ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V004T00A001. doi:10.1115/OMAE2018-NS4.

This online compilation of papers from the ASME 2018 37th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2018) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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: Advances in Life Extension Analysis

2018;():V004T03A001. doi:10.1115/OMAE2018-77280.

This research aims to explore the evolutions of the piezomagnetic field (B field) of X80 steel under cyclic tensile stress at different piezomagnetic field measurement distances. The X80 sample was subjected to cyclic preloading with a large stress amplitude. Subsequently, a series of cyclic tensile experiments were conducted to measure the piezomagnetic fields in the vicinity of the X80 sample at pre-determined distances by a fluxgate magnetometer. It was found that the magnetic parameters of X80 steel have quantitative relationship with the piezomagnetic field measurement distance. The B field-stress curves with different maximum stresses share a same evolution trajectory during the loading process when the starting point of the loading processes stays unchanged. The unloading curves have different traces with the tendency towards the starting points. Additionally, the relationship between Tail reversals and piezomagnetic field measurement distance was also discussed in this paper. Based on the piezomagnetic signals, the stresses of the pipelines can be detected and safety problems can be also prevented.

Topics: Steel , Stress
Commentary by Dr. Valentin Fuster
2018;():V004T03A002. doi:10.1115/OMAE2018-78482.

Cost efficient offshore field development often involves tiebacks to existing field infrastructure. Efficient field development requires life extension of existing production facilities and pipelines to accommodate the new field resources over their life expectation. For fields near the tail end of their production the pipelines may be close to the end of their design life, and it must be shown that they have potential for extended life beyond the original design life until the end of the period of operation of the new field.

Offshore pipelines are designed and constructed to recognized standards, such as the widely applied DNV OS-F101 2013 Submarine Pipelines Systems and earlier versions. The latest edition of the code was recently issued as a standard with some major updates and a modified code number i.e. DNVGL ST-F101 [1].

As pipelines age, they will inevitably be exposed to various types of degradation and an Operator must be able to both assess the significance of this damage and the pipeline remaining life to ensure that the pipelines do not fail as they age before the end of their design lives. Currently, many pipelines are operated far beyond the original design life and as mentioned above for cost efficient field development the pipeline operator often needs to demonstrate that the pipeline’s useful life can be extended another 10 or in some cases up to 30 years. For some pipelines, new operating conditions will be introduced by tie-in of new fields and this will impact the future rate of degradation. Hence, it cannot be assumed that the future degradation will be similar or less severe than experienced since commissioning of the pipeline.

Extension of the life of the pipeline can be demonstrated by adopting methods of analysis that show the line is safe for an extended life under the future expected operating condition.

This paper describes the risk based approach applied for pipeline remaining life and life extension analyses based on DNV GL codes and other relevant recommended practices. For illustration of the methodology a typical case of remaining life assessment of and life extension of a gas export pipeline is presented in the Case Study.

Commentary by Dr. Valentin Fuster
2018;():V004T03A003. doi:10.1115/OMAE2018-78677.

Decision making for a new pipeline’s design and provision of the most effective maintenance or repair measures for a pipeline in operation can be a long and costly process. The final decision made, whether during design or operation, may not always reduce the risk or remediate the threat. This is mainly due to the uncertainty and missing information regarding the field chemistry for current and future pipeline operating conditions, that were not considered and quantified during the assessment.

In this paper, two case studies of pipeline internal corrosion risk are presented, one for pipeline in design and the latter for pipeline in operation. Both cases were assessed using Bayesian Networks. Bayesian Networks (BN) have been used to quantify the value of information of uncertain and missing data. BN displays the cause-effect relationships of these data in the form of conditional probabilities to describe how one’s data is influencing internal corrosion rates probability. Thus, predicting the pipeline’s conditions over the design life. Operators can visualize the development of internal corrosion within a pipeline over time and gain clearer understanding of the causal relationships that could lead to pipeline failure. The results allowed operators to confirm the effects of the parameter and followed by a sensitivity analysis to find out which data to prioritize in acquisition and validation before proceeding to decide on how the pipeline should be designed and maintained/inspected in future.

Commentary by Dr. Valentin Fuster

Materials Technology: Application and Integrity of High Strength Steels

2018;():V004T03A004. doi:10.1115/OMAE2018-77282.

The objective of this research is to investigate the effect of uniaxial ratcheting on the piezomagnetic field variations of X80 pipeline steel where the loading history is a major consideration. The piezomagnetic field variations of initially undemagnetized X80 pipeline steel specimen were recorded simultaneously by an APS 428D fluxgate magnetometer during the whole asymmetrical cyclic stressing process. The variations of the ratcheting strain and its rate with the changing mean stress were obtained. The influence of the loading history on the uniaxial ratcheting behavior was investigated. It was found that the prior loading step with higher mean stress greatly restrains the ratcheting behavior of subsequent loading step with lower mean stress. On the contrary, the prior loading step with lower mean stress does not play a significant role on the subsequent ratcheting behavior. The corresponding variations of the piezomagnetic hysteresis curve with the uniaxial ratcheting proceeding were systematically studied in terms of its shape and position, and the underlying reasons were discussed based on the magnetic domain theory and the dislocation theory.

Topics: Steel , Pipelines
Commentary by Dr. Valentin Fuster
2018;():V004T03A005. doi:10.1115/OMAE2018-77471.

Hydrogen induced cracking (HIC) occurs by the poisoning effect of hydrogen sulfide (H2S) which promotes hydrogen absorption and entry at steel surface. Therefore, it is important for linepipe steels to have sufficient HIC resistance in sour environments. The HIC resistance is usually evaluated by measuring cracks after the standardized immersion test such as NACE TM0284. However, the general evaluation method cannot investigate HIC initiation and propagation behavior separately. It is necessary to understand the effect of metallurgical factors on the cracking behavior of sour service linepipe. In this study, in-situ ultrasonic inspection equipment was applied to the HIC test for the several linepipe steels with bainitic microstructure in order to clarify crack initiation and propagation behavior quantitatively. The three dimensional (3-D) distribution of cracks in the specimen was successfully captured as time sequence, and the temporal change of the crack area ratio (CAR) was investigated. It was revealed that the CAR-time curves are consist of four stages with different CAR increment rate. The first stage is the incubation of crack initiation. In the second stage, cracks occur and grow, and adjacent cracks coalesced rapidly. Regarding the first and second stages, sensitivity for the HIC initiation was well correlated with the hydrogen diffusion coefficient and the density of crack initiation site, such as MnS and Nb inclusions. In the third stage, the coalesced cracks propagate along the center segregation region. From the investigation of individual crack behavior, the crack along harder region showed higher propagation rate. In the fourth stage, the crack propagation rate was decreased to be in stasis. It can be stated that crack growth in the final stage is strongly affected by the hardness of base material and the crack easily propagate when HIC occurs in high strength steels.

Topics: Steel
Commentary by Dr. Valentin Fuster
2018;():V004T03A006. doi:10.1115/OMAE2018-77937.

Production system integrity in remote assets is a great challenge, conventionally addressed by the application of conservative standards, leading to costly construction materials. There is evidence that API RP14E standard, accepted as standard for erosional velocity estimations and some of the most extended carbon steel CO2 corrosion models, such as NORSOK, generally lead to conservative material loss rates, with strong impact on design and capital costs [1, 2, 3, 4, 5, 6]. Additionally, there is very little description in the literature of stainless steel CO2 corrosion models.

In this work, an approach to enhanced erosion-corrosion estimations is presented, to cover this gap in material selection. Conventional approaches are incomplete, as they disregard an accurate fluid definition, geometry, flow regimes, the integration of erosion-corrosion phenomena or the validation against experimental and field data. A comprehensive workflow that starts at the fluid characterization, considers the interaction between the phases, includes fluid-dynamic simulation for computation of the flow regime vs. well completion dependence, and calibrates state of art stainless steel erosion-corrosion models with dedicated lab and Calliper field data, was applied to the erosion-corrosion analysis in a gas condensate field.

The results have identified the main material loss mechanism at different points of the well completion and predicted the remaining life of the tubing in different scenarios. This approach enables a deeper insight into erosion-corrosion interaction with the fluid system and enhances the integrity management of an asset.

Topics: Fluids , Corrosion , Erosion
Commentary by Dr. Valentin Fuster
2018;():V004T03A007. doi:10.1115/OMAE2018-78296.

Blast panels are integral structures in offshore topside modules to protect personnel and safety critical equipment by preventing the escalation of events due to hydrocarbon explosions. As such, blast panels are expected to retain their integrity against any blast loading and subsequent hydrocarbon fire. Most of the blast panels currently installed in offshore structures have been designed using simplified calculation approaches such as the Single Degree of Freedom (SDOF) models, as recommended by offshore design codes and industry recommended practices.

In this paper, the Non-Linear Finite Element Analysis (NLFEA) technique is used to simulate the structural response of corrugated panels subjected to blast loading. Detailed numerical analyses allow identifying the limits of the SDOF approach, and exploring different design options to optimize the structural response of corrugated blast panels.

The blast load profile corresponding to an explosion is one of the most important factors to consider in the structural analysis. The mechanism of hydrocarbon explosions is very complex, and the corresponding blast load profile intimately depends on the type of explosion, the congestion and the structural confinement. A sensitivity analysis is performed to investigate the influence of the blast pulse shape, and in particular to evaluate the effect of the maximum peak pressure and the exposure time.

To explore the benefits of introducing higher strength steels in demanding offshore applications, pressure-impulse diagrams have been derived for different high strength steel grades. In our analysis, (ultra)high strength cut-to-length plates from hot rolled coil are proposed to optimize the design of the blast panel whilst preserving the structural performance under demanding load conditions.

Commentary by Dr. Valentin Fuster

Materials Technology: Engineering Critical Assessment: Guidelines and Applications

2018;():V004T03A008. doi:10.1115/OMAE2018-77067.

Engineering critical assessment (ECA) is increasingly being used in the offshore industry to determine the maximum tolerable initial flaw size in girth welds for pipelines and risers. To account for the effect of the stress concentration factor (SCF) at the weld toe on the stress intensity factor range, ΔK, a magnification factor, Mk, is used. The existing Mk solutions given in BS 7910 were developed for fatigue assessment of flaws at the toes of fillet and butt welds and may not be suitable for assessing flaws at girth weld root toes, where the weld width is relatively small. On the other hand, for single-sided girth welds, fatigue cracking often initiates from weld toes on the root side, rather than on the weld cap side.

Finite element (FE) modelling was performed to determine a 2D Mk solution for ECA of a flaw at the weld root bead toe. The weld root bead profile was uniquely characterised by five variables including weld root bead width, weld root bead height, hi-lo, weld root bead angle and weld root bead radius. Following a parametric sensitivity study, defect size, weld root bead height and hi-lo were identified as the governing parameters. A total of 6,000 FE simulations was performed and three types of defect models, which covered different combinations of weld root bead height and hi-lo, were generated and analysed. A series of automation scripts were developed in the Python programming language and the Mk solution for each type of defect model was developed and provided in a parametric equation. The accuracy of the 2D Mk solutions was confirmed by the experimental data, in terms of both fatigue crack growth and S-N curves. It was found that the methods and Mk solutions currently recommended in BS 7910 and DNV OS-F101 are inappropriate for assessing a flaw at a girth weld root toe.

Commentary by Dr. Valentin Fuster
2018;():V004T03A009. doi:10.1115/OMAE2018-78685.

Development of remote energy requires large pipeline networks to be placed in more challenging environments such as offshore in deeper waters or on land in Arctic or near-Arctic locations. Pipeline installed and operated in such regions may be subjected to large plastic strains. Engineering critical assessments (ECA) are commonly carried out during design, installation and operation of offshore pipelines to determine acceptable flaw sizes in pipeline girth welds. A number of fracture mechanics-based procedures are available for ECA of pipeline girth welds. Most of these methods are primarily stress-based assessments and are therefore not directly applicable to cases where the displacement-/strain-controlled loading generates large amounts of plastic deformation. For such cases, strain-based fracture assessment for pipeline/girth welds should be carried out instead. However, limited guidance on strain-based assessment is available in the current codes and standards used primarily by the oil and gas industries. This paper reviews the existing strain-based fracture assessment methods, and reports the results of preliminary studies performed to compare the methods reviewed with the available full-scale pipe test data.

Commentary by Dr. Valentin Fuster

Materials Technology: Fatigue and Fracture Assessment

2018;():V004T03A010. doi:10.1115/OMAE2018-77247.

Recently, demands for liquefied natural gas (LNG) are increased by developing countries such as China, India and Middle East area. In addition, the International Maritime Organization (IMO) reinforced regulations to avoid the serious environmental pollution. This trend has led to manufacturing and operating various LNG vessels such as liquefied natural gas carrier (LNGC), floating liquefied natural gas (FLNG) and very large gas carrier (VLGC). In the design of LNG vessels, the structural integrity of LNG storage tank is of significant importance to satisfy the service conditions. In order to secure structural integrity, LNG storage tank is fabricated with low temperature materials. In general, low temperature materials such as SUS304L, Invar alloy, Al 5083-O, nickel alloy steel and high manganese steel exhibit excellent fatigue and fracture performances at cryogenic temperature. In particular, high manganese steel has attracted interest because they are potentially less expensive than the competing other low temperature materials.

This study compares the fracture toughness and fatigue crack growth characteristics of high manganese steel with those of nickel steels. In addition, fracture toughness and fatigue crack growth rate tests for various nickel steels are conducted according to BS 7448 and ASTM E647, respectively. In order to obtain less conservative design values, the results of high manganese steel and various nickel steels were compared to those of BS7910. As a result, the CTOD value of high manganese steel is higher than that of 9% nickel steel at cryogenic temperature. In case of FCGR, the high manganese steel and 9% nickel steel are found to be similar to each other.

Commentary by Dr. Valentin Fuster
2018;():V004T03A011. doi:10.1115/OMAE2018-77952.

Environmentally assisted sub-critical static crack growth can occur in offshore pipelines exposed to aggressive production environments. Recent advances in fracture mechanics testing methods have shown that slow static crack growth rates can be reliably measured in sweet and sour environments under constant stress intensity factor (K) conditions. This has potential implications for the engineering critical assessment (ECA) of pipe girth welds subject to low cycle fatigue loading with long periods of operation under constant static load between cycles, e.g. lateral buckling. This paper demonstrates the influence of including static (i.e. time dependent) crack growth as well as fatigue crack growth in a modified pipeline ECA approach.

Commentary by Dr. Valentin Fuster
2018;():V004T03A012. doi:10.1115/OMAE2018-77960.

Floating wind turbine facilities, which are installed in the deep sea area, plays an essential role to promote the green energy application. One of the problems associated with the commercialization of facilities installed in the deep sea is the reduction of the maintenance cost of mooring chain, because breaking of the mooring chain caused by the wear between links leads to enormous economic losses. Therefore, it is necessary to establish a quantitative wear evaluation method for mooring chains.

Experimental facility to reproduce the wear caused by sliding between links in actual scale applied for floating wind turbine, which had been proposed by the authors, was updated and the wear tests was conducted by setting some tensile force conditions between the links. Besides, procedure of the nonlinear finite element analysis was improved to estimate the behaviour of wearing between links.

From experiments and numerical analysis, it has been confirmed that the tensile force between links is an important factor of the wear amount between links.

Topics: Wear , Chain , Mooring
Commentary by Dr. Valentin Fuster
2018;():V004T03A013. doi:10.1115/OMAE2018-78529.

The planned operation of natural gas liquefaction units offshore has raised many new safety issues. Cryogenic spillage is among the most critical as load bearing structures cannot be designed to resist such accidents.

In order to propose better protection at manageable cost, to enhance structural integrity and to lower the risks associated with cryogenic spillage on marine structures, TechnipFMC as FLNG market leader, has run an extensive physical test program to support the development of tailor made numerical modeling tools.

In a first step, numerous protective materials were screened and then tested to assess their performances. We found that all products currently available in the market display limitations. During the test campaign, alternate product designs were investigated to improve structural design and avoid the need for human intervention “Inspection Maintenance and Repair” during service lifetime.

In this paper, a summary of the test campaign and a complete introduction of Series of standard of ISO 20088 will be provided.

Commentary by Dr. Valentin Fuster

Materials Technology: Fatigue and Fracture Performance in Corrosive Environments

2018;():V004T03A014. doi:10.1115/OMAE2018-77002.

Corrosion-fatigue in sour brine (SB) environments is a significant design consideration in deepwater floating production systems. Extensive testing over the past 20 years has shown that sour brine environments can reduce the fatigue life of line pipe steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air; moreover, the extent of material degradation depends on a multitude of loading, environmental, and materials variables. Thus, in 2010 Southwest Research Institute (SwRI) embarked on an industry-supported Joint Industry Project (JIP) to develop a quantitative model to predict the effects of these variables on corrosion-fatigue crack growth rate (CFCGR) in offshore structure steels exposed to sour brine environments. Phase 1 of this JIP had successfully developed and validated such a model in the intermediate fatigue crack growth rate regime — i.e., with CFCGRs between 10−4 ∼ 10−2 mm/cycle. However, the Phase 1 model gave overly conservative CFCGRs at rates in the low growth rate regime below 1 × 10−4 mm/cycle, corresponding to S-N corrosion-fatigue lives in the high-cycle fatigue regime. It was hypothesized that these conservative predictions might result from the fact that the model did not consider effects of crack closure that could significantly reduce the effective crack-driving force in this low growth rate regime, a process that might also give rise to crack-size effects. Thus, the primary objective of the current study was to assess whether or not crack closure is responsible for the conservativism in the Phase 1 CFCGR model, as well as to explore related crack-size effects that in theory would not be predictable with conventional linear elastic fracture mechanics. Both of these possible effects are explored here using critical CFCGR experiments on X65 steel in sour brine under loading conditions for which the nominally applied mechanical driving force (ΔK), as well as the stress ratio (Rσ) and loading frequency were held constant, while crack closure measurements were made as the crack grew from 2 mm to about 10 mm. The crack closure measurements were made using elastic compliance measurements made with a specially designed, high-sensitivity clip gage. Results indicate that a crack-size dependence of CFCGR did occur and could be correlated using a crack-closure-corrected effective stress intensity factor (ΔKeff). These results have provided a foundation for extending the JIP’s Phase 1 CFCGR model into the low growth rate regime in the ongoing Phase 2 of the JIP.

Commentary by Dr. Valentin Fuster
2018;():V004T03A015. doi:10.1115/OMAE2018-77068.

Fatigue crack growth rates (FCGR) in corrosive environment depends on loading frequency. Frequency scanning testing is often used to determine this effect. However, it is well known that the effect of loading frequency also depends on the magnitude of stress intensity factor range, ΔK. It is generally found that, with decreasing loading frequency, FCGR decreases in the low ΔK regime, increases and then decreases after reaching the saturating loading frequency in the intermediate ΔK regime, and keeps increasing in the high ΔK regime. To accurately characterise the effect of loading frequency on FCGR, several frequency scanning tests are required for a particular application (corrosive environment, material, welding procedure etc), each at a different ΔK level. These are time consuming and expensive tests.

A novel screening frequency scanning test method has thus been developed. The method is similar to the step load fracture toughness test method often used to make a quicker estimate of fracture toughness of material in corrosive environment. In the screening frequency scanning test, both loading frequency and ΔK are changed in steps. At a relatively low and constant ΔK level, loading frequency is reduced in steps, after a certain amount of crack growth. Once the FCGR exhibits decreasing or has achieved a saturating loading frequency with decreasing loading frequency, ΔK is then increased to another higher level and the above process is repeated; the above procedures are repeated until the target maximum ΔK and the lowest loading frequency have been achieved. This method allows an estimate of the effect of loading frequency on FCGR in a large ΔK range using a single specimen. The results of the screening frequency scanning tests demonstrated that this method was feasible and provided a good and quick estimate of the effect of loading frequency on FCGR.

Commentary by Dr. Valentin Fuster
2018;():V004T03A016. doi:10.1115/OMAE2018-77252.

Duplex stainless steel has been used on subsea facilities since the mid 80-ties. The experiences with these materials have been relative good and only a few failures have been reported. However, BP and Shell experience some serious cracking of duplex steel in the mid 90-ties and in beginning of the century. The root cause of these failures was identified to be Hydrogen Induced Stress Cracking, HISC, where the hydrogen source was the cathodic protection system of the subsea facility. These and other similar failures resulted establishment of Joint Industry Projects, JIPs with financial and technical contribution from leading oil companies, contractors, material suppliers and research institutions as TWI, SINTEF and DNVGL. The objective of the JIPs was to establish practical usage limits for duplex stainless steels. The JIPs resulted in a recommended practice “DNV-RP-F112 - Design of duplex stainless subsea equipment exposed to cathodic protection.”

This document minimized the failure rate of duplex steel components used subsea. However, since duplex steels components have been used on subsea facilities long before the guidelines and recommendations were issued, there are lot of components presently in use that may be overloaded compared to guidelines and recommendations. As a part of life time extension of one of Statoil’s long time producing fields, a HISC re-calculation of spools connecting SPSs to infield pipelines showed that many of the spools were exposed to stresses above the recommended stresses given in DNV-RP-F112. Since these recommendations were primarily based on testing at ambient seabed temperature (4°C), Statoil, together with SINTEF, started in 2016 a project where the aim was to evaluate the resistance against HISC as an effect of the operation temperature.

The results of this project show that the critical net section stress/AYS (HISC resistance) increases with increasing temperature. Based on this, the before mentioned spools can be considered safe even though the spools are exposed to stresses above the recommendations in DNV-RP-F112.

Further, the investigations show that the guidelines and recommendations given in DNV-RP-F112 may be conservative for temperatures above 4°C. It is therefore recommended to perform more testing to confirm and incorporate the findings from the present investigation in future revision of DNV-RP-F112.

Commentary by Dr. Valentin Fuster
2018;():V004T03A017. doi:10.1115/OMAE2018-78672.

The recent premature failure of TMCP steel pipes handling sour gas was attributed to the presence of local hard spots. This raised concern regarding the suitability of the Thermo-Mechanically Controlled Process (TMCP) steel for sour service. The paper discusses the impact of hard spots on pipeline fracture resistance in sour service, proposes a basis for steel specification to avoid the formation of hard spots and presents a recommended qualification program to ensure the integrity of the steel pipelines despite the presence of hard spots.

Topics: Pipelines
Commentary by Dr. Valentin Fuster

Materials Technology: Fatigue Performance and Improvement

2018;():V004T03A018. doi:10.1115/OMAE2018-77530.

The use of synthetic fiber ropes for subsea installation is extending, as the offshore industry explores deeper waters, but there are few data available to evaluate the lifetime of these materials. In a previous OMAE presentation the authors described results from the first phase (2010–2013) of a JIP aiming to understand the mechanisms controlling the long term behavior of HMPE fibre ropes [1]. This presentation will describe the results from the second phase of this study (2014–2018) in which predictive models have been developed and applied to a range of improved braided rope materials. Two modeling approaches will be discussed, an empirical method based on residual strength after cycling, and a numerical approach using finite element software specifically adapted to fibre materials [2]. An extensive test program, which has generated a database of CBOS (cyclic bend over sheave) results for various grades of HMPE and different constructions, will be described. Comparisons have been made with steel wire handling lines in order to quantify the benefits of fibre ropes for these deepwater applications.

Topics: Durability , Pulleys , Ropes , Seas
Commentary by Dr. Valentin Fuster
2018;():V004T03A019. doi:10.1115/OMAE2018-77887.

Fatigue strength of welded structures is affected by welding residual stresses, which influence crack propagation at weld toe or root, considerably reducing the effectiveness of the structure and increasing its construction and maintenance costs. A mechanical post-weld treatment such as HFMI (High-Frequency Mechanical Impact) has been found to exhibit a significant fatigue life enhancement of welded joints.

The effectiveness of this mechanical impact treatment is primarily based on the combination of three effects: improvement of the local work hardening, introduction of compressive residual stresses and closing of notches at the weld toe. However, several aspects of this treatment need to be clarified or further investigated. Many researchers are focused on HFMI-treatment, simulating the impact of the peening tool at the weld toe. The present study focuses on the stability of compressive residual stresses introduced by HFMI-treatment when a cyclic load is applied on a gusset welded joint. Peening response of a flat stress-free-plate, under different peening process parameters is also investigated. Furthermore, comparisons of HFMI with other peening techniques are carried out to clarify which technique is the most reliable.

The validity of the present study is demonstrated by comparison with experimental residual stress measurements.

Topics: Stability , Stress
Commentary by Dr. Valentin Fuster
2018;():V004T03A020. doi:10.1115/OMAE2018-78024.

Tubular joints are complex geometries present in many offshore structures. Fatigue testing of tubular joints requires either downsizing of the dimensions of the joints or the use of a simplified geometry, to avoid prohibitive costs. For the development of a welding process optimized for the fatigue performance of tubular joints, one needs to use a representative sample: same material, same thickness, use of similar welding positions, same level of restraint as in a real structure, combined with the supplementary requirement of ease of manufacturing and testing.

A novel geometry was developed to fulfil all these requirements. Then, different welds were produced using robot and manual welding in different positions. The fatigue tests proved to be very reproducible, and indicate a strong influence of the welding position on the fatigue resistance. For that reason, an optimized welding procedure and sequence was developed, in which the welds in the most fatigue sensitive locations are produced first in optimum condition, while the less sensitive parts are produced by subsequent welding in position.

Commentary by Dr. Valentin Fuster
2018;():V004T03A021. doi:10.1115/OMAE2018-78056.

Tenaris and ExxonMobil conducted a qualification program focused on Heavy Wall Seamless Line Pipe (HW SMLS LP) and its girth welds, to experimentally demonstrate and validate the material potential to be used as riser or fatigue sensitive flowline for the development of challenging deep and ultra-deep offshore fields. The qualification program was divided in three phases.

Phase I [1] included: 1) HW SMLS LP, X65QS, 273 mm OD, 46 mm WT plain pipe qualification; and 2) development and qualification of a WPS (STT® process for root pass and GMAW process for hot, fill & cap passes, narrow groove J-bevel type, 1G welding torch position) and full scale fatigue testing qualification.

Phase II [2] was focused on evaluating the root girth weld fatigue endurance, taking into account the fatigue behavior observed in Phase I (fatigue crack initiation mainly occurred from the outer surface). The fatigue strength of the weld root surface is needed as the weld root surface is in direct contact with corrosive fluids and is the most sensitive area for sour service applications. Thus, to obtain the root girth weld fatigue endurance, full scale fatigue specimens were manufactured with their caps fully removed (by grinding and flapping) to generate weld cap toe free of stress raisers/imperfections. In addition, pipe outer surface was grinded to mitigate the fatigue crack nucleation from outer pipe surface. With this special preparation, the fatigue strength of weld root surface was able to be assessed.

This paper presents Phase III qualification outcomes. The objective was to develop and qualify a WPS representative of J-Lay installation, meaning 2G welding torch position (CMT® for the root pass and pGMAW process for hot, fill and cap passes, Narrow groove J3°-bevel type). The qualification included full scale fatigue assessment as well. The LP material was from the same batch used in Phase I & II (HW SMLS, X65QS, 273 mm OD, 46 mm WT).

The WPS was successfully developed and qualified. The full scale resonant fatigue testing included three specimens with cap smoothed similarly to Phase I, and three specimens with full cap removal plus outer pipe surface grinding similar to Phase II. The fatigue results surpassed the expectations (Class D, S-N curve). Most of the specimen’s fatigue lifetime is above the class B, S-N curve classification (reference for plain material). Obtaining a smooth root weld overfill was key to delaying the fatigue crack nucleation from the weld root toe.

Commentary by Dr. Valentin Fuster
2018;():V004T03A022. doi:10.1115/OMAE2018-78060.

Rigid pipelines have been widely applied in offshore oil & gas operation and transmission industries on account of their structural simplicity, cost-effectiveness, ease of installation and maintenance. However, surface cracks frequently appear in the internal surface of rigid pipes due to dynamic loads or hydrogen embrittlement, etc. Under dynamic fatigue loads, surface cracks may continue to propagate and finally develop into penetrated cracks, which may cause leakage and serious accidents.

Fiber-reinforced polymer (FRP) strengthening technology is already a reliable technique for structure maintenance in onshore pipelines and penetrated cracks in load-bearing circular hollow sections (CHS). Nevertheless, there are very limited systematic investigations of surface crack in rigid pipes reinforced with FRP, which has a remarkable significance for offshore rigid pipes.

This paper aims to understand the mechanism of semi-elliptical surface crack growth in the internal surface of rigid pipes under fatigue bending moment reinforced with FRP. Stress intensity factors along the crack front are computed through finite-element (FE) models, which are validated by experimental data from references. The influence of wrapping orientation of CFRP are discussed as well. The numerical results show that under CFRP reinforcement, surface crack growth rate decreases significantly which ensures the safety use of rigid pipes in offshore industry.

Commentary by Dr. Valentin Fuster
2018;():V004T03A023. doi:10.1115/OMAE2018-78256.

Defining the yielding point of semicrystalline polymers is a matter to be established in the literature. ASTM D638-14 and ISO 527-1 standards define the yielding point as the point where there is an increase in strain without an increase in stress, which coincides with the beginning of necking of the test samples. The literature has been reevaluating this matter, taking into account the methods used and their respective damage generation in the material. Polymer materials are used in the oil and gas industry, for example, in risers. The understanding of the transition between the elastic and plastic regions is necessary, as well as the understanding of the damage done in both regions. This study is about the effects of cyclic loadings with triangular and sinusoidal loading, with different strain levels and their effect on the mechanical behavior of a fluorinated polymer(Halar ECTFE). The cyclic loading tests were strain-controlled and done with frequencies around 0.1Hz, equivalent to a strain rate of 0.2 and 0.4%/s, and strains up to 2%, with the effects on the transition from the elastic and on the stress relaxation being observed. The results show that up to strains of 0.5% the material has elastic behavior, irrespective of the loading. When the strains are greater than 0.75%, the material shows relaxation on all loadings cycles. Between 0.5% and 0.75%, the triangular loading led to cyclic hardening, while the sinusoidal lead to stress relaxation. The stress relaxation is then related to the damage accumulation on the structure of the material, while the hardening to the chain orientation.

Topics: Polymers , Yield point
Commentary by Dr. Valentin Fuster
2018;():V004T03A024. doi:10.1115/OMAE2018-78408.

Deep-water tendon and riser systems are often subjected to severe fatigue loading from waves, currents and vessel movements. The girth welds between successive lengths of pipe or at pipe terminations represent fatigue-critical features where failure would be catastrophic. Hence, validation fatigue testing by full scale pipes of the most critical welds are often performed to ensure adequate quality and/or to document a better S-N curves than those available in standards today like DNVGL-RP-C203 [1] and BS7608 [2]. To better understand the fatigue performance with respect to identify trends, dependencies and critical features that influence the fatigue performance, a JIP on Fatigue of Girth Welds were initiated in 2011. Two phases have been conducted and a total of 1700 full scale one sided girth welds, mostly run by Stress Engineering, have been statistically analyzed. The test data has been interrogated to investigate the effect of as-welded condition, OD ground, OD/ID ground, un-reeled pipe, reeled pipe, thickness and effect of misalignment. Based on these analyses, new S-N curves for risers and pipelines have been included in DNVGL-RP-C203 for non-reeled girth welds. This paper presents the findings and trends from the JIP work which has been the rationale for the updates of girth welds in section 2.10 in DNVGL-RP-C203 2016 edition.

Commentary by Dr. Valentin Fuster
2018;():V004T03A025. doi:10.1115/OMAE2018-78752.

The hardness of a material shows its ability to resist to microplastic deformation caused by indentation or penetration and is closely related to the plastic slip capacity of the material. Therefore, it could be significant to study the resistance to microplastic deformations based on microhardness changes on the surface, and the associated accumulation of fatigue damage. 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. Here, Berkovich indentation tests were carried out in the samples previously submitted to high cycle fatigue (HCF) tests. It was observed that the major changes in the microhardness values occurred at the surface of the material below 3 μm of indentation depth, and around 20% of the fatigue life of the material, proving that microcracking is a surface phenomenon. So, the results obtained for the surface of the specimen and at the beginning of the fatigue life of the material will be considered in the proposal of a new method to estimate the fatigue life of metal structures.

Commentary by Dr. Valentin Fuster

Materials Technology: Fracture Assessment: Analytical Methods

2018;():V004T03A026. doi:10.1115/OMAE2018-77177.

Cracks caused by cold bending during shipyard construction were found to be longer in mild steel than in high tensile steel. Because mild steel has better elongation, the cause of such cracking was difficult to ascertain. Bending tests of large number of steel pieces to confirm the effects of multiple parameters, nonlinear finite-element analysis to check strain distribution, and fractography to determine the nature of the fracture indicated that ductile fracture initiated where the strain value was highest, as is generally expected. In the bending tests, the fractures were reproducibly longer in alloys with better ductility. One of the reasons for this phenomenon was that the surface was more hardened by plasma-arc cutting in mild steel because of its high carbon content according to hardness tests on the cutting surfaces. We therefore proposed a new equation to estimate crack length, taking into account three factors: strain, elongation and surface hardness. We showed that the relationship between crack length and the values by the equation explains the results of the bending tests. Additionally, we suggested the possibility that inclusions within an alloy affect the crack length. Cold forming is a general procedure used in shipyards. If inspections fail to find a crack, it may propagate to fatal damage after delivery. This study is therefore useful in preventing the initiation of cracks.

Commentary by Dr. Valentin Fuster
2018;():V004T03A027. doi:10.1115/OMAE2018-77188.

The through-thickness distribution of welding residual stress in a 30-mm-thick butt-welded Q345qD steel plate has been investigated through experimental measurements and finite-element simulations. In this paper, the weight function and finite element methods are used to investigate the stress intensity factors (SIFs) at the surface and deepest points of the semi-elliptical surface cracks, subjected to a combination of external tensile load and through-thickness welding residual stress. Different crack aspect ratios and relative depths are analyzed. The results reveal that the longitudinal residual stress is always tensile through the plate thickness, which makes the SIFs of the surface and deepest points larger than those without considering the longitudinal residual stress. However, the transverse residual stress through the thickness presents tension–compression–tension, with the tensile transverse residual stress causing the SIFs to increase. When the crack tip enters the compressive stress region, the compressive stress offsets the external load and causes the SIFs to decrease.

Commentary by Dr. Valentin Fuster
2018;():V004T03A028. doi:10.1115/OMAE2018-77496.

Chain corrosion-fatigue often governs the design of the mooring systems of floating production units, and it is assessed based on a stress-life or S-N approach. For integrity management along the lifetime of the assets, a Fracture Mechanics (FM) approach can be more appropriate, especially if cracks were detected during in-service inspections.

An essential parameter of FM is the Stress Intensity Factor (SIF), which becomes fundamental to evaluate the response of a cracked link. The aim of this paper is to present SIF results for chains under different combinations of cracks and loads. Analytic or handbook solutions exist and provide accurate SIFs for simple geometries and test specimens. However, the particular geometry of chain links makes the Finite Element Method (FEM) more appropriate.

The authors utilized the contour integral method together with the general purpose nonlinear FEM code Abaqus, to carry out a large amount of analyses to obtain SIFs for different chain sizes, material grades, crack shapes, crack depths and crack locations along the links. In addition to this, SIFs were derived in combination with other degradation phenomena such as large pitting or interlink wear.

As an outcome, empirical equations were developed to predict the SIFs of propagating cracks in mooring chains under a large variety of scenarios. This allows engineers to assess the criticality of fatigue cracks using suitable crack growth models, and hence evaluate their fitness-for-service or need to implement remedial actions.

Commentary by Dr. Valentin Fuster
2018;():V004T03A029. doi:10.1115/OMAE2018-78608.

The main aim of this paper is to perform the validation of the adaptive Gaussian process regression model (AGPRM) developed by the authors for the Stress Intensity Factor (SIF) prediction of a crack propagating in topside piping. For validation purposes, the values of SIF obtained from experiments available in the literature are used. Sixty-six data points (consisting of L, a, c and SIF values obtained by experiments) are used to train the AGPRM, while four independent data sets are used for validation purposes. The experimental validation of the AGPRM also consists of the comparison of the prediction accuracy of AGPRM and Finite Element Method (FEM) relative to the experimentally derived SIF values. Four metrics, namely, Root Mean Square Error (RMSE), Average Absolute Error (AAE), Maximum Absolute Error (MAE), and Coefficient of Determination (R2), are used to compare the accuracy. A case study illustrating the development and experimental validation of the AGPRM is presented. Results indicate that the prediction accuracy of the AGPRM is comparable with and even higher than that of the FEM, provided the training points of the AGPRM are aptly chosen.

Commentary by Dr. Valentin Fuster
2018;():V004T03A030. doi:10.1115/OMAE2018-78652.

DNV-RP-F108 [1] was first issued in 2006. The Recommended Practice was developed to provide guidance on testing and analyses for fracture control of pipeline girth welds subjected to cyclic plastic deformation, e.g. during installation by the reeling method, but also for other situations where pipelines may be subjected to large plastic strains. The Recommended Practice was based upon a Project Guideline developed within the Joint Industry Project “Fracture Control for Installation Methods Introducing Cyclic Plastic Strain - Development of Guidelines for Reeling of Pipelines”.

The new revision is based on the extensive experience and knowledge gained over the years use of the previous versions, as well as new knowledge from recent R&D projects.

The main content of Appendix A of DNV-OS-F101 (now DNVGL-ST-F101) [2] have been transferred to DNVGL-RP-F108. Only the requirements relative to ECA and testing have been retained in DNVGL-ST-F101 [2].

The new revision has got a new number and new title, i.e. DNVGL-RP-F108, “Assessment of Flaws in Pipeline and Riser Girth Welds”.

This paper lists the fundamental changes made in the new RP from the old Appendix A of the previous DNV-OS-F101 and discusses some of the changes, although within this paper it is not possible to cover all changes. The focus is on clarification of use of S-N versus the fracture mechanics approach for fatigue life computation, classification of fatigue sensitive welds, calculations of more accurate crack driving force by re-introduction of the plate solution, for which a new Lr,max (plastic collapse) calculation and a modified way to account for residual stresses have been specified. The RP presents new assessment procedures pertaining to use of finite element analyses for fracture mechanics assessments. A unique feature of the new RP is the guidance on sour service testing and assessments included in the Appendix C of the document to support pipeline/riser ECAs to develop flaw acceptance criteria for NDT.

Commentary by Dr. Valentin Fuster

Materials Technology: Materials Selection and Performance for Arctic Application

2018;():V004T03A031. doi:10.1115/OMAE2018-77723.

The new generation of value-added low carbon-low manganese-niobium microalloyed structural steels for both low and high yield strength, energy absorption, fatigue and fracture resistant applications is under development for offshore and arctic materials engineering applications. These materials engineering considerations are shifting designers to consider new lower cost and more robust construction materials even for low yield strength applications require improved fatigue, fracture arrest and toughness performance. The civil engineering and end user community demand structural reinforcing bars, shapes, beams and plates with improved energy absorption and fatigue properties. With more severe climatic conditions evolving every day, demands also necessitate improved fire and seismic resistance, yield-to-tensile ratio consistency, improved bendability and weldability. These attributes are difficult to obtain from steel producers today with their current higher carbon microalloyed steel approaches and hot rolling practices. There is a global shift in motion to low C-Nb-Mn bearing construction steels displacing traditional materials. The technological and metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) plate steels have been further developed to meet more demanding fatigue, fracture and low temperature toughness end user requirements. Niobium enables achievement of substantial grain refinement and grain size uniformity when the plate is rolled with the proper reduction, thermal schedule and process metallurgical operational practices. The effects of microalloying elements on the continuous cooling transformation behavior must be carefully controlled during the process metallurgy of the reheating and rolling process to successfully achieve the desired mechanical properties. TMCP applications have been successfully developed in numerous product sectors with thickness exceeding 120 mm. Since the very fine grained microstructure improves toughness and increases the yield strength, this Nb-TMCP process enables the required tensile properties with the growing trend to leaner chemical composition designs (less than 0.10%C) and excellent toughness properties. From an operational cost perspective, in today’s very competitive market environment, there exists a huge opportunity for structural offshore and arctic plate producing steel mills to improve their profitability by thoroughly assessing a shift to lower carbon and manganese steels in their product mix. Through the adoption of these lower carbon Nb-containing structural materials, several design and/or manufacturing companies are initiating new offshore steel designs that will further provide improved overall lifetime and cost performance at reduced maintenance expense. These high strength plate steels offer the opportunity to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the platforms.

Commentary by Dr. Valentin Fuster
2018;():V004T03A032. doi:10.1115/OMAE2018-77803.

Oil and Gas industry in the last decades has increased the use and need of heavy wall thickness line pipes, in particular for onshore / offshore high pressures and high temperatures (HP/HT) and offshore deep water / ultra-deep water applications.

The paper presents the results achieved by Tenaris on seamless line pipes in grades X65/X70, according to API 5L / ISO 3183, with wall thickness in a range from 40 to 60 mm and diameter between 6 5/8” and 16”, produced by hot rolling process followed by quenching and tempering. Such line pipes are able to withstand very demanding conditions, like sour environment, very high pressure and wide temperature range.

In this publication, the main outcomes of laboratory testing activities on the mentioned materials will be presented as part of heavy wall line pipe qualification. For this purpose, a special testing program, including mechanical and corrosion tests, has been executed. Material demonstrated an excellent behaviour, exhibiting both mechanical, toughness and stress corrosion properties suitable for the envisaged harsh applications.

Topics: Pipes , Wall thickness , Water
Commentary by Dr. Valentin Fuster
2018;():V004T03A033. doi:10.1115/OMAE2018-78426.

The main goal of the 10 years Arctic Materials KMB project run by SINTEF (2008–2017) and supported by the industry is to establish criteria and solutions for safe and cost-effective application of materials for hydrocarbon exploration and production in arctic regions. The objective of the arctic materials project guideline (PG) is to assist designers to ensure safe and robust, yet cost-effective, design of offshore structures and structural elements in arctic areas through adequate material testing and requirements to material toughness. It is well known that when the temperature decreases, steel becomes more brittle. To prevent brittle fracture in the Arctic, the structure needs adequate toughness for the loading seen at low temperatures. None of the common offshore design codes today consistently address low temperature applications. In this respect, arctic areas are defined as minimum design temperatures below what current international standards have considered per today, i.e. −10 °C to −14°C. For practical applications, the PG defines arctic areas as minimum design temperature lower than −10 °C.

It is acknowledging that design standards to a certain degree are based on operational and qualitative experiences gained by the offshore industry since the 1970’s. However, for arctic offshore facilities, limited operational experiences are gained by the industry. The basis of the guideline is that safe and robust design of structures and structural elements are ensured by combining standard industry practice today with learnings and findings from the 10 years Arctic Materials project.

This paper is concerned with the rationale behind the material and test requirements provided in the arctic material guideline. The material requirements will be discussed in detail with emphasis on toughness requirement, constraint effect, thickness effect, acceptance criteria and material qualification criteria.

Topics: Arctic region , Design
Commentary by Dr. Valentin Fuster
2018;():V004T03A034. doi:10.1115/OMAE2018-78653.

There have been recent incidents associated with cracking and leaks in C-Mn line pipe steels exposed to high H2S service. The incidents led to pipeline replacement with very expensive CRA clad pipeline causing substantial project delays and project cost escalations.

The incidents occurred when TMCP ACC steels were exposed to severe Region 3 environment as per domain diagram in NACE MR0175 (high partial pressure of H2S). The leaks were associated with longitudinal cracking which initiated at hard zones present on the parent pipe internal surface, and possibly also in girth welds.

The hard spots were observed to be contained within a very shallow depth of the ID surface of the pipe. The pipe microstructure beyond the thin layer of the hard zones at the ID surface did not contain hard material. However, the cracks propagated through the parent pipe normal microstructure in the through thickness direction.

Several of the operators are now concerned and uncertain on how to ensure the integrity of C-Mn pipelines in similar severe sour environments. Some operators have therefore introduced more stringent requirements for sour environment resulting in significant challenges to manufacture of line pipes and qualification of welding procedures that meet these new requirements. We also see different requirements being imposed by different operators. The use of CRA, clad/lined pipes or other exotic materials can solve the challenges, but are very expensive and can significantly reduce margins and make several sour service projects less viable.

Several R&D institutions have already started to study the phenomena. DNV GL have also initiated a broad JIP that will look into the challenges, with the objective of developing an industry guideline for use of C-Mn line pipe for high H2S service.

This paper will give background on the challenges associated with using C-Mn steel in high H2S service, describe the various uncertainties in detail, and describe how the JIP will address the challenges on a broad basis.

Commentary by Dr. Valentin Fuster

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