Fatigue Design Methodology for Large Mooring Chains Subjected to Out-of-Plane Bending

2014 ◽  
Author(s):  
Tom Lassen ◽  
Jan Aarsnes ◽  
Einar Glomnes
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Barents Sea ◽  
Weather Condition ◽  
Design Criterion ◽  
Fatigue Damage Accumulation ◽  
Out Of Plane ◽  
Plane Bending ◽  
Mooring Chain

The present paper presents a fatigue life prediction method for chains subjected to tension and Out-of-Plane-Bending (OPB). The investigation was carried out on a high strength mooring chain segment with a diameter of 165 mm and a steel quality R5. Such chains are used in mooring systems for large Floating Production Storage and Offloading (FPSO) units in harsh weather condition. Due to the fact that the mooring chains are pre-tensioned the wave induced displacements will introduce secondary bending effects about the weak axis in the chain links, particularly in the bilge hang-off area. In such chains both conventional tension fatigue and fatigue damage accumulation due to Out-of-Plane-Bending (OPB) have to be analyzed. Results from full scale behavior tests, Finite Element Analyses and a case study with simulation of in-service loading are included in the present study. Finally, fatigue life predictions and an operational strategy are presented for a case study for a floater in the Barents Sea. With a target service life of 30 years, the endurance with respect to fatigue damage is a major design criterion for the mooring chain in this case.

Calculation Methodology of Out of Plane Bending of Mooring Chains

2007 ◽  
Author(s):  
E. ter Brake ◽  
J. van der Cammen ◽  
R. Uittenbogaard
Keyword(s):  
Fatigue Damage ◽  
Bending Moment ◽  
Local Stress ◽  
Mooring Line ◽  
Empirical Formulas ◽  
Out Of Plane ◽  
Plane Bending ◽  
Chain Links ◽  
Mooring Chain

Recently, the phenomenon of out-of-plane bending (OPB) fatigue of mooring chain links emerged as an important parameter in the fatigue assessment of mooring lines. Vessel motions induce a bending moment at the top chain of a mooring line. This bending moment induces alternating local stresses in the link and thus contributes to fatigue damage of those links. High pretension mooring systems are particularly sensitive to this phenomenon, since a small vessel motion combined with a high tension results in a relatively large bending moment in the upper mooring chain links. In mooring systems with high pre-tensions, this damage is of much greater magnitude than the fatigue damage induced by tension-tension loading only. An extensive study has been executed to investigate the fatigue life of mooring chain in deep water systems. This paper presents the calculation procedure to include the effects of local chain bending in the overall mooring line fatigue analysis. It was concluded that despite the complexity of the OPB issue, it is a phenomenon that can be incorporated in the mooring analyses by means of numerical procedures. The developed method is based on extensive Finite Element Method (FEM) analyses of chain links. Models of multiple chain links have been used that take into account the plastic-elastic properties of the material and contact friction between chain links. The FE models are used to derive empirical relations, between load angles, interlink angles, bending moments and stresses. These calculations were made for different combinations of line tension, interlink friction and chain size. The results were stored in a database to gain insight in the out-of-plane bending phenomenon. This database provides empirical formulas to lead to the local stress in different points on a chain link. These empirical formulas are used to translate floater (vessel or buoy) motions into local stress variations and fatigue damages in chain links. The long-term motion behaviour of the floater is known, the long term tension and bending stress ranges can be obtained and thus a fatigue damage of the chain links can be calculated.

Evaluating the Coupledness of the Aerodynamics and Hydrodynamics on the Estimation of Fatigue Damage Equivalent Load for a Floating Offshore Wind Platform

2018 ◽  
Author(s):  
Samuel Kanner ◽  
Bingbin Yu
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Environmental Conditions ◽  
Offshore Wind ◽  
Coupled Analysis ◽  
Offshore Wind Turbine ◽  
Wave Conditions ◽  
Equivalent Load ◽  
The Waves

In this research, the estimation of the fatigue life of a semi-submersible floating offshore wind platform is considered. In order to accurately estimate the fatigue life of a platform, coupled aerodynamic-hydrodynamic simulations are performed to obtain dynamic stress values. The simulations are performed at a multitude of representative environmental states, or “bins,” which can mimic the conditions the structure may endure at a given site, per ABS Floating Offshore Wind Turbine Installation guidelines. To accurately represent the variety of wind and wave conditions, the number of environmental states can be of the order of 103. Unlike other offshore structures, both the wind and wave conditions must be accounted for, which are generally considered independent parameters, drastically increasing the number of states. The stress timeseries from these simulations can be used to estimate the damage at a particular location on the structure by using commonly accepted methods, such as the rainflow counting algorithm. The damage due to either the winds or the waves can be estimated by using a frequency decomposition of the stress timeseries. In this paper, a similar decoupled approach is used to attempt to recover the damages induced from these coupled simulations. Although it is well-known that a coupled, aero-hydro analysis is necessary in order to accurately simulate the nonlinear rigid-body motions of the platform, it is less clear if the same statement could be made about the fatigue properties of the platform. In one approach, the fatigue damage equivalent load is calculated independently from both scatter diagrams of the waves and a rose diagram of the wind. De-coupled simulations are performed to estimate the response at an all-encompassing range of environmental conditions. A database of responses based on these environmental conditions is constructed. The likelihood of occurrence at a case-study site is used to compare the damage equivalent from the coupled simulations. The OC5 platform in the Borssele wind farm zone is used as a case-study and the damage equivalent load from the de-coupled methods are compared to those from the coupled analysis in order to assess these methodologies.

Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) Static Test Program and OPB Interlink Stiffness

2016 ◽  
Author(s):  
Lucile Rampi ◽  
Fata Dewi ◽  
Michel Francois ◽  
Arnaud Gerthoffert ◽  
Pedro Vargas
Keyword(s):  
Fatigue Loading ◽  
Time Frame ◽  
Small Time ◽  
Static Test ◽  
Fatigue Design ◽  
Out Of Plane ◽  
Bending Stresses ◽  
Plane Bending ◽  
Mooring Chain ◽  
Fatigue Mechanism

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the quasi static OPB stiffness measurement campaign and the post processing work to derive the OPB interlink stiffness.

scholarly journals Fatigue Damage Accumulation Modeling of Metals Alloys under High Amplitude Loading at Elevated Temperatures

Metals ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 1030 ◽  
Author(s):  
Jarosław Szusta ◽  
Andrzej Seweryn
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Damage Accumulation ◽  
Elevated Temperatures ◽  
Low Cycle Fatigue ◽  
High Amplitude ◽  
Metal Alloys ◽  
Damage State ◽  
State Variable ◽  
Fatigue Damage Accumulation

This article presents an approach related to the modeling of the fatigue life of constructional metal alloys working under elevated temperature conditions and in the high-amplitude load range. The article reviews the fatigue damage accumulation criteria that makes it possible to determine the number of loading cycles until damage occurs. Results of experimental tests conducted on various technical metal alloys made it possible to develop a fatigue damage accumulation model for the LCF (Low Cycle Fatigue) range. In modeling, the material’s damage state variable was defined, and the damage accumulation law was formulated incrementally so as to enable the analysis of the influence of loading history on the material’s fatigue life. In the proposed model, the increment of the damage state variable was made dependent on the increment of plastic strain, on the tensile stress value in the sample, and also on the actual value of the damage state variable. The model was verified on the basis of data obtained from experiments in the field of uniaxial and multiaxial loads. Samples made of EN AW 2024T3 aluminum alloy were used for this purpose.

Fracture mechanics analysis for mooring chain links subjected to out-of-plane bending (OPB)

2020 ◽  
Vol 71 ◽  
pp. 102740
Author(s):  
Xutian Xue ◽  
Nian-Zhong Chen ◽  
Yongchang Pu ◽  
Xifeng Gao
Keyword(s):  
Fracture Mechanics ◽  
Mechanics Analysis ◽  
Out Of Plane ◽  
Plane Bending ◽  
Chain Links ◽  
Mooring Chain

Experimental and Numerical Investigation of Torsion Fatigue of Bearing Steel

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
John A. R. Bomidi ◽  
Nick Weinzapfel ◽  
Trevor Slack ◽  
Sina Mobasher Moghaddam ◽  
Farshid Sadeghi ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Crack Initiation ◽  
Normal Stress ◽  
Failure Mechanism ◽  
Fatigue Damage ◽  
Fatigue Failure ◽  
Damage Mechanics ◽  
Damage Model ◽  
Bearing Steel ◽  
Fatigue Damage Accumulation

This paper presents the results of torsion fatigue of widely used bearing steels (through hardening with bainite, martensite heat treatments, and case hardened). An MTS torsion fatigue test rig (TFTR) was modified with custom mechanical grips and used to evaluate torsional fatigue life and failure mechanism of bearing steel specimen. Tests were conducted on the TFTR to determine the ultimate strength in shear (Sus) and stress cycle (S-N) results. Evaluation of the fatigue specimens in the high cycle regime indicates shear driven crack initiation followed by normal stress driven propagation, resulting in a helical crack pattern. A 3D finite element model was then developed to investigate fatigue damage in torsion specimen and replicate the observed fatigue failure mechanism for crack initiation and propagation. In the numerical model, continuum damage mechanics (CDM) were employed in a randomly generated 3D Voronoi tessellated mesh of the specimen to provide unstructured, nonplanar, interelement, and inter/transgranular paths for fatigue damage accumulation and crack evolution as observed in micrographs of specimen. Additionally, a new damage evolution procedure was implemented to capture the change in fatigue failure mechanism from shear to normal stress assisted crack growth. The progression of fatigue failure and the stress-life results obtained from the fatigue damage model are in good agreement with the experimental results. The fatigue damage model was also used to assess the influence of topological microstructure randomness accompanied by material inhomogeneity and defects on fatigue life dispersion.

Fatigue Damage Accumulation Prediction for Combined Sine and Random Stresses

1988 ◽  
Vol 31 (3) ◽  
pp. 53-63
Author(s):  
Ronald Lambert
Keyword(s):  
Fatigue Life ◽  
Closed Form ◽  
Fatigue Damage ◽  
Damage Accumulation ◽  
Structural Elements ◽  
Numerical Examples ◽  
Stress Situation ◽  
Fatigue Damage Accumulation ◽  
Special Cases ◽  
Parameters Of Interest

Simple closed-form expressions have been derived to predict fatigue life, damage accumulation, and other fatigue parameters of interest for structural elements with combined sinusoidal (sine) and narrowband Gaussian random stresses. These equations are expressed in common engineering terms. The sine and random only stress situations are special cases of the more general combined sine/random stress situation. They also have application for establishing vibration workmanship screens. Numerical examples are also included.

Out-of-Plane Bending Testing of Chain Links

2005 ◽  
Author(s):  
Cecil Melis ◽  
Phillipe Jean ◽  
Pedro Vargas
Keyword(s):  
Empirical Relationship ◽  
Chain Link ◽  
Out Of Plane ◽  
Bending Stresses ◽  
Test Frame ◽  
Scale Test ◽  
Plane Bending ◽  
A Chain ◽  
Chain Links ◽  
Mooring Chain

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life or 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A full scale test frame was constructed that has the capacity of applying inter-link rotation to a pre-tensioned chain. Although the test frame limits the number of links that can be tested together as a chain, a significant amount of testing was performed for the following chain sizes: 1. 81 mm Studded Grade R3S. 2. 107 mm Studdless Grade RQ3. 3. 124 mm Studless Grade R4. 4. 146 mm Studless Grade RQ4. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper the OPB mechanism is explained, and the test frame and results are presented. An empirical relationship is found to predict the OPB stresses in the chain links as a function of pretension and inter-link rotation. The OPB stress relationship obtained was applied to the failed mooring chain of the off-loading buoy with reasonable agreement. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

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