Mooring Chain Steel Research Articles

Corrosion susceptibility of mooring chain steel under cyclic stress

Corrosion susceptibility of mooring chain steel under cyclic stress

Delayed fracture behavior of ultra-high-strength mooring chain steel evaluated by potentiostatic hydrogen-charging combined with SSRT

The influence of hydrogen charging potentials on the hydrogen embrittlement susceptibility of R6 ultra-high strength mooring chain steel was investigated via constant potential hydrogen charging slow strain rate tensile tests combined with thermal desorption analysis. The results reveal that hydrogen charging leads to a 38.94% decrease in elongation, while the impact on tensile strength is relatively minor. Furthermore, the specimens experienced intergranular cracking at the critical potential of −1150 mV, with the size of the brittle region increasing as the negative charging potential becomes more negative. And, hydrogen atoms can cause local embrittlement of materials and increase KAM value.

Effect of Niobium Microalloying on the Mechanical Properties in Grade R4 Mooring Chain Steel

Effect of Niobium Microalloying on the Mechanical Properties in Grade R4 Mooring Chain Steel

Effect of boriding on tribocorrosion behaviour of HSLA offshore mooring chain steel

This study explores the transformative potential of boriding to enhance the tribocorrosion resistance of mooring chain steel for offshore environments. The boride layers formed at various temperatures (800 °C, 900 °C, and 1000 °C) for 1 h, revealing high hardness (17–21 GPa). X-ray diffraction confirmed dual-phase FeB and Fe2B borides, contributing to superior mechanical properties. Energy-dispersive X-ray spectroscopy showed clear phase boundaries. Potentiodynamic data demonstrated improved corrosion and tribocorrosion resistance, particularly at higher boriding temperatures. Mechanical effects played a pivotal role in tribocorrosion behaviour, emphasizing the need to optimize boriding process temperatures. Electrochemical impedance spectroscopy highlights the 900 °C-1 h sample as offering excellent corrosion resistance. This study underscores boriding's potential to enhance the tribocorrosion resistance of offshore mooring chain steel.

Investigation of Friction Hydro-Pillar Processing as a Repair Technique for Offshore Mooring Chain Links

Repairing links of offshore mooring chains has presented a significant industry challenge, primarily arising from modifications in material properties, encompassing alterations in microstructure, hardness, and residual stress. In this context, the present work investigates the method of friction hydro-pillar processing (FHPP) applied to R4 grade mooring chain steel. Joints in as-repaired and post-weld heat treatment (PWHT) conditions were subjected to residual stress (RS) tests using the neutron diffraction technique, microhardness mapping, and microstructural evaluations. The process generated peaks of tensile and compressive stresses in different directions and hardness below that of the parent material in the softening zone. The friction zone promoted high hardness levels in the thermo-mechanically affected zone (TMAZ) with a maximum of 19% of the ultimate tensile strength of the parent material. As expected, the PWHT restored the RS and reduced the hardness; however, 4 h PWHT allowed the elimination of a hardness higher than that of the base material.

Crack growth in high strength mooring line steel under variable amplitude loading

Mooring lines are of key importance for the safe and reliable operation of numerous floating offshore structures. Due to the constantly changing wind and wave conditions acting on such structures, any fatigue crack growth in a mooring line will occur under cyclic loading of distinctly variable amplitude. This paper considers a long-term load history for mooring lines on a North Sea offshore platform. For fatigue testing purposes, a characteristic load sequence representing the actual load history in a condensed and simplified form is developed and presented. Fatigue crack growth rate test results for a grade R4 high strength mooring chain steel subjected to this variable amplitude load sequence is furthermore presented, including tests performed in air as well as under free corrosion conditions in artificial seawater. Based on comparison testing performed under variable- and constant amplitude conditions, the validity of a linear damage rule hypothesis for stable crack growth under the characteristic load sequence is investigated. This damage rule converts the load spectrum to an equivalent constant load range, making it a potentially useful tool for estimating fatigue crack growth in mooring chains when test data for a representative characteristic load sequence are unavailable. Results from the comparison testing are used to show that under stable fatigue crack growth conditions, while subjected to the characteristic load sequence developed for this offshore platform, the linear damage rule can be a reasonable assumption.

Fatigue crack growth in mooring chain steel at different mean load levels

The fatigue crack growth rates of R4 mooring chain steel at different R-ratios are investigated. Crack growth experiments are performed at three different R-ratios, 0.1, 0.3 and 0.7, utilizing compact tension specimens in air environment. Huang and Moan's driving force parameter ΔKE, which relates the growth rates at different R-ratios to those obtained at R=0, is explored and adapted for R4 steel.A full-scale fatigue experiment involving a chain assembly with an identified crack is presented, where the observed crack growth was successfully modelled by the proposed crack growth model.Case studies with different mean loads are explored where the crack growth model is utilized to predict the fatigue life of D = 145 mm chain links with a crack at the crown location. The crack growth simulations are compared and discussed against published S–N curves for chain links with varying mean load. The S–N curves may give very inaccurate results when extrapolated outside of the underlying test data. The crack growth model predicts longer lives at low stress ranges and low mean loads compared to the S–N curves, indicating that current guidelines may be overly conservative in these conditions.

Corrosion form transition of mooring chain in simulated deep-sea environments: Remarkable roles of dissolved oxygen and hydrostatic pressure

The corrosion form and mechanical properties deterioration of mooring chain steel in simulated deep-sea environments were investigated. With the increase of ocean depth, not only the pressure increases, but also the dissolved oxygen content decreases. These two factors affect corrosion evolution of mooring chain steel in simulated deep-sea environments, which was studied for the first time. Compared with uniform corrosion of mooring chain steel in shallow sea with sufficient oxygen, low dissolved oxygen leads to the corrosion dominated by pitting with pit covers. Meanwhile, hydrostatic pressure distinctly accelerates pitting initiation and propagation. The higher the hydrostatic pressure is, the more serious the pitting is. For failure mechanism of unstressed mooring chain steel serving in simulated deep-sea environments, both absorbed hydrogen and corrosion morphology can degrade the ductility of mooring chain steel, in which the leading factor depends on the service time. The severe pitting is the main factor and causes remarkable ductility loss of the steel after long-term immersion. But hydrogen plays an important role on elongation loss in early stage.

Remaining fatigue life assessment of corroded mooring chains using crack growth modelling

Failures caused by the combined actions of fatigue, corrosion and wear are important safety concerns for mooring chains used on floating structures in the oil and gas industry. Prediction of remaining corrosion fatigue life based on surface condition could therefore be a useful tool for the continued safe operation of corroded chains. This paper investigates the use of crack growth modelling for estimating the remaining corrosion fatigue life of mooring chains that exhibit significant pitting corrosion damage. A crack growth modelling approach is used to produce remaining fatigue life estimates for a selection of severely pitted mooring chains. Using fatigue crack growth rate test results for grade R4 high strength mooring chain steel, empirical crack growth laws are presented for free corrosion and cathodic protection conditions at load ratio R = 0.1. Two different methods for establishing equivalent cracks from surface scans of corrosion damage are presented. The mooring chains are proof loaded as part of their manufacturing process. Residual stresses introduced during this process have therefore been determined by finite element analysis and accounted for in the fatigue crack growth predictions. One of the equivalent crack models, accounting for the single dominant corrosion pit, provided quite accurate fatigue life predictions when compared with full scale test results.

ENHANCEMENT OF MARINE CORROSION AND TRIBOCORROSION RESISTANCE OF OFFSHORE MOORING CHAIN STEEL BY ALUMINIZING PROCESS

The safety of mooring systems and accessories is one of the most critical issues in the structural integrity of floating oil/gas and renewable offshore structures. Mooring chains and accessories operate under dynamic conditions in harsh marine environments. They are subject to severe wear and corrosion between their links due to relative movement from waves, wind, and ocean currents that disrupt structural integrity. To cope with this problem, the pack-aluminizing process was applied on the R4 grade offshore mooring chain steel for 2 h at 850 °C to improve corrosion and wear-corrosion (tribocorrosion) resistance in 3.5% NaCl. The tribocorrosion behaviour of untreated and aluminized samples was investigated by a tribo-electrochemical setup that simultaneously allows for collecting the wear and corrosion data. Potentiodynamic and potentiostatic corrosion and tribocorrosion tests were carried out to understand corrosion kinetics. Optical, SEM, XRD and EDS analyses were performed to characterize the aluminide layer and surface morphologies before and after tribocorrosion investigations. In polarization scans under corrosion and tribocorrosion conditions, the current showed a significant activation stretch of several orders of magnitude, with minor potential changes in the anodic region. Due to the galvanic effects of sliding under natural electrochemical conditions, the untreated R4 alloy exhibited cathodic properties in the wear track, while the aluminium coating was out of the wear track due to its oxide-forming ability. At the cathodic potential, two hard Al2O3 materials under pure mechanical effects and third bodies emerging from cracks on the coating surface increase the friction coefficient (COF), while the oxide product film, which has a lubricating ability and pits which reduces the contact area, caused a decrease in COF at the high anodic potential. The study revealed that while the aluminide layer improved the corrosion and tribological character of R4 alloy, material loss from wear track increased due to micro fractures and cracks in the coating layer during sliding tribocorrosion conditions.

Effect of cyclic softening and mean stress relaxation on fatigue crack initiation in a hemispherical notch

AbstractCorroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in three‐point bending on specimens with hemispherical notches, representing an idealized geometry of a corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim of the study was to quantify mean stress relaxation (MSR) and its influence on fatigue initiation life. MSR data were obtained from strain‐controlled fatigue tests on R4‐grade offshore mooring chain steel. Interestingly, MSR was observed even at a stress amplitude below the cyclic yield stress. For initiation lives below 100,000 cycles, significant improvement in predictions was obtained if cyclic softening and MSR were accounted for. Using only the monotonic stress‐strain curve led to less accurate predictions. For longer lives, scatter in the base material data and residual stresses from machining of the notch made predictions less accurate.

Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel

R4-grade mooring chain steel specimens were subjected to alternating phases of saltwater corrosion and air fatigue to replicate seasonal load variations on chains in service. No significant difference in fatigue lifetimes was registered between precorroded specimens subjected to continuous fatigue and those periodically interrupted by phases of accelerated corrosion. Moreover, eight months of natural corrosion of specimens with fatigue cracks did not have a large effect on their remaining fatigue lifetimes. Retardation of fatigue crack growth by crack tip blunting from corrosion is deemed unlikely based on observations from the current work and the literature. Corrosion accelerated by anodic polarization is demonstrated to be unsuitable for replicating natural corrosion of fatigue cracks.

The effect of corrosion evolution on the stress corrosion cracking behavior of mooring chain steel

The effect of corrosion evolution on the stress corrosion cracking (SCC) of mooring chain steel is investigated by adjusting the corrosion evolution degree in a temperature acceleration environment. The corrosion evolution is the transformation of pitting into uniform corrosion in simulated seawater, which affects the initiation site of microcracks and the roles of corrosion and hydrogen during SCC. When corrosion form is pitting at 25 °C, microcracks initiate at pits and hydrogen plays an important role. When corrosion form is uniform corrosion at 55 °C, Cr-rich carbides cause microcracks initiation along grain boundaries and anodic dissolution plays the dominant role.

Localized Corrosion of Mooring Chain Steel in Seawater

Corrosion of mooring chains is regarded as one of main threats to the offshore mooring systems. Localized corrosion is even more dangerous than uniform corrosion because it may not show significant mass loss but it can cause stress concentration and initiate cracks under force, leading to accelerated degradation of mooring chains. Localized corrosion of steel in seawater is influenced by many factors such as the local heterogeneities of the steel, and the local electrochemical and microbiological environments. It is difficult to predict and the mechanism is not fully understood. The aim of this work was to study the mechanism of localized corrosion on mooring chain steel in seawater which is helpful in the search for corresponding monitoring tools and mitigation methods. The corrosion behavior of chain steel grade R4 was studied in artificial seawater and artificial seawater containing microorganisms collected from a practice field. The corrosion behavior of the steel was studied using different techniques such as potentiodynamic polarization, linear polarization resistance measurements and electrochemical impedance spectroscopy. The microstructures such as inclusions and compositions of the chain steel were studied using SEM: Scanning Electron Microscope and EDS: Energy Dispersive Spectroscopy. The microbial cells were observed using epi-fluorescence microscopy. The corrosion morphology and pit geometry were investigated using photo-microscopy. The localized corrosion rate has been found to be much higher than the uniform corrosion rate of the steel in the seawater in the presence of bacteria. In the case of localized corrosion, applying uniform corrosion measurement techniques and formulas is not considered representative. The representative areas have to be introduced to match physical results with the measurements. Inclusions, such as MnS and TiVCr found in the steel have a critical influence on localized corrosion. The corrosion mechanism of the steel in seawater is discussed.

Effect of Tempering Temperature on the Microstructure and Stress Corrosion Cracing Susceptibility of Ultra-High-Strength Mooring Steel

Mooring chain steel has been widely used for stabilizing offshore platforms and suffers from stress corrosion cracking. Herein, the microstructure difference, its relation to strength and susceptibility of stress corrosion cracking after tempered at different temperatures have been studied. Result shows increasing tempering temperature increases the proportion of low value CSL boundaries, decreases the local misorientation angle and promotes the precipitation of carbides. These factors induced the decrease of SCC susceptibility at higher tempering temperature. The resistance to SCC at 640 °C tempering temperature is about 20% higher than that of 580 °C at − 1000 mVSCE. The main strengthening mechanism of the ultra-high-strength steel is ultra-fine grain strengthening mechanism, precipitation strengthening mechanism, and dislocation strengthening mechanism. Increasing tempering temperature from 580 to 640 °C decreases the yield strength by 85 MPa, which is mainly attributed to larger carbides size.

Fretting Tribocorrosion Behaviors of Marine Mooring Chain Steel 22MnCrNiMo in Artificial Seawater

Abstract The fretting wear behaviors of mooring chain steel 22MnCrNiMo specimens were evaluated in pure water and artificial seawater using a pin-on-flat configuration with 500-μm amplitude at room temperature for 2 h. Open-circuit potential tests and potentiodynamic anodic polarization were used to measure the alloy’s corrosion behaviors. Worn specimen surfaces were observed using scanning electron microscopy and wear volume loss determined using laser-scanning confocal microscopy. Comparative analyses of friction coefficient, wear volume loss, and worn surface morphologies were conducted. The results showed that the friction coefficients of 22MnCrNiMo friction pairs were in general smaller in seawater compared with that in pure water. Comparing contact stresses, sliding velocities had greater influence on tribocorrosion behavior of friction pairs in a marine environment. Moreover, wear volume loss of this steel in seawater was lower than that in pure water, indicating that corrosion had inhibitory effects on wear. With the increase of wear degree, the corrosion degree also increased, indicating that wear occurrence promoted corrosion. This also demonstrated a transformation of positive to negative interactions between corrosion and wear in the fretting corrosion and wear processes of this steel. The wear mechanism of the mooring chain steel 22MnCrNiMo was typically abrasion wear in pure water, whereas it was corrosion fatigue in seawater.

The Character of Hydrogen Embrittlement in Mooring Chain Steel

The digital image correlation method has been applied to directly observe the local strain and crack formation in mooring chain steel during tensile tests in air or when simultaneously charging hydrogen in different ways, viz. including/excluding the notch root. Interestingly, hydrogen accumulation promoted crack initiation on the surface when the local strain reached approximately 0.9%, while strain of 19% to 20% was reached in the hydrogen-free specimens before visible crack formation. Even through the stress–strain curves prior to the sudden drop indicate a negligible effect of hydrogen, its presence can greatly reduce the stress-induced crack initiation. In addition, hydrogen introduced while avoiding the notch root can disorganize the regular distribution of strain produced by the notch before crack initiation. Therefore, hydrogen-induced embrittlement below the critical stress criterion can be explained by hydrogen-enhanced localized plasticity, while above the critical stress criterion, hydrogen-enhanced decohesion provides a better explanation.

Corrosion of Copper-bearing High Strength Mooring Chain Steel Affected by Microbes in Seawater

Corrosion behavior of copper-free (BR5) and copper-bearing (BR5Cu, 0.8% Cu) bainite type grade R5 mooring chain steels is investigated in sterile and bacteria-containing seawater. Electrochemical impedance spectroscopy and weight loss results demonstrate that both steels corrode steadily with similar rates in sterile seawater, which is closely related to stable dissolved oxygen (DO) concentration and loose corrosion product layers. While in bacteria-containing seawater, corrosion is inhibited at the initial stage due to DO consumption and biofilm formation, and the inhibition efficiency declines with time because of bacteria decay, leading to a slightly smaller total weight loss than that in sterile media. Although less adhesive bacteria are observed on BR5Cu steel owing to the antimicrobial activity of copper, it does not bring a visible difference in corrosion rate. The validity of copper introduction in mooring chain steel towards seawater corrosion and MIC inhibition is expected to be checked further in different systems.

Study of the Hydrogen Delayed Fracture of Mooring Chain Steel using Strain Rate Testing

Study of the Hydrogen Delayed Fracture of Mooring Chain Steel using Strain Rate Testing

Toughness properties of a friction hydro pillar processed offshore mooring chain steel

In the past few years, solid-state welding has been a great alternative in comparison to the conventional fusion welding technologies. Solid-state welds are usually processed in lower temperatures than the other common fusion joints, tending to avoid problems such as hydrogen embrittlement, brittle microstructures and porosity. Therefore, these advantages can be useful to industrial applications which do not allow fusion welding. Offshore mooring components, which are fabricated under restricted standard requirements to ensure the floating marine integrity, is one of these applications. Thus, defects in links of mooring chains cannot be repaired by traditional welding, resulting in high operational costs. In this context, this study aims to characterize the Friction Hydro-Pillar Processing (FHPP) application to a mooring chain steel IACS UR W22 grade R4. The main process parameter studied was an axial force ranging from 30 to 60kN. The welded joints were evaluated by metallography (macro and microstructure) and microhardness mapping. Furthermore, the toughness properties were assessed by Charpy and Crack Tip Opening Displacement (CTOD) tests, and the fractures were afterwards observed by scanning electron microscopy (SEM). The results showed that an increase in the axial force enhances the Charpy and CTOD values.