Advanced Methods for ULS and FLS
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Advanced Methods for ULS and FLS

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    Advanced Methods for ULS and FLS Advanced Methods for ULS and FLS Presentation Transcript

    • Advanced Methods for Ultimate and Fatigue Strength ofFloatersDNV SoftwareTorbjørn Lindemark, Nauticus Product Manager
    • Agenda  Strength assessment of FPSOs and related software from DNV  Introduction to direct load and strength calculations  Deterministic vs. spectral analysis  Fatigue loading and critical details for FPSOs  Case study and software demo on direct strength calculations of a ship shaped FPSOAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 2
    • FPSO - What is required?  FPSO - Complex design process - Ships and Offshore Rule requirements - Regulatory requirements - Seakeeping, Hydrodynamic analysis - Long operation life without docking - Topside & Topside/Hull interaction - Turret area - Risers & Moorings - Deep water  Tools for assessment of - Conversion of tanker to FPSO - FPSO newbuilding  Tools for maintenance of FPSO’s in operation We deliver a package that ties it all together and provide a complete, integrated toolkit, tailor made for FPSOsAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 3
    • Challenge of FPSO New Build and Conversion  New Builds  Conversions - Selection corrosion protection - Increase certainty that the strategy to determine a rational chosen vessel is suitable for material thickness conversion, - Identify comprehensive - Determine how much steel analysis requirements for design should be replaced during - Develop Inspection Plans conversion/maintenance, - Choice of turret design - Identify where to focus surveys.Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 4
    • FPSO Package for design and analysis Risk Analysis Hydrodynamics Safeti Topside • Seakeeping Genie • Wave loads HydroD Main scantlings 3D Hull Nauticus Hull modelling GeniE Fatigue Turret Simplified, Local analysis Spectral GeniE Nauticus Hull Sesam/Stofat Risers DeepC Mooring Proven solutions in use Mimosa by major companies around the worldAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 5
    • Direct Calculations in an Integrated Analysis System 1. Stability and wave load 2. Pressure loads and analysis accelerations Wave scatter diagram Load transfer Local FE analysis 5. Local stress and deflection & fatigue FE analysis 4. Global stress and 3. Structural model loads (internal + external pressure) deflection & fatigue screeningAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 6
    • Wave Load Analysis  Input  Output - Models - Load transfer functions (Response Amplitude - Panel &/or Morrison model Operators – RAOs) - Mass model - Motions in 6 dof (+ derived velocities and accelerations) - Compartments - External wave pressures - Structural model for load transfer - Internal tank pressures - Loading conditions - Morrison forces - Compartment fillings, draught and trim - Sectional loads - Wave and environmental data - Load statistics - Scatter diagram - Derived by combining the load RAOs with wave data - Wave spectrum - Design values for ULS/ALS - Directionality and spreading - Long term load distribution for simplified fatigue - Current calculations - Water depth - Load files for transfer to structural model - Design waves for deterministic ULS and/or FLS analysis - Load RAOs for stochastic ULS and FLS analysis - Both containing accelerations, external and internal pressuresAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 7
    • Finite Element Analysis Deterministic Analysis Spectral Analysis  Input  Input - Global and local FE models - Global and local FE models - Design wave load transfer files (or long term - RAO based load transfer files loads by manual input) - Wave and environmental data - Scatter diagram - Wave spectrum - Directionality and spreading  Output  Output - Stress response for a given design wave/load - Stress transfer functions (Response Amplitude Operators – RAOs) - Stress statistics - Derived by combining the stress RAOs with wave data - Short and long term distribution - Design values for specified probability level/return periodAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 8
    • Fatigue Analysis by Cumulative Damage Deterministic Analysis Spectral Analysis  Input  Input - Long term stress distribution - Stress transfer functions (Response Amplitude - Described by Weibull distribution or stress histogram Operators – RAOs) - The Weibull distribution is described by - Wave and environmental data - Stress at a given probability level - Scatter diagram - Weibull parameter - Wave spectrum - Zero crossing frequency - Directionality and spreading - S-N curves - S-N curves  Output  Output - Calculated fatigue life or damage - Calculated fatigue life or damage - Fatigue calculations performed based on short term statistics by summing up part damage for each cell in the scatter diagram  the uncertainties involved in Weibull fitting are avoidedAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 9
    • Simplified vs. direct fatigue calculations Environment Simplified Spectral Analysis Long term Weibull Wave scatter diagram and distribution by rule energy spectrum formulas Wave Load Analysis: Accelerations, pressure and Direct calculated loads - moments on 10^-4 or 10^-8 3D potential theory probability level by rule formulas Stress analysis: Rule formulations for Load transfer to FE model. stresses and correlation of Stress transfer function implicit different loads in FE model Based on expected largest stress Based on summation of part damage Fatigue damage among 10^4 cycles of a rule long from each Rayleigh distributed sea analysis: term Weibull distribution state in scatter diagram.Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 10
    • Fatigue loads and stress components  Global wave bending moments  Hull girder stress  Stress in topside supports due to global hull deflections  Stress in turret and moonpool areas due to hull deflections  Wave pressure  Shell plate local bending stress  Local stiffener bending stress  Secondary stiffener bending due to deflection of main girder system  Local peak stresses in knuckles due to deflection of main girder system  Vessel motions (accelerations)  Liquid pressure in tanks  Stress in topside support from inertia forces  Mooring and riser fasteningsAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 11
    • Moonpool areas Increased plate thickness Nominal stress level Actual stress distribution CL Long. stress in deck (no Long. stress in deck Long. stress in deck shear lag effect) when plates near side uniform deck thickness are increasedAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 12
    • In-service Experience on Fatigue Critical Details  Stiffener end connections Web-plating  Root source of cracking Stiffener Global hull girder bending Longitudinal Local dynamic pressures Relative deflections caused by bending of girder system Stress concentration at stiffener toe and heelAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 13
    • In-service Experience on Fatigue Critical Details Knuckles in inner structure (hopper knuckle) Root source of cracking: Deflection on main girder system High stress concentration Cracks under development Repair exampleAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 14
    • In-service Experience on Fatigue Critical Details  Shell plating  Root source of cracking Local pressureAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 15
    • In-service Experience on Fatigue Critical Details  Main deck openings and attachments  Root source of cracking Global hull girder stress Stress due to hull girder deflection and stiff topside lattice construction Stress from topside inertia forces Local stress concentrationsAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 16
    • Summary Fatigue Critical Details  Main deck openings, attachments and topside support  Moonpool area  Knuckles and discontinuities in the main girder system  Stiffener end connections  Side shell platingAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 17
    • A few useful ratios Ratio Stress factor Fatigue Damage factor (equivalent stress reduction) Base / Weld - SN (10^12.89) / 0.83 1.74 curve (10^12.65) World wide / North 0.8 / 1.0 0.8 2.0 Atlantic ocean Non-corrosive / (10^12.65) / 0.81 2.0 corrosive environment (10^12.38) Mean / Design SN (10^12.09) / 0.7 3.0 curve (10^11.63)Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 18
    • Part 2 – Case Study and Demos Direct strength ULS and FLS calculations of a ship shaped FPSOAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 19
    • Why direct load and strength calculations  Rule loads are not always the truth  Modern 2000000 calculation tools give more accurate loads 1500000 [kNm] - Ultimate strength loads 1000000 - Fatigue loads 500000 - Phasing and simultaneity of different load effects 0 0 0.2 0.4 0.6 0.8 1  Design and strength optimizations based on analysis VBM (linear) closer to actual operating conditions 150000  Improved decision basis for 100000 [kN] - In-service structural integrity management 50000 - Life extension evaluation 0 0 0.2 0.4 0.6 0.8 1 Vertical Bending Moment VSF (linear) Sea Pressure Double Hull Bending Total Stress −−− Stress Rule Direct −−− Pressure TimeAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 20
    • Direct calculated loads vs. rule loads  Fatigue loads: 1.20 1.00 0.80 Direct 0.60 DNV Rule CSR 0.40 0.20 0.00 Vertical Horizontal Pressure WL Vert. Acc. Bending BendingAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 21
    • Spectral vs Simplified Fatigue Analysis  Comparison of fatigue damage by DNV rules and Common Scantling Rules relative to spectral fatigue calculations: 1.20 1.00 0.80 Comp. Stoch. 0.60 DNV Rule CSR 0.40 0.20 0.00 Bottom at Side at Side at T Trunk B/4 T/2 DeckAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 22
    • Analysis Overview Task Purpose Input Output Global modelling  Make global model for  Ship drawings Global FE model hydrodynamic and  Loading manual strength analysis Hydrodynamic Calculate loads for Global FE model Load files for analysis fatigue and ultimate Wave data structural analysis strength ULS analysis Calculate hull girder Global FE model Ultimate strength strength Snap shot load files results from HydroD Spectral fatigue Fatigue screening on Global FE model Calculated fatigue analysis nominal stress Frequency domain load lives Local fatigue analysis files from HydroD Spectral ULS Calculate long term Global FE model Long term stress analysis stress based on spectral Frequency domain load method files from HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 25
    • Creating the Global Model Model requirements Challenges  The global model is used to calculate  Modelling of hull form loads and strength and must represent the actual properties of the ship  Creating compartment and loads  For direct strength calculations  Mass tuning essential properties are - Buoyancy and weight distribution - Compartment loads - Structural stiffness and strengthAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 26
    • Demo – Global Modelling with GeniEAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 27
    • Benefits of GeniE for Global Modelling  One common model for hydrodynamic and structural analysis  Geometry modelling - Advanced surface modelling functions - Re-use data from CAD - Parametric modelling using JavaScript - Use of units  Compartment and loads - Compartments are created automatically - GeniE calculates tank volumes and COG - Loads are generated from compartment fillings and automatically applied to tank boundaries  Mass tuning - Scaling mass density to target massAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 28
    • Analysis Overview Task Purpose Input Output Global modelling  Make global model for  Ship drawings Global FE model hydrodynamic and  Loading manual strength analysis Hydrodynamic Calculate loads for Global FE model Load files for analysis fatigue and ultimate Wave data structural analysis strength ULS analysis Calculate hull girder Global FE model Ultimate strength strength Snap shot load files results from HydroD Spectral fatigue Fatigue screening on Global FE model Calculated fatigue analysis nominal stress Frequency domain load lives Local fatigue analysis files from HydroD Spectral ULS Calculate long term Global FE model Long term stress analysis stress based on spectral Frequency domain load method files from HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 29
    • Hydrodynamic Analysis Model requirements Challenges  Hull shape as real ship  Obtain correct weight and mass distribution  Correct draft and trim  Balance of loading conditions  Weight and buoyancy distribution according to loading manual  Mass and buoyancy in balanceAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 30
    • Demo – HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 31
    • Benefits of HydroD  One common model for - Stability calculations - Linear hydrodynamic analysis - Non-linear hydrodynamic analysis - With or without forward speed  Supports composite panel & Morrison models  Model shared with structural analysis  Loading conditions - Multiple loading conditions by changing compartment contents  Balancing the model - Auto balance of loading conditions by draft and trim or compartment fillings  Built in roll damping module - Stochastic linearization - Quadratic damping  Strong postprocessing and graphical results presentation  Load transfer to FE analysis - Snap shot or frequency domain - With splash zone correction for fatigueAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 32
    • Analysis Overview Task Purpose Input Output Global modelling  Make global model for  Ship drawings Global FE model hydrodynamic and  Loading manual strength analysis Hydrodynamic Calculate loads for Global FE model Load files for analysis fatigue and ultimate Wave data structural analysis strength ULS analysis Calculate hull girder Global FE model Ultimate strength strength Snap shot load files results from HydroD Spectral fatigue Fatigue screening on Global FE model Calculated fatigue analysis nominal stress Frequency domain load lives Local fatigue analysis files from HydroD Spectral ULS Calculate long term Global FE model Long term stress analysis stress based on spectral Frequency domain load method files from HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 33
    • Ultimate Strength Analysis  Global structural analysis with load transfer from hydrodynamic analysis  Snap shot load transfer of non linear loads for selected design conditions  Yield and buckling check with PULS  Benefits of global analysis with direct load transfer Eliminate effect of boundary conditions Loads applied as a simultaneous set of sea and tank pressures according to the calculated design wave  No need for conservative and/or uncertain assumptions Integrated buckling checkAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 34
    • Cutres - Verification of Applied Loads  Cutres calculates and integrates the force distribution of cross sections and is ideal to evaluate the hull girder structural response Vertical shear force distribution Vertical bending moment distribution 0 50 100 150 200 250 300 350 Vertical bending momentVertical shear force WASIM WASIM CUTRES CUTRES 0 50 100 150 200 250 300 350 Distance from AP Distance from AP Advanced Methods for Ultimate and Fatigue Strength of Floaters © Det Norske Veritas AS. All rights reserved. 35
    • PULS – Advanced Buckling & Panel Ultimate Limit State PULS is a code for buckling and ULS assessments of stiffened and unstiffened panelsAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 36
    • Benefits of PULS Py  Characteristics - Higher accuracy than traditional rule formulations and classic buckling theory - Quick and easy-to-use design tool for calculation of ULS capacity Px - Valuable information about failure mode and buckling pattern - Effective to evaluate  Benefits 250 Abaqus PULS DNV Rules - Design optimization with increased control of safety 200 GL Rules margins 150 τ 12 (MPa) 100 50 0 0 20 40 60 80 100 120 140 σ 2 (MPa)Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 37
    • PULS - Element library  Un-stiffened plate element  Stiffened plate element (S3)  Corrugated plate element (K3)  Stiffened plate element (T1)Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 38
    • Demo – PULS Code Check in GeniEAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 39
    • Analysis Overview Task Purpose Input Output Global modelling  Make global model for  Ship drawings Global FE model hydrodynamic and  Loading manual strength analysis Hydrodynamic Calculate loads for Global FE model Load files for analysis fatigue and ultimate Wave data structural analysis strength ULS analysis Calculate hull girder Global FE model Ultimate strength strength Snap shot load files results from HydroD Spectral fatigue Fatigue screening on Global FE model Calculated fatigue analysis nominal stress Frequency domain load lives Local fatigue analysis files from HydroD Spectral ULS Calculate long term Global FE model Long term stress analysis stress based on spectral Frequency domain load method files from HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 40
    • Stochastic Fatigue Analysis Wave Load Analysis - Input: Global model, wave headings and frequencies - Output: Load transfer functions (RAOs) Direct Load Transfer Stress Response Analysis - Input: FE models and load file from wave load analysis - Output: FE results file with load cases describing complex (real and imaginary) stress transfer functions (RAOs) Stress Transfer Functions S-N Fatigue Fatigue Damage Calculation Curves - Input: Stress transfer functions (FE results file), wave data Wave scatter - Output: Calculated fatigue life diagram Fatigue LifeAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 41
    • Global Frequency Domain Analysis  Loads from HydroD  Static load case - For verification of load balance and static shear and bending compared to loading manual - Enables automatic calculation of mean stress Head Sea effect in fatigue calculartions - Enables possibility for to calculate long term extreme loads including static stress  Dynamic load cases - Number of complex dynamic load cases = number of wave headings x number of wave periods (e.g. 12 x 25 = 300)Advanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 42
    • Demo - Stofat Calculated fatigue damage by nominal stress and user defined SCF for an LNG carrierAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 43
    • Global Screening Analysis  Fatigue calculations based on nominal stress from global analysis and stress concentration factors  Typical use - Identify fatigue sensitive areas - Determine critical stress concentration factors for deck attachment and topside supports - Determine location of local models and fine mesh areas - Decide extent of reinforcements based on SCF from local analysisAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 44
    • Local Fatigue Analysis  Local fine mesh model created from global GeniE model by changing the mesh density in the location of the crack  Hot spot stress RAOs at the location of the crack established by spectral FE calculation  Submodelling techniques is Local fine mesh model used to transfer the results from the global FE analysis to the boarders of the local model  Fatigue damage/life calculated using Stofat Concept model with mesh densities Calculated fatigue lifeAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 45
    • Fatigue Strengthening and Screening of Extent  Soft bracket added in the local model of the stringer at crack location  Re-run sub-model analysis and fatigue calculation to check effect of strengthening proposal  Necessary extent of repair evaluated by fatigue screening of global Local model with new bracket Fatigue results  Stress concentration factor used in global screening calculated by the ratio of long term stress from local and global analysis Results from fatigue screening of global model to evaluate extent of repairAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 46
    • Analysis Overview Task Purpose Input Output Global modelling  Make global model for  Ship drawings Global FE model hydrodynamic and  Loading manual strength analysis Hydrodynamic Calculate loads for Global FE model Load files for analysis fatigue and ultimate Wave data structural analysis strength ULS analysis Calculate hull girder Global FE model Ultimate strength strength Snap shot load files results from HydroD Spectral fatigue Fatigue screening on Global FE model Calculated fatigue analysis nominal stress Frequency domain load lives Local fatigue analysis files from HydroD Spectral ULS Calculate long term Global FE model Long term stress analysis stress based on spectral Frequency domain load method files from HydroDAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 47
    • Stochastic ULS Analysis Challenge: Determine ULS design wave for areas subjected to a combination of different load effects (e.g. turret area) Typical way: Selection of one or several design waves  Uncertainties New solution with Stofat: Spectral stress analysis to determine long term stress distribution directly Wave Load Analysis - Input: Global model, wave headings and frequencies - Output: Load transfer functions (RAOs) Direct Load Stress Response Analysis Transfer - Input: FE models and load file from wave load analysis - Output: FE results file with load cases describing complex (real and imaginary) stress transfer functions (RAOs) Stress Transfer Functions Long Term ULS Load Calculation Wave scatter diagram - Input: Stress transfer functions (FE results file), wave data - Output: Calculated long term stress Long term stressAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 48
    • Stofat – Features and Benefits  Features - Stochastic fatigue calculations based on wave statistics - Supports all common wave models - Predefined and user defined S-N curves - Option for implicit mean stress correction (by static load case) - Statistical stress response calculations - Calculation of long term stress and extreme response including static loads Calculated fatigue damage by nominal stress - Graphical presentation of fatigue results and long and user defined SCF for an LNG carrier term stress directly on FE model  Benefits - Unique functionality for spectral fatigue and stochastic long term stress and extreme response calculations - Flexible – support all your needs - Transparent – all calculation steps can be documented Calculated long term stress amplitude (left) and fatigue damage (right) for the hopper knuckle in an oil tankerAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 49
    • Benefits of Sesam for Advanced Analysis  Complete system – Proven Solution - Cover your needs for strength assessment of ship and offshore structures - 40 years of DNV experience and research put into software tools  Concept modelling - Minimize modelling effort by re-use of models for various analysis - Same concept model for global & local strength analysis and for hydrodynamic analysis - Same model basis for hydrostatics and frequency and time domain hydrodynamic analysis  Same system for offshore and maritime structures - Minimizes the learning period and maximizes the utilisation of your staff  Process, file and analysis management by Sesam ExplorerAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 50
    • Safeguarding life, property and the environment www.dnv.comAdvanced Methods for Ultimate and Fatigue Strength of Floaters© Det Norske Veritas AS. All rights reserved. 51