Advanced Integrated Design Method for Fatigue Assessment of Large Elastic Ship Structures

2013 ◽  
Vol 371 ◽  
pp. 443-447
Author(s):  
Ionica Rubanenco ◽  
Iulia Mirciu ◽  
Leonard Domnisoru
Keyword(s):  
Service Life ◽  
Hot Spot ◽  
Ratio Method ◽  
Wave Loads ◽  
Ship Structures ◽  
Fatigue Assessment ◽  
Integrated Method ◽  
Oscillation Analysis ◽  
Non Linear ◽  
Wave Induced

This paper is focused on an advanced method for ship structures fatigue assessment. The ships classification societies standard rules for fatigue analysis are based on simplified procedures, with wave induced loads obtained by linear oscillation analysis (low frequency, around 0.1 Hz), or equivalent statistical wave loads. In the case of large elastic ship structures, with hull length over 150 m, the global wave induced vibration response (high frequency, around 1 Hz) becomes significant. The developed integrated method for large ships fatigue assessment includes three interlinked analyses, as follows: the hot-spot stresses evaluation by 3D finite element models, wave induced loads by short term linear and non-linear hydroelastic dynamic analysis, ship service life and fatigue assessment by damage cumulative ratio method. As testing ship, it is considered a double hull LPG Liquefied Petroleum Gas carrier, with total length 239 m, for a set of structural details with stress hot-spots. Based on the non-linear hydroelastic wave loads, the integrated method of fatigue assessment becomes more accurate, predicting for the amidships structure 14 years of ship service life, instead of over 20 years according to the rules standard approach, so that the confidence on ship structure fatigue evaluation can be increased in the design process.

Fatigue Assessment of Ship Structures using Hot Spot Stress and Structural Stress Approaches with Experimental Validation

2008 ◽  
Author(s):  
Myung Hyunn Kim ◽  
Seong Minn Kim ◽  
Jae Myung Lee ◽  
Sung Wong Kang
Keyword(s):  
Experimental Investigation ◽  
Experimental Validation ◽  
Hot Spot ◽  
Initial Study ◽  
Systematic Investigation ◽  
Structural Stress ◽  
Ship Structures ◽  
Gusset Plates ◽  
Hot Spot Stress ◽  
Fatigue Assessment

The aim of this study is to investigate fatigue assessment of typical ship structures employing structural stress approach and to compare with hot spot stress approach. As an initial study of the systematic validation efforts on structural stress method, an experimental investigation is performed on a series of edge details with welded gusset plates. Extrapolation based hot spot stress using converged mesh were also calculated for each specimen types. Having validated the application of structural stress for small edge details, a systematic investigation is carried out for a fatigue assessment of typical ship structures employing structural stress approach. Fatigue strength of side shell connection of a 8,100 TEU container vessel is evaluated using hot spot stress and structural stress employing simplified fatigue analysis.

An Overview of the Reassessment Studies of Fixed Offshore Platforms in the Bay of Campeche, Mexico

2005 ◽  
Author(s):  
Partha Chakrabarti ◽  
Ibrahim Abu-Odeh ◽  
Adinarayana Mukkamala ◽  
Bidyut Majumdar ◽  
Michael Havbro Faber ◽  
...  
Keyword(s):  
Service Life ◽  
Structural Models ◽  
Gas Production ◽  
Offshore Platforms ◽  
Wave Loads ◽  
Inspection Planning ◽  
Joint Flexibility ◽  
Analysis Technique ◽  
Non Linear ◽  
Pile Interaction

Pemex Exploration Produccio´n owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of these fixed offshore platforms were built during the 70s and 80s and have already exceeded their design service life. In order to meet the growing demand for oil and natural gas it is necessary to extend the service life of these platforms by at least another 15 to 30 years. To meet this extended service life, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness and possible locations of fatigue problems. To extend the fatigue life of the welded joints, inspections are required to be performed according to a risk based inspection planning procedure. In the present paper, an overview of the reassessment study procedure is outlined and pertinent results are presented for more than twenty platforms which were studied in a recent project. The most important engineering considerations and various analyses involved in the study are discussed in detail. The platforms cover the categories of Drilling, Production, Gathering and Habitation. Depending on the category, Pemex specifications assign different levels of acceptable reliabilities and reserve strength ratios. The ultimate strength of the platforms is determined using a detailed finite element model of jacket, piles and deck structures and a state-of-the-art non-linear progressive collapse analysis technique commonly known as ‘pushover’ analysis. The analytical structural models include local joint flexibility of the jacket joints, soil-pile interaction, geometric non-linearity and material plasticity. They also include the information of damages and deterioration obtained from inspection reports, such as dents, bents, cracks etc. The joint strength modeling is performed using the latest available procedures that use non-linear load-deformation curves. Fatigue analyses are based on spectral analysis technique and include the dynamic response of the structure to wave loads. The structural models for fatigue analysis include the effects of local joint flexibility (LJF) and soil-pile interaction. Results for one typical platform are presented in complete detail to facilitate the understanding of the reassessment study procedure. A typical risk based inspection planning for extending fatigue service life is also presented. Finally, the summary of results for 28 platforms is presented to appreciate the importance of the various analytical parameters. It is hoped that this very wide database of results for platforms of different configurations can serve as a useful resource for the offshore industry in general.

Statistical Uncertainty Analysis in the Long-Term Distribution of Wind- and Wave-Induced Hot-Spot Stress for Fatigue Design of Jacket Wind Turbine Based on Time Domain Simulations

2011 ◽  
Author(s):  
WenBin Dong ◽  
Torgeir Moan ◽  
Zhen Gao
Keyword(s):  
Wind Turbine ◽  
Time Domain ◽  
Hot Spot ◽  
Statistical Uncertainty ◽  
Wave Loads ◽  
Hot Spot Stress ◽  
Tubular Joints ◽  
Stress Components ◽  
Wave Induced

The statistical uncertainty of the long-term distribution of wind- and wave-induced hot-spot stress ranges in multi-planar tubular joints of a fixed jacket offshore wind turbine designed for a North Sea site in a water depth of 70m has been assessed in this paper. The dynamic response of the jacket support structure due to wind and wave loads is calculated using a decoupled procedure. Hot-spot stresses at failure-critical locations of each reference brace for 4 different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from axial, in-plane and out-plane action. The effects of planar and non-planar braces are also considered. A two-parameter Weibull function is used to fit the long-term statistical distribution of hot-spot stress ranges by combination of time domain simulation for representative environmental conditions (wind / sea states) in operational condition of the wind turbine. The statistical uncertainty of the Weibull distribution of hot-spot stress ranges and the two parameters defining the Weibull distribution is assessed, based on 20 simulations for each representative environmental condition. The contributions to the uncertainty from wind loads and wave loads are analyzed by considering 3 different load cases: wind loads only, wave loads only and combination of wind and wave loads. The sensitivity of the long-term distribution of hot-spot stress ranges due to their stress components is also assessed.

scholarly journals Evaluation of Moisture and Decay Models for a New Design Framework for Decay Prediction of Wood

Forests ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 721
Author(s):  
Jonas Niklewski ◽  
Philip Bester van Niekerk ◽  
Christian Brischke ◽  
Eva Frühwald Hansson
Keyword(s):  
Service Life ◽  
Wood Decay ◽  
Element Model ◽  
Digital Design ◽  
Design Guideline ◽  
Digital Performance ◽  
Detail Design ◽  
Non Linear ◽  
Service Life Design ◽  
Life Design

Performance-based, service-life design of wood has been the focus of much research in recent decades. Previous works have been synthesized in various factorized design frameworks presented in the form of technical reports. Factorization does not consider the non-linear dependency between decay-influencing effects, such as between detail design and climate variables. The CLICKdesign project is a joint European effort targeting digital, performance-based specification for service-life design (SLD) of wood. This study evaluates the feasibility of using a semi-empirical moisture model (SMM) as a basis for a digital SLD framework. The performance of the SMM is assessed by comparison against a finite element model (FEM). In addition, two different wood decay models (a logistic, LM, and simplified logistic model (SLM)) are compared. While discrepancies between the SMM and FEM were detected particularly at high wood moisture content, the overall performance of the SMM was deemed sufficient for the application. The main source of uncertainty instead stems from the choice of wood decay model. Based on the results, a new method based on pre-calculated time series, empirical equations, and interpolation is proposed for predicting the service life of wood. The method is fast and simple yet able to deal with non-linear effects between weather variables and the design of details. As such, it can easily be implemented as part of a digital design guideline to provide decision support for architects and engineers, with less uncertainty than existing factorized guidelines.

Structural Safety Assessment of a 1100 TEU Container Ship, Based on a Enhanced Long Term Fatigue Analysis

2014 ◽  
Vol 1036 ◽  
pp. 935-940
Author(s):  
Leonard Domnisoru ◽  
Ionica Rubanenco ◽  
Mihaela Amoraritei
Keyword(s):  
Safety Assessment ◽  
Safety Evaluation ◽  
Irregular Waves ◽  
Structural Safety ◽  
Random Wave ◽  
Wave Loads ◽  
Strength Assessment ◽  
Integrated Method ◽  
Hydroelastic Response

This paper is focused on an enhanced integrated method for structural safety assessment of maritime ships under extreme random wave loads. In this study is considered an 1100 TEU container test ship, with speed range 0 to 18 knots. The most comprehensive criteria for ships structural safety evaluation over the whole exploitation life is based on the long term ship structures analysis, that includes: stress hot-spots evaluation by 3D/1D-FEM hull models, computation of short term ship dynamic response induced by irregular waves, long term fatigue structure assessment. The analysis is enhanced by taking into account the ships speed influence on hydroelastic response. The study includes a comparative analysis on two scenarios for the correlation between the ships speed and waves intensity. The standard constant ship speed scenario and CENTEC scenario, with total speed loss at extreme waves condition, are considered. Instead of 20 years ship exploitation life estimated by classification societies rules from the long term structural safety criteria, the enhanced method has predicted more restrictive values of 14.4-15.7 years. The numerical analyses are based on own software and user subroutines. The study made possible to have a more realistic approach of ships structural strength assessment, for elastic and faster ships as container carriers, in compare to the standard one based only on naval rules, delivering a method with higher confidence in the designed structural safety.

A Numerical Simulation of Non-Linear Air-Gap for Deepwater Semi-Submersible Platform

2021 ◽  
Author(s):  
Zhuang Kang ◽  
Yansong Zhang ◽  
Haibo Sui ◽  
Rui Chang
Keyword(s):  
Air Gap ◽  
Second Order ◽  
Difference Frequency ◽  
Hydrodynamic Performance ◽  
Wave Loads ◽  
Order Difference ◽  
Motion Response ◽  
Nonlinear Motion ◽  
Non Linear ◽  
Simulation Results

Abstract Air gap is pivotal to the hydrodynamic performance for the semi-submersible platform as a key characteristic for the strength assessment and safety evaluation. Considering the metocean conditions of the Norse Sea, the hydrodynamic performance of a semi-submersible platform has been analyzed. Based on the three-dimensional potential flow theory, and combined with the full QTF matrix and the second-order difference frequency loads, the nonlinear motion characteristics and the prediction for air gap have been simulated. The wave frequency motion response, the second-order nonlinear air gap response and nonlinear motion response of the platform have been analyzed. By comparing the simulation results, the air gap response of the platform considering the nonlinear motion is more intense than the results simulated by the first-order motion without considering the second-order difference frequency loads. Under the heavy metocean conditions, for the heave and pitch motion of the platform, the non-linear simulation values for some air gap points and areas are negative which means the wave slam has been occurred, but the calculation results of linear motion response indicate that the air gap above has not appeared the wave slamming areas. The simulation results present that the influence of the second-order wave loads is a critical part in the air gap prediction for the semi-submersible platform.

A Novel Solution to Compute Stress Time Series in Nonlinear Hydro-Structure Simulations

2015 ◽  
Author(s):  
Fabien Bigot ◽  
François-Xavier Sireta ◽  
Eric Baudin ◽  
Quentin Derbanne ◽  
Etienne Tiphine ◽  
...  
Keyword(s):  
Time Series ◽  
Frequency Domain ◽  
Time Domain ◽  
Hot Spot ◽  
Structural Response ◽  
Container Ship ◽  
Time Step ◽  
Computation Cost ◽  
Hull Girder ◽  
Non Linear

Ship transport is growing up rapidly, leading to ships size increase, and particularly for container ships. The last generation of Container Ship is now called Ultra Large Container Ship (ULCS). Due to their increasing sizes they are more flexible and more prone to wave induced vibrations of their hull girder: springing and whipping. The subsequent increase of the structure fatigue damage needs to be evaluated at the design stage, thus pushing the development of hydro-elastic simulation models. Spectral fatigue analysis including the first order springing can be done at a reasonable computational cost since the coupling between the sea-keeping and the Finite Element Method (FEM) structural analysis is performed in frequency domain. On the opposite, the simulation of non-linear phenomena (Non linear springing, whipping) has to be done in time domain, which dramatically increases the computation cost. In the context of ULCS, because of hull girder torsion and structural discontinuities, the hot spot stress time series that are required for fatigue analysis cannot be simply obtained from the hull girder loads in way of the detail. On the other hand, the computation cost to perform a FEM analysis at each time step is too high, so alternative solutions are necessary. In this paper a new solution is proposed, that is derived from a method for the efficient conversion of full scale strain measurements into internal loads. In this context, the process is reversed so that the stresses in the structural details are derived from the internal loads computed by the sea-keeping program. First, a base of distortion modes is built using a structural model of the ship. An original method to build this base using the structural response to wave loading is proposed. Then a conversion matrix is used to project the computed internal loads values on the distortion modes base, and the hot spot stresses are obtained by recombination of their modal values. The Moore-Penrose pseudo-inverse is used to minimize the error. In a first step, the conversion procedure is established and validated using the frequency domain hydro-structure model of a ULCS. Then the method is applied to a non-linear time domain simulation for which the structural response has actually been computed at each time step in order to have a reference stress signal, in order to prove its efficiency.

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