) and thus allow reel-lay installation, TechnipFMC has qualified reeling of MLPs at ambient and elevated pressures. The ambient reeling approach, where the liner thickness is increased to prevent wrinkling during reeling, is appropriate for smaller diameter MLPs. For larger pipelines, it is generally more cost-effective to pressurise the MLP during reeling.

Concerns have been expressed that liner imperfections such as small dents or wrinkles, introduced during manufacturing, installation or service, may compromise the integrity of the MLP subjected to high in-service cyclic loading. Therefore, this study was undertaken to examine the criticality of such flaws and determine the low cycle fatigue endurance of reeled MLPs with imperfections. First, a numerical study was undertaken to estimate in-service stress/strain ranges in the MLPs with liner flaws. Subsequently, small scale tests were carried out to quantify the fatigue performance of such MLPs. The obtained results confirmed that there is a negligible risk of failure of MLP flowlines due to crack initiation at liner imperfections and subsequent breach of the CRA layer, even for pipelines subjected to very severe inservice cyclic loading.

Commentary by Dr. Valentin Fuster
2017;():V004T03A017. doi:10.1115/OMAE2017-62495.

Friction welding is being performed offshore in environments where arc welding may be difficult and where fatigue performance is critical. Friction welding underwater with Remotely Operated Vehicles (ROVs) can greatly reduce the cost of a project compared with using divers and arc welding because the support vessel, which is the major cost component in such an operation, is smaller. This paper describes two different programs of experimental work in which the fatigue endurance of friction welds were found to be better than that which could be expected from arc welded joints of similar geometry. The first program involved experimental work done with 25mm diameter steel bars. It found that, in the as-welded condition, friction welds have high fatigue strength. Residual stress measurements showed that this was due to a beneficial residual stress distribution in which compressive stresses are present at the surface adjacent to the failure site. Further evidence of this was obtained by subjecting some specimens to thermal stress relief. The fatigue strength of the stress relieved specimens was reduced compared with the as-welded joints but nevertheless the fatigue strength of these specimens was still high. The second program involved fatigue tests on friction stud welds in which the friction welding equipment was deployed offshore by divers or ROVs. The test specimens were made up of 19mm diameter studs friction welded onto structural steel plate. As with the first program, the specimens showed high fatigue endurance with results approximating to a DNV Class C1 curve. In some of the tests, the studs were preloaded in tension and results from specimens that were preloaded to the correct value specified for the joint were all stopped as run-outs, with specimens remaining unbroken.

Commentary by Dr. Valentin Fuster
2017;():V004T03A018. doi:10.1115/OMAE2017-62677.

Fatigue is a major cause of failures concerning metal structures, being capable of causing catastrophic damage to the environment and considerable financial loss. Steel pipelines used in oil and gas industry for hydrocarbon transportation, for instance, are submitted to the action of cyclic loads, being susceptible to undergo fatigue failures. The phenomenon of metal fatigue is a complex process comprising different successive mechanisms. In general, four stages can be identified, representing microcrack initiation (nucleation), microcracking, macrocrack propagation, and final fracture. Fatigue damage prior to nucleation of microcracks is primarily related to localized plastic strain development at or near material surface during cycling. The microhardness of the material shows its ability to resist microplastic deformation caused by indentation or penetration, and is closely related to the material plastic slip capacity. Therefore, the study of changes in material surface microhardness during the different stages of fatigue process can estimate the evolution of the material resistance to microplastic deformations and, consequently, provide relevant information about the cumulated fatigue damage on the surface. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. In a previous work, the X-ray diffraction technique was used to evaluate these changes. This technique presents several advantages, since it is non-destructive and concerns the surface and subsurface of the material, where major microstructural changes take place during fatigue. The most important parameter obtained by this technique is the full width at half maximum (FWHM) of the diffraction peak, which can provide information about the dislocation network density and estimate microdeformations. It was found that the evolution of this parameter with cycling presents three different stages, associated to the mechanisms of microcrack initiation, microcracking, macrocrack propagation, respectively. Here, the fatigue damage of pipeline steels is evaluated through microhardness testing. Different stages of changes in microhardness are also found and they are correlated to those observed with the X-ray technique and also with transmission electron microscopic (TEM) images from experimental tests performed with a similar material. This correlation can help to corroborate the X-ray diffraction results previously obtained and recommend then this non-destructive technique as the base of the method for predicting fatigue life of steel structures proposed here.

Commentary by Dr. Valentin Fuster
2017;():V004T03A019. doi:10.1115/OMAE2017-62696.

Fatigue failures during offshore drilling operations is still a very costly problem. The fatigue behavior of drill pipes is reviewed, and typical failure modes are identified. The effects of drill string curvature during directional drilling on pipe body stress and on the fatigue life is examined. Effects on applied mean stress from drill string weight are discussed. Interaction effects of degradation mechanisms such as fatigue, wear and corrosion are evaluated. Experimental background data and statistical evaluation that form the basis for the current design practice issued by American Petroleum Institute (API) and other guidance in codes and standards is reviewed. Results from several recent testing programs performed under rotating bending of pipes with threaded connections, and tests involving the pipe body under resonance conditions are presented. The tests were made with pipe sizes from 2 7/8 in. to 5 7/8 in. in Grade S-135 pipes. The results are compared with published test data and design guidance such as API Recommended Practice G7 [1]. Recommendations are given for research and testing to improve reliability and the safe operation of drill strings.

Commentary by Dr. Valentin Fuster

Materials Technology: Fracture Assessment — Analytical Methods

2017;():V004T03A020. doi:10.1115/OMAE2017-61091.

Evaluation of the stress intensity factor (SIF) for a crack propagating in a structural component is the analytical basis of linear elastic fracture mechanics (LEFM) approach. Handbook solutions give accurate SIF results for simple crack geometries. For intricate crack geometries and complex loading conditions finite element method (FEM), is used to predict SIF. The main drawback of FEM techniques is that they are prohibitively expensive in terms of computing cost and also very time consuming. In this manuscript, authors have presented a Gaussian Process Regression Model (GPRM), which may be used as an alternative to FEM for predicting SIF of a propagating crack. The GPRM is firstly trained using 70 SIF values obtained by FEM, and then validated by comparing the values of SIF predicted by GPRM and FEM for 30 data points (i.e. combination of crack size and loading). On comparing the aforementioned values the average residual percentage between the two is 2.57%, indicating good agreement between GPRM and FEM model. Also, the time required to predict SIF of 30 data points is reduced from 30 mins (for FEM) to 10 seconds with the help of proposed GPRM.

Commentary by Dr. Valentin Fuster
2017;():V004T03A021. doi:10.1115/OMAE2017-61166.

In this paper, the evolution of ductile damage in pipe girth welds for offshore pipelines application is studied with the use of Finite Element Analysis. To this purpose, the Gurson-Tvergaard-Needlmen (GTN) model is calibrated and applied to different finite element simulations. Saipem internally developed software, written in Matlab® environment, is used to automatically calibrate the Gurson model from experimental data and run several Finite Element Models of the pipe with different flaws in the girth weld.

The objective of this paper is to present and discuss this numerical tool, developed by Saipem coupling a Matlab® GUI and the ABAQUS FE solver. The tool allows speeding up and automatizing the calibration procedure of the Gurson model and making a rapid evaluation in terms of the Crack Driving Force (CDF) for defective girth weld. The tool allows also a quick and efficient model preparation, full FE analysis and post-processing, with saving of engineering hours.

Commentary by Dr. Valentin Fuster
2017;():V004T03A022. doi:10.1115/OMAE2017-61425.

The local approach to modelling ductile tearing is a useful technique to give insight into fracture mechanics. However, applications of the local approach have been stymied by the high cost of finding the parameters that characterize it because of the number of specimens and expensive post-processing that the testing requires. In this paper, a novel iterative method to extract a failure locus from one Crack Tip Opening Displacement (CTOD) specimen is presented. Material points fail under various different stress states in a CTOD specimen, so many different points on the failure locus can be found through thoughtful post-processing in FEA. A phenomological ductile failure locus is fitted through the stress triaxiality, Lode angle, and plastic strains that cause failure at material points in the CTOD test. Simulating a CTOD test with a different aspect ratio has shown that the failure locus found by this method can be predictive, giving both accurate force versus Crack Mouth Opening Displacement (CMOD) curves and realistic fracture surfaces featuring separate tunnelling and shear lips.

Commentary by Dr. Valentin Fuster
2017;():V004T03A023. doi:10.1115/OMAE2017-61475.

In this paper, extensive three-dimensional finite element analysis is conducted to study the asymmetric four-point shear (AFPS) specimen: a widely used mixed mode I/II fracture test specimen. Complete solutions of fracture mechanics parameters KI, KII, KIII, T11, and T33 have been obtained for a wide range of a/W and t/W geometry combinations. It is demonstrated that the thickness of the specimen has a significant effect on the variation of fracture parameter values. Their effects on crack tip plastic zone are also investigated. The results presented here will be very useful for the toughness testing of materials under mixed-mode loading conditions.

Commentary by Dr. Valentin Fuster
2017;():V004T03A024. doi:10.1115/OMAE2017-62162.

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.

Topics: Welded joints , Pipes
Commentary by Dr. Valentin Fuster
2017;():V004T03A025. doi:10.1115/OMAE2017-62333.

This paper examines the applicability of the different meta-models (MMs) to predict the Stress Intensity Factor (SIF) of a semi-elliptic crack propagating in topside piping, as an inexpensive alternative to the Finite Element Methods (FEM). Five different MMs, namely, multi-linear regression (MLR), second order polynomial regression (PR-2) (with interaction), Gaussian process regression (GPR), neural networks (NN) and support vector regression (SVR) have been tested. Seventy data points (SIF values obtained by FEM) are used to train the aforementioned MMs, while thirty data points are used as the testing points. In order to compare the accuracy of the MMs, four metrics, namely, Root Mean Square Error (RMSE), Average Absolute Error (AAE), Maximum Absolute Error (AAE), and Coefficient of Determination (R2) are used. Although PR-2 emerged as the best fit, GPR was selected as the best MM for SIF determination due to its capability of calculating the uncertainty related to the prediction values. The aforementioned uncertainty representation is quite valuable, as it is used to adaptively train the GPR model, which further improves its prediction accuracy.

Commentary by Dr. Valentin Fuster

Materials Technology: Fracture Assessment — Experimental

2017;():V004T03A026. doi:10.1115/OMAE2017-61144.

Conical connections are important structural members for the integrity of most types of welded tubular structures. They are for example used in traditional jacket structures for oil and gas production and in monopiles for support of wind turbines where an optimal design is aimed for. From contact with the industry it is noted that there is uncertainty about the basis for the stress concentration factors for conical connections in design standards for fatigue assessment. This is related to how fabrication tolerances are accounted for and how a transition in thickness from the cone to the tubular or the cylinder should be made to minimise stresses due to thickness transitions and fabrication tolerances.

Analytical expressions for stress concentrations at conical transitions are outlined in this paper to get a better understanding of the thickness of the cone and the cylinder. By a proper basis for fatigue design it is possible to control additional stresses from thickness transitions and fabrication tolerances at these connections.

Topics: Fatigue design
Commentary by Dr. Valentin Fuster
2017;():V004T03A027. doi:10.1115/OMAE2017-61420.

It is sometimes necessary to find the toughness of existing structures without damaging them. Examples of this scenario include situations in which the material is suspected of being brittle or service life extensions. However, fracture testing is inherently destructive. Removing material for Charpy or Crack Tip Opening Displacement (CTOD) specimens can result in expensive repairs. The Small Punch Test (SPT), which has been developed for monitoring programs in the nuclear industry, offers a test method that requires such small amounts of material that the test can be performed in a practically non-destructive way.

A pilot project was conducted to determine if the SPT can be applied to steels of use in maritime and offshore applications. The results of the pilot project showed that the SPT can identify behavior related to the ductile to brittle transition for an example S355 steel. Therefore, the SPT can provide valuable information for predicting fracture properties relevant to structural-level behavior of steel, such as Charpy transition and estimates of CTOD values in the lower shelf and lower portion of the ductile to brittle transition curve.

In the end of this paper, a theoretical framework for transferring results from SPT to CTOD or Charpy testing is outlined.

Commentary by Dr. Valentin Fuster
2017;():V004T03A028. doi:10.1115/OMAE2017-61728.

Effects by intercritical quenching, which is quenching from dual-phase of ferrite (α) and austenite (γ) region from 953 to 1068 K, on mechanical properties and microstructures of Cu-containing low alloy steel based on ASTM A707 5L grade (hereafter called A707 modified steel) were investigated using 50 kg test ingots. The mechanical properties of the A707 modified steel, i.e. strength at room temperature and fracture toughness at low temperature, were significantly improved by intercritical quenching. This is probably because its effective grain size decreased by intercritical quenching. Then, the optimum temperature of intercritical quenching for A707 modified steel was 1068 K near the AC3 point.

Based on the experimental results of the test ingot, we applied intercritical quenching to a trial full-size forging production of about 20,000 mm in length, and researched the tensile, Charpy impact, crack tip opening displacement (CTOD) and drop weight test (DWT) properties across whole length of the trial production. It was found that the trial production has good mechanical properties across whole length. From the present work, an appropriate intercritical quenching is considered to apply for improvement method of the mechanical properties in A707 modified steel forgings.

Commentary by Dr. Valentin Fuster

Materials Technology: Impact of Steel and Construction Technologies on Structural Integrity

2017;():V004T03A029. doi:10.1115/OMAE2017-61176.

The process piping on subsea production systems (SPS) is normally made of solid corrosion resistant alloys (CRAs). However, some process components are made of low alloyed steels (LASs) which are internally cladded with a CRA. These components require post weld heat treatment (PWHT) to improve the properties in the LAS heat affected zone (HAZ). In order to avoid PWHT during on-site welding to adjoining piping systems, it has been common to weld a buttering layer (e.g. 15 – 20mm long) on to the connecting end of the LAS. The buttering layer consumable has traditionally been an austenitic nickel alloy, Alloy 625/725. The LAS HAZ and the buttering layer are thereafter PWHT’d and machined prior to on-site welding to the adjoining piping system. By this, it is not necessary to perform PWHT on the on-site (e.g. tie-in or closure) dissimilar welds. In the beginning of the century, some operators experienced cracking along the fusion line interface between the nickel alloy buttering and the LAS. These problems were typically experienced during start-up or prior to first production. An extensive research programme was established in order to determine the causes and remedial actions. A group sponsored project led by TWI was performed to understand the failure mechanisms and essential parameters leading to hydrogen assisted cracking, (HAC) of dissimilar metal welds (DMWs). Recommendations were made related to LASs chemistry, welding parameters, bevel geometry and especially PWHT time and temperature. Based on these recommendations there have been only a few incidents with cracking of such welded combinations before 2013 and onwards. Since then Statoil has experienced four off incidents with cracking of dissimilar welds on subsea LAS components. Common for these incidents are that they have been in operation for about 15 years and the cracking happened during cold shut-down periods.

This paper presents key observations made and lessons learnt from the incidents summarized above. The main focus has been on environmental fracture mechanics-based testing of samples charged with hydrogen by cathodic protection (CP). Variables have been pre-charging temperature and time, as well as testing temperature. The testing has revealed strong dependency between the operating temperature (i.e. shutdown versus operation) and the sensitivity to HAC. Further, the investigations have shown that the integrity of the coating, as an effective barrier to hydrogen ingress, is the main feature to prevent HAC on this kind of DMWs. The investigation of the four off cracked welds showed clearly that the insulating polyurethane (PU) coating was heavily degraded by hydrolysis at higher temperatures. This exposed the dissimilar weldments to CP which contributed to the hydrogen charging of the weldments. The paper gives also result that show that it is not only PWHT’d LAS (e.g. type 8630M, 4130 and F22M) with dissimilar welds that may suffer from this failure mechanism. Testing has shown that as-welded F65 steel /Alloy 59 combinations may also suffer when charged with hydrogen and tested at low temperatures (e.g. shut down temperature).

Commentary by Dr. Valentin Fuster
2017;():V004T03A030. doi:10.1115/OMAE2017-62179.

The topic on performance evaluation of air-backed metallic structures subjected to close-in underwater explosion is of interest to protecting construction designers. The focus of performance includes not only the deformation/failure modes but also the energy absorption capability. This paper presented a blast experiment to investigate the blast resistance of circular solid plate. The deformation and failure modes were classified. The energy absorption of the blast-loaded plate was quantified by the response of another plate, which was arranged below the target. Particular attention was paid to discussing the effects of the charge mass and stand-off distance (SoD) on the blast performance. Results showed that the target plates appeared to experience petalling failure in case of contact tests and large inelastic deformation in case of noncontact. The shock waves induced by the blast explosion and the fragments teared from the target plates caused a capping or some permanent deformation on rear plates. Damages and deformations of target and rear plates were strongly correlated with the explosion intensity. As the increase of stand-off distance, the failure mode of target plates transitioned from petalling to large inelastic deformation. Experimental results presented in this paper provided valuable guidance for the following research on sandwich structures.

Commentary by Dr. Valentin Fuster
2017;():V004T03A031. doi:10.1115/OMAE2017-62188.

It is well known that when the temperature decreases, steel becomes more brittle. In order to prevent brittle fracture in the Arctic, the structure needs adequate toughness for the loading seen at these low temperatures. None of the common offshore design codes today consistently addresses low temperature applications. Generally the design codes and steel construction material with reference to EN 10225 is applied down to −10°C, while below this temperature it is up to the designer to show fit for purpose of the selected material. This paper is concerned with discussion regarding model to potentially rationalize overall temperature and wall thickness effects on fracture toughness for low temperature application. The proposed model is linked up to results in the Arctic Material research project (2008–2017) and used for support of guideline formulations. This paper is a continuation of the work presented in by the authors in 2016 [1] with the aim to support a more design friendly approach addressing both temperature effect and material utilization. In addition, the paper will sum up findings and conclusions from the overall research work carried out in this project with focus on the implication on design for Arctic applications.

Commentary by Dr. Valentin Fuster
2017;():V004T03A032. doi:10.1115/OMAE2017-62195.

To produce offshore wind power generation plants, deep-sea floating wind turbine facilities are required. Commercial installation of floating wind turbine facilities requires a reduction of the mooring cost. Mooring chain breaks due to progressive wear will lead to enormous damages. Therefore, a quantitative wear evaluation method for mooring chains needs to be established. In this study, an experimental setup was constructed to reproduce the wearing phenomenon in mooring chains due to the motion of the floating body induced by waves, and its usefulness was confirmed. The result of the wear test conducted in this study suggests that the tensile force between links affects the degree of wear. Additionally, numerical simulations were performed using a finite element model with measured wear characteristics of the link material to reproduce the phenomenon of wear between links and confirmed that the wear phenomenon could be represented by numerical simulation.

Commentary by Dr. Valentin Fuster
2017;():V004T03A033. doi:10.1115/OMAE2017-62646.

The effect of heat input on the microstructure and mechanical properties of dissimilar welds of direct-quenched ultrahigh strength steel (Optim 960 QC) and duplex stainless steel (UNS S32205) was studied. The effect of heat input on grain coarsening and the proportion of bainite-martensite on the ferritic side and ferrite-austenite on the duplex side was clearly evident. The hardness profile showed a trend of increasing hardness with lower heat input. Enhancement in tensile strength corresponded to lower heat input and increase in hardness. The elongation values and bend behavior were roughly the same for all the specimens; there was no clear link to the heat input. Moderate heat input gave optimum impact toughness in the weld and the ferritic HAZ. The fatigue performance of the specimens demonstrated the profound influence of geometrical effects. However, the effect of microstructural characteristics on fracture location was also observed.

Commentary by Dr. Valentin Fuster

Materials Technology: Performance and Application of Non-Metallics

2017;():V004T03A034. doi:10.1115/OMAE2017-62091.

Outer sheath wear of polymers in flexible risers has been experienced with risers anchored with guide tubes and bellmouths. In addition, wear damages have been observed at touch down points and on subsea arch support structures.

A wear test methodology was developed with the purpose to mimic the observations from pipes in operation. The test apparatus used was similar to a pin-on-disc measurement method, but with emphasis on reproducing the exact environment and temperature.

Comparative tests have been done between different materials, and for some conditions the differences are significant.

Topics: Wear , Pipes , Testing
Commentary by Dr. Valentin Fuster
2017;():V004T03A035. doi:10.1115/OMAE2017-62579.

Conventional steel coiled tubing cannot reach along the entire length of very long horizontal oil wells. A lighter and more buoyant coiled tube is made possible using composite materials. The high stiffness to weight ratio of fiber reinforced polymers, coupled with a lower coefficient of friction, has the potential of greatly extending the reach in horizontal oil wells. This study shows how to design composite coiled tubing and gives a comprehensive discussion about the most influential parameters. Several solutions, using glass-fiber and carbon are considered. Finite element models are used to calculate the buckling loads and the corresponding interlaminar stresses. The very positive results obtained during this study show that composite coiled tubing systems are vastly superior to their steel counterparts, and that in the future, these will become the new industry standard.

Commentary by Dr. Valentin Fuster

Materials Technology: Special Session Honoring Profs Stig Berge and Per Haggansen

2017;():V004T03A036. doi:10.1115/OMAE2017-61143.

Fatigue is a governing limit state for design of wind turbine structures. This implies use of thicker plates in these structures than that used as basis for derivation of standard design S-N curves. Due to a significant number of wind turbine support structures designed in a similar way it is important to use a reliable design procedure that does not require use of unnecessary steel costs or fabrication costs. There are also questions related to calculation of structural stresses to be entered the different S-N curves for calculation of fatigue lives. Therefore an alternative assessment method based on notch stress and very fine element meshes has been used to assess target hot spot stress values for calculation of best estimates of fatigue lives.

Commentary by Dr. Valentin Fuster
2017;():V004T03A037. doi:10.1115/OMAE2017-61181.

Girth welded pipes, such as those located offshore on platforms in the North Sea, are subjected to highly corrosive environment. The need to consider welding residual stresses in the assessment of the fitness for service and damages to these pipes when investigating local corrosion damages across a welded region is therefore important for the operators of the platforms and the manufacturers of the pipes. This paper presents a review of work carried out to ascertain the welding residual stresses present within a thin-walled girth welded pipe mock-up made from steel API 5LX Grade 52 before and after reduction of the wall thickness. The mock-up was manufactured to replicate typical pipes used to convey gas, oil and water through the platforms. The mock-up was of diameter 30” and of thickness 19mm. The incremental deep hole drilling (iDHD), contour, hole drilling, XRD, and ultrasonic technique were applied to characterise the residual stresses in the weld and heat affected zone of the specimen. The residual stresses were then measured during the manufacture of a groove located on the weld at the ID and were compared to an FE prediction. Ultrasonic measurements were then carried out on the outer surface of the pipe and show a significant increase in the residual stress and could be used to monitor the changes in the residual stress caused by internal corrosion.

Commentary by Dr. Valentin Fuster
2017;():V004T03A038. doi:10.1115/OMAE2017-62169.

This research aims to investigate the relationship between the metal magnetic memory (MMM) signals of Q345 welded steel and its mechanical characteristics including the range of stress concentration zone and the tensile stress. A series of tensile experiments were carried out to measure the tangential residual magnetic field (RMF) on the surface of Q345 steel welded specimens under the action of stress. The variation of tangential RMF and the characteristic parameters (the peak-peak width and the peak-peak amplitude) of tangential RMF gradient curve were investigated. It was found that the tangential magnetic field curve of welded specimen has abnormal magnetic changes in the weld joint area. An analysis of the tangential RMF gradient curve shows that the range of stress concentration zone of welded specimen can be evaluated by the peak-peak width which is nearly constant under different tensile stress. Furthermore, the peak-peak amplitude of the tangential RMF gradient curve has quartic polynomial relationship with tensile stress and it is a potentially useful indictor of the tensile stress in welded steel.

Commentary by Dr. Valentin Fuster
2017;():V004T03A039. doi:10.1115/OMAE2017-62189.

The motivation of this paper is to highlight the importance of the work carried out during the last decade by Prof. Haagensen and Prof. Berge at the Norwegian University of Science and Technology NTNU, with the aim to inspire and motivate young engineers to continue their important and valuable research within fatigue and fracture. This paper will focus on their historical contribution to the research within fatigue and fracture of offshore and ship structures.

Stig Berge is a professor of Marine Technology at the Norwegian University of Science and Technology NTNU. He has spent his academic carrier focusing on fatigue of offshore and ship structures; he has published more than 70 papers and articles in well-known journals and conferences since 80’s.

Per Jahn Haagensen is currently an Emeritus professor at Department of Mechanical Engineering and Logistic Faculty of Technology. He has spent his whole research carrier within fatigue and fracture related topics mainly for the offshore industry. He is especially known for the different fatigue improvement methods which have been developed since the 90’s. He has until recently been an active member of the International Institute of Welding (IIW).

This article aims to present their main findings and conclusions from their long academic carrier. While the authors have strived to convey in a single paper an overview of the careers and important contributions, the Professors themselves may well have chosen to place a different emphasis on their work.

Commentary by Dr. Valentin Fuster
2017;():V004T03A040. doi:10.1115/OMAE2017-62212.

The offshore environment contains many sources of cyclic loading. Standard design S-N curves, such as those in DNVGL-RP-C203, are usually assigned to ensure a particular design life can be achieved for a particular set of anticipated loading conditions. Girth welds are often the ‘weak link’ in terms of fatigue strength and so it is important to show that girth welds made using new procedures for new projects that are intended to be used in fatigue sensitive risers or flowlines do indeed have the required fatigue performance. Alternatively, designers of new subsea connectors, used for example in tendons for tension leg platforms, mooring applications or well-heads which will experience cyclic loading in service, also wish to verify the fatigue performance of their new designs. Often operators require contractors to carry out resonance fatigue tests on representative girth welds in order to show that girth welds made using new procedures qualify to the required design S-N curve. Operators and contractors must then interpret the results, which is not necessarily straightforward if the fatigue lives are lower than expected.

Many factors influence a component’s fatigue strength so there is usually scatter in results obtained when a number of fatigue tests are carried out on real, production standard components. This scatter means that it is important first to carry out the right number of tests in order to obtain a reasonable understanding of the component’s fatigue strength, and then to interpret the fatigue test results properly. A working knowledge of statistics is necessary for both specifying the test programme and interpreting the test results and there is often confusion over various aspects of test specification and interpretation.

This paper describes relevant statistical concepts in a way that is accessible to non-experts and that can be used, practically, by designers. The paper illustrates the statistical analysis of test data with examples of the ‘target life’ approach (that is now included in BS7608:2014 + A1) and the equivalent approach in DNVGL-RP-C203, which uses the stress modification factor. It gives practical examples to designers of a pragmatic method that can be used when specifying test programmes and interpreting the results obtained from tests carried out during qualification programmes, which for example, aim to determine whether girth welds made using a new procedure qualify to a particular design curve. It will help designers who are tasked with specifying test programmes to choose a reasonable number of test specimens and stress ranges, and to understand the outcome when results have been obtained.

Commentary by Dr. Valentin Fuster

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