Fatigue Life Assessment of Offshore Patrol Vessel

2009 ◽  
Author(s):  
Asokendu Samanta ◽  
P. Kurinjivelan
Keyword(s):  
Fatigue Life ◽  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Design Guidelines ◽  
Life Assessment ◽  
Strength Analysis ◽  
Hot Spot Stress ◽  
Critical Locations

Fatigue is a phenomenon, which needs to be considered in the present day’s vessel design. The welded joints are particularly affected by the fatigue damage due to high stress concentrations caused by the metallurgical discontinuities present in the weld. For oil tankers and bulk carriers adequate guidelines for the fatigue strength assessment have been established by the classification societies. But for navy vessel, like offshore patrol vessel, the design guidelines for the fatigue strength analysis are not widely available. In the present paper, an attempt has been made to calculate the fatigue life of offshore patrol vessel (OPV). In general five stages of work is involved in calculating fatigue life of any ship structure. These are, load calculation, nominal and hot spot stress computation, long-term stress distribution, selection of S-N curve and the fatigue damage calculation. In the present study, the wave loads are obtained by the rule based estimation. The finite element analysis with the submodeling approach has been used to get the hot spot stress at critical locations. The two-parameter Weibull curve has been used to get the long-term distribution of stress. And at the end, the fatigue damage and the fatigue life have been computed using the Palmgren-Miner linear cumulative damage theory at the critical locations of the vessel.

Calibration of Long-Term Time-Domain Load Generation for Fatigue Life Assessment of Offshore Wind Turbine

2017 ◽  
Author(s):  
Bryan Nelson ◽  
Yann Quéméner ◽  
Tsung-Yueh Lin ◽  
Hsin-Haou Huang ◽  
Chi-Yu Chien
Keyword(s):  
Fatigue Life ◽  
Wind Turbine ◽  
Fatigue Damage ◽  
Time Domain ◽  
Hot Spot ◽  
Life Assessment ◽  
Offshore Wind ◽  
Wind Loads ◽  
Offshore Wind Turbine

This study evaluated, by time-domain simulations, the fatigue life of the jacket support structure of a 3.6 MW wind turbine operating in Fuhai Offshore Wind Farm. The long-term statistical environment was based on a preliminary site survey that served as the basis for a convergence study for an accurate fatigue life evaluation. The wave loads were determined by the Morison equation, executed via the in-house HydroCRest code, and the wind loads on the wind turbine rotor were calculated by an unsteady BEM method. A Finite Element model of the wind turbine was built using Beam elements. However, to reduce the time of computation, the hot spot stress evaluation combined FE-derived Closed-Form expressions of the nominal stresses at the tubular joints and stress concentration factors. Finally, the fatigue damage was assessed using the Rainflow Counting scheme and appropriate SN curves. Based on a preliminary sensitivity study of the fatigue damage prediction, an optimal load setting of 60-min short-term environmental conditions with one-second time steps was selected. After analysis, a sufficient fatigue strength was identified, but further calculations involving more extensive long-term data measurements are required in order to confirm these results. Finally, this study highlighted the sensitivity of the fatigue life to the degree of fluctuation (standard deviation) of the wind loads, as opposed to the mean wind loads, as well as the importance of appropriately orienting the jacket foundations according to prevailing wind and wave conditions.

scholarly journals Improvement of trawler hull structure under condition of ensuring fatigue strength

2021 ◽  
Vol 4 (7(112)) ◽  
pp. 50-59
Author(s):  
Leontii Korostylov ◽  
Dmytro Lytvynenko ◽  
Hryhorii Sharun ◽  
Ihor Davydov
Keyword(s):  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Bending Moment ◽  
Theoretical Method ◽  
Corrosive Wear ◽  
Hot Spot Stress ◽  
Hull Structure ◽  
Internal Forces

The structure of the hull of the project 1288 trawler in a region of fore hold was improved to ensure fatigue strength of assemblies of the intersection of main frames with the second bottom. To this end, a study of the fatigue strength of these assemblies was carried out for the original side structure and two versions of its modernization. Values of internal forces at the points of appearance of fatigue cracks in the compartment have been determined for three design versions of the side. It was found that the greatest forces act in the middle of the fore half of the compartment. Calculations of parameters of the long-term distribution of magnitudes of ranges of total equivalent operating stresses according to the Weibull law in the points of occurrence of fatigue cracks for different design versions of the side grillage have been performed. These parameters were determined for the middle of the fore hold of the vessel and for the areas in which maximum values of bending moment ranges are in effect with and without corrosive wear. Values of total fatigue damage and durability of the studied assemblies were determined. Calculations were carried out by nominal stress method, hot spot stress method, and experimental and theoretical method. It was shown that in order to ensure fatigue strength of the assembly under consideration, it is necessary to extend the intermediate frames of the original version of the side structure to the level of the second bottom fixing them to the deck. It is also necessary to attach a cargo platform to the side thus reducing the frame span. As a result, the level of fatigue damage over 25 years of operation will decrease by about 3.5 times. As it was found, approximate consideration of the slamming effect does not significantly increase the amount of fatigue damage to the assembly. The results of the development of recommendations for modernization of the side structure can be implemented both on ships of the 1288 project and on other ships with a transverse side framing system.

Fatigue strength assessment of ship structures accounting for a coating life and corrosion degradation

2016 ◽  
Vol 7 (2) ◽  
Author(s):  
Yordan Garbatov
Keyword(s):  
Finite Element ◽  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Limit State ◽  
State Function ◽  
Fatigue Reliability ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Fatigue Limit State

Purpose Fatigue strength and reliability assessment of complex double hull oil tanker structures, based on different local structural finite element approaches, is performed accounting for the uncertainties originating from load, nominal stresses, hot spot stress calculations, weld quality estimations and misalignments and fatigue S-N parameters including the correlation between load cases and the coating life and corrosion degradation. Design/methodology/approach Ship hull wave-induced vertical and horizontal bending moments and pressure are considered in the analysis. Stress analyses are performed based on the nominal, local hot spot and notch stress approaches. A linear elastic finite element analysis is used to determine the stress distribution around the welded details and to estimate structural stresses of all critical locations. Fatigue damage is estimated by employing the Palmgren-Miner approach. The importance of the contribution of each random variable to the uncertainty of the fatigue limit state function is also estimated. The probability of fatigue damage of hot spots is evaluated taking into account random coating life and corrosion wastage. Fatigue reliability, during the service life, is modelled as a system of correlated events. Findings The fatigue analysis showed that the fatigue damage at the hotspot, located at the flange of the stiffener close to the cut-out, is always highest in the cases of the structural hot spot stress and effective notch stress approaches, except for the one of the nominal stress approach. The sensitivities of the fatigue limit state function with respect to changes in the random variables were demonstrated showing that the uncertainty in the fatigue stress estimation and fatigue damage are the most important. Fatigue reliability, modelled as a parallel system of structural hot spots and as a serial system of correlated events (load cases) was evaluated based on the Ditlevsen bounds. As a result of the performed analysis, reliability and Beta reliability indexes of lower and upper bounds were estimated, which are very similar to the ones adopted for ultimate strength collapse as reported in literature. Originality/value This paper develops a very complex fatigue strength and reliability assessment model for analysing a double hull oil tanker structure using different local structural finite element approaches accounting for the associated uncertainties and the correlation between load cases and the coating life and corrosion degradation. The developed model is flexible enough to be applied for analysing different structural failure modes.

Automated Identification of Critical Tubular Joints of Offshore Jacket Structure by Deterministic Fatigue Analysis

2017 ◽  
Author(s):  
Shrikarpagam Dhandapani
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Optimized Design ◽  
Base Shear ◽  
Hot Spot Stress ◽  
Environmental Loads ◽  
Tubular Joints

Fatigue occurs in structures due to the stresses from cyclic environmental loads. Offshore environmental loads being highly cyclic and recurring in nature, fatigue analysis with high degree of accuracy is required for reliable and optimized design of offshore structures. The main aim of this paper is to automate the process of identification of fatigue critical tubular joints of an offshore jacket structure using deterministic fatigue analysis with emphasis on the Hot Spot Stress Range (HSSR), an important measure in estimating fatigue damage, calculated using three different approaches for each tubular joint. The first approach determines HSSR at the time of maximum base shear of the jacket, the second, by calculating the difference between maximum and minimum Hot Spot Stress (HSS) and the third, at all time-instants of the wave cycle. Thus fatigue damage and fatigue life of the tubular joints are estimated using the highest HSSR value and the joints with lower fatigue life are identified as fatigue sensitive joints. This ensures effective identification of critical tubular joints of the offshore jacket structure which needs detailed investigation or redesign for fatigue. The deterministic approach discussed in this paper is applicable to large jackets which contains more number of tubular joints where sophisticated fatigue assessment at the preliminary stage is computationally intensive and manual identification of fatigue critical joints is laborious.

Fatigue Life Assessment for Variable Strain in a Mixing Tee by Use of Effective Strain Range

2021 ◽  
Author(s):  
Koji Miyoshi ◽  
Masayuki Kamaya
Keyword(s):  
Fatigue Life ◽  
Crack Growth ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Effective Strain ◽  
Strain Range ◽  
Life Assessment ◽  
Miner’S Rule ◽  
Loading Sequence ◽  
Miner's Rule

Abstract Mixing flow causes fluctuations in fluid temperature near a pipe wall and may result in fatigue crack initiation. Movement of the hot spot, at which the pipe inner surface was heated by hot flow from the branch pipe, causes thermal stress fluctuations. In this study, the effect of the loading sequence on thermal fatigue in a mixing tee was investigated. In addition, the prediction method of the fatigue life for the variable thermal strain in the mixing tee was discussed. The time histories of the strain around the hot spot were estimated by finite element analysis for which the temperature condition was determined by wall temperature measured in a mock-up test. The accumulated fatigue damage around the hot spot obtained by Miner's rule was less than 1.0. Since the strain around the hot spot had waveforms with periodic overload, the loading sequence with periodic overload caused reduction of the fatigue life around the hot spot. Crack growth tests showed that a single overload decreased crack opening strain and increased the effective strain range. The increment of the effective strain range accelerated the crack growth rate after the overload. The accumulated fatigue damage for the strain in the mixing tee was calculated using Miner's rule and the strain ranges which added the maximum increment of the effective strain range. The accumulated fatigue damage was larger than 1.0 under most conditions. The proposed procedure is suitable to predict the conservative fatigue life in a mixing tee.

Fatigue Reliability Analysis for Brace–Column Connection Details in a Semisubmersible Hull1

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Dilnei Schmidt ◽  
Lance Manuel ◽  
Hieu H. Nguyen ◽  
Luis V. S. Sagrilo ◽  
Edison C. Prates de Lima
Keyword(s):  
Reliability Analysis ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Response Spectra ◽  
Life Assessment ◽  
Response Analysis ◽  
Fe Model ◽  
Fatigue Reliability ◽  
Hot Spot Stress ◽  
Axial Forces

Semisubmersible floating platforms used in offshore deep or ultradeep water environments have hull structures that are comprised of vertical cylinders (columns) connected by braces, pontoons, etc. Several of the connections between these various members are susceptible to fatigue damage. In fatigue damage assessment or fatigue reliability analysis, a global structural response analysis is typically carried out using a finite element (FE) model where internal forces or stresses in the various members are evaluated for specified sea states measured at the site. Of specific interest in the present study is the fatigue reliability analysis of brace-column connection details in a semisubmersible hull unit for selected Brazilian environmental conditions. Stress concentration factors (SCFs) for the selected critical hot spots are applied to the nominal component stresses due to axial forces and biaxial bending. The hot-spot stress response spectra are used with various spectral methods—referred to as Rayleigh, modified Rayleigh (with bandwidth correction), and Dirlik—to estimate fatigue damage using Miner's rule. Uncertainties in some parameters used in the fatigue life assessment are considered and the probability of fatigue failure in the last operational year of the structure is estimated.

Uncertainty of Long Term Fatigue Load of Subsea Well Heads

2012 ◽  
Author(s):  
Guttorm Grytoyr ◽  
Oddrun Steinkjer
Keyword(s):  
Fatigue Life ◽  
Dynamic Loading ◽  
Fatigue Damage ◽  
Life Assessment ◽  
Statistical Uncertainty ◽  
Design Codes ◽  
Fatigue Load ◽  
Short Term ◽  
Damage Estimates

Well heads (WH) are exposed to dynamic loading from waves and floater motions through the marine drilling riser and the drill string. Risers are known to have a pronounced nonlinear response characteristic, this influences the dynamic loading of the well heads. Non-linear time-domain finite element analyses are in general required to give an adequate description of the non-linearities involved. Analyses of a large number of short-term environmental conditions considering stochastic wave loading are required to give a representative description of the long-term fatigue loading on the structure. The objective of this paper is to give an assessment of the sufficient simulation time of each short term seastate in order to get a required level of the statistical uncertainty of the long term fatigue load of the well head and conductor. The analyst can actually influence this statistical uncertainty by selecting proper analysis methodology. A statistical uncertainty measure can be used to evaluate the robustness in the estimated fatigue life. Assessment of statistical uncertainty in fatigue damage estimate is demonstrated by a case study, using typical North Sea conditions, and for varying boundary conditions. Short term fatigue damage is established by means of rainflow cycle (RFC) counting in each stationary short-term condition. It has been experienced that significant statistical uncertainties can be present in the short-term fatigue damage estimates. This is because the accumulated fatigue damage in a stationary condition normally has significant contribution from the largest stress cycles in the realization. Selection of sufficient simulation length is hence essential to obtain reliable fatigue life estimates. Applicable codes and standards for risers provide Design Fatigue Factors (DFF) to secure adequate safety against failure due to wave induced fatigue. The total uncertainty in the calculated fatigue damage comes from various sources and the DFFs in e.g. DNV-OS-F201 “Dynamic Risers” and API-RP-2RD corresponds to a certain uncertainty level in the fatigue damage estimate. A recommended target value for the statistical uncertainty of the fatigue damage estimates is given with basis in these design codes. There are no similar target values for statistical uncertainty of long term fatigue damage given in the applicable design codes for well heads and drilling equipment, hence the values from the riser codes have been selected as a starting point. The recommendations and methodologies presented in this paper, will be included in the upcoming DNV RP “Fatigue design and analysis of drilling and well equipment”, at present this is available only as a draft, ref. [1].

Load Uncertainties Effects on the Fatigue Life Evaluation by the Common Structural Rules

2015 ◽  
Author(s):  
Po-Kai Liao ◽  
Yann Quéméner ◽  
Chi-Fang Lee ◽  
Kuan-Chen Chen
Keyword(s):  
Fatigue Life ◽  
International Association ◽  
Hot Spot ◽  
Structural Response ◽  
Response Assessment ◽  
Life Assessment ◽  
Wave Loads ◽  
Hot Spot Stress ◽  
Structural Rules ◽  
Common Structural Rules

This study evaluated the fatigue life of various hot spots located amidship a handy size oil tanker and a capesize bulk carrier. Specifically, the fatigue was evaluated accordingly to the harmonized common structural rules for bulk carriers and oil tankers recently released by the international association of classification societies. This study examined the stillwater and wave loads uncertainties effect on the fatigue life assessment. Hydro-structure coupling analyses were thus carried out enabling direct hydrodynamic load computations and accurate structural response assessment by finite element analyses. The comparison between direct and rules assessment allowed to identify the load uncertainties effect on the fatigue evaluation. As a result, the fatigue life evaluated by both approaches was significantly different, as expected with regard to the stillwater and dominant wave loads deviations. In addition, the study showed that the influence of the subjected loads was underestimated by the rules, leading to overestimated hot spot stress.

Study on the Remaining Fatigue Life of FPSO Based on Spectral Analysis

2017 ◽  
Author(s):  
Lei Yu ◽  
Huilong Ren ◽  
Xudong Liu ◽  
Xiaoxiong Sun ◽  
Yakang Peng
Keyword(s):  
Spectral Analysis ◽  
Fatigue Life ◽  
Fatigue Damage ◽  
Failure Modes ◽  
Hot Spot ◽  
Stress Transfer ◽  
Three Dimensional Model ◽  
Hot Spot Stress ◽  
Hull Structure ◽  
Remaining Fatigue Life

The fatigue failure, as one of the traditional failure modes of ship hull structures [1], has been widely concerned in recent years. For FPSO converted from large oil tankers, it is critical to predict and extend their service life. The analysis has been finished in compliance with the fatigue damage calculation procedure on the basis of spectral method. A three dimensional model has been performed to represent the entire hull structure. The Hot-Spot Stress Approach is employed to determine the stress transfer function for a location where the fatigue strength is to be evaluated. The fatigue damage resulting from combining the damage from each of the short-term conditions can be accomplished by the use of a weighted liner summation technique. The remaining fatigue life of the FPSO is calculated by the method of spectral analysis to determine the fatigue damage of the oil tanker during the operation period and the FPSO working period respectively. According to the results, the inspection and maintenance of hull structures can be effectively carried out.

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