Comparative Study on Fatigue Damage Assessment of a Structure Member in a Bulk Carrier Using Various Environmental Conditions

2019 ◽  
Author(s):  
Fredhi Agung Prasetyo ◽  
Naoki Osawa ◽  
Mohammad Arif Kurniawan ◽  
Siti Komariyah
Keyword(s):  
Fatigue Damage ◽  
Environmental Conditions ◽  
Damage Assessment ◽  
North Atlantic Ocean ◽  
Environmental Condition ◽  
Scatter Diagram ◽  
Sea State ◽  
Fatigue Design ◽  
Design Life

Abstract Specific design life could be identified by using fatigue damage assessment in the structure engineering field as well as in the maritime sector. Fatigue assessment is one of the assessments to be conducted during review of ship structure design. Fatigue assessment of ship structural member is mainly conducted based on specific environmental condition. In general, specific environmental condition, which is provided by Classification Society rules, is a long term sea-state data of North Atlantic Ocean. The wave scatter diagram presents the tabulation of a long term data of sea state history in the specific ocean. Therefore, a realistic encounter of wave scatter diagram is essential to simulate the variation of wave loadings applied on the ship structure in determination of fatigue design life. Since the application of North Atlantic ocean environmental condition is commonly used by major Classification societies, this condition might give the substantial deterioration on the fatigue design life of the ship that specially operate only in specific ocean area, i.e. South East Asia area. In this work, the wave scatter diagram of various environmental conditions is chosen and the statistical characteristic is compared. The wave load sequence that is used on the fatigue damage assessment are generated by using the concept of storm model, so that the changing nature of sea state could be emulated as in real ocean. Fatigue damage of a structure member of 220 meter Bulk Carriers is calculated based on various environmental conditions.

scholarly journals Representative Environmental Condition for Fatigue Analysis of Offshore Jacket Substructure

Energies ◽  
2020 ◽  
Vol 13 (20) ◽  
pp. 5494
Author(s):  
Tsung-Yueh Lin ◽  
Yi-Qing Zhao ◽  
Hsin-Haou Huang
Keyword(s):  
Fatigue Damage ◽  
Environmental Conditions ◽  
Element Model ◽  
Computational Time ◽  
Wave Load ◽  
Sea State ◽  
Stress Signals ◽  
Cumulative Fatigue Damage ◽  
The Taiwan Strait

The 20-year cumulative fatigue damage of an offshore jacket substructure was estimated under the long-term local environmental conditions in the Taiwan Strait. Because of the nonlinearity of wave load for slender members of the structure, time-domain simulations of the dynamic finite element model were conducted for each sea state. By utilizing the Dirlik method to process the stress signals, the fatigue damages of joints were computed. Concerning the computational time, we propose a probability-based method of using a representative combination of environmental conditions in this study, which can considerably reduce the required number of evaluations prior to determining fatigue damage, thereby improving the process of preliminary design. The results show that only three sea states among 120 can represent 28% of the average damage ratio, and up to 17 sea states fully resolved the fatigue life.

Sensitivity Study of Calculated Jacket Fatigue Damage due to Long Term Distribution of Wave Heights

2017 ◽  
Author(s):  
Hege Halseth Bang ◽  
Siri Hoel Smedsrud ◽  
Øistein Hagen ◽  
Terje Nybø
Keyword(s):  
Fatigue Damage ◽  
Wave Height ◽  
Time Domain ◽  
Height Distribution ◽  
Scatter Diagram ◽  
Sea State ◽  
Jacket Structures ◽  
Wave Heights ◽  
The Individual

Marine structures like jacket structures are often highly utilized structures operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is to a large extent governing the structural dimensions and the amount of resources utilized in inspection and maintenance of members and joints. There is a considerable degree of uncertainty related to the parameters determining the fatigue damage. The models applied, both for describing the fatigue driving mechanisms e.g. the wave-description and load modeling and the deterioration mechanism, are always compromises between the ability to accurately describe the nature and computationally efficiency. The main focus in this paper is to show how sensitive the calculated fatigue damage of a jacket is to different models for the short term variability of wave heights. To obtain consistent basis for comparison a deterministic fatigue analysis is considered and a potential structural dynamic amplification is not included in the comparison study. Sensitivity to selection of wave spectra will not be addressed. In a deterministic approach the long term distribution of individual wave heights is used to calculate the stress ranges occurring in the joints and butt welds. Typically, the long term variability of sea state conditions is given by a scatter diagram of significant wave height (Hs) and the peak period (Tp). When converting the scatter diagram of sea states to the long term distribution of wave heights, it is common to assume that the individual waves in the sea states are Rayleigh distributed. Later developments indicate that a Forristall distribution may be a more accurate assumption. The following cases have been considered: 1. Assuming that the individual waves in each sea state are Rayleigh distributed. 2. Assuming that the individual waves in each sea state follows a Forristall distribution. 3. Calculating the long term wave height distribution from time domain simulations. In the third method, second order wave theory was used to simulate all sea states in the Hs/Tp scatter diagram. I.e. extensive time domain simulations were carried out to cover the complete scatter diagram of possible sea states. The study is performed for an 8-legged jacket. The analyses are performed for a typical North Sea wave environment for water depth about 110 m. The objective of this study is to investigate the robustness in the current design practice for jacket structures where the individual waves in the sea states are Rayleigh distributed. The paper documents the calculated fatigue lives for main joints along the height of the jacket for the three wave height distributions. Further, the paper gives advice on application of wave distribution models for design of new structures and reassessment of existing structures.

The Consequence Method: An Approach for Estimating Roll Damping in Transportation Fatigue Analyses

2017 ◽  
Author(s):  
Tormod Bøe ◽  
Limin Yang ◽  
Erik Falkenberg
Keyword(s):  
Fatigue Damage ◽  
Viscous Damping ◽  
Scatter Diagram ◽  
Roll Damping ◽  
Time Of Year ◽  
Correct Estimation ◽  
Wave Induced ◽  
Partial Damage ◽  
Transportation Route

In order to compute fatigue damage during offshore transports it is necessary to assume a description of the sea states encountered during the voyage. In recent years, it has become a common approach to apply directional long-term scatter diagrams for the transportation route, taking into account vessel speed, course and time of year for the departure. An important contribution to the transportation fatigue damage is usually the wave induced inertia load. For ship shaped vessels additional viscous damping needs to be included in order to estimate correct roll response. However, since viscous roll damping is non-linear, correct estimation of fatigue damage can only be obtained by computing partial damage for all individual sea states in the scatter diagram. This becomes very time-consuming and is usually not done. Instead, the roll damping level is tuned to match typical mean sea states in the scatter diagram. The roll damping will then be too low for higher sea states and too large for smaller sea states. When choosing the roll damping level, the aim should be to obtain an overall error in transportation fatigue damage which is minimized. This paper describes a method to estimate a representative viscous roll damping level for transportation fatigue analyses.

A Fatigue Design for Large Container Ship Taking Long-Term Environmental Condition Into Account

2011 ◽  
Author(s):  
Masayoshi Oka ◽  
Yoshitaka Ogawa ◽  
Ken Takagi
Keyword(s):  
Fatigue Damage ◽  
Element Model ◽  
Full Scale ◽  
Elastic Model ◽  
Direct Computation ◽  
Wave Condition ◽  
Irregular Wave ◽  
Fatigue Design ◽  
Tank Test

In order to promote the reliable evaluation for the fatigue strength of ships, the validation of a direct load and strength computation was performed based on the tank test and the full scale measurement. The fatigue damage in short term sea state under various operation parameters was indicated quantitatively by the tank test in irregular wave utilizing the elastic model. The long term fatigue damage was evaluated based on the full scale measurement [1]. The fatigue damage inferred from the measured stress on deck structure is quite small compared with the direct computation utilizing a full ship finite element model. That is mainly caused by the difference of environmental wave condition. Moreover, the effect of operational condition through whole life was indicated by the direct computation quantitatively. To make more rational fatigue design, it is important to take the long-term wave condition into account.

scholarly journals Fatigue Analysis of Deepwater Hybrid Mooring Line Under Corrosion Effect

2014 ◽  
Vol 21 (3) ◽  
pp. 68-76 ◽  
Author(s):  
Dongsheng Qiao ◽  
Jun Yan ◽  
Jinping Ou
Keyword(s):  
Fatigue Damage ◽  
Oil And Gas ◽  
Line Tension ◽  
Chain Structure ◽  
Mooring Line ◽  
Mooring System ◽  
Load Spectrum ◽  
Sea State ◽  
Mooring Lines

Abstract In the deepwater exploitation of oil and gas, replacing the polyester rope by a wire in the chain-wire-chain mooring line is proved to be fairly economic, but this may provoke some corresponding problems. Te aim of this paper is to compare the fatigue damage of two mooring system types, taking into account corrosion effects. Using a semi-submersible platform as the research object, two types of mooring systems of the similar static restoring stiffness were employed. Te mooring lines had the chain-wire-chain and chain-polyester-chain structure, respectively. Firstly, the numerical simulation model between the semi-submersible platform and its mooring system was built. Te time series of mooring line tension generated by each short-term sea state of South China Sea S4 area were calculated. Secondly, the rain flow counting method was employed to obtain the fatigue load spectrum. Thirdly, the Miner linear cumulative law model was used to compare the fatigue damage of the two mooring system types in long-term sea state. Finally, the corrosion effects from zero to twenty years were considered, and the comparison between the fatigue damage of the two mooring system types was recalculated.

How to Account for Short-Term and Long-Term Variability in the Prediction of the 100 Years Response?

2017 ◽  
Author(s):  
Quentin Derbanne ◽  
Guillaume de Hauteclocque ◽  
Martin Dumont
Keyword(s):  
Return Period ◽  
Environmental Conditions ◽  
Gumbel Distribution ◽  
Accurate Method ◽  
Design Methods ◽  
Short Term ◽  
Sea State ◽  
Reliability Method ◽  
Unit Design

Current practices in offshore unit design are based on the prediction of the 100 years response (tension, offset, stress...). The methodologies described in various standards (ISO, API...) are all very similar: several design environments are described with a combination of sea state, wind and current. Usually envelope contours are used, describing a set of environmental conditions corresponding to a 100 years return period. These design conditions are supposed to produce the highest responses. A time domain (or sometimes frequency domain) simulation is done on each of these short-term conditions, and the 3h most probable maximum (MPM) is computed for each. The highest MPM over all the design conditions is taken as the 100 years response. This approach completely neglects the short-term variability of the response. This paper compares several design methods with the exact 100 years response. The exact 100 years response is computed by integrating the conditional short-term distributions with respect to the probability density function of the environmental conditions. The various design methods are all based on a simplification of an Inverse First Order Reliability Method (IFORM) approach, which requires computing one or several design conditions corresponding to one or several return periods, each of these conditions being associated with a given short-term quantile. Computations are done using two datasets. At first realistic line tensions of 7 offshore units are used, based on a large number of simulations with a mooring software. On a second stage a more general parametric model using a Weibull distribution to describe the long-term variability and a Gumbel distribution to describe the short-term distribution of the 3h maximum is used. It is shown that the current methods are unconservative with respect to the exact 100 years response. A more accurate method is proposed, based on a 40 years return period associated with the quantile 90% and a correction factor of 1.04.

Flexible Riser Fatigue Procedure Using a Long Term Distribution of FPSO’s Heading Direction

2002 ◽  
Author(s):  
Carlos Alberto Duarte de Lemos ◽  
Murilo Augusto Vaz
Keyword(s):  
Environmental Conditions ◽  
Environmental Data ◽  
Mooring Lines ◽  
Fatigue Design ◽  
Beam Seas ◽  
Heading Direction ◽  
Campos Basin ◽  
Directional Waves ◽  
Flexible Risers

The intensive use of FPSOs in Campos Basin, offshore Brazil, has demanded more attention on the fatigue design of flexible risers. Field experience and a new set of environmental data collected in the last years have shown a large number of swell waves reaching the floating units in quartering and beam seas, which has amplified the motions at the risers’ top connections. Hence, it is necessary to evaluate the occurrence probability of wave heading direction relative to the vessel, especially for beam seas, but also noting the probability increase of quartering seas considering the existence of bi-modal and bi-directional wave spectra. The simultaneous environmental conditions — wind, current and bi-directional waves (sea and swell) — registered in Campos Basin, were condensed in a few hundreds environmental conditions to enable its utilization on a simulation program, where the FPSO, mooring lines and risers were modeled to obtain the mean heading and offsets. This new set of FPSO headings were then used to define a new proposal of load conditions to evaluate the fatigue life of flexible risers. This new procedure was then applied to a case study for the FPSO P-33 employing the tools available at PETROBRAS research center. The results have confirmed the importance of beam and quarter seas, which respectively cause heavy roll and vertical motions at the turret and then, affecting the fatigue of the risers.

Statistical Methods for Prediction of Characteristic Loads for Free Fall Lifeboats Based on CFD Screening Results

2013 ◽  
Author(s):  
Vidar Tregde ◽  
Arne Nestegård
Keyword(s):  
Environmental Conditions ◽  
Time History ◽  
Free Fall ◽  
Response Analysis ◽  
Short Term ◽  
Sea State ◽  
Screening Process ◽  
History Of ◽  
Term Analysis

Computational Fluid Dynamics (CFD) has been used in a screening process to calculate characteristic loads for a Free Fall Lifeboat (FFLB) during impact and submergence. The link between various input, e.g. environmental conditions and host specific data, resulting structural loads and motion of the lifeboat is explored. The screening can be used together with host specific environmental conditions to find structural design loads and motion restrictions. Response based analysis have been developed for both short term and long term predictions. For the short term predictions a sea state given by (Hs, Tp) on the 100-year contour line is identified and a three hour irregular sea state is simulated. This time history of surface elevations is used for a large number of random lifeboat drops. From these random drops a distribution of wave height and corresponding wave steepness is derived which is then input to an interpolation in the database of CFD screening results. The resulting responses are fitted to a Weibull distribution and the 90% quantile in this short term load distribution is determined. The long term response analysis is further developed from the short term analysis. The short term distributions for each (Hs, Tp) are combined with the probability of occurrence of the sea state, and long term distributions are derived for the responses similar to the short term analysis. The screening results are used to identify critical load cases which are further investigated.

Variance of Fatigue Damage During Transportation

2018 ◽  
Author(s):  
Limin Yang ◽  
Tormod Bøe ◽  
Erik Falkenberg ◽  
Florus Korbijn
Keyword(s):  
Monte Carlo Simulation ◽  
Fatigue Damage ◽  
Offshore Structures ◽  
Mean Value ◽  
Scatter Diagram ◽  
Large Variability ◽  
Damage Prediction ◽  
Structural Fatigue ◽  
The Mean

Fatigue damage prediction of offshore structures during transportation is today commonly based on long term scatter diagram for the sea route. This gives a mean value for the expected fatigue. The deviation from the mean value may however be significant due to the large variability in metocean conditions for the typical short duration of the transport. The actual fatigue damage experienced during the voyage may therefore be much higher than predicted. Hence, it is recommended to take into account the variability of environmental conditions in the calculation of transportation fatigue. The aim of this paper is to establish a method for estimating transportation fatigue where the fatigue damage is computed with a specified level of non-exceedance. The fatigue damage is quantified by a measure as defined in OMAE2017 /1/. The distribution of this measure is obtained through Monte Carlo simulation of around 300 separate sea route scatter diagrams derived from hindcast data. The 90% percentile value (10% probability of exceedance) of the fatigue measure is then taken as target value and used for establishing a limited set of design fatigue sea states to be used in the structural fatigue damage calculations.

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