Statistical Method to Requalify Steel Grades During Conversion of Tankers to FPSO

During ship life, operating conditions may change, tanker may be converted into FPSO, and flag requirements may be modified. Generally these modifications have few impacts on existing structures; flag requirements only rarely are to be applied retroactively. Nevertheless in some cases modifications of operating condition may induce considerable consequences, making in the worst cases impossible any reengineering. For example converting a common tanker, built with plain steel of grade A into an Offshore Floating Unit able operating in cold region, may require a grade change corresponding to a grade B. It is obviously meaningless to replace all material just because material certificates. Steels used by shipyards have to fulfill Classification society’s requirements dealing with mechanical strength; generally shipbuilding corresponds to a small part of steelmaker’s production. For this reason steelmakers are reluctant to produce steels with mechanical properties corresponding exactly to the minima required. They generally deliver steels already in stock, with higher mechanical characteristics than required. In this case it can be taken advantage of this common practice. In order to demonstrate that the material fulfill the requirements of grade B it has been decided to adopt a statistic approach. At this stage there are two main issues, the first one is that it is needed to provide evidences that the actual material Charpy V characteristics fulfill the requirements of grade B; the second one is to provide these evidences with a minimum testing. To assess this assumption a random check has been carried out. Different probabilistic model have been tested in order to check common approaches and probabilistic model based on physical considerations. In the paper the main assumptions for estimating the minimum Charpy value main assumption in the probabilistic models are recalled, the behavior of empirical sample is examined, the parameters of probability laws fitting the empirical distribution and definitely as accuracy of probability law parameters determination is not perfect with a finite number of specimens the uncertainty in the determination of parameters is taken into account with confidence limits. According to the selected probabilistic model the minimum value corresponds to an acceptable probability of failure, taking into account the target confidence level, or is independent of any acceptable probability of failure and is defined with the same confidence level. At the end it is concluded that a random check with a data treatment assuming a random distribution of Charpy V test results distributed according to a Weibull probability law of the minimum allows providing evidences that with a sufficient confidence level the steel used for the considered structure fulfill the requirements of the new operating conditions.

Study on the Piezomagnetic Response to Low-Cycle Fatigue of X80 Pipeline Steel

The objective of this work is to explore the relationship between the progressive material degradation process of fatigue and the evolution of the piezomagnetic field surrounding a ferromagnetic sample. The continuous examination of changes in the magnetomechanical behavior during a series of strain-controlled fatigue tests was performed on X80 pipeline steel samples. Analysis of the data obtained allowed the division of the fatigue life of the investigated steel into three stages: incipient stage, steady stage and terminal stage. Furthermore, the piezomagnetic field evolution demonstrates conspicuous changes in the initial stage of fatigue loading, then reverts to a relatively stable phase, and finally, drastic variations appear again before terminal failure. The progressive degradation of the steel under cyclic loading can therefore be tracked by following the evolution of the piezomagnetic field. The characteristics of the evolution of the piezomagnetic responses are also discussed in terms of the mechanical and microstructural condition of the steel during the fatigue process.

A Review of the Metal Magnetic Memory Technique

Metal magnetic memory (MMM) is a newly developed non-destructive testing (NDT) technique. It has potentials to detect early failure, such as stress concentration, micro-crack and fatigue damage of ferromagnetic components. This paper outlines the recent developments of the mechanism studies on the MMM technique. Several advances in experimental investigations on the MMM technique are also summarized, e.g. the factors which can influence the detection signals, the criteria for judging the damage state and the magnetic parameters which can be used to assess the testing results. Finally, some future development trends about this technique are suggested.

Cofferdam and Hyperbaric “Live” Repair of Gas Pipeline Leaks

Offshore pipelines may face many threats. Apart from internal and external corrosion third party threats represent major hazards to submarine pipelines. Recent pipeline leaks caused by third party as well as construction and installation have been repaired online in a two-step process involving installation of a leak clamp followed by a permanent repair by installation of a welded stand-off sleeve. The welding has depending on the water depth been executed in a hyperbaric habitat or in shallow water using purpose built cofferdams. A concept proposed by DNV GL has been successfully applied to repair of leaking submarine pipelines. To ensure the safety of the repair crew the concepts involves using a gas containment barrier installed over a traditional mechanical leak clamp. The gas containment barrier is either purged with inert gas or nitrogen or it is maintained with a constant inert gas pressure that is monitored continuously during the repair. In the event of a sudden gas leak into the gas containment barrier a pre-set pressure relief valve will open and dump the gas leak outside the habitat. This paper describes the details of a few cases of leaking submarine gas pipelines and the immediate causes of the leak, the repair method selection, the repair method details, cofferdam or hyperbaric welding qualification and execution. The paper also describes the various steps in the process to ensure that the pipeline damage is stable and that the repairs can be safely undertaken to restore the pipelines to their original design condition without reduction of pressure or flow rate. The paper describes the method of global and local finite element analyses as well as fracture mechanics assessment by FEA to assess the stability of the flaws causing the gas leaks. The pipelines in question have all been gas transmission lines carrying gas to gas fired power plants for which gas pressure reduction or shutdown were completely unacceptable. Future development is expected to involve development of remotely controlled repairs using similar concepts at water depths where diver/welders cannot be employed due to the various country regulations or simply because the water depths are too deep for saturation divers. Methodology according to DNV RP-A203 [1] is described for qualification of new technology for underwater pipeline repairs. Further references are made to the recent updates to the DNV RP-F113 Pipeline Subsea Repair [2] with regards to requirements for “live” pipeline repairs. The DNV RP-F113 refers to the PRCI Weld Thermal analyses [3] and requirements to perform full scale mock-up tests of the repair as part of the repair method qualification based on DNV OS-F101 Submarine Pipelines [4].

Full Scale Reeling Simulation Tests of Girth Welded X60 HFW Linepipe

The reel-lay method is a fast and cost efficient installation process for offshore rigid steel pipelines. Pipelines installed by the reel-lay method are plastically deformed due to reeling, unreeling, aligning and straightening during pipeline installation. Therefore, local buckling is one of the major concerns from a view point of integrity in linepipes, especially around girth welds where strength mismatching arises due to adjacent pipes with different yield strength. One the other hand, the change in mechanical properties of linepipes during reel-lay, including coating process (e.g. 250°C) and long time exposure (e.g. 250°C aging) after installation is also important in order to guarantee safety of linepipes. Furthermore fracture toughness at girth weld Coarse-grain HAZ (CGHAZ) after reeling and aging should be clear to prevent brittle fracture of offshore linepipes in service. In this study, full scale reeling simulation tests of girth welded X60 HFW (High Frequency electric resistance Welded) linepipes with OD; 323.9mm and WT;15.9mm after full body heat treatment (coating simulation) were conducted at 5cycles and 2cycles reeling and straightening situations when yield strength mismatches are present between adjacent pipes around girth welds. Localized strain concentration was observed near girth welds by strength mismatching of adjacent pipes. DNV ovality increased with increasing reeling and straightening cycles, however the ovality did not exceed 10%, which was a criterion value for local buckling, after 5cycles reeling simulation test. The change ratio of wall thickness after full scale reeling simulation tests were about ±2% (within DNV-OS-F101 tolerance) regardless of circumferential and longitudinal direction of pipes. Longitudinal tensile properties could be characterized by axial last introduced plastic strain. That is, in the positive number of last introduced plastic strain, YS and Y/T increased, while uEl decreased by work hardening effect. On the other hand YS and Y/T decreased, while uEl increased at the negative number of last introduced plastic strain by Bauschinger effect. Circumferential tensile properties could be also characterized by axial last introduced plastic strain. Yield strength and Y/T slightly increased while uniform elongation slightly decreased by aging at longitudinal and circumferential direction. Tensile properties did not change irrespective of the number of cycles of reeling simulation tests. After 5cycles reeling simulation test and aging, girth welded CGHAZ CTOD values were over 0.4mm at −20°C and Charpy absorbed energy were over 200J at −30°V, therefore, it was considered that the brittle fracture of the girth welded linepipe unlikely occur at reeling and aging situation in this study.

Development of Under-Film Corrosion Simulation Method Based on Cellular Automaton

A simulation method for under-film corrosion has been developed for epoxy coated steel panels within a ship’s Water Ballast Tank (WBT) environment. The incubation and extension of coating failure is simulated by using two-dimensional cellular automaton, and the steel diminution is simulated by modifying IACS CSR-H’s 3-phases probabilistic model. Analysis parameters are determined by using the results of onboard exposure and cyclic corrosion tests performed by Shiotani et al. (2012, 2015). The change in corroded surface shape of epoxy coated scribed steel panels made of conventional steel and corrosion resistant steel (CRS) exposed in an ore carrier’s WBT for 4.8 years is simulated. The simulated coating deterioration (blister) area and the corroded surface profile agree well with those measured. This demonstrates the effectiveness of the developed simulation method and the determined parameters. The differences in analysis parameters between conventional steel and CRS suggest that CRS can reduce the harmful effect of the active corrosion region on the remaining coating life at the blister’s frontline and the corrosion under the blister.

Effect of Sour Acidizing Treatments on the Fatigue Crack Growth and Fracture Toughness Behavior of C-Mn Line Pipe Steels

Acidizing treatments are typically performed intermittently during the life of a well. However, more recently there has been a desire to perform an increased number of acidizing treatments in order to improve production. The acidizing treatments typically involve highly corrosive acids, such as hydrofluoric (HF), hydrochloric (HCl) and acetic acid, which are known to cause significant corrosion. In the presence of hydrogen sulfide (H2S), these acidizing treatments could cause environmentally assisted fatigue and fracture (i.e. increased fatigue crack growth rates and reduced fracture toughness). A test program is underway to evaluate and quantify the effect of sour acidizing treatments on the fatigue and fracture behavior of welded C-Mn line pipe steels. This paper describes the preliminary findings from fatigue crack growth rate (FCGR) and fracture toughness (FT) tests on as-welded (i.e. unstrained) pipe. All tests were conducted at room temperature (RT) using compact tension (CT) specimens notched in the parent pipe (PP). Frequency scan FCGR tests were performed in the following sour acid conditions: simulated production environment (PE), spent acid without inhibitor and spent acid with residual corrosion inhibitor. The PE consisted of a simulated brine with pH = 4.5 and partial pressure of H2S (pH2S) = 0.21psia. FCGRs in the sour PE were of the order of 20 times faster than in air. The pH2S was the same for the tests in spent acid environments, but the pH was lower (approximately 3.5). As would be expected, the FCGRs were much higher in the low pH environment. The highest FCGRs were observed in the inhibited sour spent acid environment and were up to 100 times faster than in air. Sour FT tests were also conducted in the PE and in spent acid with and without inhibitor. In all cases, the measured FT values were significantly lower than in air. The test in PE exhibited higher FT than in the sour acidizing environment. The lowest FT values were observed in spent acid with inhibitor. Future work will investigate the effect of reeling on the fatigue and FT performance of pipe girth welds in sour acidizing environments.

Study on Weld Fatigue Evaluation Incorporating Welding Induced Residual Stress Effect

For fatigue assessment of a welded structure, an important characteristic is the failure location. The fatigue crack begins from the pre-existing crack-like flaw which is an inherent feature of a weld toe or root. Residual stress in the welded joints is another important characteristic. It is also well known that tensile residual stress in welded structures can be as high as the material yield strength level and so the fatigue strength of the welded joint is governed by the applied stress range regardless of the applied load ratio. However, when the presumed conditions with high tensile residual stress in the weld regime are not satisfied, the fatigue behavior in welded structure deviates from the general weld fatigue behavior. In this paper, firstly the applied load ratio effect on fatigue behavior is investigated for two different residual stress levels of welds, i.e., as-welded and stress relieved. It has been observed that stress-relieving effect of welds is apparent when the applied loading introduces stress fluctuations in compressive loading. When the load ratio effect is considered in Battelle structural stress based fatigue parameter, the stress relieved weld fatigue data was consolidated within the master S-N curve. Based on this investigation, the Battelle structural stress based effective load ratio considering both applied loading and detailed residual stress is introduced for the weld fatigue behavior. The two-stage crack growth model is reformulated with the structural stress based effective load ratio, which is a function of crack length. Lastly, a fatigue life prediction procedure incorporating residual stress effect is proposed and validated using the existing fatigue test results. The proposed procedure clearly shows that the compressive residual stress distributions at the weld prone to crack provide great benefits for fatigue life improvement.

The Fracture Resistance Approach in Order to Prevent Brittle Failure of Offshore Structures Under Arctic Environments

This paper is concerned with the challenges related to steel design under Arctic conditions where both loading and temperature have been discussed in relation to material requirements. Today there is a lack of rules and standards for selecting steel materials for bulk engineering for a lower design temperature than −10°C (NORSOK N-004 [1] allows down to −14°C). Both ISO 19902 Steel Structures [2] and NORSOK N-004 Design of steel structures make reference to EN10225 “Weldable structural steels for fixed offshore structures technical delivery conditions [5]” where steel materials are Charpy tested at a lowest test temperature of −40°C and proven for a design of −10°C. Hence, one major challenge for designers are to specify adequate toughness requirements at an early stage of the design process for low temperature applications. Both NORSOK N-004[1] and ISO 19902[2] provide requirements to load combinations that need to be fulfilled, however the relationship between various load types and temperature is not mentioned in any of these standards. Thus, in the design stage the material needs to demonstrate adequate toughness where loading and temperature are treated independently. For the offshore industry, the main question is the balance between materials requirements and cost-effective solutions, and how to address this within an overall design perspective in order to avoid brittle failure. This paper discusses some of these challenges with the aim of starting a focused process leading up to a clear interpretation of the implications of overall design philosophies, necessary in order to define consistent materials requirements ensuring that brittle fracture is not going to represent a significant threat to the structural integrity. The material recommendations provided in the paper are based on the latest research results from the Arctic Materials project (2008–2017) managed by SINTEF and supported by the industry.

Fatigue Strength and Angular Distortion of the Full-Penetration Tee Type Joint Fabricated by One-Side Single-Pass Laser-Arc Hybrid Welding

Laser-arc hybrid welding is a high-quality welding technology and is expected to improve the productivity of manufacturing hull and offshore structures. Application of this technology allows the replacement of fillet-welded joints in structures with full-penetration welded joints. The fatigue performance and deformation caused by welding will be improved by this replacement. The present study experimentally investigates the fatigue performance and deformation due to welding. Two types of tee joints, which penetrate from one side and both sides, were applied. The investigations confirm the superiority of full-penetration tee joints fabricated by laser-arc hybrid welding over conventional fillet-welded joints.