Offshore Mooring Chains Research Articles

Analysis in Mooring Chain Links Tensioned on Curved Surface

The development of offshore energy resources, specifically oil and gas, is increasing. Consequently, there is an increasing demand for state-of-the-art technologies and concepts to enhance productivity in this sector. One critical component of deep and ultra-deep water floating platforms is the mooring system, which requires rigorous design considerations. The motivation behind this lies in the substantial repair costs associated with unexpected failures. Structural failure in anchor chain links is primarily attributed to fatigue damage. While extensive studies have explored this phenomenon, the impact of mooring equipment with curved surfaces – responsible for guiding or restricting the upper end of mooring lines – on chain link behavior remains somewhat overlooked. In this study, we investigate the effects of ungrooved curved surfaces on the fatigue life of tensioned offshore mooring chain links. Using Finite Element Analysis (FEA), we analyze the behavior of mooring chain links when subjected to tension on curved surfaces. Our findings identify stress hotspots and calculate the corresponding Stress Concentration Factors (SCFs). Notably, the presence of curved surfaces significantly influences stress distribution and fatigue life. Additionally, we explore the effects of varying nominal stress levels applied during assembly and changes in the radius of the curved surface. These results have important implications for the design of mooring systems used in oil and gas exploration and production.

An Out-of-Plane Bending Fatigue Assessment Approach for Offshore Mooring Chains Considering the Real-Time Updating of Interlink Bending Stiffness

Fatigue failure caused by frequent tension and bending loads is a crucial safety concern for mooring chains used on floating structures in the oil and gas industry. The bending effect for a chain’s fatigue is usually not considered by existing fatigue analysis methods, and even if it is considered, existing commercial software can only calculate the constant interlink bending stiffness without consideration of the stiffness variations due to tension and friction. To address this issue, this study develops an in-plane bending (IPB)/out-of-plane bending (OPB) fatigue assessment approach for offshore mooring chains considering the time-varying nonlinear interlink bending stiffness. Initially, the mooring tension and IPB/OPB angles are calculated by an in-house code MeCAP (multi-element coupled analysis program), considering the time-varying interlink stiffness between the chain links. The fatigue life of mooring chains for pure tension–tension (TT) and OPB combined fatigue at different hotspot locations (A, B, and C) is calculated by MeCAP-fatigue, a newly-developed module in MeCAP. Based on the analysis model, a comparative study is implemented to investigate the effects of interlink stiffness on fatigue damage. The differences and advantages of the combined fatigue calculation method over the pure TT fatigue assessment method are discussed. The results illustrate that combined fatigue would be underestimated significantly if zero interlink stiffness (hinge joint assumption) is applied or even constant interlink stiffness (calculated by mooring pretension), while pure TT fatigue will be largely unaffected. Moreover, compared to pure TT fatigue, the OPB combined fatigue assessment method, considering the effects of OPB/IPB and the time-varying properties of the connection between chain links, could evaluate fatigue life more comprehensively.

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.

General point load weight function for semi-elliptical surface cracks in round bars

Offshore mooring chains are continuously subjected to cyclic environmental loadings, resulting in fatigue cracks that always develop at the surface and grow across the circular cross-sections. To analyse the fatigue crack growth in a mooring chain based on a fracture mechanics-based approach, the evaluation of stress intensity factors (SIFs) of surface cracks in the chain segments becomes essential. A general point load weight function of SIFs for semi-elliptical surface cracks in straight and curved round bars was derived in this study. Parametric three-dimensional finite element (FE) models were developed to calculate the SIF solutions for determining the unknown parameters in the weight function. The analysis matrix covered wide ranges of aspect ratios 0.1 ≤ a/c ≤ 2.0, relative crack depths 0.03 ≤ a/D ≤ 0.6 and relative bar curvatures 0.0 ≤ D/Ri ≤ 1.5. The developed weight function was verified against the solutions available in the literature and FE results, indicating good accuracy. The proposed weight function can be used in SIF calculation and remaining fatigue life prediction of surface cracked round bar components under complex stress distributions, such as chain links.

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.

Effect of Boron-Aluminide Coating Applied on R4 Grade Offshore Mooring Chain Steel on Pitting and Tribo-Corrosion Behaviour

Offshore mooring systems are susceptible to wear, corrosion, and fatigue damage, making improving their tribocorrosion resistance essential. This study aims to evaluate the effects of boron-aluminizing treatment on the corrosion and tribocorrosion resistance of R4 steel in a marine environment. The boron-aluminide coating was characterized by SEM, EDS, and XRD, revealing the presence of FeAl as the dominant phase with minor amounts of FeB, Fe2B, and Fe2Al5 phases. Results showed that the boron-aluminide coating improved the corrosion resistance of R4 steel, acting as a barrier between the coating and the steel substrate and maintaining its corrosion resistance even under wear conditions. The findings suggest that the boron-aluminide coating can potentially improve the corrosion and tribocorrosion resistance of R4 steel in marine environments, making it a cost-effective alternative to passive materials for the thermochemical coating of low-alloy steels.

Analysis of S–N data for new and corroded mooring chains at varying mean load levels using a hierarchical linear model

Results from full scale fatigue tests of offshore mooring chains are analyzed. The data set includes new and used chains, tested at a variety of mean load levels. The used chains have been retrieved after operation offshore and include samples with varying surface conditions, ranging from as-new to heavily corroded. Based on a parameterized S–N curve intercept parameter, the effects of mean load and chain condition are estimated empirically by regression analysis. A hierarchical linear model is used, to account for and quantify correlations within subsets of the data. The choice of grouping criterion for the hierarchical model is discussed, and assessed based on the current data. Results show that the mean load and corrosion effects are both significant. Differences in the fatigue performance of new versus used chains are quantified and discussed.

Characteristics of Grade R4 Steel Manufactured by Ingot Casting and Used in the Production of Offshore Mooring Chains

The objective of this study is to characterise grade R4 steel manufactured by ingot casting that is used as the raw material for the production of offshore mooring chains. Rolled bars of grade R4 steel have been selected and metallographic analyses and micro hardness, tensile and Charpy V-notch impact tests performed. Tensile and Charpy V-notch impact tests have also been carried out on mooring links manufactured from the same grade R4 steel. Based on the results presented, the influence of the rolling reduction ratio of the bars and the influence of the ingot casting cooling and reheating process have been observed. Both factors have been found to increase product quality, particularly with respect to the central segregation and homogeneity of the rolled bars.

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.

Fatigue crack propagation analysis in offshore mooring chains and the influence of manufacturing residual stresses

The main objective of this work has been the validation of a methodology to assess fatigue crack propagation in offshore mooring chains under service conditions. For this purpose, analytical Stress Intensity Factor (SIF) solutions with step-by-step application of the Paris law and the Extended Finite Element Method (XFEM) implemented in the Abaqus® 2018 software have been studied. The fatigue crack propagation analysis is divided in two stages. First, a static analysis -pre-stretch and subsequent unloading-representative of the manufacturing process is performed. Next, the fatigue crack propagation simulation is performed under simplified loading conditions and taking into account the residual stresses induced in the previous analysis. Finally, analytical solutions, numerical simulations and experimental results have been compared. Results justify the use of the Extended Finite Element Method (XFEM) for fatigue crack propagation analysis in mooring chains.

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.

Digital image correlation for continuous mapping of fatigue crack initiation sites on corroded surface from offshore mooring chain

The full-field digital image correlation (DIC) technique was used to identify crack initiation and early crack growth on irregular, corroded surfaces from offshore mooring chain. Finite element analysis was paired with the DIC measurements, which made it possible to correlate initiation events with local stress concentrations due to corrosion pits. An initiation S-N curve was established based on this correlation. It included 14 crack initiation events from 4 specimens, in addition to statistical treatment of regions where no initiation occurred. The first cracks initiated at 13–24% of the total lifetimes of the specimens.

Mode I stress intensity factors for semi-elliptical fatigue cracks in curved round bars

Some cyclically loaded components such as mooring chains can develop fatigue cracks in locations where the shape of the part is equivalent to that of a curved or bent round bar. Here we consider a semi-elliptical crack growing from the surface of a curved round bar. This geometry can for example represent a chain link segment with a crack located at its inner- or outer radius. The surface crack can be either almond shaped, sickle shaped or straight-fronted. Stress intensity factors (SIFs) over the fronts of such crack geometries are in the present work investigated for several elementary mode I stress distributions. Finite element analysis and linear elastic fracture mechanics methods are used to develop semi-analytical solutions for the SIF at any point on the crack front. Effects of relative bar curvature on numerical results are demonstrated. Relative to otherwise identical cracks in straight bars, SIFs for cracks in the curved bars considered here are found to differ by up to 8%. With an offshore mooring chain model as a case example, the estimation of SIFs for cracks in a complex residual stress field is furthermore demonstrated using a cubic polynomial stress approximation.

Low-cycle fatigue assessment of offshore mooring chains under service loading

The integrity of mooring chains is essential to the safety of a range of offshore platforms. However, mooring line failures are occurring earlier than their design lives, with a high number of these failures occurring due to fatigue. Early in the fatigue life of the component fatigue initiation processes occur, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stress-strain responses of the material, leading to cyclic plastic damage accumulation. The traditional SN approach suggested by mooring standards does not consider these effects, and it is proposed that this lack of consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. This paper aims to develop a fatigue approach based on a critical plane multiaxial fatigue criterion for mooring chains that can consider the damage-induced by the cyclic plasticity and the mean load effect, to investigate the importance of incorporating low-cycle fatigue into the mooring chain life prediction. To develop the critical plane approach, the multiaxial stress-strain states are extracted for the critical plane at the fatigue hotspot from a finite element model of a mooring chain. This is then correlated with a fatigue life prediction provided by conventional fatigue design data. It uses a simulation of an FPSO as a case study to demonstrate the importance of low cycle fatigue, which shows that the mean load effect is significant in reducing the fatigue life for mooring chain applications, while the effect of fatigue damage-induced cyclic plasticity is limited. The fatigue damage accumulation predicted by the critical plane approach is significantly higher than that of the traditional SN approach and should be accounted for in mooring line design.

Calculation of stress intensity factors in offshore mooring chains

The main objective of this work has been the validation of a methodology to calculate the Stress Intensity Factors (SIFs) in prospective cracks of offshore mooring chains under service conditions. For this purpose, the analytic methods described in the BS7910 have been compared with those calculated by the conventional Finite Element Method (FEM) by means of contour integrals and by the Extended Finite Element Method (XFEM) implemented in the Abaqus 2018 software. First, an axially loaded cylinder has been studied in order to determine the correlation between different methods for a simple case, as well as to establish the most suited finite element methodology. Then, the real case of a mooring chain has been studied, including the residual stresses induced in the manufacturing process. Finally, numerical simulations and experimental results have been compared. Results justify the use of numerical methods, specifically the use of contour integrals, for the calculation of Stress Intensity Factors (SIFs) in offshore mooring chains.

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.

Out-Of-Plane Bending Stiffnesses in Offshore Mooring Chain Links Based on Conventional and Advanced Numerical Simulation Techniques

Out-Of-Plane Bending Stiffnesses in Offshore Mooring Chain Links Based on Conventional and Advanced Numerical Simulation Techniques

A robot for non-destructive testing weld inspection of offshore mooring chains

Welding flaw detection is a key step in manufacturing many components. In offshore chains, every link is manufactured from a steel bar that is bent and the ends joined by flash butt welding. Ultrasonic inspection of the welded area is required for classification. Defects, if any, are parallel to the welded area, which do not favour detection by manual inspection with 45° beams, as per usual practice . This article reports on CIRUS, a robot developed for automatic inspection of the weld area using a combination of pulse-echo and pitch-catch ultrasonic testing. The robot kinematic structure includes global positioning, local positioning and inspection subsystems, and each subsystem design is described in detail. A data acquisition system processes ultrasonic inspection results and provides visual information for the inspector as well as traceability for quality manufacturing.

Study on prediction of stresses by out-of-plane and in-plane bending moments in offshore mooring chain

ABSTRACTA fatigue evaluation due to an out-of-plane bending (OPB) and in-plane bending (IPB) moments has been a key engineering issue in the mooring design of a floating offshore unit. Choung and Han recently proposed a numerical approach to predict the fatigue damage which was dependent on the maximum principal stresses at final load step without using linearly superposed stresses at each load step for a hotspot. The proposed approach does not use the nominal OPB/IPB moments and the nominal OPB/IPB stresses therefrom, but it is important to compare them for verification of the proposed approach. Based on the mid-span elastic deflection, the nominal OPB/IPB moments are shown in this study. Since the elastic deflection-based nominal moment increased as the OPB tension angle develops, it was proven that the assumption of the elastic deflection was not suitable. By introducing BV guideline formulas, we show the OPB/IPB moments change along the OPB/IPB tension angles.