Mooring Chain Research Articles

Numerical Investigation of Hydrodynamic Performance of a Light Buoy With Different Mooring Configurations in Regular Waves

Abstract The single-moored light buoys employed in the lower reaches of the Yangtze River play an important role in indicating ship navigation and ensuring safety. To clarify the interaction between waves and floating buoys moored to the riverbed, this paper applies a numerical approach to investigate the wave-induced motion performance of a light buoy and reveal the effects of different mooring configurations to extend its service life. An open-source smoothed particle hydrodynamics (SPH)-based numerical model named dualsphysics coupled with MoorDyn is implemented. This coupled model is validated by simulating the motion of a moored rectangle buoy in regular waves, and compared with experimental data and the numerical results of reef3d code, a mesh-based computational fluid dynamics (CFD) model. The validation results show that the coupled model reproduces experimental data well and has a smaller deviation in comparison with reef3d. Then, the coupling model is applied to simulate the hydrodynamic performance of the real-size light buoy employed in Yangtze River and investigate the effects of encounter angle between wave propagation direction and mooring chain. The results demonstrate the capability of this coupled mooring model to simulate the motion of a moored buoy in regular waves, and this numerical approach will be extended to simulate the light buoy in more complex environments such as irregular waves, flow or extreme weather in further work.

Fracture mechanics assessment for mooring chain links tensioned over a curved surface

A fracture mechanics (FM) based investigation on the effects of curved surfaces on fatigue lives of mooring chain links tensioned over curved surfaces is conducted in this paper. Three typical curved surfaces including ungrooved surface, shallow grooved surface, and deep grooved surface are investigated. The mooring chain links tensioned over an ungrooved surface are considered under the combined action of in-plane bending (IPB), out-of-plane bending (OPB), torque, and tension. The chain links supported by the bottom of a shallow groove surface are assumed to be influenced by extra IPB. The chain links laying on the upper surface of a deep grooved surface are considered to be subjected to OPB. The results show that fatigue lives of mooring chain links tensioned over the three curved surfaces are significantly shorter than those of mooring chain links subjected to pure tension. Fatigue lives of mooring chain links tensioned over the three curved surfaces would decrease with the decrease of the diameter of the curved surface. For mooring chain links tensioned over an ungrooved surface, the fatigue lives of mooring chain links would decrease with the increase of the angle between the transverse axis of mooring chain link and the surface.

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.

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.

Water Depth Variation Influence on the Mooring Line Design for FOWT within Shallow Water Region

The objective of this paper was to present the modeling and optimization of mooring lines for floating offshore wind turbines (FOWT) located in various water depths from 50 m to 100 m in Taiwan western offshore areas. A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCwind platform with the National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbine. The mooring lines proposed consist of a catenary mooring with studless chains. Three nominal sizes of the mooring chain links are taken into account with diameters of 95 mm, 115 mm and 135 mm. According to this configuration, a total of five mooring designs for different water depths (i.e., 50 m, 60 m, 70 m, 80 m, 100 m) are analyzed according to the rules and regulations of the two certification institutions, Det Norske Veritas (DNV) and American Petroleum Institute (API). Considering ultimate limit state (ULS), fatigue limit state (FLS) and maximum operating sea state (MOSS) based on a typhoon with a 50-year return period and current with a 10-year return period, 25-year design life, as well as 1-year return period, respectively, long-term predictions of breaking strength, fatigue and stability are performed. The software OrcaFlex version 10.3 d is used to simulate and design the mooring lines. The obtained results show that the shallow mooring design of 50 m water depth case presents the heaviest chains among the other water depths, increasing their mooring costs. On the other hand, the 100 m water design has much longer mooring lines, making this parameter the cost driving one. Thus, the minimum mooring cost range is from 60 m to 80 m water depth.

Feature recognition and strength estimation of chain links by 3D inspections

In the past mooring chain failure probabilities have been much higher than the design failure probabilities. The high failure rates are attributed to prolonged exposure to various degradation mechanisms in harsh offshore conditions. In addition to understanding failure mechanisms, researchers have stressed on reducing the risk of failures through regular and reliable inspections. Current inspection techniques involve both in-air and in-water inspections. However, current in-air inspections are time consuming and expensive, while in-water inspection techniques are not yet reliable. The present paper proposes a condition monitoring and strength estimation methodology based on three-dimensional (3D) scans of links. The study first identifies and estimates changes in simple geometrical features of chain links using finite element (FE) modeling for condition monitoring. The methodology has two stages: 3D general imaging (3D-GI) to identify degraded chain links using feature monitoring, followed by 3D detailed imaging (3D-DI) of the identified weak links. The weak links 3D point clouds could be further converted to solid FE models for residual strength estimation by employing a suitable material degradation model. The geometrical feature monitoring methodology is verified by 3D scans of an undeformed and a deformed studded link.

The motion response and hydrodynamic performance comparisons of the new subsea suspended manifold with two mooring scenarios

The subsea manifold as the hub oil and gas transportation equipment is a critical part for the layout optimization of subsea production system and reducing the number of pipelines. The traditional subsea manifold is fixed on seabed by its foundation, so traditional manifold system is hard to be discarded and recovered although it has many advantages and is applied widely in the deep water development scenario. In this paper, a new subsea suspended manifold concept is proposed to discuss its feasibility by the hydrodynamic performance state in vertical tension and catenary mooring methods. A mathematical model is established to solve the chain tension distribution and displacement of the subsea suspended manifold. Meanwhile, the motion response of the subsea suspended manifold under different influence factors, tension distribution of the mooring chains and the flexible jumpers in two mooring schemes are simulated, respectively, through OrcaFlex software. The research results will provide an important reference for the optimization of the preliminary scenario design of the subsea suspended manifold mooring system.

Development of interlink wear estimation method for mooring chain of floating structures: Validation and new approach using three-dimensional contact response

Long-term operation of mooring systems is one of the challenging issues of floating structures such as floating offshore wind turbines (FOWTs). For integrity assessment, fatigue and its affecting factors have generated considerable recent research interest as the occurrence of a large number of mooring chain failures at a high rate has been reported. By contrast, only few studies on the effect of nonuniform volume loss of mooring chain links due to wear can be found because of difficulties to estimate wear amounts quantitatively. Considering this issue, in this paper, validation of the quantitative interlink wear estimation method is investigated by applying to a spar-type floating structure. Firstly, the method is presented which consists of the material test, derivation of an interlink wear estimation formula with FE analysis, and calculation of mooring chain response with coupled dynamic analysis using a mass-spring model. To improve insufficient accuracy due to the mass-spring model around a clump weight and the touchdown point, the method is further modified by using a 3-D rigid-body link model. The estimation results and comparison show that the modified method distinguishing between rolling and sliding can calculate the interlink wear amount closer to the chain diameter measurements and more reasonable than the method using the conventional mass-spring model.

Value of information of in situ inspections of mooring lines

Mooring systems that are used to secure position keeping of floating offshore oil and gas facilities are subject to deterioration processes, such as pitting corrosion and fatigue crack growth. Past investigations show that pitting corrosion has a significant effect on reducing the fatigue resistance of mooring chain links. In situ inspections are essential to monitor the development of the corrosion condition of the components of mooring systems and ensure sufficient structural safety. Unfortunately, offshore inspection campaigns require large financial commitments. As a consequence, inspecting all structural components is unfeasible. This article proposes to use value of information analysis to rank identified inspection alternatives. A Bayesian Network is proposed to model the statistical dependence of the corrosion deterioration among chain links at different locations of the mooring system. This is used to efficiently update the estimation of the corrosion condition of the complete mooring system given evidence from local observations and to reassess the structural reliability of the system. A case study is presented to illustrate the application of the framework.

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.

In-plane and out-of-plane bending moments and local stresses in mooring chain links using machine learning technique

This paper proposes an efficient approach based on a machine learning technique to predict the local stresses on mooring chain links. Three-link and multi-link finite element analyses were conducted for a target chain link of D107 with steel grade R4; 24,000 and 8000 analyses were performed, respectively. Two serial Artificial Neural Network (ANN) models based on a deep multi-layer perceptron technique were developed. The first ANN model corresponds to multi-link analyses, where the input neurons were the tension force and angle and the output neurons were the interlink angles. The second ANN model corresponds to the three-link analyses with the input neurons of the tension force, interlink angle, and the local stress positions, and the output neurons of the local stress. The predicted local stresses for the untrained cases were reliable compared to the numerical simulation results.

Numerical modeling of the interaction between submerged floating tunnel and surface waves

This paper presents a numerical model for analyzing the nonlinear interaction between the moored Submerged Floating Tunnel (SFT) and surface waves. The mechanics model of the moored floating body driven by wave forces is built, and an efficient mesh update method is employed to dynamically configure the computational meshes solving the Navier-Stokes equations for viscous and incompressible free surface flows with the volume of fluid (VOF) method. Two laboratory experiments are used for validating the numerical model in terms of surface elevations, motion responses and mooring forces of the SFT, indicating the proposed model is capable of simulating the dynamics of the moored floating body under the wave action. This hydrodynamic model is then utilized to simulate the wave-structure interaction of the prototype SFT designed for Funka Bay, Hokkaido located in Japan. A total of 49 cases are designed for the numerical simulation to investigate the characteristics of the wave-tunnel interaction for different hydrodynamic parameters, including wave height, wave period, immersion depth and buoyancy-weight ratio (BWR). The numerical experiments not only shed light on the mooring forces, as well as pitch, sway and heave responses of the SFT in different wave conditions, but also provide guidance for the choice of BWR in engineering design. A medium value of BWR is suggested to be suitable, which is useful for avoiding the happening of snap forces in mooring chains and preventing SFT from experiencing large movement under external forces in severe wave conditions. As the correlation between the motion responses and BWR is not merely linear or quadratic but parabolic with a peak value, the design of BWR should avoid the case where peak motion responses of SFT happen.

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.

Numerical investigations on load transfer of mooring line considering chain–seabed dynamic interaction

Taut or semi-taut mooring lines are often used in deep-sea area to connect the floating structures to the anchors in the seabed. In previous studies, the quasi-static analysis method was adopted to assess the load transfer of the mooring line. However, the inertia effect of mooring line and chain–seabed dynamic interaction cannot be neglected in practice. In this paper, the soil strain rate effect was incorporated in the chain–seabed dynamic analyses, and the load transfer mechanism of mooring line was investigated based on the proposed model. The numerical model was firstly verified with the quasi-static analysis results and experimental data under dynamic condition, and then the contributions of different forces to the variations of line tension were evaluated. It is found that two factors (inertia force and soil resistance) have a great influence on the mooring line tension variation. The inertial effect is dominated under high-frequency condition, and the maximum line tension at the fairlead is larger than that under static condition. However, the tension at the pad-eye under dynamic condition always has a certain increase. The reason is that partial reverse soil resistance under dynamic condition increases the tension at the pad-eye.

Numerical investigation of dynamic responses and mooring forces of submerged floating tunnel driven by surface waves

Based on Navier–Stokes equations, a numerical model for studying the dynamic responses and mooring forces of the moored Submerged Floating Tunnel (SFT) driven by surface waves is presented in this paper. The mechanics models of the vertically and inclinedly moored floating body under wave forces are built, and the overset meshing method is employed to dynamically configure the computational meshes. Two laboratory experiments are used for validating the numerical model in terms of motion responses and mooring forces of the SFT, indicating the proposed model is capable of accurately simulating the instantaneous position of the body under the wave action. This hydrodynamic model is then utilized to simulate the wave–structure interaction of the prototype SFT designed for Qiongzhou Strait located between Mainland China and Hainan Island. The effects of the fundamental structure parameter, or the inclined mooring angle (IMA), on the dynamic responses of SFT are analyzed. The numerical experiments not only shed light on the mooring forces, as well as pitch, sway and heave responses of the SFT with various values of IMA, but also provide guidance for the choice of IMA in engineering design. The range of IMA is separated into five zones, and Zone 2 is regarded as the best choice for the design of IMA for both motion displacements and mooring forces are relatively small in this zone. Zone 3 is considered to be the worst choice as not only are motion responses of SFT severe in this zone, but also the mooring chains are at the risk of going slack under severe wave conditions.

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.

Experimental evaluation of a mooring system simplification methodology for reducing mooring lines in a VLFS model testing at a moderate water depth

The scale model experiment for typical offshore floating structures with increasingly excessive mooring lines is usually complex and difficult to set up, which often results in low reliability of the experimental results. In the previous paper, Liang et al. (2019) proposed a mooring system simplification methodology for the scale model experiment based on the equivalent of the vessel/mooring coupled dynamics for vessel with original and simplified mooring systems and demonstrated its effectiveness by numerical studies In this paper, the feasibility of the mooring system simplification methodology is further evaluated by a model test for a very large floating structure’s (VLFS’s) mooring system comprising 20 mooring chains. A 10 chains simplified mooring system is obtained with the correlation coefficients of coupled vessel/mooring dynamics larger than 0.9. Thereafter, the equivalence of the original and simplified mooring systems is validated by performing the decay test, white noise test, regular and irregular wave tests. The coincidence of the model test results, including natural period, damping coefficient, response amplitude operator (RAO), motion power spectral density and motion time series of the VLFS model, confirms the equivalence of the simplified mooring system with the original mooring system and indicates that the mooring system simplification methodology is effective in designing the simplified mooring system to replace the original mooring system in a scale model experiment.

Selection of appropriate numerical models for modelling the stresses in mooring chains

Mooring chains are key components for floating platforms. The failure of these components can be catastrophic in terms of the economic and environmental impacts, especially when dealing with the potential failure of FPSOs. However, mooring failures have been regularly occurring much earlier in their service lives than expected, with almost 50% of the reported failures happening in the first 3 years of 20-year design lives. Although the operating stresses play a major role in determining the failure mechanisms of mooring chains, the methods of predicting the operating stresses in mooring chains vary in the openly available literature, and the accuracy of these different numerical methods for predicting types of mooring failures is unknown. There is currently little evidence provided for when one model is appropriate for a particular scenario. Therefore, this paper benchmarks the different available methods for modelling mooring chains under tension, including FE models found in the literature. These models are calibrated and verified against previous studies and compared with experiments and a developed FE explicit model. There is a significant difference in the way that the numerical models behave, which are discussed in terms of their applicability and limitations in modelling mooring chains. The results of this study show that the explicit modelling approach should be utilised for accurate assessment of mooring lines, as it provides the most realistic response, with a substantial reduction in the computational cost and without any convergence problems.

Investigation on Temper Embrittlement of TS1100 MPa Grade Ultra-High Strength Steel

Temper embrittlement is one of the major challenges encountered during the heat treatment of ultra-high strength steels. In this work, the microstructural evolution and mechanical properties during the austenite region quenching and tempering (Q-T) and the intercritical quenching and tempering (IQ-T) processes in a tensile strength (TS) 1100 MPa grade steel targeted for the manufacturing of a mooring chain were investigated. The temper embrittlement presented in the Q-T sample was attributed to the carbide network decorating the prior austenite grain and martensitic lath boundaries. In contrast, the IQ-T sample, where intercritical holding before quenching at the temperature of 12 °C lower than Ac3 was applied, exhibited a reasonable CVN impact energy of 188 J at − 20 °C with a slightly reduced strength. The ferrite formed during intercritical holding not only added extra interphase boundaries, but also refined the substructure of the quenched martensite. These increased boundary areas provided more nucleation sites for carbide precipitation during tempering. Meanwhile, the refined martensitic substructure promoted the diffusion of Cr, encouraging the fast coarsening of Cr-rich carbides. The formation of a carbide network was eventually prevented in the IQ-T sample. Thus, the IQ-T process was an effective approach to eliminate temper embrittlement caused by the carbide network.

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.